Carnitine palmitoyltransferase: separation of enzyme activity and malonyl-CoA binding in rat liver mitochondria

Substitution of glutamate-3, valine-19, leucine-23, and serine-24 with alanine in the N-terminal region of human heart muscle carnitine palmitoyltransferase I abolishes malonyl CoA inhibition and binding

The muscle isoform of carnitine palmitoyltransferase I (M-CPTI) is 30- to 100-fold more sensitive to malonyl CoA inhibition than the liver isoform (L-CPTI). We have previously shown that deletion of the first 28 N-terminal amino acid residues in M-CPTI abolished malonyl CoA inhibition and high-affinity binding [Biochemistry 39 (2000) 712-717]. To determine the role of specific residues within the first 28 N-terminal amino acids of human heart M-CPTI on malonyl CoA sensitivity and binding, we constructed a series of substitution mutations and a mutant M-CPTI composed of deletion 18 combined with substitution mutations V19A, L23A, and S24A. All mutants had CPT activity similar to that of the wild type. A change of Glu3 to Ala resulted in a 60-fold decrease in malonyl CoA sensitivity and loss of high-affinity malonyl CoA binding. A change of His5 to Ala in M-CPTI resulted in only a 2-fold decrease in malonyl CoA sensitivity and a significant loss in the low- but not high-affinity malonyl CoA binding. Deletion of the first 18 N-terminal residues combined with substitution mutations V19A, L23A, and S24A resulted in a mutant M-CPTI with an over 140-fold decrease in malonyl CoA sensitivity and a significant loss in both high- and low-affinity malonyl CoA binding. This was further confirmed by a combined four-residue substitution of Glu3, Val19, Leu23, and Ser24 with alanine. Our site-directed mutagenesis studies demonstrate that Glu3, Val19, Leu23, and Ser24 in M-CPTI are important for malonyl CoA inhibition and binding, but not for catalysis.

Functional characterization of mammalian mitochondrial carnitine palmitoyltransferases I and II expressed in the yeast Pichia pastoris

Mitochondrial carnitine palmitoyltransferases I and II (CPTI and CPTII), together with the carnitine carrier, transport long-chain fatty acyl-CoA from the cytosol to the mitochondrial matrix for beta-oxidation. Recent progress in the expression of CPTI and CPTII cDNA clones in Pichia pastoris, a yeast with no endogenous CPT activity, has greatly facilitated the characterization of these important enzymes in fatty acid oxidation. It is now well established that yeast-expressed CPTI is a catalytically active, malonyl CoA-sensitive, distinct enzyme that is reversibly inactivated by detergents. CPTII is a catalytically active, malonyl CoA-insensitive, distinct enzyme that is detergent stable. Reconstitution studies with yeast-expressed CPTI have established for the first time that detergent inactivation of CPTI is reversible, suggesting that CPTI is active only in a membrane environment. By constructing a series of deletion mutants of the N-terminus of liver CPTI, we have mapped the residues essential for malonyl CoA inhibition and binding to the conserved first six N-terminal amino acid residues. Mutation of glutamic acid 3 to alanine abolished malonyl CoA inhibition and high affinity malonyl CoA binding, but not catalytic activity, whereas mutation of histidine 5 to alanine caused partial loss in malonyl CoA inhibition. Our mutagenesis studies demonstrate that glutamic acid 3 and histidine 5 are necessary for malonyl CoA inhibition and binding to liver CPTI, but not catalytic activity.

