Importance of acyl-CoA availability in interpretation of carnitine palmitoyltransferase I kinetics

Personalised modelling of clinical heterogeneity between medium-chain acyl-CoA dehydrogenase patients

BACKGROUND

Monogenetic inborn errors of metabolism cause a wide phenotypic heterogeneity that may even differ between family members carrying the same genetic variant. Computational modelling of metabolic networks may identify putative sources of this inter-patient heterogeneity. Here, we mainly focus on medium-chain acyl-CoA dehydrogenase deficiency (MCADD), the most common inborn error of the mitochondrial fatty acid oxidation (mFAO). It is an enigma why some MCADD patients-if untreated-are at risk to develop severe metabolic decompensations, whereas others remain asymptomatic throughout life. We hypothesised that an ability to maintain an increased free mitochondrial CoA (CoASH) and pathway flux might distinguish asymptomatic from symptomatic patients.

RESULTS

We built and experimentally validated, for the first time, a kinetic model of the human liver mFAO. Metabolites were partitioned according to their water solubility between the bulk aqueous matrix and the inner membrane. Enzymes are also either membrane-bound or in the matrix. This metabolite partitioning is a novel model attribute and improved predictions. MCADD substantially reduced pathway flux and CoASH, the latter due to the sequestration of CoA as medium-chain acyl-CoA esters. Analysis of urine from MCADD patients obtained during a metabolic decompensation showed an accumulation of medium- and short-chain acylcarnitines, just like the acyl-CoA pool in the MCADD model. The model suggested some rescues that increased flux and CoASH, notably increasing short-chain acyl-CoA dehydrogenase (SCAD) levels. Proteome analysis of MCADD patient-derived fibroblasts indeed revealed elevated levels of SCAD in a patient with a clinically asymptomatic state. This is a rescue for MCADD that has not been explored before. Personalised models based on these proteomics data confirmed an increased pathway flux and CoASH in the model of an asymptomatic patient compared to those of symptomatic MCADD patients.

CONCLUSIONS

We present a detailed, validated kinetic model of mFAO in human liver, with solubility-dependent metabolite partitioning. Personalised modelling of individual patients provides a novel explanation for phenotypic heterogeneity among MCADD patients. Further development of personalised metabolic models is a promising direction to improve individualised risk assessment, management and monitoring for inborn errors of metabolism.

Effects of high-intensity interval versus continuous moderate-intensity aerobic exercise on apoptosis, oxidative stress and metabolism of the infarcted myocardium in a rat model

The optimal aerobic exercise training (AET) protocol for patients following myocardial infarction (MI) has remained under debate. The present study therefore aimed to compare the effects of continuous moderate-intensity training (CMT) and high-intensity interval training (HIT) on cardiac functional recovery, and to investigate the potential associated mechanisms in a post-MI rat model. Female Sprague Dawley rats (8-10 weeks old) undergoing MI or sham surgery were subsequently submitted to CMT or HIT, or kept sedentary for eight weeks. Prior to and following AET, echocardiographic parameters and exercise capacity of the rats were measured. Western blotting was used to evaluate the levels of apoptosis and associated signaling pathway protein expression. The concentrations of biomarkers of oxidative stress were also determined by ELISA assay. Messenger (m)RNA levels and activity of the key enzymes for glycolysis and fatty acid oxidation, as well as the rate of adenosine triphosphate (ATP) synthesis, were also measured. Compared with the MI group, exercise capacity and cardiac function were significantly improved following AET, particularly following HIT. Left ventricular ejection fraction and fraction shortening were further improved in the MI-HIT group in comparison to that of the MI-CMT group. The two forms of AET almost equally attenuated apoptosis of the post-infarction myocardium. CMT and HIT also alleviated oxidative stress by decreasing the concentration of malondialdehyde and increasing the concentration of superoxide dismutase and glutathione peroxidase (GPx). In particular, HIT induced a greater increase in the concentration of GPx than that of CMT. AET, and HIT in particular, significantly increased the levels of mRNA and the maximal activity of phosphofructokinase-1 and carnitine palmitoyl transferase-1, as well as the maximal ratio of ATP synthesis. In addition, compared with the MI group, the expression of signaling proteins PI3K, Akt, p38mapk and AMPK was significantly altered in the MI-CMT and MI-HIT groups. HIT was superior to CMT in its ability to improve cardiac function and exercise capability in a post-MI rat model. HIT was also superior to CMT with regard to attenuating oxidative stress and improving glucolipid metabolism of the post-MI myocardium.

