From the
The expression pattern of mitochondrial carnitine
palmitoyltransferase (CPT) enzymes was examined in the developing rat
heart. Whereas the specific activity of CPT II increased
Because the myocardial carnitine
content is very low at birth and rises dramatically over the next
several weeks, it can be estimated that L-CPT I
( K
The carnitine palmitoyltransferase (CPT)
Efforts to dissect the
mitochondrial CPT system
In examining the
features of CPT I in adult rat heart mitochondria we made the
unexpected observation that this tissue expresses both the M- and
L-type enzymes
(16, 17) . The fact that the L variant
contributed only
Animals were bred in-house,
with the day of birth designated as day 0 and weaning at day 21. The
rats were killed by cervical dislocation, adult males (250-450 g)
being first anesthetized by intraperitoneal injection of Nembutal. To
obtain fetal tissue, timed pregnant females were purchased, and hearts
were removed 1-2 days prepartum. The mothers were rendered
unconscious by inhalation of Metofane prior to sacrifice.
Hearts
from 1-3 litters of mixed sex were used for each analysis in
animals between the fetal and 29-day-old stages of life. Individual
males were used for adult time points.
For measurement of CPT II activity
mitochondria were pelleted by centrifugation and stored at -20
°C (studies not shown had established that CPT II is stable for
months under these conditions). At the time of assay, the pellets were
resuspended in buffer A (4 ml/g of original tissue), octyl glucoside
was added to a concentration of 1% (w/v), and the mixtures were kept on
ice for 30 min. Such treatment causes inactivation of CPT I and
solubilization of CPT II in active form
(21) .
CPT activity
was assayed at 30 °C in the direction of palmitoylcarnitine
formation
(18) in the presence of 50 µ
M
palmitoyl-CoA, 1% bovine serum albumin (w/v) and the indicated
concentrations of
L-carnitine (containing
L-[ methyl-
Protein
was measured by the method of Lowry et al. (22) .
Fig. 1
depicts the profile of CPT I and CPT II
activities in heart mitochondria from rats at various stages of
development. These assays were conducted using our standard procedure
in which carnitine was present at a concentration of 200
µ
M. It is seen that the activity of CPT I (measured in
intact mitochondria) remained at its prepartum level until 2 days after
birth. Over the next 5 days the value rose by
In order to quantify changes in the actual activities of the two CPT
I isoforms, mitochondria were preincubated in the absence or presence
of DNP-etomoxir (together with ATP and CoASH), a maneuver that
completely inhibits L-CPT I while leaving the M variant unaffected
(17) . Because the K
In light of
the fact that the L- and M-isoforms of CPT I have such different
affinities for carnitine (but similar K
The present study was prompted by our recent observation that
the adult rat heart expresses both liver and muscle isoforms of
mitochondrial CPT I
(16, 17) . Here we have explored the
development of the cardiac CPT enzymes with emphasis on the
distribution of the two CPT I variants.
Early reports had described
a major increase in CPT activity in rat heart during the postnatal
period
(27, 28) . However, at the time of those studies
the complexities of the CPT system were not as well understood than
they are now, and the assay procedures employed did not adequately
discriminate between the CPT I and CPT II components. By carrying out
the CPT measurements in intact and octyl glucoside-solubilized
mitochondria it is now possible to make this distinction
(21) .
Using this methodology it is evident that while the specific activity
of both CPT I and CPT II increases between the time of birth and
weaning there is a greater -fold change for CPT II (
The data of Fig. 1, however, do not describe
the situation fully, since the activity noted for CPT I represents the
sum of two kinetically different components, the L and M isoforms of
the enzyme. We took two approaches to examining the apparent CPT I
profile more closely by analyzing the contribution of each variant to
total CPT I activity. Using [
To gain some appreciation of what these
findings might mean as regards the quantitative contribution of L-CPT I
to cardiac fatty acid oxidation in the developing rat, the data must be
viewed in the context of the carnitine content of the heart at any
given stage of life. We had previously shown that total (free plus
esterified) carnitine levels in rat heart are low at birth and increase
rapidly during the suckling period
(19) . Here we confirm this
finding and extend it to the status of the free carnitine pool, which
is also seen to be very low at birth, rising some 5-fold during
suckling and by a further 3-fold in adult animals. When the changes in
carnitine concentration are considered together with the shifting ratio
of L- to M-CPT I, it can be estimated that the functional contribution
of L-CPT I to overall cardiac CPT flux, while only
The presence of L-CPT I in rat heart might also have
relevance to the still puzzling question of how fatty acid oxidation
can ever proceed in cardiac tissue given the fact that its content of
malonyl-CoA has been found to fluctuate between
Although the present work
adds CPT I to the growing list of proteins known to undergo isoform
switching during cardiac development
(25) , we do not yet
understand which signals are responsible for the phenomenon. Some
regulatory factors must exist that are specific to cardiac tissue.