A single amino acid change (substitution of glutamate 3 with alanine) in the N-terminal region of rat liver carnitine palmitoyltransferase I abolishes malonyl-CoA inhibition and high affinity binding

We have recently shown by deletion mutation analysis that the conserved first 18 N-terminal amino acid residues of rat liver carnitine palmitoyltransferase I (L-CPTI) are essential for malonyl-CoA inhibition and binding (Shi, J., Zhu, H., Arvidson, D. N. , Cregg, J. M., and Woldegiorgis, G. (1998) Biochemistry 37, 11033-11038). To identify specific residue(s) involved in malonyl-CoA binding and inhibition of L-CPTI, we constructed two more deletion mutants, Delta12 and Delta6, and three substitution mutations within the conserved first six amino acid residues. Mutant L-CPTI, lacking either the first six N-terminal amino acid residues or with a change of glutamic acid 3 to alanine, was expressed at steady-state levels similar to wild type and had near wild type catalytic activity. However, malonyl-CoA inhibition of these mutant enzymes was reduced 100-fold, and high affinity malonyl-CoA binding was lost. A mutant L-CPTI with a change of histidine 5 to alanine caused only partial loss of malonyl-CoA inhibition, whereas a mutant L-CPTI with a change of glutamine 6 to alanine had wild type properties. These results demonstrate that glutamic acid 3 and histidine 5 are necessary for malonyl-CoA binding and inhibition of L-CPTI by malonyl-CoA but are not required for catalysis.

Functional studies of yeast-expressed human heart muscle carnitine palmitoyltransferase I

Long-chain fatty acids are the primary source of energy production in the heart. Carnitine palmitoyltransferase I (CPT-I) catalyzes the first reaction in the transport of long-chain fatty acids from the cytoplasm to the mitochondrion, a rate-limiting step in beta-oxidation. In this study, we report the functional expression of the human heart/skeletal muscle isoform of CPT-I (M-CPT-I) in the yeast Pichia pastoris. Screening of a human heart cDNA library with cDNA fragments encoding the rat heart M-CPT-I resulted in the isolation of a single full-length human heart M-CPT-I cDNA clone. The clone has an open reading frame of 2316 bp with a 5' untranslated region of 38 bp and a 256-bp 3' untranslated region with the poly(A)+ addition sequence AATAAA. The predicted protein has 772 amino acids and a molecular mass of 88 kDa. Northern blot analysis of mRNAs from different human tissues using the human M-CPT-I cDNA as a probe revealed an abundant transcript of approximately 3.1 kb that was only present in human heart and skeletal muscle tissue. Expression of the human M-CPT-I cDNA in P. pastoris, a yeast with no endogenous CPT activity, produced an 80-kDa protein that was located in the mitochondria. Isolated mitochondria from the M-CPT-I expression strain exhibited a malonyl-coenzyme A (CoA)-sensitive CPT activity that was detergent labile. The I50 for malonyl-CoA inhibition of the yeast-expressed M-CPT-I was 69 nM, and the Kms for carnitine and palmitoyl-CoA were 666 and 42 microM, respectively. The I50 for malonyl-CoA inhibition of the heart enzyme is 30 times lower than that of the yeast-expressed liver CPT-I, and the Km for carnitine is more than 20 times higher than that of the liver CPT-I. This is the first report of the expression of a heart CPT-I in a system devoid of endogenous CPT activity and the functional characterization of a human heart M-CPT-I in the absence of the liver isoform and CPT-II.

Reconstitution of highly expressed human heart muscle carnitine palmitoyltransferase I

The human heart muscle carnitine palmitoyltransferase I (M-CPTI) gene was expressed at high levels from a strain of the methylotrophic yeast Pichia pastoris containing approximately 24 copies of the expression vector. Levels of M-CPTI were more than ten-fold higher than previously reported by our group with a single-copy strain (Arch. Biochem. Biophys., in press) and were sufficient to perform reconstitution studies on the membrane protein, a key step in purification and structural analysis of the enzyme. Solubilization of yeast mitochondria containing M-CPTI in 5% Triton X-100 abolished M-CPTI activity. The detergent-inactivated M-CPTI was then reconstituted by removal of the detergent in the presence of phospholipids. The reconstituted proteoliposomes exhibited M-CPTI activity of 2.4 nmol palmitoylcarnitine formed/mg protein/min, a recovery of 23% of the activity present in the starting mitochondrial preparation. The malonyl-CoA sensitivity of the reconstituted reactivated M-CPTI was 88%. This is the first demonstration of direct reactivation of malonyl-CoA-sensitive M-CPTI activity from solubilized materials from any organism. Previously, M-CPTI was presumed to be irreversibly inactivated by detergents.