Ultra-fast simultaneous detection of obesity-related coenzymes in mice using microchip electrophoresis with a LIF detector

Hepatic acyl-coenzyme A synthetase (ACS), carnitine palmitoyltransferase-I (CPT-I) and acetyl coenzyme A carboxylase (ACC) are coenzymes associated with the genetic type of obesity in animal models. This paper reports the use of microchip electrophoresis (ME) with a laser-induced fluorescence (LIF) detector based on a reverse transcriptase-polymerase chain reaction (RT-PCR) to detect the amplified DNA fragments of these coenzymes (ACS, CPT-I and ACC) in the mRNA extracted from mice. DNA fragments ranging from 50 to 2652 bp were well resolved using this procedure with a running buffer (1x TBE), 0.5% polyvinylpyrrolidone (M(r) 1,000,000) as the coating gel and 0.7% polyethyleneoxide (M(r) 8,000,000) as the sieving gel at pH 8.30. The separation of the three RT-PCR products was achieved by ME in a single-run within 17 s using programmed field strength gradients (PFSG) (470 V cm(-1) for 9 s, 205.8 V cm(-1) for 2 s, 411.6 V cm(-1) for 4 s, 117.6 V cm(-1) for 2 s and 470.4V cm(-1) for 8 s). The ME-PFSG method was found to be 4 times faster than the method using a constant field and 138 times faster than slab gel electrophoresis. Moreover, the amplified RT-PCR products of the obesity-related coenzymes in C57BL/6J mice were analyzed using only sub-micro liter samples.

Rat Liver Carnitine Palmitoyltransferase 1 Forms an Oligomeric Complex within the Outer Mitochondrial Membrane

Carnitine palmitoyltransferase (CPT) 1A catalyzes the rate-limiting step in the transport of long chain acyl-CoAs from cytoplasm to the mitochondrial matrix by converting them to acylcarnitines. Located within the outer mitochondrial membrane, CPT1A activity is inhibited by malonyl-CoA, its allosteric inhibitor. In this study, we investigate for the first time the quaternary structure of rat CPT1A. Chemical cross-linking studies using intact mitochondria isolated from fed rat liver or from Saccharomyces cerevisiae expressing CPT1A show that CPT1A self-assembles into an oligomeric complex. Size exclusion chromatography experiments using solubilized mitochondrial extracts suggest that the fundamental unit of its quaternary structure is a trimer. When studied in blue native-PAGE, the CPT1A hexamer could be observed, however, suggesting that under these native conditions CPT1A trimers might be arranged as dimers. Moreover, the oligomeric state of CPT1A was found unchanged by starvation and by streptozotocin-induced diabetes, conditions characterized by changes in malonyl-CoA sensitivity of CPT1A. Finally, gel filtration analysis of several yeast-expressed chimeric CPTs demonstrates that the first 147 N-terminal residues of CPT1A, encompassing its two transmembrane segments, trigger trimerization independently of its catalytic C-terminal domain. Deletion of residues 1-82, including transmembrane 1, did not abrogate oligomerization, but the latter is limited to a trimer by the presence of the large catalytic C-terminal domain on the cytosolic face of mitochondria. Based on these findings, we proposed that the oligomeric structure of CPT1A would allow the newly formed acylcarnitines to gain direct access into the intermembrane space, hence facilitating substrate channeling.