While the L-CPT I level in the heart changes little during the first 48
h after birth (Fig. 4), expression of the enzyme in liver rises
4-fold during this time
(31) . Conversely, whereas hepatic
expression of CPT II remains constant in the neonatal period
(31) , the present study shows a substantial up-regulation in
the heart. The differential response of the two tissues likely involves
dietary components and changes in circulating hormone levels. The
intriguing possibility exists that carnitine itself plays a role, in
light of the fact that its concentration in liver and heart changes
reciprocally in the neonatal rat (Ref. 19 and Fig. 5). It remains
to be established whether skeletal muscle also expresses some L-CPT I
at an early age, since carnitine levels in this tissue would also be
predicted to be low at birth.
Apart from liver, heart, and, as
recently documented, fibroblasts
(14) , the range of tissues
expressing L-CPT I is not known. However, we suspect that kidney and
the pancreatic
3-fold
during the first month of life, the profile for CPT I, which is
composed of both liver (L) and muscle (M) isoforms, was more complex.
Exposure of mitochondria to [
H]etomoxir (a
covalent ligand for CPT I), followed by fluorographic analysis of the
membrane proteins, established that while in the adult heart L-CPT I
represents a very minor constituent, its contribution is much greater
in the newborn animal. Use of the related inhibitor,
2-[6-(2,4-dinitrophenoxy)hexyl]oxirane-2-carboxylic acid
(specific for L-CPT I), allowed the activities of the two CPT I
variants to be quantified separately. The results showed that in the
neonatal heart, L-CPT I contributes
25% to total CPT I activity
(in V
terms), the value falling during growth of
the pups (with concomitant increasing expression of the M isoform) to
its adult level of 2-3%.
for carnitine of only 30
µ
M compared with a value of 500 µ
M for M-CPT
I) is responsible for some 60% of total cardiac fatty acid oxidation in
the newborn rat; the value falls to
4% in adult animals. Should
these findings have a parallel in humans, they could have important
implications for understanding the pathophysiological consequences of
inherited L-CPT I deficiency syndromes.
(
)
enzyme system effects the entry of long chain fatty acids
into the mitochondrial matrix for
-oxidation. CPT I, located on
the outer membrane, catalyzes the transfer of acyl groups from coenzyme
A to carnitine, the acylcarnitine so formed then traversing the inner
membrane by means of a specific transporter. CPT II, on the matrix side
of the inner membrane, reverses the transacylation reaction,
regenerating acyl-CoA
(1) . Overall control of fatty acid
transport, and thus of
-oxidation, is exerted at the level of CPT
I by virtue of its unique inhibitability by malonyl-CoA, the product of
the acetyl-CoA carboxylase reaction
(1) . Although first
recognized in the context of hepatic ketogenesis and its regulation
(2) , the malonyl-CoA/CPT I interaction has since emerged as a
key component of fuel ``cross-talk'' in a variety of
non-hepatic tissues such as heart
(3) , skeletal muscle
(4) , and the pancreatic
-cell
(5, 6) . In
addition, CPT I has attracted attention as a potential site of
pharmacological intervention in poorly controlled diabetes mellitus
where fatty acid oxidation is excessive and has a detrimental effect on
glucose homeostasis
(7, 8) .
(
)
in terms of its
structure/function/regulatory characteristics have been greatly
enhanced by the recent isolation of cDNAs corresponding to the rat and
human CPT II proteins
(10, 11, 12) as well as
those encoding rat and human liver CPT I
(13, 14) .