Effect of pH and acyl-CoA chain length on the conversion of heart mitochondrial CPT-I/CPTo to a high affinity, malonyl-CoA-inhibited state

The effect of pH and acyl-CoA chain length on the conversion of the malonyl-CoA-sensitive carnitine palmitoyltransferase (CPT-I/CPTo) to a high-affinity, malonyl-CoA-inhibited state using a particle derived from rat heart mitochondria was determined. Preincubation with malonyl-CoA for one minute in the absence of acyl-CoA substrate lowers the IC50 for malonyl-CoA from 2 microM, 14 microM, and 15 microM at pH 7.4 to 15 nM, 14 nM, and 14 nM for decanyl-, lauryl-, and palmitoyl-CoA, respectively. Reducing the pH to 7.1 and 6.8 had little effect on the transition to the high affinity, malonyl-CoA-inhibited state. Preincubation of malonyl-CoA with the acyl-CoA, but not with L-carnitine, prevented the transition to the high affinity, malonyl-CoA-inhibited state.

Temperature effects on malonyl-CoA inhibition of carnitine palmitoyltransferase I

Malonyl-CoA inhibition of hepatic mitochondrial carnitine palmitoyltransferase I and malonyl-CoA binding were measured at temperatures ranging from 0 degrees C to 37 degrees C. Protease treatment of mitochondria resulted in greatly diminished malonyl-CoA binding, indicating that the method used detected malonyl-CoA binding sites located on the outer surface of the mitochondrial outer membrane as expected. The apparent Ki for malonyl-CoA inhibition was found to increase with increasing temperature. Arrhenius plots for the initial velocity of the enzymatic reaction and for the Ki for malonyl-CoA both indicated a transition temperature between 20 and 25 degrees C with the transition for the malonyl-CoA interaction being more pronounced. Total specific binding of malonyl-CoA to mitochondrial proteins increased with increasing temperature, and Kd values decreased. The opposite effect of temperature on Kd values and Ki values was surprising because it was expected that these equilibrium constants would be identical. These observations indicate that Kd values for malonyl-CoA binding and Ki values for inhibition of carnitine palmitoyltransferase I by malonyl-CoA represent two significantly different binding phenomena. These data suggest that either: (a) malonyl-CoA binding measurements are unrelated to malonyl-CoA inhibition, or (b) inhibition of carnitine palmitoyltransferase I by malonyl-CoA involves more complex relationships than binding of malonyl-CoA to a single protein.

Diabetes and proteolysis: effects on carnitine palmitoyltransferase-I and malonyl-CoA binding

Malonyl-CoA binding and malonyl-CoA inhibition of carnitine palmitoyltransferase-I (CPT-I) were measured in hepatic mitochondria from normal and diabetic rats and in protease-treated mitochondria from fed rats to test the hypothesis that proteolysis represents a mechanism by which diabetes produces changes in the sensitivity of CPT-I to inhibition by malonyl-CoA. As in diabetes, protease treatment increased the apparent Ki values for malonyl-CoA. Palmitoyl-CoA greatly diminished malonyl-CoA specific binding in the mitochondrial system being studied, suggesting strong competition at the malonyl-CoA binding site. Proteolysis decreased capacity for specific binding of malonyl-CoA by 60-80%, but it had no effect on binding affinity. In contrast, the decreased specific binding of malonyl-CoA seen in the diabetic state is accompanied by increased binding affinity. Furthermore, observed Kd values differed from Ki values by a factor of 10 or more, suggesting that measured Kd and Ki may represent different ligand-protein complexes. These data suggest that alterations in inhibition of CPT-I by malonyl-CoA occurring in the diabetic state may involve mechanisms other than simple proteolytic removal of malonyl-CoA binding sites.

