Nutritive metal uptake in teleost fish
King's College London, School of Health and Life Sciences, Franklin-Wilkins Building, 150 Stamford Street, London, SE1 9NN, UK
* Author for correspondence (e-mail:nic.bury{at}kcl.ac.uk)
Accepted 10 October 2002
![]() |
Summary |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: divalent metal transporter (DMT1), ferroportin, epithelial sodium channel, epithelial calcium channel, Cu-ATPase, ZnT1, rainbow trout, zebrafish
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Fish are unique among the vertebrates, a consequence of having two routes
of metal acquisition, from the diet and from the water. This review will focus
on the uptake processes present in the gill and intestinal epithelium of
teleost fish for the three most abundant nutritive metals: iron, copper and
zinc. The majority of the available literature concerns metal uptake processes
in freshwater teleosts, but where appropriate examples exist, information on
seawater teleosts will be reviewed. Molecular evidence indicates that
transporters for these metals identified in yeast, plants or mammals all show
high sequence homology in key functional regions
(Rolfs and Hediger, 2001),
but to date, none of these transporters have been characterised in fish.
However, due to the evolutionary conservation of these proteins between yeast,
plants and mammals, it is envisaged that fish metal transporters will also
belong to the large iron, copper or zinc metal transporter protein families
already identified. This review will combine physiological and molecular data
to provide an overview of metal uptake mechanisms in teleost fish.
![]() |
Iron |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Teleost fish iron homeostasis
The iron content of fish is, in general, considerably lower than that of
other vertebrates (Van Dijk et al.,
1975), but the precise daily iron requirements for fish are at
present unknown. Aside from the generally lower levels of iron, it is widely
assumed that iron metabolism and function in teleost fish is similar to that
in other vertebrates (Lall,
1989
). Animals lose iron through defecation and epithelial
sloughing, and this loss is compensated for by absorption from the diet. In
fact, the regulation of iron homeostasis is governed by intestinal absorption,
as a regulated excretory mechanism is not known for iron in higher vertebrates
(Andrews, 2000
).
Branchial versus intestinal iron uptake
The role of the gill or gut in iron uptake is dominated by the chemistry of
this compound in the natural environment. Iron is one of the most abundant
elements on Earth, but in aerobic environments it is predominantly found as
ferric (hydro)oxides that are relatively insoluble at neutral pH, and thus,
ionic ferric (Fe3+) concentrations are exceedingly low
(Stumm and Morgan, 1996).
Consequently, unicellular aquatic organisms have evolved specialised transport
mechanisms to obtain sufficient iron to meet metabolic demand. This is
particularly pertinent in the expanses of the oceans where free iron may be
incredibly low (Martin and Fitzwater,
1988
). Marine bacteria and blue-green algae have been shown to
excrete extracellular chelators of iron, known as siderophores, with
exceedingly high affinities for iron (logKcond=19-23;
Wilhelm, 1995
), forming part
of a high-affinity iron-uptake process (see review by
Braun and Killmann, 1999
).
Iron's insolubility and hydrophilic nature in the aquatic environment would
suggest that it is relatively unavailable for uptake by fish from the water
via the gills, and it has been suggested that the diet meets daily
iron requirements (Watanabe et al.,
1997
).
Despite the improbability of aqueous iron acquisition by fish, a number of
reports have indicated that fish can obtain iron from the water. Early work by
Roeder and Roeder (1966) on
swordtail (Xiphorous helleri) and platyfish (X. maculatus)
showed that rearing of newly hatched fry in water of a low iron content
[<18 nmol (1 µg) l-1 bioavailable iron], at pH 7-8, and a
daily ration that contained <0.07 mg iron, resulted in retarded growth
rates. However, if the water was supplemented with >25 µmol (3.7 mg)
FeSO4 l-1, the fry showed an enhanced growth rate. This
response was not observed if the water was spiked with a similar concentration
of ferric nitrate, suggesting that the reduced ferrous form is more
bioavailable.
There is only one piece of direct evidence for iron uptake across the gill
epithelium using radiotracers. Andersen
(1997) exposed brown trout
Salmo trutta larvae (developmental stages of late-eyed eggs, yolk-sac
larvae or start-fed fry) to 6.4 or 636 µmol (0.35 or 35 mg) Fe
l-1, added as a combination of 59FeCl3 and
ferric ammonium citrate. Waterborne iron was unavailable to late-eyed eggs and
yolk-sac larvae with low bioconcentration factors (tissue-to-water
concentration), indicating that the developing embryos receive sufficient iron
from their maternal stores, the yolk. The ferroportin transcript (an
intestinal basolateral membrane iron transporter identified in zebrafish
Brachydanio rerio; see Intestinal iron uptake, below, for
more details) has been located just below the membrane (syntical layer) of the
yolk cell (Donovan et al.,
2000
), suggesting that it is responsible for iron transport from
the yolk to the embryo. In the start-fed fry, the gills begin to develop,
taking on a prevalent role in cation acquisition from the water
(Li et al., 1995
). It is the
start-fed fry that accumulate 59Fe added to the water. Mortality
was seen in the start-fed fry high-iron group, but it is unclear whether this
was due to an enhanced iron uptake from the water, or the precipitation of
iron resulting in respiratory perturbations (Peurannen et al., 1994;
Dalzell and MacFarlane,
1999
).
The nutritional value of waterborne iron compared to dietary iron has not been elucidated, but the gills may play a vital role in iron homeostasis at times of developmental need, for example, after yolk-sac absorption and prior to feeding. How fish acquire this iron, despite the constraint of unfavourable water chemistry, has not been determined and requires further investigation.
Intestinal iron uptake
The form in which iron is presented in the feed has a profound effect on
bioavailability. For example, Andersen et al.
(1997) have shown that
haem-bound iron may be more bioavailable than inorganic iron. In mammals, the
haem-iron derived from recycled proteolysis of haemoglobin from the bile may
be reabsorbed (Conrad et al.,
1999
). In mammals a considerable amount of iron is still lost
via the faeces. This deficit is overcome by acquisition of non-haem
bound iron from the diet (Andrews,
2000
).
Despite very few mechanistic studies of piscine intestinal iron uptake, it
may be possible to predict how iron is taken up from the diet. This assumption
is based on molecular evidence. cDNAs have been cloned from fish with high
sequence similarity with those genes that encode for iron membrane transport
proteins in yeast and mammalian systems (see review by
Andrews, 2000).
cDNAs whose sequences show high similarity to the ferrous iron transporters
termed solute carrier 11a1 (Slc11a1) and solute carrier 11a2 (Slc11a2),
formally known as natural resistance associated macrophage protein 1 (NRAMP1)
and NRAMP2, have been cloned in a number of fish species, including carp
Cyprinus cyprinus (Saeij et al.,
1999), rainbow trout Oncorhynchus mykiss
(Dorschner and Phillips,
1999
), zebrafish Brachydanio rerio (GenBank accession
number AF190508) and sea bass Morone saxatilis (GenBank accession
number AY008746). But, to date, no definitive proof that these sequences
encode for an iron transporter has been provided. Slc11a1 is restricted to the
cells of the myeloid lineage and is involved in resistance to pathogens
(Forbes and Gros, 2001
). The
role of Slc11a2 in intestinal iron uptake was identified in two separate
laboratories using different methods. Gunshin et al.
(1997
) used expression-cloning
techniques in African clawed frog Xenopus laevis oocytes to identify
intestinal mRNA that conferred iron uptake. Conversely, Fleming et al.
(1997
) undertook a positional
cloning approach to identify genes responsible for microcytic anaemia
(mk) in mice, a syndrome characterised by defective intestinal iron
transport. Functional studies of this gene revealed that the transporter was a
Fe2+/H+ symporter, operational in the range of pH 5.5-7.
This has subsequently been confirmed in a number of studies using the Caco2
cell line, a model cell culture system for mammalian intestinal function
(Han and Wessling-Resnick,
2002
; Zerounian and Linder,
2002
). The Fe2+/H+ symporter also transports
other divalent metals such as Mn2+, Co2+,
Cu2+, Zn2+, as well as the non-essential Cd2+
and Pb2+ (Gunshin et al.,
1997
). Due to its metal promiscuity, the transporter is referred
to as Divalent Metal Transporter 1 (DMT1), and this terminology will be used
throughout the rest of this review.
The rainbow trout DMT1 transcripts are located in most tissues
(Dorschner and Phillips, 1999)
including the transport epithelia of the gill, intestine and kidney (N. R. B.,
personal observation). In mammals, the DMT1 transcript is also found in most
tissues, but predominantly in the duodenum
(Gunshin et al., 1997
). This
transcript profile corresponds to the anatomical pattern of mammalian
intestinal iron uptake (Gunshin et al.,
1997
). Furthermore, the lumen fluids of the duodenum are slightly
below neutral pH, favouring the functioning of a proton symporter. DMT1
transcript is upregulated in iron-deficient mice and is most abundant in the
villus-crypt of the duodenal enterocytes, with transcript levels decreasing
along the crypttip axis (Trinder
et al., 1999
). The identification of a 3' UTR iron response
element (IRE) associated with the DMT1 gene gives credence to this protein
being regulated by cellular iron levels (Gunshin et al.,
1997
,
2001
).