Available evidence indicates that in both species CPT II (
71 kDa)
is expressed as the same protein throughout the entire body
(12, 15) . By contrast, the regulated enzyme, CPT I,
exists in at least two isoforms. These have been designated L-CPT I and
M-CPT I, indicating their association with liver and skeletal muscle,
respectively
(16, 17) . The two proteins differ not only
in monomeric size (
88 and 82 kDa, respectively) but also in their
profoundly different kinetic characteristics ( K
for carnitine,
30 and 500 µ
M, respectively;
I
(
)
for malonyl-CoA,
2.7 and
0.03 µ
M, respectively
(18) ).
2-3% to total CPT I activity was puzzling
and difficult to rationalize teleologically. In the present study,
therefore, we asked the following question: is the 98:2 ratio of M- to
L-CPT I activity a fixed property of rat cardiac tissue, or is it
possible that the L isoform (low K
for
carnitine) makes a more significant contribution in the newborn period
when the carnitine content of the heart is known to be low
(19) ? As outlined below, it turns out that soon after birth
L-CPT I is probably responsible for a major fraction of fatty acid
oxidation in rat heart and that its decline during growth of the pups
represents another striking example of developmental isoform switching
in heart tissue. The findings might have pathophysiological
significance in situations of inherited CPT I deficiency syndromes.
Animals
Sprague-Dawley rats (Harlan
Sprague-Dawley, Inc., Indianapolis, IN) were maintained on regular
Purina chow (4% fat) with lighting from 1000 to 2200 h. Organ harvest
was performed between 1100 and 1200 h.
Mitochondrial Preparation and Treatment
Hearts
were rapidly removed, trimmed of visible blood vessels, and homogenized
in 10 ml/g, wet weight, of 250 m
M sucrose, 5 m
M
Tris-HCl, pH 7.2 (Tissumizer; Tekmar Co., Cincinnati, OH; large probe,
10 s, setting 50). Mitochondria were prepared as described in method A
of Ref. 18 and finally suspended in 150 m
M KCl, 5 m
M
Tris-HCl, pH 7.2 (buffer A) at 4 ml/g of original tissue. For inhibitor
treatment, the suspension was diluted with an equal volume of
activation mixture (200 m
M Tris-HCl, pH 7.2, 100 m
M
KCl, 12 m
M MgCl
, 12 m
M ATP, 0.7
m
M glutathione, 100 µ
M CoASH, and 1.2% bovine
serum albumin) before addition of [
H]etomoxir or
unlabeled DNP-etomoxir to a concentration of 3 or 10 µ
M,
respectively. Control samples lacked only the inhibitor. The tubes were
then rocked gently for 1 h at room temperature (during this time the
inhibitors become activated to their CoA derivatives, and it is in this
form that they interact with CPT I, bonding covalently). Subsequently,
the mitochondria were centrifuged at 12,000
g for 15
min at 4 °C, resuspended in the same volume of 0.9% NaCl, 0.4%
bovine serum albumin, and similarly resedimented. For samples treated
with or without DNP-etomoxir, the final pellet was brought to 4 ml/g of
original tissue in buffer A for assay of CPT I. The washed
[
H]etomoxir-labeled mitochondrial pellets were
resuspended to 4 ml/g of original tissue in 5 m
M potassium
phosphate, pH 7.2, twice frozen and thawed in liquid N
, and
then centrifuged at 100,000
g for 30 min at 4 °C.
The labeled membranes were suspended to
1 mg of protein/ml in SDS
sample buffer and boiled for 3 min, and an aliquot of each sample
containing
5,000 cpm of protein-bound
H of the mixture
was subjected to polyacrylamide gel electrophoresis followed by
fluorography
(20) .
C]-carnitine).
Reaction rates were linear over the time period studied, usually 4 min.
With intact mitochondria, the assay monitors only CPT I, the activity
of which is essentially completely inhibited by malonyl-CoA.
Myocyte Isolation
Cardiac myocytes were isolated
from 7-day-old litters or adult male rats by collagenase digestion of
diced heart tissue and purification of released myocytes by
centrifugation through 3% Ficoll as described
(16, 23) except for the inclusion of 10 µg/ml DNase I during
collagenase treatment. Preparations contained >90% myocytes as
judged by microscopic examination. The cells, suspended in buffer A at
a concentration of 0.4-1.0 mg of protein/ml, were broken using
eight cycles of a tightly fitting glass homogenizer. The homogenate was
then incubated in the absence or presence of DNP-etomoxir and assayed
for CPT I as described above for heart mitochondria.