Cysteine-scanning mutagenesis of muscle carnitine palmitoyltransferase I reveals a single cysteine residue (Cys-305) is important for catalysis

Carnitine palmitoyltransferase (CPT) I catalyzes the conversion of long-chain fatty acyl-CoAs to acyl carnitines in the presence of l-carnitine, a rate-limiting step in the transport of long-chain fatty acids from the cytoplasm to the mitochondrial matrix. To determine the role of the 15 cysteine residues in the heart/skeletal muscle isoform of CPTI (M-CPTI) on catalytic activity and malonyl-CoA sensitivity, we constructed a 6-residue N-terminal, a 9-residue C-terminal, and a 15-residue cysteineless M-CPTI by cysteine-scanning mutagenesis. Both the 9-residue C-terminal mutant enzyme and the complete 15-residue cysteineless mutant enzyme are inactive but that the 6-residue N-terminal cysteineless mutant enzyme had activity and malonyl-CoA sensitivity similar to those of wild-type M-CPTI. Mutation of each of the 9 C-terminal cysteines to alanine or serine identified a single residue, Cys-305, to be important for catalysis. Substitution of Cys-305 with Ala in the wild-type enzyme inactivated M-CPTI, and a single change of Ala-305 to Cys in the 9-residue C-terminal cysteineless mutant resulted in an 8-residue C-terminal cysteineless mutant enzyme that had activity and malonyl-CoA sensitivity similar to those of the wild type, suggesting that Cys-305 is the residue involved in catalysis. Sequence alignments of CPTI with the acyltransferase family of enzymes in the GenBank led to the identification of a putative catalytic triad in CPTI consisting of residues Cys-305, Asp-454, and His-473. Based on the mutagenesis and substrate labeling studies, we propose a mechanism for the acyltransferase activity of CPTI that uses a catalytic triad composed of Cys-305, His-473, and Asp-454 with Cys-305 serving as a probable nucleophile, thus acting as a site for covalent attachment of the acyl molecule and formation of a stable acyl-enzyme intermediate. This would in turn allow carnitine to act as a second nucleophile and complete the acyl transfer reaction.

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.

Leucine-764 near the extreme C-terminal end of carnitine palmitoyltransferase I is important for activity

Muscle carnitine palmitoyltransferase I (M-CPTI) catalyzes the conversion of long-chain fatty acyl-CoAs to acylcarnitines in the presence of L-carnitine. To determine the role of the C-terminal region of M-CPTI in enzyme activity, we constructed a series of deletion and substitution mutants. The mutants were expressed in the yeast Pichia pastoris, and the effect of the mutations on M-CPTI activity and malonyl-CoA sensitivity was determined in isolated mitochondria prepared from the yeast strains expressing the wild-type and deletion mutants. Deletion of the last 210, 113, 44, 20, 10, and 9 C-terminal amino-acid residues resulted in an inactive M-CPTI, but deletion of the last 8, 7, 6, and 3 C-terminal residues had no effect on activity, demonstrating that leucine-764 (L764) is essential for catalysis. Substitution of L764 with alanine caused a 40% loss in catalytic activity, but replacement of L764 with arginine resulted in an 84% loss of activity; substitution of L764 with valine had no effect on catalytic activity. The catalytic efficiency for the L764R mutant decreased by 80% for both substrates. Secondary structure prediction of the M-CPTI sequence identified a 21-amino-acid residue, 744-764, predicted to fold into a coiled-coil alpha-helix in the extreme C-terminal region of M-CPTI that may be important for native folding and activity. In summary, our data demonstrate that deletion of L764 or substitution with arginine inactivates the enzyme, suggesting that L764 may be important for proper folding of M-CPTI and optimal activity.

Continuous recording of long-chain acyl-coenzyme a synthetase activity using fluorescently labeled bovine serum albumin

The fluorescence-based long-chain fatty acid probe BSA-HCA (bovine serum albumin labeled with 7-hydroxycoumarin-4-acetic acid) is shown to respond to binding of long-chain acyl-CoA thioesters by quenching of the 450 nm fluorescence emission. As determined by spectrofluorometric titration, binding affinities for palmitoyl-, stearoyl-, and oleoyl-CoA (Kd = 0.2-0.4 microM) are 5-10 times lower than those for the corresponding nonesterified fatty acids. In the presence of detergent (Chaps, Triton X-100, n-octylglucoside) above the critical micelle concentration, acyl-CoA partitions from BSA-HCA and into the detergent micelles. This allows BSA-HCA to be used as a fluorescent probe for continuous recording of fatty acid concentrations in detergent solution with little interference from acyl-CoA. Using a calibration of the fluorescence signal with fatty acids in the C14 to C20 chain-length range, fatty acid consumption by Pseudomonas fragi and rat liver microsomal acyl-CoA synthetase activities are measured down to 0.05 microM/min with a data sampling rate of 10 points per second. This new method provides a very promising spectrofluorometric approach to the study of acyl-CoA synthetase reaction kinetics at physiologically relevant (nM) aqueous phase concentrations of fatty acid substrates and at a time resolution that cannot be obtained in isotopic sampling or enzyme-coupled assays.