Cholate extracts of mitochondrial outer membranes increase inhibition by malonyl-CoA of carnitine palmitoyltransferase-I by a mechanism involving phospholipids

It has been reported that sodium cholate can separate the catalytic component of carnitine palmitoyltransferase-I (CPT-I) from a putative malonyl-CoA-binding regulatory protein capable of conferring sensitivity to malonyl-CoA on CPT-II. We found that cholate preferentially extracted a contaminating malonyl-CoA-sensitive CPT from mitochondrial inner membranes. When cholate extracts of outer membranes were incubated either with cholate extracts of inner membranes or with osmotically swollen mitochondria, inhibition of CPT by malonyl-CoA was increased. Treatment of intact mitochondria with subtilisin abolished the increased inhibition by malonyl-CoA, suggesting that the outer-membrane CPT-I was responsible for the increased inhibition. Incubation of cholate extracts with proteinase K did not prevent the increased inhibition. Fractionation of the cholate extract indicated the presence of phospholipids. Addition of cardiolipin or phosphatidylglycerol to osmotically swollen mitochondria increased sensitivity of CPT to malonyl-CoA, but several other phospholipids did not. When cardiolipin was added to intact mitochondria from either starved or fed rats, there were large increases in inhibition by malonyl-CoA; sensitivity in mitochondria from starved rats increased to that normally observed with mitochondria from fed rats. These results suggest that phospholipids are responsible for the increased inhibition of CPT by malonyl-CoA with added cholate extracts and that changes in membrane composition may be involved in the physiological regulation of CPT-I.

Malonyl-CoA inhibition of peroxisomal carnitine octanoyltransferase

Although the malonyl-CoA sensitivity of peroxisomal carnitine octanoyltransferase (COT) is reportedly lost on solubilization, we show that malonyl-CoA does inhibit the purified enzyme. Assay conditions such as buffer composition, pH, acyl-CoA substrate and the presence or absence of BSA can affect the observed inhibition. When assayed in the absence of BSA, COT shows simple competitive inhibition by malonyl-CoA. The Ki value for inhibition of purified COT is high (106 microM) compared with physiological concentrations (1-6 microM) and other short-chain acyl-CoA esters inhibit COT to the same degree. However, when COT is assayed in intact peroxisomes, the Ki for malonyl-CoA is almost 20-fold lower than found with the purified enzyme, whereas inhibition by other short-chain acyl-CoA esters does not change significantly. Several features of the inhibition of peroxisomal COT, including the specificity of malonyl-CoA over other short-chain acyl-CoA esters, resemble those of carnitine palmitoyltransferase (CPT)-I, suggesting that the regulation of COT and CPT-I in parallel may be necessary for the control of cellular fatty acid metabolism.

Physiological state and the sensitivity of liver mitochondrial outer membrane carnitine palmitoyltransferase to malonyl-CoA. Correlations with assay temperature, salt concentration and membrane lipid composition

1. Liver mitochondrial outer membranes were pre-exposed to media of low (20 mM phosphate) or high salt concentration (20 mM phosphate + 0.3 M KCl) before assay of carnitine palmitoyltransferase (CPT) at 25 degrees C. 2. With membranes from fed rats, exposure to high salt decreased sensitivity of CPT to malonyl-CoA whereas high salt increased sensitivity of CPT to malonyl-CoA in membranes from 48 hr-fasted rats. These changes were paralleled by alterations in the KD for high affinity binding of [14C]malonyl-CoA to outer membranes. 3. Decreasing the CPT assay temperatures from 25 to 10 degrees C caused qualitatively similar changes to those seen on exposure to high salt. 4. The relative content of sphingomyelin was increased 2-fold and 4-fold in liver mitochondrial outer membranes from fasted and diabetic rats respectively. Fasting had no effect on the content of cholesterol whereas diabetes decreased this by a third.