Measurement of Heart Carnitine Content
Hearts were
rapidly excised, frozen in liquid N, and stored at
-70 °C until the assay. At that time, samples of frozen
tissue (250-500 mg) were pulverized under liquid N
,
and the powder was immediately homogenized in 3 ml of ice-cold 3%
perchloric acid. After centrifugation, the supernatant was neutralized
with KOH and used for the measurement of free and total carnitine (the
difference representing short chain acylcarnitines, primarily
acetylcarnitine) using an isotopic assay
(24) .
Materials
Sources have been given in Refs.
16-18 and 24.
50% and remained
constant out to the 29-day time point. By contrast, CPT II activity,
measured in octyl glucoside-solubilized mitochondria, increased
steadily throughout the course of development, reaching a value at day
29 that was almost 3 times that seen at day -1. The levels of
both enzymes declined somewhat between day 29 and
adulthood.
(
)
The CPT II component is not
considered further here.
Figure 1:
Activities of mitochondrial CPT I and
CPT II in the developing rat heart. Mitochondria were prepared and
assayed for CPT I and CPT II in the presence of 200 µ
M
carnitine as described under ``Experimental Procedures.''
Values are means ± S.E. for three or four independent
experiments at each time point. Where error bars are
not shown they lie within the
symbol.
The data for CPT I shown in Fig. 1do
not discriminate between the two isoforms of the enzyme, M and L,
previously shown to be expressed in adult rat heart
(16, 17) . To obtain qualitative insight into whether
the ratio of these isozymes might vary during development we made use
of the irreversible inhibitor, [H]etomoxir,
which, in its CoA ester form, interacts covalently with both proteins
(causing complete inactivation) and thus serves as a radiolabeled
tagging agent
(16, 17) . Accordingly, heart mitochondria
from juvenile and adult rats were exposed to
[
H]etomoxir in the presence of ATP and CoASH,
after which the membranes were subjected to SDS-polyacrylamide gel
electrophoresis followed by fluorography. As seen from Fig. 2, both L-
and M-CPT I were present at all stages of postnatal development. This
was also true in late fetal tissue (results not shown). The important
point is that the relative labeling intensity of the higher
M
L-CPT I compared with the faster migrating M-CPT
I can clearly be seen to diminish with increasing age of the litters.
of L-CPT I
for carnitine is substantially lower than that of M-CPT I (30
versus 500 µ
M(18) ) the presence of the
former is more readily detected in assays conducted at low levels of
this substrate
(17) . Thus, a concentration of 20
µ
M was chosen for the studies illustrated in Fig. 3.
As seen in panel A, while DNP-etomoxir caused
23% inhibition of CPT I activity in adult hearts, this value was
in the region of 70-75% in late fetal and newborn animals and
fell progressively with postnatal development. The actual activity of
each CPT I isoform under these assay conditions is seen in panel B, which reveals their distinct developmental profiles.
Over the first month postpartum the level of M-CPT I rises rapidly and
changes little thereafter. L-CPT I displays the converse pattern. All
of the assays in the experiment of Fig. 3were repeated using 10
µ
M carnitine. Qualitatively similar results were obtained,
although, as expected, the percentage of inhibition of CPT I activity
by DNP-etomoxir at each time point was even more pronounced (data not
shown).
Figure 3:
Use of
DNP-etomoxir to discriminate between L- and M-CPT I in heart
mitochondria. Mitochondria were preincubated with ATP and CoASH in the
absence or presence of DNP-etomoxir, washed, and then assayed for CPT I
activity using a carnitine concentration of 20 µ
M. The
percentage of suppression of overall activity by the inhibitor (due to
selective loss of L-CPT I) is shown in panel A.
Panel B depicts the actual activities of the two CPT
I isoforms. The contribution of L-CPT I represents the difference in
activity between untreated samples (L-CPT I + M-CPT I) and those
treated with DNP-etomoxir (M-CPT I). Values are means ± S.E. for
three to five independent experiments at each time
point.
Based on the measured activity of the two CPT I isoforms at
10 and 20 µ
M carnitine (Fig. 3) and on the known
Kvalue of each adult enzyme for this
substrate (see above), it is possible, using the Michaelis-Menten
equation, to transform the data into a V
format
if it is assumed (not unreasonably) that throughout development the
K
of each enzyme remains unchanged. The
picture obtained is shown in Fig. 4. It is seen that the combined
maximal capacity of L-CPT I plus M-CPT I rises from
20 to 45 nmol
min
mg of protein
during the suckling period and falls back to
30 nmol
min
mg of protein
in adult
animals. This is accompanied by a marked change in the contribution of
L-CPT I to overall enzyme activity, the value being
20-25%
in the perinatal stage and declining to
2-3% in the adult
heart.