The way in which the intestine maintains iron in a bioavailable ferrous
form is uncertain. During digestion of food the acidic environment of the
stomach releases ferric iron from the ingested matrix
(Powell et al., 1999a;
Whitehead et al., 1996
). The
ferric iron may be bound to mucin that may act to maintain metal solubility in
the small intestine (Whitehead et al.,
1996
). How Fe3+ is physically presented to the
intestinal tissue is unclear, given that it would first have to traverse the
mucus layer covering the epithelium before being taken up. The identification
of a mammalian ferric reductase present on the apical membrane of the duodenal
enterocytes provides further evidence that iron is imported into these cells
via a Fe2+ transport process
(McKie et al., 2001
). In
addition, ferric iron may also be reduced via the presence of
reducing agents in the diet, such as ascorbate
(Raja et al., 1992
).
Maintaining an environment that aids ferrous iron uptake via a
Fe2+/H+ symporter will be particularly pertinent to
marine fish whose intestinal lumen chemistry differs from that of freshwater
fish and terrestrial vertebrates (Wilson,
1999
).
The intestine of marine fish secretes large quantities of bicarbonate,
resulting in the precipitation of divalent cations
(Walsh et al., 1991). This
secretion may play a role in osmoregulation of marine teleosts (see review by
Wilson, 1999
). The presence
of HCO3- at concentrations in excess of 50 mmol
l-1 (Wilson, 1999
)
may limit the bioavailability of Fe2+, via the
precipitation of Fe(HCO3)2. In addition, a consequence
of a large HCO3- secretion is an alkaline lumen, which
would result in a proton gradient incapable of providing the driving force for
Fe2+ uptake via a proton symporter. Despite such an
adverse environment we recently showed that the European flounder
Platichthys flesus intestine preferentially absorbed ferrous iron
when compared to ferric iron (Bury et al.,
2001
). Flounder intestinal Fe2+ uptake occurred
predominantly in the posterior region, which differs from the scenario in
mammals where uptake is in the anterior region
(Trinder et al., 1999
). This
ferrous iron uptake process was enhanced in fish with low iron status (i.e.
low haematocrit), indicating a physiologically regulated process
(Bury et al., 2001
). It is not
known how marine fish maintain Fe2+ availability, but it is
hypothesised that epithelial mucus secretion may play a role in maintaining
metal solubility in fish (Glover and
Hogstrand, 2002a
), as well as a key role in modulating the
microclimate adjacent to the tissue, making this environment suitable for
metal transport (Powell et al.,
1999a
,b
).
The passage of iron from the enterocyte into the blood has recently been
discerned. It consists of an iron-regulated transporter, which was initially
identified by three independent groups, and thus has been termed IREG1
(McKie et al., 2000), MTP1
(Abboud and Haille, 2000) or ferroportin
(Donovan et al., 2000
).
Ferroportin was identified by positional cloning of the gene responsible for
hypochromic anaemia in the zebrafish mutant weissherbst
(Donovan et al., 2000
).
The study of Donovan et al.
(2000) was originally devised
to utilise the concept of `model hopping'. Here genetic information from
zebrafish was used to identify the genes in humans that are responsible for
iron deficiency or overload disorders. The success of this study provides
strong evidence that the machinery for cellular iron export is evolutionarily
conserved between fish and mammals. Ferroportin is located on the basolateral
membrane of the enterocytes (McKie et al.,
2000
), and export of iron via this transporter depends on
the presence of a membrane-associated copper containing oxidase, termed
haephestin (Vulpe et al.,
1999
). Iron is transported out of the cell as Fe2+,
haephestin oxidises Fe2+ to Fe3+, which then binds to
transferrin. Transferrin is present in fish
(Tange et al., 1997
) and in
this form the iron is transported to other tissues in the body
(McKie et al., 2000
). The
presence of a 5'-UTR IRE associated with the IREG1/ferroportin gene
demonstrates that expression may be regulated via cellular iron
concentrations (Donovan et al.,
2000
; McKie et al.,
2000
).
Branchial iron uptake
The localisation of the DMT1 transcript to the gill epithelium (N. R. B.,
personal observation) and the evidence for iron being taken up by the gill
(see Branchial versus intestinal iron uptake, above) would
suggest that the machinery for iron uptake is present. It is not clear,
however, how fish acquire iron with remarkably low concentrations in the
water. The gills do not secrete siderphore `like' proteins, but the bacteria
(Vibro sp.) present on the gills do
(Muiño et al., 2001).
It is possible that the compounds that make up branchial mucus play a key role
in sequestering waterborne iron, but this needs validation. Mucus does act as
a barrier on the gill enabling a microclimate close to the tissue to form, and
this may be sufficiently different from the surrounding water to enable apical
membrane iron uptake.
A diagrammatic representation of the generic cellular iron uptake pathways in teleost fish, which combines information for both the branchial and intestinal uptake routes described above, is given in Fig. 1.
|
![]() |
Copper |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Teleost fish copper homeostasis
Plasma copper levels are tightly regulated in the freshwater rainbow trout
(Grosell et al., 1997). As
with mammals (Harris, 2000
),
the liver is the major organ involved in copper homeostasis (Grosell et al.,
1997
,
2000
; Kamunde et al.,
2001
,
2002a
). The liver accumulates
a large proportion of the copper absorbed from the diet or water, and is the
site for synthesis of the most abundant copper-containing protein in the body,
ceruloplasmin. Ceruloplasmin is secreted into the blood and acts as a source
of copper to extrahepatic organs (Harris,
2000
). Copper may also circulate in the body bound to albumin and
other low-molecular mass proteins (Harris,
2000
). The main site for secretion of excess copper in teleost
fish is via the bile (Grosell et al.,
1997
,
2000
) and, in the case of the
European eel Anguilla anguilla, very little copper is found in the
urine (Grosell et al., 1998
).
The gills of fish have also been implicated in copper excretion
(Handy, 1996
), but this has
yet to be fully characterised. In fish, the mechanism by which excess copper
is transported across the cannicular membrane of the liver into the bilary
ducts has not been ascertained. In mammals there are three candidate secretory
pathways: (1) a Cu-ATPase, identified in patients suffering from Wilson's
disease, which is a hereditary disorder that results in elevated plasma copper
concentration, due to the inability of the body to secrete copper via
the Cu-ATPase (termed Wilson's protein or ATP7B)
(Bull et al., 1993
;
Harris, 2000
;
Puig and Thiele, 2002
); (2) a
multiorganic cation transporter (cMoat)
(Elferink and Jansen, 1994
)
and (3) lysosomal secretion (Gross et al.,
1989
).
Branchial versus intestinal copper uptake
The diet is the major source of copper for fish under optimal growth
conditions (Handy, 1996;
Kamunde et al., 2002a
,
b
). It is evident from
waterborne toxicity studies that the gill can also contribute considerably to
copper uptake (Taylor et al.,
2000
). Studies by Miller et al.
(1993
) and Kamunde et al.
(2002a
) have highlighted the
significance of waterborne copper as a potential nutritional source to rainbow
trout. In the latter study, rainbow trout fry were fed either a low [12.5 nmol
(0.8 µg) Cu g-1], normal [50 nmol (3.2 µg) Cu
g-1], or high [4390 nmol (281 µg) Cu g-1] copper
diet, in combination with either low [6.25 nmol (0.4 µg) Cu l-1]
or normal [47 nmol (3 µg) Cu g-1] waterborne copper regimes. By
utilising the copper radionuclide, 64Cu, the investigators were
able to ascertain the relative significance of dietary or waterborne routes
for newly accumulated copper in these various groups. The fish fed with a
low-copper diet and kept in low-copper water showed a marked reduction in
growth over a 50-day experimental period. The growth rate of those fish fed
the same diet but reared in normal waterborne copper levels showed no changes
compared to the other groups, and 60% of the copper accumulated by these fish
was from waterborne copper. In contrast, the dietary source of copper
accounted for 99% of the accumulated copper in those fish fed on a high-copper
diet. Miller et al. (1993
)
concluded that the diet was also the major source of copper for rainbow trout,
the copper accumulated from the waterborne route accounting for 37% of the
liver copper burden. These studies demonstrate the significance of waterborne
copper for fish health at times when the dietary source of copper may be
inadequate.
Intestinal copper uptake
In fish there is evidence that apical entry of copper into the intestinal
epithelium is a passive process, and the rate-limiting step of intestinal
copper uptake is basolateral membrane extrusion
(Clearwater et al., 2000;
Handy et al., 2000
). This
conclusion is supported by two independent observations. Clearwater et al.
(2000
) noted a Q10
ratio of <1 for copper accumulation into the epithelium of rainbow trout
intestine, and Handy et al.
(2000
) observed a
dose-dependent accumulation of copper into the intestinal mucosa of the
African walking catfish Clarias gariepinus, but no such relationship
between lumen copper concentrations and the appearance of copper into the
blood. Passive diffusion may also occur in mammals
(Crampton et al., 1965
). Other
uptake pathways may be present, such as the copper entry via an
amiloride-sensitive Na+ pathway observed in rat intestine
(Wapnir, 1991
). Caution is
required, however, when interpreting metal uptake studies in the presence of
amiloride, because this drug may form metal-complexes that are unavailable to
the organism (viz Bury and Wood,
1999
; Grosell and Wood,
2002
).
It is of interest that mammalian intestinal copper uptake primarily occurs
in the small intestine (Wapnir and Stiel,
1987), whereas in fish, copper uptake is found on the
mid/posterior region (Clearwater et al.,
2000
; Handy et al.,
2000
). The same disparity between the positioning of the iron
uptake pathways in fish and mammals has also been observed
(Bury et al., 2001
).
