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INTRODUCTION |
Long chain polyunsaturated fatty acids such as
20:4(n-6) and 22:6(n-3) play pivotal roles in a
number of biological functions including brain development, cognition,
inflammatory responses, and hemostasis (1-4). Over 30% of the fatty
acid in brain phospholipid consists of 20:4(n-6) and
22:6(n-3), and approximately 50% of the fatty acid in the
retina is 22:6(n-3) (5, 6). An inadequate availability of
20:4(n-6) is associated with impaired nerve transmission, reduced eicosanoid synthesis, and impaired fetal growth (7-9). Recently, premature infants were found to have reduced cognitive development, apparently because they could not synthesize adequate quantities of 22:6(n-3) to meet the biological demands for
proper retina function (1, 10). In addition to being vital components of membrane phospholipids and functioning in key steps of cell signaling, 20- and 22-carbon polyunsaturated fatty acids govern the
expression of a wide array of genes, including those encoding proteins
involved with lipid metabolism, thermogenesis, and cell differentiation
(11-14).
The availability of 20- and 22-carbon (n-6) and
(n-3) polyenoic fatty acids is greatly dependent upon the
rate of desaturation of 18:2(n-6) and 18:3(n-3)
by
-6 desaturase (15).
-6 Desaturase is a microsomal enzyme (15)
and is thought to be a component of a three-enzyme system that includes
NADH-cytochrome b5 reductase, cytochrome
b5, and
-6 desaturase (16). The enzymatic
activity for
-6 desaturase is reportedly low in most tissues except
the liver (16). Consequently, the liver has been considered the primary
site for the production of long chain polyenoic fatty acids (17, 18).
Numerous dietary studies indicate that hepatic
-6 desaturase
activity is induced by diets low in essential fatty acids and
suppressed by diets rich in vegetable or marine oils (19, 20). In
addition,
-6 desaturase activity is induced by peroxisome
proliferators and by the administration of insulin to diabetic rats
(21, 22). Unfortunately, defining the molecular determinants of
-6
desaturase activity, as well as characterizing its developmental
pattern and tissue distribution, has been hampered by the fact that
-6 desaturase has been neither cloned nor reproducibly purified.
Therefore, our objective was to clone the mammalian
-6 desaturase
and utilize the cDNA to examine the tissue distribution and
nutritional regulation of
-6 desaturase mRNA.
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EXPERIMENTAL PROCEDURES |
Cloning of the Mouse
-6 Desaturase cDNA--
A murine
cDNA (GenBank accession number W53753) displaying high homology to
the amino acid sequence of
-6 desaturase from Synechocystis sp. was acquired and sequenced. Subsequently,
a 23-base oligonucleotide primer (5'-CTTGGCATCGTGGGGAAGAGGTG-3') specific for the 5' end of murine cDNA W53753 was synthesized and
utilized to screen a mouse adaptor-ligated liver cDNA library (Marathon-Ready cDNA; CLONTECH) by rapid
amplification of cDNA ends-PCR.1 The PCR conditions
consisted of an initial denaturation step of 94 °C for 1 min,
followed by 5 cycles of 94 °C for 10 s and 72 °C for 4 min,
5 cycles of 94 °C for 10 s and 70 °C for 4 min, and,
finally, 20 cycles of 94 °C for 10 s and 68 °C for 4 min. The resulting rapid amplification of cDNA ends-PCR product was cloned into pBluescript (Stratagene) and sequenced by the dideoxy chain
termination method (23).
The nucleotide sequence of the PCR product was utilized to BLAST search
the mouse EST database. Two mouse cDNAs (GenBank accession numbers
AA237892 and AA250162) possessing 100% nucleotide homology with our
PCR product were identified and acquired from Genome Systems. Clone
AA250162 contained two possible AUG start codons, and the EST cDNA
AA237892 contained an apparent stop codon. The two EST cDNAs were
fused at the StyI restriction site, and the product was
inserted into the cytomegalovirus promoter expression vector
pcDNA3.1 (Invitrogen). Sequence analysis and prediction of amino
acid sequence were performed using MacDNASIS pro (Hitachi), and a
translation initiator codon was determined based on Kozak's rule
(24).