Figure 4:
Calculated Vactivities of L- and M-CPT I in heart mitochondria.
V
values for liver ( L) and muscle
( M) CPT I isoforms are derived from the data of Fig. 3 as
explained under ``Results.'' % L represents the
contribution of the liver isoform.
Since changes in the cellular structure of cardiac muscle are
also occurring in the neonatal period
(25) , the possibility
that the high level of L-CPT I detected during this time (Figs.
1-4) derived from non-myocyte cell types had to be considered. We
therefore prepared myocytes that were >90% in purity from 7-day-old
and adult hearts. The cells were then broken (under conditions that
maintained mitochondrial integrity); incubated with DNP-etomoxir, ATP,
and CoASH; and subsequently assayed for CPT I at 20 µ
M
carnitine. In two separate experiments the percentage of inhibition of
enzyme activity by the etomoxir derivative was 44.6 and 50.2 in
7-day-old cells and 19.4 and 24.2 in adult cells. These values are very
similar to those obtained with mitochondria isolated from whole heart
tissue (Fig. 3 A), lending confidence that the changing
ratio of L- to M-CPT I observed in the experiments of Figs. 1-4
did indeed reflect events in the cardiac myocyte itself.
values for palmitoyl-CoA
(26) ), it follows that the
contribution of each enzyme to the process of fatty acid oxidation in
the developing heart will be a function not only of its relative
abundance but also of the tissue carnitine concentration at any given
time. To explore this matter further we measured both the
total
(
)
and free carnitine content of the heart
throughout the course of development (Fig. 5). The data for total
carnitine are very similar to those reported by us earlier
(19) and reaffirm the fact that this is low in the fetal heart
and rises rapidly in the postnatal period. A similar profile was found
with regard to free (unesterified) carnitine, although at all time
points between days -1 and +29 it represented less than half
of the total pool. Both free and total carnitine increased further
between the 29-day-old and adult stage of life. By dividing the values
for free carnitine in nmol/g, wet weight, of tissue by 0.7 (to obtain
an approximation of the cytosolic carnitine concentration in
µ
M) and combining these with the data of
Fig. 4
(showing the percentage of contribution of each CPT I
isoform to total enzyme activity) it is possible, again using the
Michaelis-Menten equation, to estimate the relative functional contribution of L-CPT I to overall cardiac fatty acid oxidation in
the developing rat. The composite data are shown in Fig. 6. The
key finding is that in the immediate newborn period the calculated
fraction of total fatty acid flux through the
-oxidation pathway
of heart mitochondria that is supported by L-CPT I approaches 60%. This
value falls sharply during suckling as the relative expression of L-CPT
I diminishes and the tissue carnitine content rises, the majority of
theoretical flux shifting to the high K
M-CPT I, which itself is increasing in expression over this time
frame. In the adult rat,
96% of CPT I catalysis would be predicted
to occur via the muscle isoform.
Figure 5:
Carnitine content of the developing rat
heart. Free and total (free plus esterified) carnitine was assayed in
neutralized perchloric acid extracts of heart tissue. Values are means
± S.E. for three or four independent measurements at each time
point.
Figure 6:
Theoretical contribution of L-CPT I to
overall CPT I flux in the developing rat heart. Results are expressed
as the percentage of CPT I flux estimated to occur via L-CPT I.
Derivation of data is detailed under
``Results.''
3) compared
with CPT I (
1.5). It should be noted, however, that the
mitochondrial content of the heart cells is also increasing during this
period
(25) , which would exaggerate the changes in CPT I and
CPT II levels when data are expressed on a per gram of tissue basis.
The most striking aspect of the experiment depicted in Fig. 1is
the marked change in the ratio of CPT II to CPT I when assays are
performed at our standard carnitine concentration of 200
µ
M.
H]etomoxir to label
both proteins, fluorographic analysis revealed that they do not follow
identical paths of developmental expression. The results with adult
heart confirmed our previous finding
(16, 17) that both
L- and M-CPT I are present but that the former is a very minor
constituent. In contrast, the relative intensity of the L-CPT I signal
was clearly seen to be higher in the perinatal period. Quantification
of the catalytic activity of each isoform was achieved by the use of
DNP-etomoxir, which selectively inhibits the liver enzyme
(17) .