At present there are two proposed mechanisms of basolateral Cu transport in
fish: (1) a Cu P-type ATPase and (2) a Cu/anion symporter
(Handy et al., 2000). The
Cu-ATPase involved in mammalian intestinal copper uptake was identified from
patients with Menkes (MNK) syndrome. This genetic condition results in low
plasma copper levels due to a defect in the MNK protein (termed ATP7A)
involved in copper transport from the enterocytes to the blood
(Vulpe et al., 1993
). The MNK
cDNA shows similarities to a number of other Cu-ATPases in bacteria
(Solioz and Odermatt, 1995
;
Mandal et al., 2002
), yeast
(Riggle and Kumamoto, 2000
)
and mammals (Vulpe et al.,
1993
; Qian et al.,
1998
). The evolutionarily conserved nature of this protein would
suggest its presence in fish, and support for this comes from the recent
identification of a partial cDNA homologue to the MNK protein in the Gulf
toadfish Opsanus beta (Grosell et
al., 2001
).
Under normal conditions, copper that enters cells from the lumen is bound
to intracellular metallochaperones, resulting in intracellular `free' copper
levels as low as 10-18 mol l-1 (1 attomole;
Huffman and O'Halloran, 2000).
Metallochaperones traffic the metal to sites within the cell where it is
incorporated into cuproproteins (see reviews by
O'Halloran and Culotta, 2000
;
Puig and Thiele, 2002
). An
example is the human metallochaperone, HAH1
(Klomp et al., 1997
), which
delivers monovalent Cu [Cu(I)] to the Golgi apparatus, where it donates Cu(I)
to the MNK protein (Huffman and
O'Halloran, 2000
). Cu(I) is transported via this
Cu-ATPase into the lumen of the Golgi. Vesicles containing Cu(I) bud off the
Golgi network and are redistributed to the basolateral membrane where Cu(I) is
secreted from the cell (Petris et al.,
1996
; Francis et al.,
1999
). The MNK protein is then recycled
(Petris and Mercer, 1999
). A
similar trafficking process is probably present in fish.
In the presence of excessive (possibly toxic) dietary levels, copper is
prevented from entering the body by retention in the gut tissue bound to the
small molecular mass cysteine-rich proteins, i.e. metallothionein (MT)
(Olsen et al., 1996).
Potentially, this MT-bound copper may then be excreted into the faeces
via sloughing of the epithelial membrane
(Handy, 1996
;
Clearwater et al., 2000
).
Evidence for a Cu/anion symporter extrusion process in fish intestine comes
from experiments performed on isolated everted gut sacs from the African
walking catfish (Handy et al.,
2000). In these studies, applications of drugs designed to inhibit
P-type ATPases, vanadate (Cantely et al., 1978), and a
Cl-/HCO3- antiporter, DIDS, stimulated Cu
transport from the tissue to the serosal medium. At first the lack of
inhibition by vanadate appears puzzling, because the MNK protein is a P-type
ATPase. However, very little vanadate may have been in contact with functional
Cu-ATPases at the Golgi membrane because of the slow movement of vanadate
across the intestine where the muscle layer is still intact
(Handy et al., 2000
), and
intracellular bioreactive vanadate concentrations may be reduced due to
chelation (Edel and Sabbioni,
1993
). The stimulation of copper efflux by this drug was proposed
to be due to a reduction in the transepithelial potential in the presence of
vanadate. This resulted in a reduction in the electrochemical gradient leading
to enhanced copper movement (Handy et al.,
2000
). The stimulation by DIDS was proposed to be a consequence of
the rise in intracellular [Cl-] resulting from inhibition of the
basolateral membrane Cl-/HCO3- antiporter.
This observation, along with the fact that copper efflux reduction is coupled
to a decrease in mucosal [Cl-], suggests the presence of a
basolateral Cu/Cl- symporter. Metal ion/Cl- symporters
have been observed in other cell types
(Torrubia and Garay, 1989
;
Alda and Garay, 1990
;
Endo et al., 1998
;
Ödblom and Handy, 1999
).
This may be a novel mechanism by which copper traverses vertebrate intestine,
and further research is required to determine the precise mechanism(s) of
teleost fish intestinal basolateral membrane Cu extrusion.
Branchial copper uptake
A recent paper by Grosell and Wood
(2002) has identified two
branchial apical copper uptake processes, a sodium-sensitive and a
sodium-insensitive pathway. Both uptake pathways showed saturation kinetics
with similar low affinities for Cu (Km 7.1 nmol
l-1 for the sodium-sensitive and 9.5 nmol l-1 Cu for the
sodium-insensitive pathways). The sodium-sensitive copper uptake pathway was
characterised by an IC50 of 104 µmol l-1 sodium, but
copper uptake was not completely inhibited in the presence of 20 mmol
l-1 sodium. In addition, the sodium-sensitive pathway was inhibited
by the drugs phenamil (an amiloride analogue that is an irreversible inhibitor
with high affinity to epithelial sodium channels, ENaCs) (Kleymann and Cragoe,
1988) and bafilomycin A (a proton pump inhibitor)
(Drose and Altendorf, 1997
).
This suggests that copper is entering via a putative ENaC.
Coincidently, the non-essential metal monovalent Ag [Ag(I)], which has been
shown to mimic Cu(I) in various transport processes
(Solioz and Odermatt, 1995
;
Havelaar et al., 1999
;
Riggle and Kumamoto, 2000
;
Mandal et al., 2002
) has also
been shown to enter fish via a sodium uptake pathway
(Bury and Wood, 1999
).
However, the biophysical characteristics of known ENaCs show that they allow
only the passage of Na+ and the smaller Li+
(Garty and Palmer, 1997
).
Consequently, the proposition that apical copper or Ag(I) entry is
via a branchial Na+ channel
(Bury and Wood, 1999
),
suggests that the teleost ENaC possesses unique characteristics.
The nature of the branchial sodium-insensitive copper uptake pathway is
unclear, but the identification of high-affinity copper importers (the Ctr
family of proteins) in evolutionarily distinct organisms, such as yeast
(Dancis et al., 1994) and
mammals (Zhou and Gitschier,
1997
; Lee et al.,
2002
) may provide clues. The human Ctr1 (hCtr1) has, however, a
much lower affinity (Km 1.71-2.54 µmol l-1
Cu, based on vectorcell transfection studies;
Lee et al., 2002
), than the
uptake of Cu across the fish gill (9.5 nmol l-1 Cu;
Grosell and Wood, 2002
). The
disparity may be simply because the assays performed on fish were carried out
in ion-poor water where the copper is present almost exclusively (90%) in the
ionic form, whereas in cell culture conditions copper will be bound to various
components of the culture medium (cf.
Grosell and Wood, 2002
).
Re-examination of apical Ag(I) uptake in rainbow trout suggests that there is
also a proportion of Ag(I) uptake that is sodium-insensitive
(Bury and Wood, 1999
). A
260,000-fold excess of water Na+ could not exclusively prevent
branchial Ag(I) uptake. This may suggest that Ag(I) and Cu(I) share both the
sodium-sensitive and sodium-insensitive uptake pathways. However, a 100-fold
excess of copper is required to prevent Ag(I) from entering rainbow trout,
which suggests that if this uptake pathway is shared, it has a higher affinity
for Ag(I) compared to copper (Bury and
Hogstrand, 2002
).
Interestingly, the close relationship between copper and silver uptake is
also seen with hCtr1, where copper uptake is significantly blocked by Ag(I)
(Lee et al., 2002). This
implies that copper is entering via the hCtr1 in the monovalent form.
Copper is predominantly found as the Cu(II) valency in water. Thus, to enter
via a putative Na+-channel or Ctr carrier, Cu(II) must be
reduced to Cu(I). The presence of a copper-reductase on the gills of fish
however, has not been shown.
Branchial basolateral copper extrusion occurs via a carrier
mediated process (Grosell et al.,
1997; Campbell et al.,
1999
). Using an in situ perfused head technique, Campbell
and coworkers demonstrated second-order reaction kinetics for the movement of
copper from the gills of rainbow trout into the perfusate. This copper
transport was inhibited by vanadate, suggesting the involvement of a P-type
ATPase. The concern over whether vanadate is bioreactive within the cell (see
Intestinal copper uptake, above, for details) means it is unclear
whether this active branchial copper ATPase is resident at the Golgi (i.e.
akin to the MNK protein) or at the basolateral membrane. However, a
Ag(I)stimulated ATPase has been identified in basolateral membrane vesicles
(BLMV) prepared from the gills of rainbow trout
(Bury et al., 1999
).
Inhibition studies of BLMV Ag(I) uptake by various metals (Cu, Pb, Cd, Zn, Fe)
show that copper is the only antagonist
(Fig. 2A). This is further
verified by the inhibition of dose-dependent BLMV Ag(I) uptake by copper
(Fig. 2B). The inference from
these studies is that the fish gill basolateral membrane Ag(I) transporter
(Bury et al., 1999
) is in fact
a Cu(I) P-type ATPase, and Ag(I) may simply be mimicking Cu(I). Considering
that there is only partial contamination of fish gill BLMV with the Golgi
membrane marker thiamine pyrophosphatase
(Perry and Flik, 1988
) this
would argue against the possibility of Ag(I) mimicking Cu(I) for transport
via a MNK ATPase residing in the trans-Golgi network, and it may be
possible that a teleost MNK-`like' protein is functional at the basolateral
membrane. Fig. 3 combines
information for both the branchial and intestinal uptake routes described
above, and represents the generic epithelial copper uptake pathways in teleost
fish.
|
|
![]() |
Zinc |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The ubiquity of zinc is governed by its ability to form a wide range of
coordination geometries, allowing it to interact with a wide range of cellular
entities (Vallee and Falchuk,
1993; McCall et al.,
2000
). Furthermore zinc is redox inert, enabling the formation of
relatively stable associations within the cellular environment
(Vallee and Falchuk, 1993
).