Cloning of the Human
-6 Desaturase cDNA--
Using the
nucleotide sequence of mouse liver
-6 desaturase, the EST human
database was searched for a human homologue cDNA. The search
identified a highly homologous human brain EST cDNA (GenBank
accession number Z44979), which was purchased from Genome Systems and
sequenced for verification. The 5' end of the human cDNA was
extended by PCR using a human brain cDNA library (Marathon-Ready
cDNA; CLONTECH). The forward oligonucleotide
primer (5'-AGACTGGCAGCATGGGGAAG-3') was prepared using the 5' end of the mouse
-6 desaturase and designed to include the putative start
codon. The reverse primer (5'-CATGGTGGGGAAGAGGTGGTG-3') was prepared
from the sequence derived from human Z44979 cDNA.
Expression of the Mouse
-6 Desaturase--
Cellular
expression of the mouse
-6 desaturase was performed in rat primary
hepatocytes and CHO cells. Rat primary hepatocytes were isolated by
collagenase perfusion and allowed to attach to a collagen-coated 60-mm
culture plate in 3 ml of Waymouth 752 medium supplemented with 0.5%
fetal bovine serum and 1 µM insulin and dexamethasone
(25). After a 6-h attachment period, the cells were washed with
phosphate-buffered saline and transfected with 6 µg of the mouse
-6 desaturase expression plasmid or the pcDNA3.1 expression
vector alone using 6.6 µl of Lipofectin per µg of DNA (Life
Technologies, Inc.). Transfection was conducted by adding the mixture
of Lipofectin and DNA in the absence of fetal bovine serum. After the
12-h transfection period, the medium was replaced with the one
containing either 200 µM albumin-bound 18:2, n-6 (molar
ratio of fatty acid to albumin, 4:1) or albumin alone. CHO cells were
grown in Kaighn's modification of Ham's F-12 medium supplemented with
10% fetal bovine serum in a 25-cm2 flask. At 80%
confluence, the serum-containing medium was removed, and cells were
washed with phosphate-buffered saline for transfection. A mixture of 2 µg of the mouse
-6 desaturase expression plasmid, 12 µl of
LipofectAMINE, and 8 µl of Plus reagent (Life Technologies, Inc.) was
added to cells without serum for 4 h. Subsequently, 10% serum was
added to the transfection media for 8 h. After a total 12-h
transfection period, the CHO cells were treated with either 200 µM albumin-bound 18:3(n-3),
20:3(n-6), or albumin alone. The hepatocytes and CHO cells
were incubated with the treatment medium for 24 h and then used
for fatty acid analysis.
Fatty Acid Extraction and Analysis--
Cellular fatty acid was
extracted by saponifying fatty acids using 1 ml of 30% KOH and 1 ml of
ethanol. Fatty acids from the treatment medium were also extracted and
analyzed after 24 h of incubation. Heptadecanoic acid was added to
the saponification mixture as an internal standard. After
saponification, the nonsaponifiable lipids were removed by extraction
with petroleum ether. Subsequently, the solution was acidified, and the
fatty acids were extracted with petroleum ether. The extract was dried
under nitrogen, and the residue was methylated using 14% boron
trifluoride in methanol (Sigma). Methylated fatty acids were separated
and quantified by gas chromatography using a fused silica glass
capillary column (50 m × 530 µm internal diameter; Quadrex).
The column temperature program was composed of an initial hold at
140 °C for 5 min, ramping at 5 °C per min to 220 °C, and a
final hold at 220 °C for 7 min. The injector temperature was
250 °C, and the flame ionization detector temperature was
260 °C.
Nutritional Regulation of
-6 Desaturase Expression--
Male
BALB/c mice were fed a high-glucose, fat-free diet for a 7-day
adaptation period. After this period, the fat-free diet was
supplemented with either 10% corn oil or 10% triolein (Sigma; 99%
purity), and the mice (n = 4 mice/group) were fed for
an additional 5 days. Liver tissues were removed, and microsomes were
isolated by differential centrifugation. One g of liver was homogenized in 4 ml of homogenization buffer containing 50 mM potassium
phosphate, pH 7.4, and 0.25 M sucrose. After a 10-min
centrifugation of the homogenate at 10,000 × g, the
resulting supernatant was spun at 100,000 × g for 60 min to isolate a microsomal pellet. After resuspending the pellet in
homogenization buffer, 3 mg of microsomal protein were incubated in a
37 °C shaking water bath for 5 min with 1 ml of reaction mixture
including 1.2 mM NADH, 3.6 mM ATP, 0.5 mM coenzyme A, 4.8 mM MgCl2, 72 mM phosphate buffer, pH 7.4, and 50 nmol of
1-14C-labeled 18:2(n-6). The reaction was
stopped by adding saponification reagent, and fatty acids were
saponified and methylated as described above. Radioactive
18:2(n-6) and 18:3(n-6) were separated by silver nitrate-impregnated thin layer chromatography. The radioactivity was
quantified using an Ambis radio-imager.