Molecular enzymology of carnitine transfer and transport

Carnitine (L-3-hydroxy-4-N-trimethylaminobutyric acid) forms esters with a wide range of acyl groups and functions to transport and excrete these groups. It is found in most cells at millimolar levels after uptake via the sodium-dependent carrier, OCTN2. The acylation state of the mobile carnitine pool is linked to that of the limited and compartmentalised coenzyme A pools by the action of the family of carnitine acyltransferases and the mitochondrial membrane transporter, CACT. The genes and sequences of the carriers and the acyltransferases are reviewed along with mutations that affect activity. After summarising the accepted enzymatic background, recent molecular studies on the carnitine acyltransferases are described to provide a picture of the role and function of these freely reversible enzymes. The kinetic and chemical mechanisms are also discussed in relation to the different inhibitors under study for their potential to control diseases of lipid metabolism.

The first 28 N-terminal amino acid residues of human heart muscle carnitine palmitoyltransferase I are essential for malonyl CoA sensitivity and high-affinity binding

Heart/skeletal muscle carnitine palmitoyltransferase I (M-CPTI) is 30-100-fold more sensitive to malonyl CoA inhibition than the liver isoform (L-CPTI). To determine the role of the N-terminal region of human heart M-CPTI on malonyl CoA sensitivity and binding, a series of deletion mutations were constructed ranging in size from 18 to 83 N-terminal residues. All of the deletions except Delta83 were active. Mitochondria from the yeast strains expressing Delta28 and Delta39 exhibited a 2.5-fold higher activity compared to the wild type, but were insensitive to malonyl CoA inhibition and had complete loss of high-affinity malonyl CoA binding. The high-affinity site (K(D1), B(max1)) for binding of malonyl CoA to M-CPTI was completely abolished in the Delta28, Delta39, Delta51, and Delta72 mutants, suggesting that the decrease in malonyl CoA sensitivity observed in these mutants was due to the loss of the high-affinity binding entity of the enzyme. Delta18 showed only a 4-fold loss in malonyl CoA sensitivity but had activity and high-affinity malonyl CoA binding similar to the wild type. Replacement of the N-terminal domain of L-CPTI with the N-terminal domain of M-CPTI does not change the malonyl CoA sensitivity of the chimeric L-CPTI, suggesting that the amino acid residues responsible for the differing sensitivity to malonyl CoA are not located in this N-terminal region. These results demonstrate that the N-terminal residues critical for activity and malonyl CoA sensitivity in M-CPTI are different from those of L-CPTI.

Evidence for triacylglycerol synthesis in the lumen of microsomes via a lipolysis-esterification pathway involving carnitine acyltransferases

In this study a pathway for the synthesis of triacylglycerol (TAG) within the lumen of the endoplasmic reticulum has been identified, using microsomes that had been preconditioned by depleting their endogenous substrates and then fusing them with biotinylated phosphatidylserine liposomes containing CoASH and Mg(2+). Incubating these fused microsomes with tri[(3)H] oleoylglycerol and [(14)C]oleoyl-CoA yielded microsome-associated triacylglycerol, which resisted extensive washing and had a [(3)H]:[(14)C] ratio close to 2:1. The data suggest that the precursor tri[(3)H]oleoylglycerol was hydrolyzed by microsomal lipase to membrane-bound di[(3)H]oleoylglycerol and subsequently re-esterified with luminal [(14)C]oleoyl-CoA. The accumulation of TAG within the microsomes, even when overt diacylglycerol acyltransferase (DGAT I) was inactive, is consistent with the existence of a latent diacylglycerol acyltransferase (DGAT II) within the microsomal lumen. Moreover, because luminal synthesis of TAG was carnitine-dependent and markedly reduced by glybenclamide, a potent carnitine acyltransferase inhibitor, microsomal carnitine acyltransferase appears to be essential for trafficking the [(14)C]oleoyl-CoA into the microsomal lumen for subsequent incorporation into newly synthesized TAG. This study thus provides the first direct demonstration of an enzymatic process leading to the synthesis of luminal triacylglycerol, which is a major component of very low density lipoproteins.