Consequently, in contrast to copper and iron, zinc does not form free radical
ions, and in fact has antioxidant properties
(Powell, 2000
). Zinc may,
however, generate toxicity to fish by interfering with calcium homeostasis
(Spry and Wood, 1985
;
Hogstrand and Wood, 1996
).
Teleost fish zinc homeostasis
At both organismal and cellular levels zinc status is tightly controlled.
Surplus zinc is either excreted via the bile, intestinal sloughing
(Handy, 1996) or the gills
(Hardy et al., 1987
), whilst
urinary loss of zinc in fish is minimal
(Spry and Wood, 1985
). Even
though it has been proposed that excretion is the main means by which fish
control body zinc homeostasis (Shears and
Fletcher, 1983
; Hardy et al.,
1987
), they are also able to regulate zinc acquisition. The
proportion of zinc absorbed from the diet decreases as the dietary zinc load
increases (Shears and Fletcher,
1983
; Hardy et al.,
1987
; Glover and Hogstrand,
2002b
), suggesting the presence of a mechanism for regulating
uptake of dietary zinc. Branchial zinc accumulation is also regulated, and
rainbow trout exposed to elevated waterborne zinc levels show alterations in
zinc uptake mechanisms that limit the amount of zinc accumulating on the gill
(see Branchial zinc uptake; Hogstrand et al.,
1994
,
1995
,
1996
,
1998
). In a similar way,
mammals adjust zinc absorption and endogenous intestinal zinc excretion to
maintain zinc status (King et al.,
2000
).
Branchial versus intestinal zinc uptake
The major routes of zinc assimilation in fish are the gills and the gut.
The relative importance of these routes has been the focus of much research in
both marine (e.g. Pentreath,
1973; Renfro et al.,
1975
; Milner,
1982
; Willis and Sunda,
1984
) and freshwater fish
(Spry et al., 1988
). The
consensus is that the gut is the dominant pathway of absorption in the natural
environment. With decreasing dietary zinc levels, however, the gill may become
increasingly important, especially when waterborne zinc levels are elevated
(Spry et al., 1988
). Hence
the intestine appears to act as the bulk pathway for uptake, whereas the gills
may act to supplement absorption when required.
The relative zinc uptake affinities and capacities of gill and gut appear
to confirm this scenario in freshwater rainbow trout. The affinity
(Km) for branchial zinc uptake lies between 3.6 and 7.9
µmoll-1 (Spry and Wood,
1989; Hogstrand et al.,
1998
). The corresponding constant for the intestine is 309
µmoll-1 (Glover and
Hogstrand, 2002b
), indicating a lesser affinity for zinc. However
the gut appears to have a much greater capacity for zinc uptake with a maximal
rate (Jmax) of 933 nmol kg-1 h-1
(Glover and Hogstrand, 2002b
),
compared to 240-410 nmol kg-1 h-1 for the gill
(Spry and Wood, 1989
;
Hogstrand et al., 1998
).
Interestingly, the unicellular organism yeast has been demonstrated to have
independently regulated high- and low-affinity zinc transporters (Zhao and
Eide,
1996a
,b
),
a cellular equivalent of the organ-level patterns observed in fish.
Intestinal zinc uptake
In general, the site of gastrointestinal zinc absorption appears conserved
between fish and mammals. Pentreath
(1976) and Shears and
Fletcher (1983
) determined
that the anterior intestine was the most important region for zinc absorption
in winter flounder Pseudopleuronectes americanus and plaice
Pleuronectes platessa, respectively. This is consistent with the
scenario in human intestine, which exhibits a jejunal-biased absorptive
pattern (Lee et al.,
1989
).
Shears and Fletcher (1983)
described two components of uptake in winter flounder. One saturable component
dominated at low zinc levels, with a diffusive pathway more dominant at higher
zinc concentrations. This mimics the mechanism of zinc uptake in mammals
(Lönnerdal, 1989
).
However, in freshwater rainbow trout, only a saturable component of uptake was
discerned using an in vivo perfusion technique
(Glover and Hogstrand, 2002b
).
It was proposed that any potential diffusive uptake pathway was blocked as a
consequence of increased epithelial mucus secretion in intestine perfused with
high zinc concentrations (Glover and
Hogstrand, 2002b
). In contrast, at low zinc levels, mucus may in
fact enhance zinc uptake by trapping zinc close to the epithelial surface, and
potentially increasing bioavailability
(Powell et al., 1999a
). But,
at environmentally relevant intestinal zinc concentrations (i.e. up to approx.
50 µmoll-1; Turner and
Olsen, 2000
; Farag et al.,
2000
), any diffusive component of zinc uptake is unlikely to be of
importance for nutritive zinc uptake.
The apical entry steps in fish intestinal zinc absorption have not been
elucidated. In recent years the molecular characterisation of zinc metal
importers from evolutionary diverse organisms (yeast, plants and mammals) has
been achieved (Zhao and Eide,
1996a,b
;
Grotz et al., 1998
;
Gaither and Eide, 2001a
), and
these proteins form the large ZIP family of transporters (derived from Zrt,
Irt-like proteins; Lioumi et al.,
1999
). It is highly likely that teleosts possess ZIP homologues.
An alternative candidate for intestinal apical zinc absorption has recently
been identified by Cragg et al.
(2002
) termed hZTL, and is
related to the zinc transporter (ZnT-1) involved in zinc export described
below.
A number of small molecular mass ligands in the enterocyte cytoplasm may
modulate piscine zinc uptake. A role for metallothionein (MT) in cellular zinc
uptake and metabolism has been proposed (Shears and Fletcher,
1979,
1983
,
1984
), and evidence from
mammalian systems suggests that MT functions in nutritive uptake in
zinc-deficient animals (Hoadley et al.,
1988
; Coyle et al.,
2000
). In addition, glutathione also has an important role in zinc
uptake (Jiang et al., 1998
),
and the presence of zinc bound to low molecular mass ligands following dietary
zinc exposure has been noted in freshwater rainbow trout
(Spry et al., 1988
). The
sequestering molecules such as MT and glutathione act to maintain
intracellular `free' zinc (Zn2+) concentrations in the femtomolar
range (Outten and O'Halloran,
2001
).
Recently, Glover et al.
(2002) have showed that
intestinal basolateral transfer of zinc is via a saturable pathway in
the Gulf toadfish. This contrasts to the finding of Shears and Fletcher
(1983
) that demonstrated a
passive movement of zinc across the winter flounder intestine. A facilitated
zinc export process in fish is supported by the cloning of a full-length cDNA
in the puffer fish Fugu ribrepes with amino acid sequence
similarities to the Zinc transporter-1 (ZnT-1) protein of mammals
(Balesaria and Hogstrand,
2001
). ZnT-1 is localised to the basolateral membrane of
enterocytes and is involved in the export of zinc from the intestine into the
blood stream (Cousins and McMahon,
2000
). Whether the piscine ZnT1 is involved in the regulation of
zinc uptake awaits verification.
Aquaculture studies have tended to focus more on endpoints of zinc uptake
(i.e. growth) rather than on the mechanism of uptake. These studies, however,
have provided interesting information from a mechanistic perspective. In
particular, the chemical form of zinc added to diets has been the focus of a
number of investigations. Some authors describe enhanced body zinc status with
amino acid chelates (Hardy et al.,
1987; Paripatananont and
Lovell, 1995
; Apines et al.,
2001
), whereas others report no effect
(Li and Robinson, 1996
). Amino
acids with high affinity for zinc enhance bioavailability, and physiological
studies have shown that histidine and cysteine may increase zinc acquisition,
probably via specific uptake pathways related to the formation of bis
complexes with zinc [Zn(His)2, Zn(Cys)22-;
Glover and Hogstrand, 2002a
].
In addition, the chelation of zinc by amino acids may, by altering the
distribution of internal zinc, have nutritional benefits
(Glover and Hogstrand,
2002a
).
Branchial zinc uptake
The mechanism of freshwater branchial zinc uptake is now well understood.
Many investigations have shown that hardness (i.e. water [Ca2+])
offers a protective effect against waterborne zinc toxicity (e.g.
Eisler, 1993). The
relationship between calcium and zinc homeostasis is also apparent at the
branchial apical uptake step. Numerous studies have shown that calcium
inhibits branchial zinc uptake (Spry and
Wood, 1989
; Bentley,
1992
; Hogstrand et al.,
1996
), and correspondingly, that zinc competes with calcium
uptake. Injection of stanniocalcin, a hypocalcaemic hormone in fish
(Wagner et al., 1986
),
downregulates both calcium (Flik et al.,
1993
) and zinc uptake from the water in rainbow trout
(Hogstrand et al., 1996
). In
addition, lanthanum, a calcium channel blocker, also inhibits both calcium
(Perry and Flik, 1988
) and
zinc uptake (Hogstrand et al.,
1996
). Calcium has also been observed to compete for zinc uptake
via a channel present in the brush border membrane of the pig
intestine (Bertolo et al.,
2001
). It would thus appear that zinc uptake occurs via a
lanthanum-sensitive Ca2+-channel, which has been located in the
branchial chloride cells (Perry and Flik,
1988
). It is of interest that another essential metal, cobalt, has
also been shown to enter carp Cyprinus carpio gills via a
Ca2+-channel (Comhaire et al.,
1994
), suggesting that this channel may discriminate between
various divalent cations. It is likely, however, that alternative zinc uptake
pathways across the apical surface exist, and interestingly, the affinity
constant of in vitro zinc transport by ZIPs (see Intestinal zinc
uptake) (apparent Km=3-3.5 µmol l-1;
Gaither and Eide, 2001b
)
corresponds closely to that determined for teleost freshwater fish gill uptake
(3.7 µmol l-1; Hogstrand et
al., 1995
).