-6 Desaturase enzyme activity is expressed as the percentage of 18:2(n-6)
converted to 18:3(n-6) per mg of protein/min.
RNA Extraction and Northern Blot Analysis--
Total RNA was
isolated from the liver of mice in the dietary study using the
phenol-guanidinium isothiocyanate method (26). Twenty µg of total RNA
were size-fractionated on a 1% formaldehyde gel and then transferred
to a Zeta probe nylon membrane (Bio-Rad). The mouse
-6 desaturase
probe was prepared by incorporating [32P]dCTP by PCR. The
forward primer was 5'-GGACATAAAGAGCCTGCATG-3', and the reverse primer
was 5'-ACTGGAAGTACATAGGGATG-3'. The Northern membrane of human tissues
was purchased from Invitrogen. The radiolabeled probe for the human
tissue blot was a 200-base pair PCR fragment of human
-6 desaturase
using primers of 5'-GGCAAGAACTCAAAGATCAC-3' and
5'-GAGAGGTAGCAAGAACAAAG-3'. The autoradiographic signal was quantified
using Instant Imager (Packard).
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RESULTS |
Cloning Mouse and Human
-6 Desaturase--
The mouse EST
database was searched for mammalian homologues using the amino acid
sequence for
-6 desaturase from the photosynthetic cyanobacterium
Synechocystis sp. (27). A mouse cDNA that had a 60%
similarity to a 46-amino acid sequence of Synechocystis
-6 desaturase was identified. A 1508-base pair cDNA sequence for
mouse liver
-6 desaturase was acquired using a combination of
ligation-mediated PCR screening of a mouse liver cDNA library and
BLAST searches of the mouse EST database (Fig.
1A). Sequence analysis
revealed the presence of two in-frame methionine codons located at
positions 75 and 126. In addition, a TGA termination sequence was
identified at position 1407. Kozak's rule, which predicts that the
favored eukaryotic translation initiation sequence resides in the
sequence of AXXATGG (24), indicated that the first of the
two ATG codons was the preferred initiation codon for the putative
-6 desaturase. The apparent ORF between the first ATG codon and the
TGA termination codon predicted a peptide consisting of 444 amino acids
and having a size of 52.2 kDa. The human brain cDNA homologue for
-6 desaturase contains an initiation codon and a termination codon
that are perfectly aligned with the initiation and termination codons
of the mouse cDNA (Fig. 1A). Moreover, the amino acid
sequence derived from the ORF revealed that 87% of the amino acid
sequence for the mouse and human homologues was identical, and 96% of
the sequence had similarity (Fig. 1B). A search of the Swiss
Protein Database indicated that the putative
-6 desaturase sequence
was unique and shared very little amino acid homology with any other
mammalian proteins including the murine stearoyl-CoA desaturase (
-9
desaturase) (28).

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Fig. 1.
Alignment of the predicted amino acid
sequences for mouse and human -6 desaturase. A, schematic
diagram of the ORF and untranslated regions for mouse and human -6
desaturase. The hatched box indicates an ORF of 1332 nucleotides, and the open boxes represent untranslated
regions. The human cDNA contains a polyadenylation signal AATAAA at
the 3' end. B, a comparison of the amino acid sequences for
mouse and human -6 desaturase predicted by the nucleotide sequence
of the ORFs. Both mouse and human ORFs encode 444 amino acids.
Identical amino acids are paired by vertical lines, and
conserved amino acids are matched by colons. The cytochrome
b5-like domain is underlined.
Transmembrane domains are shown in shaded areas, and three
histidine-rich domains are in bold.
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Structural Characteristics of Mammalian
-6 Desaturase--
The
enzymatic activity of mammalian
-6 desaturase is associated with the
microsomal membrane fraction (15). Consistent with such membrane
involvement, the predicted amino acid sequence for
-6 desaturase
indicated that the peptide contains 52% nonpolar amino acids, and a
hydropathy profile revealed the presence of two membrane-spanning
domains that are characteristic of membrane-anchored proteins (Figs.
1B and 2). In addition, the
amino terminus of the
-6 desaturase peptide contains a hydrophilic
domain of 54 amino acids that is highly homologous with the
heme-binding domain of cytochrome b5 (Fig.