Restoration of malonyl-CoA sensitivity of soluble rat liver mitochondria carnitine palmitoyltransferase by reconstitution with a partially purified malonyl-CoA binding protein

Solubilization of rat liver mitochondria in 5% Triton X-100 followed by chromatography on a hydroxylapatite column resulted in the identification of malonyl-CoA binding protein(s) distinct from a major carnitine palmitoyltransferase activity peak. Further purification of the malonyl-CoA binding protein(s) on an acyl-CoA affinity column followed by sodium dodecyl sulfate gel electrophoresis indicated proteins with Mr mass of 90 and 45-33 kDa. A purified liver malonyl-CoA binding fraction, which was devoid of carnitine palmitoyltransferase, and a soluble malonyl-CoA-insensitive carnitine palmitoyltransferase were reconstituted by dialysis in a liposome system. The enzyme activity in the reconstituted system was decreased by 50% in the presence of 100 microM malonyl-CoA. Rat liver mitochondria carnitine palmitoyltransferase may be composed of an easily dissociable catalytic unit and a malonyl-CoA sensitivity conferring regulatory component.

Use of mitochondrial inner membrane proteins and phospholipids to facilitate disengagement of the catalytic and malonyl-CoA binding components of carnitine palmitoyltransferase from liver mitochondrial outer membranes

1. It was shown by Ghadiminejad and Saggerson (1991) that the anionic detergent cholate caused disengagement of the malonyl-CoA binding entity from the catalytic entity of outer membrane carnitine palmitoyltransferase (CPT1). 2. This disengagement was only observed if inner membrane material was present. 3. It is now shown that this effect is mimicked by a CPT-free inner membrane protein fraction together with an inner membrane lipid extract or with individual phospholipids (phosphatidylcholine, phosphatidylethanolamine or diphosphatidylglycerol). 4. The lipids alone have no effect but act synergistically with the inner membrane protein fraction.

Regulation of mitochondrial carnitine palmitoyl transferases from liver and extrahepatic tissues

Developments in our understanding of the complex CPT enzyme system over the past ten years have been reviewed. Liver CPT1, which is probably distinct from that in several extrahepatic tissues, is subject to up- or down-regulation of its activity and kinetic properties with changing physiological state. Evidence is now accumulating to support the notion that the catalytic and malonyl-CoA-binding entities of CPT1 are separate polypeptides.

Cholate separates the catalytic and malonyl-CoA-binding components of carnitine palmitoyltransferase from liver outer mitochondrial membranes

Sodium cholate was used as an anionic detergent to discriminate the two components of liver overt carnitine palmitoyltransferase (CPT1); namely a catalytic entity and a regulatory component that bound malonyl-CoA. Cholate solubilized approx. 40% of the malonyl-CoA binding entity from mitochondrial outer membranes without appreciable solubilization of CPT1 activity. Cholate did not interfere with binding of [14C]malonyl-CoA to outer membranes or to crude total mitochondrial membrane fractions. By contrast, the non-ionic detergent Tween-20 was ineffective in solubilizing the malonyl-CoA binding entity and also substantially interfered with the binding of [14C]malonyl-CoA. Both detergents appeared to cause total disengagement of the malonyl-CoA binding entity from the catalytic entity of CPT1 only when some inner membrane material was present. 'Reconstitution' experiments were performed in which a malonyl-CoA sensitivity conferring factor in cholate extracts from outer membranes was associated with CPT derived from inner membranes (CPT2). The IC50 for inhibition of CPT2 by malonyl-CoA in this artificial system was similar to that observed with CPT1 in situ in outer membranes. Extracts containing malonyl-CoA sensitivity conferring factor derived from outer membranes of fed or 48 h fasted rats were associated with CPT2 derived from fed rats. The outer membrane extracts from fasted animals conferred a lower maximum responsiveness to malonyl-CoA, but appeared to have a higher affinity for CPT2 than the extracts from fed rats. These results suggest that physiological state can alter the intrinsic properties of the malonyl-CoA sensitivity confering factor.