The generic cellular zinc uptake pathways in epithelial cells of teleost fish is given in Fig. 4.
|
![]() |
Conclusions |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The presence of teleost fish homologues of metal transporters (DMT1, ferroportin, Cu-ATPase and ZnT-1) suggests an evolutionarily conserved mechanism of nutrient metal uptake. The development and utilisation of molecular techniques that are currently being applied in mammalian systems should facilitate functional characterisation of the uptake process in fish. In addition, novel piscine uptake processes (i.e. Cu+/2+/Cl- symporter; a putative Cu/Na epithelial sodium channel) may provide insights into alternative transport mechanisms of these metals in other vertebrates.
![]() |
Acknowledgments |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Abboud, S. and Haile, D. J. (2000). A novel
mammalian iron-regulated protein involved in intracellular iron metabolism.
J. Biol. Chem. 275, 19906
-19912.
Alda, J. O. and Garay, R. (1990). Chloride (or
bicarbonate)-dependent copper uptake through the anion exchanger in human red
blood cells. Am. J. Physiol.
259, C570
-576.
Andersen, O. (1997). Accumulation of waterborne iron and expression of ferritin and transferrin in early developmental stages of brown trout (Salmo trutta). Fish Physiol. Biochem. 16, 223 -231.[CrossRef]
Andersen, F., Lorentzer, M., Waagbo, R. and Maage, A. (1997). Bioavailability and interactions with other micronutrients of three dietary iron sources in Atlantic salmon Salmo salar L. smolts. Aquacult. Nut. 3, 239-246.
Andrews, N. C. (2000). Iron homeostasis: insights from genetics and animal models. Nat. Rev. Genet. 1, 208 -217.[CrossRef][Medline]
Apines, M. J., Satoh, S., Kiron, V., Watanabe, T., Nasu, N. and Fujita, S. (2001). Bioavailability of amino acids chelated and glass embedded zinc to rainbow trout, Oncorhynchus mykiss, fingerlings. Aquacult. Nutr. 7, 221-228.[CrossRef]
Balesaria, S. and Hogstrand, C. (2001). Molecular approaches to investigate cellular responses to zinc in fish . Society for Experimental Biology, Annual Meeting, University of Kent, UK, England, 2-6 April 2001.
Bentley, P. J. (1992). Influx of zinc by channel catfish (Ictalurus punctatus): uptake from external environmental solutions. Comp. Biochem. Physiol. 101C, 215 -217.
Bertolo, R. F., Bettger, W. J. and Atkinson, S. A. (2001). Calcium competes with zinc for a channel mechanism on the brush border membrane of piglet intestine. J. Nutr. Biochem. 12, 66 -72.[CrossRef][Medline]
Braun, V. and Killmann, H. (1999). Bacterial solutions to the iorn supply problem. Trends. Biochem. Sci. 24, 104 -109.[CrossRef][Medline]
Bull, P. C., Thomas, G. R., Rommens, J. M., Forbes, J. R. and Cox, D. W. (1993). The Wilson disease gene is a putative copper transporting P-Type ATPase similar to the Menkes gene. Nat. Genet. 6, 214 -214.
Bury, N. R. and Hogstrand, C. (2002). Influence of chloride and metals on silver bioavailablity to Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) yolk-sac fry. Environ. Sci. Technol. 36, 2884 -2888.[CrossRef][Medline]
Bury, N. R. and Wood, C. M. (1999). Mechanism
of branchial apical silver uptake by rainbow trout is via the
proton-coupled Na+ channel. Am. J. Physiol.
277, R1385
-R1391.
Bury, N. R., Grosell, M., Grover, A. K. and Wood, C. M. (1999). ATP-dependent silver transport across the basolateral membrane of rainbow trout gills. Toxicol. Appl. Pharmacol. 159, 1 -8.[CrossRef][Medline]
Bury, N. R., Grosell, M., Wood, C. M., Hogstrand, C., Wilson, R.
W., Rankin, J. C., Busk, M., Lecklin, T. and Jensen, F. B.
(2001). Intestinal iron uptake in the European flounder
(Platichthys flesus). J. Exp. Biol.
204, 3779
-3787.
Campbell, H. A., Handy, R. D. and Nimmo, M. (1999). Copper uptake kinetics across the gills of rainbow trout (Oncorhynchus mykiss) measured using an improved isolated perfused head technique. Aquat. Toxicol. 46, 177 -190.[CrossRef]
Cantley, L. C., Cantley, L. G. and Josephson, L. (1978). A characterization of vanadate interactions with the (Na.K)-ATPase. J. Biol. Chem. 253, 7361 -7368.[Medline]
Clearwater, S. J., Baskin, S. J., Wood, C. M. and McDonald, D.
G. (2000). Gastrointestinal uptake and distribution of copper
in rainbow trout. J. Exp. Biol.
203, 2455
-2466.
Clearwater, S. J., Farag, A. M., and Meyer, J. S. (2002). Bioavailability and toxicity of dietborne copper and zinc to fish. Comp. Biochem. Physiol. 132C, 269 -313.
Comhaire, S., Blust, R., Van Ginneken, L. and Van der Borghtm, O. L. J. (1994). Cobalt uptake across the gills of the common carp, Cyprinus carpio, as a function of calcium-concentration in the water of acclimation and exposure. Comp. Biochem. Physiol. 109C, 63 -76.
Conrad, M. E., Umbreit, J. N. and Moore, E. G. (1999). Iron absorption and transport. Am. J. Med. Sci. 318, 213 -229.[Medline]
Cousins, R. J. and McMahon, R. J. (2000).
Integrative aspects of zinc transporters. J. Nutr.
130, 1384S
-1387S.
Coyle, P., Philcox, J. C. and Rofe, A. M.
(2000). Zn-depleted mice absorb more of an intragastric Zn
solution by a metallothionein-enhanced process than do Zn-replete mice.
J. Nutr. 130, 835
-842.
Cragg, R. A., Christie, G. R., Phillips, S. R., Russi, R. M., Kury, S., Mathers, J. C., Taylor, P. M. and Ford, D. (2002). A novel zinc-regulated human zinc transporter, hZTL1, is localised to the enterocyte apical membrane. J. Biol. Chem. (in press).
Crampton, R. F., Matthews, D. M. and Poisner, R. (1965). Observations on the mechanism of absorption of copper by the small intestine. J. Physiol., Lond. 178, 111 -126.
Dalzell, D. J. B. and MacFarlane, N. A. A. (1999). The toxicity of iron to brown trout and effects on the gills: a comparison of two grades of iron sulphate. J. Fish Biol. 55, 301 -315.[CrossRef]
Dancis, A., Haile, D., Yuan, D. S. and Klausner, R. D.
(1994). The Saccharomyces-cerevisiae copper transport
protein (Ctr1p) biochemical, characterization, regulation by copper,
and physiological-role in copper uptake. J. Biol.
Chem. 269, 25660
-25667.
Donovan, A., Brownlie, A., Zhou, Y., Shepard, J., Pratt, S. J., Moynihan, J., Paw, B. H., Drejer, A., Barut., B., Zapata, A., Law, T. C., Brugnara, C., Lux, S. E., Pinkus, J. L., Kinsley, P. D., Palis, J., Fleming, M. D., Andrews, N. C. and Zon, L. I. (2000). Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter. Nature 403, 776 -781.[CrossRef][Medline]
Dorschner, M. O. and Phillips, R. B. (1999). Comparative analysis of two Nramp loci from rainbow trout. DNA Cell. Biol. 18, 573 -583.[CrossRef][Medline]
Drose, S. and Altendorf, K. (1997).
Bafilomycins and concanamycins as inhibitors of V-ATPases and P-ATPases.
J. Exp. Biol. 200, 1
-8.
Edel, J. and Sabbioni, E. (1993). Accumulation, distribution and form of vanadate in the tissues and organelles of the mussel Mytilus edulis and the goldfish Carassius auratus. Sci. Total. Environ. 133, 139 -151.
Eisler, R. (1993). Zinc hazards to fish, wildlife, and invertebrate: a synoptic review. In Biological Report 10. Washington, DC: US Department of Interior, Fish and Wildlife Service.