Three important points emerged. First, it could now been seen that the
sum of the V
for total CPT I increased only
2-fold between days 1 and 29 after birth, returning to an
intermediate value in the adult rat. Second, the V
capacity of L-CPT I remained fairly consistent from the time of
birth to weaning and then fell by a factor of
3 as the animals
reached maturity, in keeping with the fluorographic data alluded to
above. Importantly, however, the contribution of L-CPT I to total
enzyme activity in V
terms was as high as 25% in
the perinatal period, falling precipitously during suckling and to its
low point of
2-3% in the adult rat. Third, this change in
the ratio of L- to M-CPT I in the developing heart was shown to be a
characteristic of the cardiac myocytes themselves and not of
non-myocyte cell type(s).
4% in the adult
rat, is almost 60% in 1-day-old pups. Whereas in the adult animal
carnitine is largely synthesized in liver (both de novo and
from butyrobetaine supplied from skeletal muscle), the major source of
this material for the newborn rat is the mother's milk
(19) . This may provide a teleological explanation for the high
expression of L-CPT I in the neonatal heart. After birth, the
animal's diet is rich in fat (derived from maternal milk), and
fatty acids, presumably together with ketone bodies, represent
important energy sources for heart function
(19) . However, the
M isoform of CPT I would be unable to operate efficiently at this time,
since sufficient carnitine has not yet accumulated in the tissue. A
transient high expression of the low K
L-CPT I would provide a mechanism for fatty acylcarnitine
synthesis despite the low tissue content of carnitine. As suckling
continues and the heart carnitine pool expands, the L form of CPT I
gradually disappears, with concomitant increased expression of the high
K
M variant, which then becomes the near
exclusive vehicle for initiating the fatty acid oxidation process. In
Fig. 6
we have provided a theoretical estimate of the contribution
of L-CPT I to cardiac fatty acid oxidation as the rat pup matures. It
must be emphasized that nonideal kinetic behavior of the CPT I isozymes
with regard to substrates, as well as subcellular compartmentation of
the latter, would influence the actual values, but the trend is solidly
apparent.
1 and 4 nmol/g,
wet weight, under various conditions
(18, 29, 30) and that M-CPT I displays such a low I
value
for the inhibitor (
0.03 µ
M). Presumably, most of the
cellular malonyl-CoA is in a bound state and unavailable for
interaction with CPT I. The additional possibility is now raised that
expression of some L-CPT I, whose I
for malonyl-CoA is
100-fold higher than that of M-CPT I
(18) , may permit fatty
acid oxidation to occur at a basal level even if the free concentration
of malonyl-CoA were to rise into an inhibitory range for the muscle
enzyme. In the rabbit, the heart content of malonyl-CoA has been
reported to decrease after birth in association with a lowered activity
of acetyl-CoA carboxylase and acceleration of fatty acid oxidation
(30) . A greater expression of L-CPT I in the perinatal period
(if indeed this applies to rabbit myocardium) would presumably offset
the higher malonyl-CoA level and allow fatty acid oxidation to occur at
a measurable rate.
(
)
-cell fall into this category.
(
)
Nor has it been established at this juncture that the
changing pattern of cardiac CPT I isoforms noted here in the developing
rat has a parallel in humans (although we have detected both L and M
forms of CPT I in adult human heart mitochondria labeled with
[
H]etomoxir). If it does, it could have important
implications for individuals with the homozygous form of the so-called
``hepatic'' CPT deficiency syndrome where CPT I has been
found to be deficient both in liver and fibroblasts
(32) . The
classic symptoms emphasized to date have been severe hypoketotic
hypoglycemia during episodes of starvation in infancy, sometimes
resulting in death
(33) . Is it possible that disturbances of
heart and kidney function, both of which have been reported in such
patients
(34) , have resulted, at least in part, from diminished
CPT I activity in these tissues? Finally, the fact that CPT I is now
known to exist in at least two different forms (and possibly more) will
need to be considered in the design of CPT I inhibitors should such
agents be employed for therapeutic purposes.
refers to the concentration of malonyl-CoA required to inhibit
CPT I activity by 50% under defined assay conditions.
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.