cDNA cloning, sequence analysis, and chromosomal localization of the gene for human carnitine palmitoyltransferase

We have cloned and sequenced a cDNA encoding human liver carnitine palmitoyltransferase (CPTase; palmitoyl-CoA:L-carnitine O-palmitoyltransferase, EC 2.3.1.21), an inner mitochondrial membrane enzyme that plays a major role in the fatty acid oxidation pathway. Mixed oligonucleotide primers whose sequences were deduced from one tryptic peptide obtained from purified CPTase were used in a polymerase chain reaction, allowing the amplification of a 0.12-kilobase fragment of human genomic DNA encoding such a peptide. A 60-base-pair (bp) oligonucleotide synthesized on the basis of the sequence from this fragment was used for the screening of a cDNA library from human liver and hybridized to a cDNA insert of 2255 bp. This cDNA contains an open reading frame of 1974 bp that encodes a protein of 658 amino acid residues including 25 residues of an NH2-terminal leader peptide. The assignment of this open reading frame to human liver CPTase is confirmed by matches to seven different amino acid sequences of tryptic peptides derived from pure human CPTase and by the 82.2% homology with the amino acid sequence of rat CPTase. The NH2-terminal region of CPTase contains a leucine-proline motif that is shared by carnitine acetyl- and octanoyltransferases and by choline acetyltransferase. The gene encoding CPTase was assigned to human chromosome 1, region 1q12-1pter, by hybridization of CPTase cDNA with a DNA panel of 19 human-hamster somatic cell hybrids.

Sensitivity of inhibition of rat liver mitochondrial outer-membrane carnitine palmitoyltransferase by malonyl-CoA to chemical- and temperature-induced changes in membrane fluidity

We have tested the possibility that alterations in the fluidity of the outer membrane of rat liver mitochondria could result in changes in the sensitivity of overt carnitine palmitoyltransferase (CPT I) to malonyl-CoA [Zammit (1986) Biochem. Soc. Trans. 14. 676-679]. The sensitivity of CPT I to malonyl-CoA inhibition was measured by using highly purified mitochondrial outer membranes prepared from fed or 48 h-starved rats in the presence and absence of agents that increase membrane fluidity by perturbing membrane lipid order [benzyl alcohol, isoamyl alcohol (3-methylbutan-l-ol) and 2-(2-methoxyethoxy)ethyl-8-(cis-2-n-octylpropyl)octanoate (A2C)]. All these agents resulted in marked decreases in the ability of malonyl-CoA to inhibit CPT I. This effect was accompanied by a modest increase in the absolute activity of CPT I in the absence of malonyl-CoA when the short-chain alcohols were used, but not when A2C was used, suggesting that the effect of increased membrane fluidity to decrease the malonyl-CoA sensitivity of CPT I may occur independently from other actions that may affect more directly the active site of the enzyme. In confirmation of the potential importance of fluidity changes, we showed that a marked increase in sensitivity of CPT I to malonyl-CoA could be produced when assays were performed at lower temperatures than those normally employed. These observations are discussed in the context of the slowness of the changes in CPT I sensitivity to malonyl-CoA inhibition that are induced by physiological perturbations.