Elferink, R. P. J. O. and Jansen, P. L. M. (1994). The role of the cannicular multispecific organic anion trnasporter in the disposal of endobiotics and xenobiotics. Pharmacol. Ther. 64, 77 -97.[CrossRef][Medline]
Endo, T., Kimura, O. and Sakata, M. (1998). Cadmium uptake from apical membrane of LLC-PK1 cells via inorganic anion exchanger. Pharmacol. Toxicol. 82, 230 -235.[Medline]
Farag, A. M., Suedkamp, M. J., Meyer, J. S., Barrows, R. and Woodward, D. F. (2000). Distribution of metals during digestion by cutthroat trout fed benthic invertebrates contaminated in the Clark Fork River, Montana and the Coeur d'Alene River, Idaho, USA, and fed artificially contaminated Artemia. J. Fish Biol. 56, 173 -190.[CrossRef]
Fleming, M. D., Trenor, C. C., Su, M. A., Foernzler, D., Beier, D. R., Dietrich, W. F. and Andrews, N. C. (1997). Microcytic anaemia mice have a mutation in Nramp2, a candidate iron transporter gene. Nat. Genet. 16, 383 -386.[Medline]
Flik, G., Van der Velden, J. A., Dechering, K. J., Verbost, P. M., Schoenmakers, T. J. M., Kolar, Z. I. and Wendelaar Bonga, S. E. (1993). Ca2+ and Mg2+ transport in gills and gut of tilapia Oreochromis mossambicus: a review. J. Exp. Zool. 265, 356 -365.
Forbes, J. R. and Gros, P. (2001). Divalent-metal transport by NRAMP proteins at the interface of host-pathogen interactions. Trends Microbiol. 9, 397-403.[CrossRef][Medline]
Francis, M. J., Jones, E. E., Levy, E. R., Martin, R. L.,
Ponnambalam, S., and Monaco, A. P. (1999). Identification of
a di-leucine motif within the C terminus domain of the Menkes disease protein
that mediates endocytosis from the plasma membrane. J. Cell.
Sci. 112, 1721
-1732.
Gaither, L. A. and Eide, D. (2001a). Eukaryotic zinc transporters and their regulation. Biometals 14, 251 -270.[CrossRef][Medline]
Gaither, L. A. and Eide, D. (2001b). The human
ZIP1 transporter mediates zinc uptake in human K562 erythroleukaemia cells.
J. Biol. Chem. 276, 22258
-22264.
Garty, H. and Palmer, L. G. (1997). Epithelial
sodium channels: Function, structure, and regulation. Physiol.
Rev. 77, 359
-396.
Gatlin, D. M. and Wilson, R. P. (1983). Dietary zinc requirement of fingerling channel catfish. J. Nutr. 113, 630 -635.[Medline]
Glover, C. N. and Hogstrand, C. (2002a). In
vivo characterisation of intestinal zinc uptake in freshwater rainbow
trout. J. Exp. Biol.
205, 141
-150.
Glover, C. N. and Hogstrand, C. (2002b). Amino
acid modulation of in vivo intestinal zinc absorption in freshwater
rainbow trout. J. Exp. Biol.
205, 151
-158.
Glover, C. N., Balesaria, S., Mayer, G. D., Thompson, E. D., Walsh, P. J. and Hogstrand, C. (2002). Intestinal zinc uptake in the marine teleosts, squirrelfish (Holocentrus adscensionis) and Gulf toadfish (Opsanus beta). Physiol. Biochem. Zool. (in press).
Grosell, M., Hansen, H. J. and Rosenkilde, P. (1998). Cu uptake, metabolism and elimination in fed and starved European eels (Anguilla anguilla) during adaptation to waterborne Cu exposure. Comp. Biochem. Physiol. C 120, 295 -305.[Medline]
Grosell, M. and Wood, C. M. (2002). Copper
uptake across rainbow trout gills: mechanisms of apical entry. J.
Exp. Biol. 205, 1179
-1188.
Grosell, M. H., Hogstrand, C. and Wood, C. M. (1997). Cu uptake and turnover in both Cu-acclimated and non-acclimated rainbow trout (Oncorhynchus mykiss). Aquat. Toxicol. 38, 257 -276.[CrossRef]
Grosell, M., Boetius, I., Hansen, H. J. M. and Rosenkilde, P. (1996). Influence of preexposure to sublethal levels of copper on Cu-64 uptake and distribution among tissues of the European eel (Anguilla anguilla). Comp. Biochem. Physiol. 114C, 229 -235.
Grosell, M., Kamunde, C., Wood, C. M. and Walsh, P. J. (2001). Copper transport across fish gills. Society of Experimental Biology, Annual Meeting, University of Kent, Canterbury, UK, 2-6 April 2001.
Grosell, M., O'Donnell, M. J. and Wood, C. M.
(2000). Hepatic versus gallbladder bile composition: in vivo
transport physiology of the gallbladder in rainbow trout. Am. J.
Physiol. Reg. I. 278, R1674
-R1684.
Gross, J. B., Jr, Myers, B. M., Kost, L. J., Kuntz, S. M. and La Russo, N. F. (1989). Bilary copper excretion by hepatocyte lysosomes in the rat major excretory pathway in experimental copper overload. J. Clin. Invest. 83, 30-39.[Medline]
Grotz, N., Fox, T., Connolly, E., Park, W., Guerinot, M. L. and
Eide, D. (1998). Identification of a family of zinc
transporter genes from Arabidopsis that respond to zinc deficiency.
Proc. Natl. Acad. Sci. USA
95, 7220
-7224.
Gunshin, H., Allerson, C. R., Polycarpou-Schwarz, M., Rofts, A., Rogers, J. T., Kishi, F., Hentze, W., Rouault, T. A., Andrews, N. C. and Hediger, M. A. (2001). Iron-dependent regulation of the divalent metal ion transporter. FEBS Lett. 509, 309 -316.[CrossRef][Medline]
Gunshin, H., MacKenzie, B., Berger, U. V., Gunshin, Y., Romero, M. F., Boron, W. F., Nussberger, S., Gollan, J. L. and Hediger, M. A. (1997). Cloning and characterization of a mammalian proton-coupled metal-ion transporter. Nature 388, 482 -488.[CrossRef][Medline]
Han, O. H. and Wessling-Resnick, M. (2002). Copper supplementation stimulates apical iron uptake and transepithelial transport in intestinal cells. FASEB J. 16, A375 -A375.
Handy, R. D. (1996). Dietary exposure to toxic metals in fish. In Toxicology of Aquatic Pollution (ed. E. W. Taylor), pp. 29-60. Cambridge: Cambridge University Press.
Handy, R. D., Musonda, M. M., Phillips, C. and Falla, S. J.
(2000). Mechanisms of gastrointestinal copper absorption in the
African walking catfish: Copper dose-effects and a novel anion-dependent
pathway in the intestine. J. Exp. Biol.
203, 2365
-2377.
Hardy, R. W., Sullivan, C. V. and Koziol, A. M. (1987). Absorption, body distribution, and excretion of dietary zinc by rainbow trout (Salmo gairdneri). Fish Physiol. Biochem. 3, 133 -143.
Harris, E. D. (2000). Cellular copper transport and metabolism. Annu. Rev. Nutr. 20, 291 -310.[CrossRef][Medline]
Havelaar, A. C., Beerens, C. E. M. T., Mancini, G. M. S. and Verheijen, F. W. (1999). Transport of organic anions by the lysosomal sialic acid transporter: a functional approach towards the gene for sialic acid storage disease. FEBS Lett. 446, 65-68.[CrossRef][Medline]
Hoadley, J. E., Leinart, A. S. and Cousins, R. J. (1988). Relationship of 65Zn absorption kinetics to intestinal metallothionein in rats: Effects of zinc depletion and fasting. J. Nutr. 118, 497 -502.[Medline]
Hogstrand, C. and Wood, C. M. (1996). The physiology and toxicology of zinc in fish. In Toxicology of Aquatic Pollution (ed. E. W. Taylor), pp. 61-84. Cambridge: Cambridge University Press.
Hogstrand, C., Reid, S. D. and Wood, C. M.
(1995). Ca2+ versus Zn2+ transport
in the gills of freshwater rainbow trout and the cost of adaptation to
waterborne Zn2+. J. Exp. Biol.
198, 337
-348.
Hogstrand, C., Verbost, P. M., Wendelaar Bonga, S. E. and Wood,
C. M. (1996). Mechanisms of zinc uptake in gills of
freshwater rainbow trout: interplay with calcium transport. Am. J.
Physiol. 270, R1141
-R1147.
Hogstrand, C., Webb, N. and Wood, C. M. (1998).
Covariation in regulation of affinity for branchial zinc and calcium uptake in
freshwater rainbow trout. J. Exp. Biol.
201, 1809
-1815.
Hogstrand, C., Wilson, R. W., Polgar, D. and Wood, C. M.
(1994). Effects of zinc on the kinetics of branchial
calcium-uptake in fresh water rainbow trout during adaptation to waterborne
zinc. J. Exp. Biol. 186, 55
-73.
Huffman, D. L. and O'Halloran, T. V. (2000).
Energetics of copper trafficking between the Atx1 metallochaperone and the
intracellular copper transporter, Ccc2. J. Biol. Chem.
275, 18611
-18614.
Jiang, L. J., Maret, W. and Vallee, B. L.
(1998). The glutathione redox couple modulates zinc transfer from
metallothionein to zinc-depleted sorbitol dehydrogenase. Proc.
Natl. Acad. Sci. USA 95, 3483
-3488.
Kamunde, C. N., Clayton, C. and Wood, C. M. (2002a). Waterborne vs. dietary copper uptake in rainbow trout and the effects of previous waterborne copper exposure. Am. J. Physiol. 283, R69 -R78.
Kamunde, C. N., Grosell, M., Lott, J. N. A. and Wood, C. M. (2001). Copper metabolism and gut morphology in rainbow trout (Oncorhynchus mykiss) during chronic sublethal dietary copper exposure. Can. J. Fish. Aquat. Sci. 58, 293 -305.[CrossRef]
Kamunde, C., Grosell, M., Higgs, D. and Wood, C. M.