Evaluation of the affinity and turnover number of both hepatic mitochondrial and microsomal carnitine acyltransferases: relevance to intracellular partitioning of acyl-CoAs

Mitochondrial carnitine palmitoyltransferase I (CPT I) and microsomal carnitine acyltransferase I (CAT I) regulate the entry of fatty acyl moieties into their respective organelles. Thus, CPT I and CAT I occupy prominent positions in the pathways responsible for energy generation in mitochondria and the assembly of VLDL in the endoplasmic reticulum, respectively. Previous attempts to determine the intrinsic kinetic properties of CPT I and CAT I have been hampered by the occurrence of sigmoidal velocity curves. This was overcome, in this study, by the inclusion of recombinant acyl-CoA binding protein in the assay medium. For the first time, we have determined the concentrations of total functional enzyme (E(t)) by specific radiolabeling of the active site, the dissociation constants (K(d)) and the turnover numbers of CPT I and CAT I toward the CoA esters of oleic acid (C18:1) and docosahexaenoic acid (C22:6). The data show that carnitine inhibits CAT I at physiological concentrations which are not inhibitory to CPT I. Thus, carnitine concentration is likely to be a significant factor in determining the partitioning of acyl-CoAs between mitochondria and microsomes, a role which has not been previously recognized. Moreover, the finding that CAT I elicits a lower turnover toward the CoA ester of C22:6 (25 s(-)(1)) than toward that of C18:1 (111 s(-)(1)), while having similar K(d) values, suggests the use of this polyunsaturated fatty acid to inhibit VLDL biosynthesis.

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.

Topological and functional analysis of the rat liver carnitine palmitoyltransferase 1 expressed in Saccharomyces cerevisiae

The rat liver carnitine palmitoyltransferase 1 (L-CPT 1) expressed in Saccharomyces cerevisiae was correctly inserted into the outer mitochondrial membrane and shared the same folded conformation as the native enzyme found in rat liver mitochondria. Comparison of the biochemical properties of the yeast-expressed L-CPT 1 with those of the native protein revealed the same detergent lability and similar sensitivity to malonyl-CoA inhibition and affinity for carnitine. Normal Michaelis-Menten kinetics towards palmitoyl-CoA were observed when careful experimental conditions were used for the CPT assay. Thus, the expression in S. cerevisiae is a valid model to study the structure-function relationships of L-CPT 1.

Influence of diet on the kinetic behavior of hepatic carnitine palmitoyltransferase I toward different acyl CoA esters

The influence of diet on the kinetics of the overt form of rat liver mitochondrial carnitine palmitoyltransferase (CPT I; EC 2.3.1.21) was studied using rats fed either a low-fat diet (3% w/w fat), or diets which were supplemented with either olive oil (OO), safflower oil (SO) or menhaden (fish) oil (MO) to 20% w/w of fat (high fat diets). When animals were fed each of these four diets for 10 days, the order of the apparent maximal activity (Vmax) of CPT I toward various individual fatty acyl CoA, when measured under a fixed molar ratio of acyl CoA/albumin, was 16:1 n-7 > 18:1 n-9 > 18:2 n-6 > 16:0 > 22:6 n-3, and was thus not affected by the fat composition of the diet. However, in all but one case, the SO and MO diets elicited a higher Vmax for each substrate than either the LF diet or the high fat OO diet. The apparent K0.5 for the different acyl CoA esters was generally lowest in LF-fed animals, and highest in those fed the high-fat SO diet. Moreover, when compared with the situation of animals fed high-fat diets, the K0.5 values of CPT I in LF-fed animals for palmitoyl CoA and oleoyl CoA were low. This possession by CPT I of a high "affinity" toward these nonessential fatty acyl CoAs, but a lower "affinity" toward linoleoyl CoA, the ester of an essential fatty acid, may enable this latter fatty acid to be spared from oxidation when its concentration in the diet is low. The data also emphasize that palmitoleoyl CoA, if available in the diet, is likely to be utilized by CPT I at a high rate.