The relationship of rat liver overt carnitine palmitoyltransferase to the mitochondrial malonyl-CoA binding entity and to the latent palmitoyltransferase

1. Confirming previous work [Murthy & Pande (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 378-382], malonyl-CoA-inhibitable carnitine palmitoyltransferase (CPT1) from rat liver was found to be localized in outer rather than in inner mitochondrial membranes. 2. Antisera were raised against a liver mitochondrial CPT of Mr 68,000, which was presumed to be the latent from of the enzyme (CPT2). These antisera cross-reacted with solubilized CPT extracted from liver inner mitochondrial membranes and with polypeptides of Mr 68,000 and 60,000 in immunoblots of both inner and outer mitochondrial membranes. The antisera also precipitated CPT activity from detergent-treated total membrane and outer-membrane preparations. 3. The antisera did not precipitate [14C]malonyl-CoA binding material obtained either from total membranes or from outer membranes. 4. It was concluded that liver CPT1 and CPT2 have some epitopes in common and may have a similar subunit size. In addition, CPT1 and the entity that binds malonyl-CoA must be separated polypeptides.

Evidence that the sensitivity of carnitine palmitoyltransferase I to inhibition by malonyl-CoA is an important site of regulation of hepatic fatty acid oxidation in the fetal and newborn rabbit. Perinatal development and effects of pancreatic hormones in cultured rabbit hepatocytes

The temporal changes in oleate oxidation, lipogenesis, malonyl-CoA concentration and sensitivity of carnitine palmitoyltransferase I (CPT 1) to malonyl-CoA inhibition were studied in isolated rabbit hepatocytes and mitochondria as a function of time after birth of the animal or time in culture after exposure to glucagon, cyclic AMP or insulin. (1) Oleate oxidation was very low during the first 6 h after birth, whereas lipogenesis rate and malonyl-CoA concentration decreased rapidly during this period to reach levels as low as those found in 24-h-old newborns that show active oleate oxidation. (2) The changes in the activity of CPT I and the IC50 (concn. causing 50% inhibition) for malonyl-CoA paralleled those of oleate oxidation. (3) In cultured fetal hepatocytes, the addition of glucagon or cyclic AMP reproduced the changes that occur spontaneously after birth. A 12 h exposure to glucagon or cyclic AMP was sufficient to inhibit lipogenesis totally and to cause a decrease in malonyl-CoA concentration, but a 24 h exposure was required to induce oleate oxidation. (4) The induction of oleate oxidation by glucagon or cyclic AMP is triggered by the fall in the malonyl-CoA sensitivity of CPT I. (5) In cultured hepatocytes from 24 h-old newborns, the addition of insulin inhibits no more than 30% of the high oleate oxidation, whereas it stimulates lipogenesis and increases malonyl-CoA concentration by 4-fold more than in fetal cells (no oleate oxidation). This poor effect of insulin on oleate oxidation seems to be due to the inability of the hormone to increase the sensitivity of CPT I sufficiently. Altogether, these results suggest that the malonyl-CoA sensitivity of CPT I is the major site of regulation during the induction of fatty acid oxidation in the fetal rabbit liver.

Evidence for distinct functional molecular sizes of carnitine palmitoyltransferases I and II in rat liver mitochondria

1. Estimates of the functional sizes of the molecular species responsible for the overt (I) and latent (II) activities of carnitine palmitoyltransferase (CPT) in 48 h-starved rat liver mitochondria were obtained from radiation inactivation experiments. 2. The decay in the activity of total CPT and that of CPT II only (after inhibition of CPT I) was measured in mitochondrial samples exposed to different doses of high-energy ionizing radiation. 3. The decay curves obtained by plotting residual activity of total CPT as a logarithm function of irradiation dose suggested the contribution of more than one target towards total CPT activity. 4. By contrast, in mitochondria in which CPT I activity was approximately 95% inhibited, the activity of CPT decayed in a simple mono-exponential manner. Target-size analysis yielded an approximate Mr of 69,700 for this component (CPT II). 5. This information, as well as that on the relative non-irradiated activities of CPT I and CPT II, was used in graphical and statistical methods to obtain the parameters of the decay curve for CPT I. These analyses yielded an approximate Mr of 96,700 for CPT I.