(2002b). Copper metabolism in actively growing rainbow, trout
(Oncorhynchus mykiss): interactions between dietary and waterborne
copper uptake. J. Exp. Biol.
205, 279
-290.
King, J. C., Shames, D. M. and Woodhouse, L. R.
(2000). Zinc homeostasis in humans. J.
Nutr. 130, 1360S
-1366S.
Kleyman, T. R. and Cragoe, E. J., Jr (1988). Amiloride and its analogs as tools in the study of ion transport. J. Membr. Biol. 105, 1 -21.[Medline]
Klomp, L. W., Lin, S. J., Yuan, D. S., Klausner, R. D., Culotta,
V. C. and Gitlin, J. D. (1997). Identification and functional
expression of HAH1, a novel human gene involved in copper homeostasis.
J. Biol. Chem. 272, 9221
-9226.
Lall, S. P. (1989). The minerals. In Fish Nutrition (ed. J. E. Halver), pp. 219 -257. Academic Press, New York.
Lanno, R. P., Slinger, S. J. and Hilton, J. W. (1985). Maximum tolerable and toxicity levels of dietary copper in rainbow-trout (Salmo gairdneri Richardson). Aquaculture 49, 257 -268.[CrossRef]
Laurén, D. J. and McDonald, D. G. (1985). Effects of copper on branchial ionoregulation in the rainbow trout, Salmo gairdneri Richardson modulation by water hardness and pH. J. Comp. Physiol. B 155, 635 -644.
Lee, H. H., Prasad, A. S., Brewer, G. J. and Owyang, C.
(1989). Zinc absorption in human small intestine. Am.
J. Physiol. 256, G87
-G91.
Lee, J., Pena, M. M. O., Nose, Y. and Thiele, D. J.
(2002). Biochemical characterization of the human copper
transporter Ctr1. J. Biol. Chem.
277, 4380
-4387.
Li, J., Eygensteyn, J., Lock, R. A. C., Verbost, P. M.,
Vanderheijden, A. J. H., Bonga, S. E. W. and Flik, G. (1995).
Branchial chloride cells in larvae and juveniles of fresh-water tilapia
Oreochromis mossambicus. J. Exp. Biol.
198, 2177
-2184.
Li, M. H. and Robinson, E. H. (1996). Comparison of chelated zinc and zinc sulfate as zinc sources for growth and bone mineralization of channel catfish (Ictalurus punctatus) fed practical diets. Aquaculture 146, 237 -243.[CrossRef]
Lioumi, M., Ferguson, C. A., Sharpe, P. T., Freeman, T., Marenholz, I., Mischke, D., Heizmann, C. and Ragoussis, J. (1999). Isolation and characterization of human and mouse ZIRTL, a member of the IRT1 family of transporters, mapping within the epidermal differentiation complex. Genomics 62, 272 -280.[CrossRef][Medline]
Lönnerdal, B. (1989). Intestinal absorption of zinc. In Zinc in Human Biology (ed. C. F. Mills), pp. 33-55. Berlin: Springer-Verlag.
Mandal, A. K., Cheung, W. D. and Arguello, J. M.
(2002). Characterization of a thermophilic P-type
Ag+/Cu+-ATPase from the extremophile Archaeoglobus
fulgidus. J. Biol. Chem.
277, 7201
-7208.
Martin, J. H. and Fitzwater, S. E. (1988). Iron-deficiency limits phytoplankton growth in the northeast pacific subarctic. Nature 331, 341 -343.[CrossRef]
McCall, K. A., Huang, C.-C. and Fierke, C.
(2000). Function and mechanism of zinc metalloenzymes.
J. Nutr. 130, 1437S
-1446S.
McKie, A. T., Barrow, D., Latunde-Dada, G. O., Rolfs, A., Sager,
G., Mudaly, E., Mudaly, M., Richardson, C., Barlow, D., Bomford, A., Peters,
T. J., Raja, K. B., Shirali, S., Hediger, M. A., Farzaneh, F. and Simpson, R.
J. (2001). An iron regulated ferric reductase associated with
the absorption of dietary iron. Science
291, 1755
-1759.
McKie, A. T., Marciani, P., Rolfs, A., Brennan, K., Wehr, K., Barrow, D., Miret, S., Bomford, A., Peters, T. J., Farzaneh, F., Hediger, M. A., Hentze, M. W. and Simpson, R. J. (2000). A novel duodenal iron-regulated transporter, IREG1, implicated in the basolateral transfer of iron to the circulation. Mol. Cell 5, 299-309.[Medline]
Miller, P. A., Lanno, R. P., McMaster, M. E. and Dixon, D. G. (1993). Relative contribution of dietary and waterborne copper to tissue copper burdens and waterborne copper uptake in rainbow trout (Oncorhynchus mykiss). Can. J. Fish. Aquat. Sci. 50, 1683 .
Milner, N. J. (1982). The accumulation of zinc by O-group plaice, Pleuronectes platessa (L.), from high concentrations in seawater and food. J. Fish. Biol. 21, 325 -336.
Milton, D. A. and Chenery, S. R. (2001). Sources and uptake of trace metals in otoliths of juvenile barramundi (Lates calcarifer). J. Exp. Mar. Biol. Ecol. 264, 47-65.[CrossRef]
Muiño, L., Lemos, M. L. and Santos, Y. (2001). Presence of high-affinity iron uptake systems in fish-isolated and environmental strains of Vibrio anguillarum serotype O3. FEMS Microbiol. Lett. 202, 79-83.[CrossRef][Medline]
O'Halloran, T. V. and Culotta, V. C. (2000).
Metallochaperones, an intracellular shuttle service for metal ions.
J. Biol. Chem. 275, 25057
-25060.
Ödblom, M. P. and Handy, R. D. (1999). A novel DIDS-sensitive anion-dependant Mg2+ efflux pathway in rat ventricular myocytes. Biochem. Biophys. Res. Commun. 264, 334 -337.[CrossRef][Medline]
Ogino, C. and Yang, G.-Y. (1978). Requirement of rainbow trout for dietary zinc. Bull. Jpn Soc. Sci. Fish. 44, 1015 -1018.
Olsen, P.-E. (1996). Metallothioneins in fish: induction and use in environmental monitoring. In Toxicology of Aquatic Pollution (ed. E. W. Taylor), pp. 187 -204. Cambridge University Press, Cambridge, UK.
Outten, C. E. and O'Halloran, T. V. (2001).
Femtomolar sensitivity of metalloregulatory proteins controlling zinc
homeostasis. Science
292, 2488
-2492.
Paripatananont, T. and Lovell, R. T. (1995). Chelated zinc reduces the dietary requirement of channel catfish, Ictalurus punctatus. Aquaculture 133, 73-82.[CrossRef]
Pentreath, R. J. (1973). The accumulation and retention of 65Zn and 54Mn by the plaice, Pleuronectes platessa L. J. Exp. Mar. Biol. Ecol. 12, 1 -18.
Pentreath, R. J. (1976). Some further studies on the accumulation and retention of 65Zn and 54Mn by the plaice, Pleuronectes platessa L. J. Exp. Mar. Biol. Ecol. 21, 179 -189.
Perry, S. F. and Flik, G. (1988).
Characterization of branchial trans-epithelial calcium fluxes in fresh-water
trout, Salmo gairdneri. Am. J. Physiol.
254, R491
-R498.
Petris, M. J. and Mercer, J. F. B. (1999). The
Menkes protein (ATP7A; MNK) cycles via the plasma membrane both in
basal and elevated extracellular copper using a C-terminal di-leucine
endocytic signal. Hum. Mol. Genet.
8, 2107
-2115.
Petris, M. J., Mercer, J. F. B., Culvenor, J. G., Lockhart, P., Gleeson, P. A. and Camakaris, J. (1996). Ligand-regulated transport of the Menkes copper P-type ATPase efflux pump from the Golgi apparatus to the plasma membrane: A novel mechanism of regulated trafficking. EMBO J. 15, 6084 -6095.[Abstract]
Peuranen, S., Vuorinen, P. J., Vuorinen, M. and Hollender, A. (1994). The effects of iron, humic acids and low pH on the gills and physiology of brown trout, Salmo trutta. Annales Zoologica Fennici 31, 389 -396.
Powell, J. J., Jugdaohsingh, R. and Thompson, R. P. H. (1999a). The regulation of mineral absorption in the gastrointestinal tract. Proc. Nutr. Soc. 58, 147 -153.[Medline]
Powell, J. J., Whitehead, M. W., Ainley, C. C., Kendall, M. D., Nicholson, J. K. and Thompson, R. P. H. (1999b). Dietary minerals in the gastrointestinal tract: hydroxypolymerisation of aluminum is regulated by luminal mucins. J. Inorg. Biochem. 75, 167 -180.[CrossRef][Medline]
Powell, S. R. (2000). The antioxidant
properties of zinc. J. Nutr.
130, 1447S
-1454S.