Carnitine palmitoyltransferase activities: effects of serum albumin, acyl-CoA binding protein and fatty acid binding protein

The carnitine palmitoyltransferase activity of various subcellular preparations measured with octanoyl-CoA as substrate was markedly increased by bovine serum albumin at low microM concentrations of octanoyl-CoA. However, even a large excess (500 microM) of this acyl-CoA did not inhibit the activity of the mitochondrial outer carnitine palmitoyltransferase, a carnitine palmitoyltransferase isoform that is particularly sensitive to inhibition by low microM concentrations of palmitoyl-CoA. This bovine serum albumin stimulation was independent of the salt activation of the carnitine palmitoyltransferase activity. The effects of acyl-CoA binding protein (ACBP) and the fatty acid binding protein were also examined with palmitoyl-CoA as substrate. The results were in line with the findings of stronger binding of acyl-CoA to ACBP but showed that fatty acid binding protein also binds acyl-CoA esters. Although the effects of these proteins on the outer mitochondrial carnitine palmitoyltransferase activity and its malonyl-CoA inhibition varied with the experimental conditions, they showed that the various carnitine palmitoyltransferase preparations are effectively able to use palmitoyl-CoA bound to ACBP in a near physiological molar ratio of 1:1 as well as that bound to the fatty acid binding protein. It is suggested that the three proteins mentioned above affect the carnitine palmitoyltransferase activities not only by binding of acyl-CoAs, preventing acyl-CoA inhibition, but also by facilitating the removal of the acylcarnitine product from carnitine palmitoyltransferase. These results support the possibility that the acyl-CoA binding ability of acyl-CoA binding protein and of fatty acid binding protein have a role in acyl-CoA metabolism in vivo.

Beta-oxidation as channeled reaction linked to citric acid cycle: evidence from measurements of mitochondrial pyruvate oxidation during fatty acid degradation

1. The kinetics of mitochondrial mammalian pyruvate dehydrogenase multienzyme complex (PDHC) is studied by the formation of CO2 using tracer amounts of [1-14C]pyruvate. It is found that the Hill plot results in a (pseudo-)cooperativity with a transition of n-1----3 at a pyruvate concentration about Ks. 2. Addition of L-carnitine, octanoate, palmitoyl-CoA or palmitate + L-carnitine + fatty acid-binding protein results in a Hill coefficient of n = 2 following the kinetics of pyruvate oxidation. 3. Addition of fatty acid-binding protein to an assay system oxidizing palmitate in presence of L-carnitine alters the pattern of the kinetics in the Hill plot so that an apparently lower level of L-carnitine is necessary for the reaction course of beta-degradation. 4. It is concluded that beta-degradation is a coordinated, multienzyme-complex based mechanism tightly linked to citric acid cycle and it is proposed that L-carnitine is actively involved into the reaction and not only functioning as carrier-molecule for transmembrane transport.

Phospholipid acylation by mouse sciatic nerve microsomes

The partition of 0.3 nmol of [1-14C]oleoyl-CoA in the microsomes (10 micrograms proteins) from mouse sciatic nerves is unaffected by the presence of lysophospholipids and is about 45% of the total oleoyl-CoA (77% of the acylglycerophosphocholine partition in the membrane). The concentration of both oleoyl-CoA and acylglycerophosphocholine is over 1 mM in the membrane. There is a selective acyl transfer from acyl-CoA to lysolipid acceptors (oleoyl greater than myristoyl, palmitoyl, stearoyl much greater than eicosanoyl greater than docosanoyl, tetracosanoyl). The exogenous acyl acceptors are acylglycerophosphocholine and acylglycerophosphoinositol and to a lesser extent acylglycerophosphoethanolamine, but not acylglycerophosphoserine. A PC formation from acylGPC in the absence of exogenous acyl donors or from oleoyl-CoA in the absence of exogenous acyl acceptor was also observed.