Effect of malonyl-CoA on the kinetics and substrate cooperativity of membrane-bound carnitine palmitoyltransferase of rat heart mitochondria

The effect of malonyl-CoA on the kinetic parameters of carnitine palmitoyltransferase (outer) the outer form of carnitine palmitoyltransferase (palmitoyl-CoA: L-carnitine O-palmitoyltransferase, EC 2.3.1.21) from rat heart mitochondria was investigated using a kinetic analyzer in the absence of bovine serum albumin with non-swelling conditions and decanoyl-CoA as the cosubstrate. The K0.5 for decanoyl-CoA is 3 microM for heart mitochondria from both fed and fasted rats. Membrane-bound carnitine palmitoyltransferase (outer) shows substrate cooperativity for both carnitine and acyl-CoA, similar to that exhibited by the enzyme purified from bovine heart mitochondria. The Hill coefficient for decanoyl-CoA varied from 1.5 to 2.0, depending on the method of assay and the preparation of mitochondria. Malonyl-CoA increased the K0.5 for decanoyl-CoA with no apparent increase in sigmoidicity or Vmax. With 20 microM malonyl-CoA and a Hill coefficient of n = 2.1, the K0.5 for decanoyl-CoA increased to 185 microM. Carnitine palmitoyltransferase (outer) from fed rats had an apparent Ki for malonyl-CoA of 0.3 microM, while that from 48-h-fasted rats was 2.5 microM. The kinetics with L-carnitine were variable: for different preparations of mitochondria, the K0.5 ranged from 0.2 to 0.7 mM and the Hill coefficient varied from 1.2 to 1.8. When an isotope forward assay was used to determine the effect of malonyl-CoA on carnitine palmitoyltransferase (outer) activity of heart mitochondria from fed and fasted animals, the difference was much less than that obtained using a continuous rate assay. Carnitine palmitoyltransferase (outer) was less sensitive to malonyl-CoA at low compared to high carnitine concentrations, particularly with mitochondria from fasted animals. The data show that carnitine palmitoyltransferase (outer) exhibits substrate cooperativity for both acyl-CoA and L-carnitine in its native state. The data show that membrane-bound carnitine palmitoyltransferase (outer) like carnitine palmitoyltransferase purified from heart mitochondria exhibits substrate cooperativity indicative of allosteric enzymes and indicate that malonyl-CoA acts like a negative allosteric modifier by shifting the acyl-CoA saturation to the right. A slow form of membrane-bound carnitine palmitoyltransferase (outer) was not detected, and thus, like purified carnitine palmitoyltransferase, substrate-induced hysteretic behavior is not the cause of the positive substrate cooperativity.

Carnitine palmitoyltransferase: characterization of a labile detergent-extracted malonyl-CoA-sensitive enzyme from rat liver mitochondria

Rat liver mitochondria were preextracted with Triton X-100 in the absence of salts to remove malonyl-CoA-insensitive carnitine palmitoyltransferase. From the remaining membrane residues a malonyl-CoA-sensitive enzyme was solubilized with octyl glucopyranoside in the presence of KCl. Significant enzyme activity, [2-14C]malonyl-CoA binding and malonyl-CoA inhibition of this enzyme was present only after removal of detergent by precipitation with poly(ethylene glycol). The enzyme activity was rapidly lost in the solubilized form. High concentrations of glycerol protected the enzyme. The alkylating irreversible inhibitor, S-(4-bromo-2,3-dioxobutyl)-CoA, strongly inhibited the malonyl-CoA-sensitive enzyme in the membrane residues. The enzyme was protected against this inhibitor by malonyl-CoA and palmitoyl-CoA. The more loosely membrane-bound malonyl-CoA-insensitive enzyme failed to bind malonyl-CoA, was stable in the presence of detergents and was not inhibited by S-(4-bromo-2,3-dioxobutyl)-CoA. It is suggested that two different carnitine palmitoyltransferase proteins exist in the inner mitochondrial membrane and that the detergent-labile malonyl-CoA-sensitive enzyme is the less easily extracted of the two.