Puig, S. and Thiele, D. J. (2002). Molecular mechanisms of copper uptake and distribution. Curr. Opin. Chem. Biol. 6, 171 -180.[CrossRef][Medline]
Qian, Y., Tiffany-Castiglioni, E. and Harris, E. D. (1998). Sequence of a Menkes-type Cu-transporting ATPase from rat C6 glioma cells: comparison of the rat protein with other mammalian Cu-transporting ATPases. Mol. Cell. Biochem. 181, 49-61.[CrossRef][Medline]
Rainbow, P. S. (1995). Physiology, physicochemistry and metal uptake a crustacean perspective. Mar. Pollution Bull. 31, 55 -59.[CrossRef]
Raja, K. B., Simpson, R. J. and Peters, T. J. (1992). Investigation of a role for reduction in ferric iron uptake by mouse duodenum. Biochim. Biophys. Acta 1135, 141 -146.[CrossRef][Medline]
Renfro, W. C., Fowler, S. M., Heyraud, M. and La Rosa, J. (1975). Relative importance of food and water in long-term zinc65 accumulation by marine biota. J. Fish. Res. Bd Can. 32, 1339 -1345.
Riggle, P. J. and Kumamoto, C. A. (2000). Role
of a Candida albicans P1-type ATPase in resistance to copper and
silver ion toxicity. J. Bacteriol.
182, 4899
-4905.
Roeder, M. and Roeder, R. H. (1966). Effect of iron on the growth rate of fishes. J. Nutr. 90, 86-90.[Medline]
Rolfs, A. and Hediger, M. A. (2001). Intestinal metal ion absorption: an update. Curr. Opin. Gastroenterol. 17, 177 -183.[CrossRef][Medline]
Saeij, J. P. J., Wiegertjes, G. F. and Stet, R. J. M. (1999). Identification and characterization of a fish natural resistance-associated macrophage protein (NRAMP) cDNA. Immunogenetics 50, 60 -66.[CrossRef][Medline]
Shears, M. A. and Fletcher, G. L. (1979). The binding of zinc to soluble proteins of intestinal mucosa in winter flounder (Pseudoplueronectes americanus). Comp. Biochem. Physiol. 64B, 297 -299.
Shears, M. A. and Fletcher, G. L. (1983). Regulation of Zn2+ uptake from the gastrointestinal tract of a marine teleost, the winter flounder (Pseudopleuronectes americanus). Can. J. Fish. Aquat. Sci. 40 (Suppl. 2), 197 -205.
Shears, M. A. and Fletcher, G. L. (1984). The relationship between metallothionein and intestinal zinc absorption in the winter flounder. Can. J. Zool. 62, 2211 -2220.
Solioz, M. and Odermatt, A. (1995). Copper and
silver transport by CopB-ATPase in membrane-vesicles of Enterococcus
hirae. J. Biol. Chem. 270, 9217
-9221.
Spry, D. J. and Wood, C. M. (1985). Ion flux rates, acidbase status, and blood gases in rainbow trout, Salmo gairdneri, exposed to toxic zinc in natural soft water. Can. J. Fish. Aquat. Sci. 42, 1332 -1341.
Spry, D. J. and Wood, C. M. (1989). A kinetic method for the measurement of zinc influx in vivo in the rainbow trout, and the effects of waterborne calcium on flux rates. J. Exp. Biol. 142, 425 -446.
Spry, D. J., Hodson, P. V. and Wood, C. M. (1988). Relative contributions of dietary and waterborne zinc in the rainbow trout, Salmo gairdneri. Can. J. Fish. Aquat. Sci. 45, 32 -41.
Stagg, R. M. and Shuttleworth, T. J. (1982). The effects of copper on ionic regulation by the gills of the seawater-adapted flounder (Platichthys flesus L.). J. Comp. Physiol. 149, 83 -90.
Stumm, W. and Morgan, J. J. (1996). Aquatic Chemistry, 3rd Edition. John Wiley & Sons, New York, USA.
Tange, N., JongYoung, L., Mikawa, N., Hirono, I. and Aoki, T. (1997). Cloning and characterization of transferrin cDNA and rapid detection of transferrin gene polymorphism in rainbow trout (Oncorhynchus mykiss). Mol. Mar. Biol. Bio. Tech. 6, 351 -356.
Taylor, L. N., McGeer, J. C., Wood, C. M. and McDonald, D. G. (2000). Physiological effects of chronic copper exposure to rainbow trout (Oncorhynchus mykiss) in hard and soft water: Evaluation of chronic indicators. Environ. Toxicol. Chem. 19, 2298 -2308.
Torrubia, J. and Garay, R. (1989). Evidence for a major route for zinc uptake in human red blood-cells [Zn(HCO3)2Cl]- influx through the [Cl-/HCO3-] anion-exchanger. J. Cell. Physiol. 138, 316 -322.[Medline]
Trinder, D., Oates, P. S., Thomas, C., Sadleir, J. and Morgan,
E. H. (1999). Localisation of divalent metal transporter 1
(DMT1) to the microvillus membrane of rat duodenal enterocytes in iron
deficiency, but to hepatocytes in iron overload. Gut
46, 270
-276.
Turner, A. and Olsen, Y. S. (2000). Chemical versus enzymatic digestion of contaminated estuarine sediment: relative importance of iron and manganese oxides in controlling trace metal bioavailability. Estuar. Coast. Shelf Sci. 51, 717 -728.[CrossRef]
Vallee, B. L. and Falchuk, K. H. (1993). The
biochemical basis of zinc physiology. Physiol. Rev.
73, 79-118.
Van Dijk, J. P., Lagerwerf, A. J., van Eijk, H. G. and Leijnse, B. (1975). Iron metabolism in the tench (Tinca tinca L.). Studies by means of intravascular administration of 59Fe(III) bound to plasma. J. Comp. Physiol. 99, 321 -330.
Vercauteren, K. and Blust, R. (1999). Uptake of cadmium and zinc by the mussel Mytilus edulis and inhibition by calcium channel and metabolic blockers. Mar. Biol. 135, 615 -626.[CrossRef]
Vulpe, C. D., Kuo, Y. M., Murphy, T. L., Cowley, L., Askwith, C., Libina, N., Gitschier, J. and Anderson, G. J. (1999). Hephaestin, a ceruloplasmin homologue implicated in intestinal iron transport, is defective in the sla mouse. Nat. Genet 21, 195 -199.[CrossRef][Medline]
Vulpe, C., Levinson, B., Whitney, S., Packman, S. and Gitschier, J. (1993). Isolation of a candidate gene for Menkes disease and evidence that it encodes a copper-transporting ATPase. Nat. Genet. 3, 7 -13.[Medline]
Wagner, G. F., Hampong, M., Park, C. M. and Copp, D. H. (1986). Purification, characterization, and bioassay of teleocalcin, a glycoprotein from salmon corpuscles of Stannius. Gen. Comp. Endocrinol. 63, 481 -491.[Medline]
Walsh, P. J., Blackwelder, P., Gill, K. A., Danult, E. and Mommsen, T. P. (1991). Carbonate deposits in marine fish intestine: a new source of biomineralization. Limnol Ocenogr. 36, 1227 -1232.
Wapnir, R. A. (1991). Copper-sodium linkage during intestinal-absorption inhibition by amiloride. P. Soc. Exp. Biol. Med. 196, 410 -414.[Abstract]
Wapnir, R. A. and Stiel, L. (1987). Intestinal absorption of copper: effect of sodium. Proc. Soc. Exp. Biol. Med. 185, 277 -282.[Abstract]
Watanabe, T., Kiron, V. and Satoh, S. (1997). Trace minerals in fish nutrition. Aquaculture 151, 185 -207.[CrossRef]
Whitehead, M. W., Thompson, R. P. H. and Powell, J. J. (1996). Regulation of metal absorption in the gastrointestinal tract. Gut 39, 625 -628.[Medline]
Wilhelm, S. W. (1995). Ecology of iron-limited cyanobacteria; a review of physiological responses and implications for aquatic systems. Aquat. Microb. Ecol. 9, 295-303.
Willis, J. N. and Sunda, W. G. (1984). Relative contributions of food and water in the accumulation of zinc by two species of marine fish. Mar. Biol. 80, 273 -279.
Wilson, R. W. (1999). A novel role for the gut of seawater teleosts in acid-base balance. In Regulation of Tissue pH in Plants and Animals (ed. S. Egginton, E. W. Taylor and J. A. Raven), pp. 257-274. Cambridge University Press, Cambridge, UK.
Wood, C. M. (2001). Toxic responses of the gill. In Target Organ Toxicity in Fresh and Marine Teleosts (ed. D. Schlenk and W. H. Benson), pp. 1 -89. Taylor and Francis, London, UK.
Woodward D. F., Farag, A. M., Bergman, H. L., Delonay, A. J., Little, E. E., Smith, C. E. and Barrows, F. T. (1995). Metals-contaminated benthic invertebrates in the Clark Fork River, Montana: effects on age-0 brown trout and rainbow trout. Can. J. Fish. Aquat. Sci. 52, 1994 -2004.
Zerounian, N. R. and Linder, M. C. (2002). Effects of copper and ceruloplasmin on iron transport in the Caco 2 cell intestinal model. J. Nutr. Biochem. 13, 138 -148.[CrossRef][Medline]
Zhao, H. and Eide, D. (1996a). The yeast ZRT1
gene encodes the zinc transporter protein of a high-affinity uptake system
induced by zinc limitation. Proc. Natl. Acad. Sci. USA
93, 2454
-2458.
Zhao, H. and Eide, D. (1996b). The ZRT2 gene
encodes the low-affinity zinc transporter in Saccharomyces cerevisiae. J. Biol. Chem. 271, 23203
-23210.
Zhou, B. and Gitschier, J. (1997). hCTR1: a
human gene for copper uptake identified by complementation in yeast.
Proc. Natl. Acad. Sci. USA
94, 7481
-7486.