Interaction of amphiphilic substrates (acyl-CoAs) and their metabolites (free fatty acids) with microsomes from mouse sciatic nerves

We have measured the partition of stearoyl-CoA and oleoyl-CoA between an aqueous phase and the microsomes from mouse sciatic nerves. A method of microultracentrifugation was used which allowed us to study separately the aqueous phase and the biological membranes. We observed that the partition is dependent upon the amount of acyl-CoAs and membrane proteins but seems to be independent of time. A theoretical analysis of these data allowed interpretation of the binding and release in terms of acyl-CoA surface density in the vesicles. We have also analyzed the fate of the membrane-bound acyl-CoAs. We show that, whereas the apparent partition does not seem to vary, the hydrolysis of the membrane-bound acyl-CoAs followed by the release of free fatty acids from the membrane leads to a modification of the partition of acyl-CoAs between the membrane and the aqueous phase. We propose that there is a constant partition of the aliphatic chains (acyl-CoAs + free fatty acids).

Effects of cholesterol loading of mouse macrophages on carnitine palmitoyltransferase activity and sensitivity to inhibition by malonyl-CoA

Scavenger receptor-mediated uptake of acetylated low density lipoprotein-derived cholesterol by peritoneal mouse macrophages resulted in decreased activity of the malonyl-CoA inhibitable carnitine palmitoyltransferase and a decrease in the sensitivity of this enzyme to inhibition by malonyl-CoA.

Carnitine palmitoyltransferase in cardiac ischemia. A potential site for altered fatty acid metabolism

The sensitivity of carnitine palmitoyl coenzyme A (CoA) transferase I to inhibition of its activity by malonyl-CoA is progressively reduced in mitochondria isolated from ischemic cardiac cells as blood flow decreases to 30% or less of the preocclusion flow. The activity of carnitine palmitoyl-CoA transferase I in mitochondria isolated from nonischemic cardiac cells demonstrates incomplete inhibition, even at high concentrations of malonyl-CoA. Kinetic analyses of these data gave results most consistent with the expression of two overt enzyme activities: one activity that is sensitive to inhibition by malonyl-CoA and one activity that demonstrates little or no sensitivity to such inhibition. The decrease in malonyl-CoA-sensitive activity associated with ischemia results from a 13% decrease in the activity of the sensitive component and a corresponding 13% increase in the activity of the insensitive component. Decreased sensitivity of ischemic carnitine palmitoyl-CoA transferase I to inhibition by malonyl-CoA, together with potential fluctuations in the content of malonyl-CoA in tissue, would increase the synthesis of palmitoylcarnitine during ischemia and facilitate return to the use of fatty acid as a preferred metabolic fuel on reperfusion. This apparent conversion occurs concomitantly with a decrease in the free protein thiol content of the mitochondrial membranes isolated from ischemic cardiac cells. Treatment of the mitochondria from ischemic cardiac cells with dithiothreitol in vitro partially reverses the loss in sensitivity to malonyl-CoA, suggesting the possible role of thiol oxidation in the altered metabolism of ischemic mitochondria. Western blot analysis of these mitochondria using an antibody against carnitine palmitoyltransferase II purified from beef heart demonstrates a 68-kDa protein, which under ischemic conditions apparently is decreased by 2 kDa. These results are more indicative of a modification in protein folding of carnitine palmitoyltransferase than proteolytic changes during ischemia.

Rat liver outer mitochondrial carnitine palmitoyltransferase activity towards long-chain polyunsaturated fatty acids and their CoA esters

The activity of the overt form of rat liver mitochondrial carnitine palmitoyltransferase or CPT0 (EC 2.3.1.21) towards different fatty acid substrates was studied. The following non-esterified fatty acids (NEFA) and their CoA esters in the presence of 1% bovine serum albumin (BSA) were tested: 16:0, 18:0, 18:1, 18:2, 18:3 omega 3, 20:4, 20:5 omega 3 and 22:6 omega 3. The data fit a square hyperbolic model for enzyme catalysis (p less than 0.001, non-linear regression). Asymptotic Vmax and K0.5, substrate concentration at one-half Vmax, were calculated using total concentrations of acyl-CoA, or unbound concentrations of NEFA. BSA was found to act as a true substrate reservoir for NEFA in that the dissociation of the NEFA-BSA complex was 10-330 times faster than the CPT0 reaction. Regardless of form (NEFA or CoA ester), 18:3 omega 3 gave the highest, while 22:6 omega 3 and 18:0 gave the lowest rates of acylcarnitine synthesis. Except for 18:3 omega 3 and 18:2, Vmax for NEFA was generally lower than for acyl-CoA, with the greatest differences observed for 20:4, 20:5 omega 3 and 22:6 omega 3, suggesting that acyl-CoA synthesis may also be important in the control of the entry of these fatty acids into the mitochondria. The data provide an enzymatic rationale for the relatively low content of 18:3 omega 3 in esterified lipid.