3A). This cytochrome
b5-like domain is also found in the
-6
desaturases from Borago officinalis (29) and
Caenorhabditis elegans (30) (Fig. 3A). The
His53 and His76 residues located within this
domain of the mammalian
-6 desaturase are exactly aligned with the
two heme-binding histidines in cytochrome b5
(31). Moreover, these two histidines are surrounded by charged amino
acids that may contribute to the stabilization of the heme-histidine complex (31). In addition, the sequence
53HPGG56 predicts the existence of a dramatic
-turn that may render His53 more accessible to heme iron
binding (31).

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Fig. 2.
Hydropathy profile of mouse (A)
and human (B) -6 desaturase. The hydropathic
pattern for -6 desaturase was plotted using the method of
Kyte-Doolittle, and the amino acid sequences were predicted by the
respective ORFs. Bars,the transmembrane regions. Boxed
H, locations of histidine-rich regions.
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Fig. 3.
A comparison of the cytochrome
b5-like and histidine-rich domains for
mammalian and nonmammalian -6 desaturases. A, a
comparison of the cytochrome b5-like domain for
mammalian and nonmammalian -6 desaturase (D6D). A
comparison of the amino acid sequence within the cytochrome
b5-like domain of mouse, human, plant (B. officinalis; Ref. 29), and C. elegans (30) -6
desaturases reveals a high level of homology with a comparable domain
within mammalian cytochrome b5
(Cyto.b5) (31). Amino acids that are identical between the
-6 desaturases and cytochrome b5 are
highlighted in black; amino acids that are highly homologous
between the desaturases and cytochrome b5 are
highlighted in gray. Asterisk, two heme-binding
histidines found in cytochrome b5 (31).
B, the three histidine-rich regions conserved in membrane
desaturases. The histidines within these regions are highlighted in
black. The amino acids that are identical in all the -6
desaturases listed are highlighted in dark gray.
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A second noteworthy feature of the mammalian
-6 desaturase is the
presence of three histidine-rich regions (Fig. 3B). Regions I (HX3H) and II (HX2HH)
are located between the two transmembrane domains, and region III (HH)
is located near the carboxyl terminus of the peptide. These
histidine-rich regions are also found in plant membrane desaturases and
mammalian stearoyl-CoA desaturase and reportedly bind non-heme iron
that is required for enzymatic activity (32).
Expression of
-6 Desaturase--
The predicted structural
characteristics of the mouse and human peptides strongly suggested that
the cDNAs did in fact correspond to mammalian
-6 desaturase. To
confirm this conclusion, the ORF for the mouse
-6 desaturase was
expressed in primary cultures of rat hepatocytes and in CHO cells.
Fatty acid analysis revealed that hepatocytes transfected with the
vector containing the
-6 desaturase ORF were capable of synthesizing
the
-6 desaturase product 18:3(n-6) from
18:2(n-6) (Fig.
4A). On the other hand, hepatocytes transfected with vector alone produced no detectable 18:3(n-6) product (Fig. 4B). Similarly, CHO cells
expressing
-6 desaturase readily converted the
-6 desaturase
substrate 18:3(n-3) to the
-6 desaturase product
18:4(n-3), whereas nontransfected CHO cells were unable to
produce detectable levels of 18:4(n-3) (Fig. 4, C
and D). In contrast, providing CHO cells with the
-5 desaturase substrate 20:3(n-6) did not lead to the
production of the
-5 desaturase product 20:4(n-6) (data
not shown). These data conclusively demonstrate that the mouse and
human ORFs do in fact encode mammalian
-6 desaturase.

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Fig. 4.
Expression of mouse -6 desaturase in rat
hepatocytes and CHO cells. A shows the conversion of
18:2(n-6) to 18:3(n-6) by hepatocytes that were
transfected with the pcDNA3.1 vector containing the mouse -6
desaturase (D6D) ORF. When hepatocytes were transfected with
pcDNA3.1 vector lacking the -6 desaturase ORF, there was no
detectable conversion of 18:2(n-6) to 18:3(n-6)
(B). The media of CHO cells incubated with albumin-bound
18:3(n-3) and transfected with the -6 desaturase
expression vector pcDNA3.1 contained 18:4(n-3)
(C), whereas the media of nontransfected CHO cells contained
no detectable 18:4(n-3) (D). Retention times for
the fatty acids are shown above the respective peaks. The
identity of each peak was confirmed using individual fatty acid methyl
ester standards.