The effect of palmitoyl-CoA binding to albumin on the apparent kinetic behavior of carnitine palmitoyltransferase I

Substrate saturation plots of carnitine palmitoyltransferase I activity from isolated rat liver mitochondria vs. palmitoyl-CoA concentration in the presence of bovine serum albumin have been reported to yield sigmoidal kinetics. Under identical assay conditions we have confirmed these observations as reflected by nonlinear Lineweaver-Burke plots (1/vi vs. 1/[S]) an average Hill coefficient of napp. = 1.98 +/- 0.09 (Mean +/- S.E. from four separate experiments). For these determinations the enzyme activity was plotted against the total [palmitoyl-CoA] in the presence of 0.13% bovine serum albumin. Utilizing the total [palmitoyl-CoA] to determine the kinetic properties of carnitine palmitoyltransferase I would be valid only if the relationship between total and free [palmitoyl-CoA] was linear, which is not the case as we have previously shown. When carnitine palmitoyltransferase I substrate saturation kinetics were reanalyzed using the previously determined free [palmitoyl-CoA]'s, the plots revealed a shift to standard hyperbolic kinetics. This observation was confirmed by an average Hill coefficient of napp. = 1.04 +/- 0.10 (Mean +/- S.E.) and linear Lineweaver-Burke plots. The double-reciprocal plots from these analyses yielded an average S0.5 of 2.55 +/- 0.82 microM (Mean +/- S.E.) palmitoyl-CoA and Vmax of 19.69 +/- 5.48 nmol/min per mg protein. These studies clearly demonstrate the importance of defining the free [palmitoyl-CoA] when analyzing the kinetics of carnitine palmitoyltransferase I in the presence of bovine serum albumin.

The binding of palmitoyl-CoA to bovine serum albumin

Bovine serum albumin (BSA) is routinely utilized in vitro to prevent the adverse detergent effects of long-chain acyl-CoA esters (i.e., palmitoyl-CoA) in enzyme assays. Determination of substrate saturation kinetics in the presence of albumin would only be valid if the relationship between bound and free substrate concentrations was known. To elucidate the relationship between bound and free palmitoyl-CoA concentrations in the presence of BSA, several different techniques including equilibrium dialysis, equilibrium partitioning, fluorescence polarization and direct fluorescence enhancement were investigated. Direct fluorescence enhancement using a custom synthesized fluorescent probe, 16-(9-anthroyloxy)palmitoyl-CoA (AP-CoA), was the best approach to this question. Measurement of the relationship between mol of palmitoyl-CoA bound per mol of BSA (nu) versus -log[free palmitoyl-CoA] revealed that the binding of palmitoyl-CoA to BSA, like palmitate was nonlinear, suggesting the presence of more than one class of acyl-CoA binding sites. Computer analyses of the binding data gave a best fit to the 2,4 two-class Scatchard model, suggesting the presence of two high-affinity primary binding sites (k1 = (1.55 +/- 0.46) x 10(-6) M-1) and four lower affinity secondary binding sites (k2 = (1.90 +/- 0.09) x 10(-8) M-1). Further analyses using the six parameter stoichiometric (stepwise) ligand binding model supports the existence of six binding sites with the higher affinities associated with the binding of the first mole of palmitoyl-CoA and weaker binding occurring after the first two sites are occupied. The association constants from this model of multiple binding diminish sequentially (i.e., K1 greater than K2 greater than K3 greater than...greater than or equal to K6), suggesting that each mol of long-chain acyl-CoA binds to BSA with decreasing affinities.