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Nutritional Regulation of
-6 Desaturase Expression--
The
enzymatic activity of
-6 desaturase increases when animals are fed
an essential fatty acid-deficient diet, whereas it decreases when
polyunsaturated fatty acids are ingested (16, 19, 20). Using the mouse
cDNA for
-6 desaturase, we have found that the suppression of
hepatic
-6 desaturase enzymatic activity associated with the
ingestion of polyunsaturated fat (i.e. corn oil) is
paralleled by a comparable reduction in
-6 desaturase mRNA
abundance (Fig. 5, A and
B). Interestingly, whereas the dominant transcript of
hepatic
-6 desaturase is approximately 4.0 kb in size, the mouse
liver also contains a minor transcript that is approximately 2.2 kb
(Fig. 5B). Both transcripts appeared to be suppressed by
dietary corn oil to the same degree. In addition, hybridizing the
Northern blot with sequences from the 5', middle, and 3' regions of the
-6 desaturase ORF yielded the same outcomes with respect to the
abundance and dietary response of the 2.2-kb transcript (data not
shown). The reason for these two different transcripts remains
unknown.

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Fig. 5.
Nutritional regulation and tissue
distribution of mammalian -6 desaturase. Mice were fed a
high-glucose diet containing 10% corn oil or 10% triolein. Hepatic
-6 desaturase activity, which is expressed in A as
means ± S.E., was significantly lower in mice fed corn oil than
in mice fed triolein (p < 0.001). The abundance of
hepatic -6 desaturase mRNA was determined by Northern analysis
(B). The abundance of the 4.0- and 2.2-kb -6 desaturase
transcripts was quantified by radio-imaging. The cpm of the
32P-labeled probe associated with the 4.0- and 2.2-kb
transcripts was 2509 ± 154 and 327 ± 17; it was 1264 ± 66 and 185 ± 9 for the triolein and corn oil groups,
respectively (p < 0.003). C depicts the
abundance of -6 desaturase mRNA found in a variety of adult male
human tissues. Each lane contains 20 µg of total RNA. Unlike mice,
only one -6 desaturase transcript with an approximate size of 3.2 kb
was detected in human tissues. Comparable results were obtained with
three different Northern blots.
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-6 Desaturase mRNA Distribution in Human
Tissues--
Northern analysis of
-6 desaturase expression revealed
that human
-6 desaturase mRNA is a single transcript of
approximately 3.2 kb and is expressed in a wide array of tissues
including the brain, liver, lung, and heart (Fig. 5C). The
level of
-6 desaturase mRNA in the liver was approximately the
same as that found in the lung and heart, but the abundance of
-6
desaturase in the human brain was severalfold higher (Fig.
5C). In addition to the tissues examined by Northern
analysis, a search of the EST database revealed that
-6 desaturase
mRNA is expressed in the human fetus and fetal heart as well as in
the 13-day-old mouse embryo heart.
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DISCUSSION |
The purification and characterization of mammalian
-6
desaturase have been difficult because of its instability. In fact, there has been only one report, in 1981, that describes the
purification of a putative linoleoyl-CoA desaturase from rat liver
(33). Because of the problems encountered in the purification of
-6 desaturase, we have used the EST database to clone and characterize the
mouse and human
-6 desaturase enzyme. Interestingly, a comparison of
the rat liver linoleoyl-CoA desaturase with the
-6 desaturase peptide predicted by the ORF of both the mouse and human cDNAs indicates that the two proteins are markedly different. First, the ORF
for mouse and human
-6 desaturase predicts a protein that is 52.2 kDa, whereas the size of the linoleoyl-CoA desaturase was cited to be
66 kDa (33). Second, the nucleotide sequence of the mouse and human
-6 desaturase ORFs predicts that these peptides contain 30 histidines (Fig. 1B). Moreover, many of these histidines are
organized into distinct histidine-rich domains. Such domains are
characteristic of all membrane-associated desaturases (32). In
contrast, the reported amino acid composition of linoleoyl-CoA desaturase indicates that it contains only 15 histidine residues (33).
Unfortunately, sequence information for linoleoyl-CoA desaturase is not
available, because the purification of linoleoyl-CoA desaturase has
never been replicated since the initial report. Clearly, the
-6
desaturase and the putative linoleoyl-CoA desaturase are distinctly
different proteins. It is possible the liver contains two
-6
desaturase enzymes. In fact, metabolic studies suggest that there may
be two isoforms of
-6 desaturase (34, 35): (a) one that
catalyzes the initial desaturation of 18:2(n-6) or 18:3(n-3), and (b) another that catalyzes the
conversion of 24:5(n-3) to 24:6(n-3). The cloning
of the
-6 desaturase should now permit us to determine whether
isoforms of
-6 desaturase do exist.
In addition to the histidine-rich domains, the mammalian
-6
desaturase contains a distinct cytochrome
b5-like domain that is also characteristic of
plant (Borage) and C. elegans
-6 desaturases (29, 30) but is not a component of the mammalian
-9 desaturase (36).
Early reconstitution studies with
-9 desaturase indicated that the
conversion of 18:0(n-9) to 18:1(n-9) required
-9 desaturase, cytochrome b5 reductase, and
cytochrome b5 itself (36). It has been assumed
from these early studies that all mammalian desaturases require
cytochrome b5 for enzymatic activity (16, 36).
However, the cytochrome b5-like domain of yeast
OLE1 was recently reported to replace the requirement for cytochrome
b5; i.e. desaturation occurred in the
absence of cytochrome b5, and removal of the
cytochrome b5-like domain rendered the OLE1
enzyme inactive (37). This observation raises the possibility that
cytochrome b5 reductase transfers electrons to
the catalytic domain of the
-6 desaturase via the cytochrome
b5-like domain, and not via cytochrome
b5 per se.
Hepatic
-6 desaturase enzymatic activity varies with hormonal and
nutritional manipulation (15, 16, 20, 38). For example, insulin
deficiency and fasting reduce
-6 desaturase enzymatic activity,
whereas the administration of insulin or refeeding increases its
activity (39). In addition to being affected by fasting and feeding,
hepatic
-6 desaturase enzymatic activity is highly dependent upon
the composition of dietary fat (16). Specifically, the ingestion of
fats that are low in essential fatty acids (e.g. butter)
results in higher levels of enzyme activity than the consumption of
fats (e.g. corn oil) that are rich in essential fatty acids
(16). Northern analysis indicates that the increase in hepatic
-6
desaturase activity associated with the consumption of an essential
fatty acid-deficient diet is paralleled by a comparable increase in the
hepatic abundance of
-6 desaturase mRNA (Fig. 5). Thus, it
appears that the activity of hepatic
-6 desaturase is largely
regulated by pretranslational events. However, this may not be the case
in all tissues. Specifically,
-6 desaturase activity is reportedly
very low in nonhepatic tissues (16-18). Because of this low enzymatic
activity in nonhepatic tissues, the liver has been considered to be the
primary site of 20:4(n-6), 20:5(n-3), and
22:6(n-3) production for peripheral tissue utilization (17).
However, Northern analysis of RNA from a number of different human
tissues challenges this concept (Fig. 5C). For example, the
level of
-6 desaturase mRNA in the human liver was comparable to
that found in the human lung and heart. Moreover, the abundance of
-6 desaturase mRNA in the adult human brain was severalfold greater than that in the human liver (Fig. 5C). This high
level of expression is certainly very consistent with the fact that >30% of the human brain phospholipid consists of 20- and 22-carbon polyenoic fatty acids (5, 6, 40). However, such high expression is in
conflict with the reports that brain microsomes have a rate of
-6
desaturation that is only 10-15% of that found in the liver (18, 41).
These data suggest that
-6 desaturase enzymatic activity may be
determined by tissue-specific mechanisms that involve both pre- and
post-translational events.
In conclusion,
-6 desaturase catalyzes the rate-limiting step in the
conversion of 18:2(n-6) and 18:3(n-3) to the long
chain polyenoic fatty acids 20:4(n-6) and
20:5(n-3) and 22:6(n-3), respectively (15). These
long chain polyenoic fatty acids are essential for a large number of
biological functions including inflammatory responses (4), brain
development (2), retina function and cognition (1, 3), signal
transduction (42, 43), reproduction (4), fetal growth (9), cell
differentiation (14), and gene regulation (11-13). Not surprisingly,
physiological conditions that are associated with low levels of
-6
desaturase activity may have a pronounced impact on a wide array of
biological functions. For example, an impaired conversion of
18:2(n-6) to 18:3(n-6) appears to be associated
with reduced nerve conductivity in human diabetics (7). Similarly, the
low rate of 18:3(n-3) conversion to 20:5(n-3) and
22:6(n-3) observed in newborn infants is highly correlated
with impaired retina function and reduced cognitive development (1).
Now that the
-6 desaturase has been cloned, we can begin to define
the role that
-6 desaturation may play in an apparently wide array
of physiological processes.