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

Association of NMT2 with the acyl-CoA carrier ACBD6 protects the N-myristoyltransferase reaction from palmitoyl-CoA

The covalent attachment of a 14-carbon aliphatic tail on a glycine residue of nascent translated peptide chains is catalyzed in human cells by two N-myristoyltransferase (NMT) enzymes using the rare myristoyl-CoA (C(14)-CoA) molecule as fatty acid donor. Although, NMT enzymes can only transfer a myristate group, they lack specificity for C(14)-CoA and can also bind the far more abundant palmitoyl-CoA (C(16)-CoA) molecule. We determined that the acyl-CoA binding protein, acyl-CoA binding domain (ACBD)6, stimulated the NMT reaction of NMT2. This stimulatory effect required interaction between ACBD6 and NMT2, and was enhanced by binding of ACBD6 to its ligand, C(18:2)-CoA. ACBD6 also interacted with the second human NMT enzyme, NMT1. The presence of ACBD6 prevented competition of the NMT reaction by C(16)-CoA. Mutants of ACBD6 that were either deficient in ligand binding to the N-terminal ACBD or unable to interact with NMT2 did not stimulate activity of NMT2, nor could they protect the enzyme from utilizing the competitor C(16)-CoA. These results indicate that ACBD6 can locally sequester C(16)-CoA and prevent its access to the enzyme binding site via interaction with NMT2. Thus, the ligand binding properties of the NMT/ACBD6 complex can explain how the NMT reaction can proceed in the presence of the very abundant competitive substrate, C(16)-CoA.

Ligand binding to the ACBD6 protein regulates the acyl-CoA transferase reactions in membranes

The binding determinants of the human acyl-CoA binding domain-containing protein (ACBD) 6 and its function in lipid renewal of membranes were investigated. ACBD6 binds acyl-CoAs of a chain length of 6 to 20 carbons. The stoichiometry of the association could not be fitted to a 1-to-1 model. Saturation of ACBD6 by C16:0-CoA required higher concentration than less abundant acyl-CoAs. In contrast to ACBD1 and ACBD3, ligand binding did not result in the dimerization of ACBD6. The presence of fatty acids affected the binding of C18:1-CoA to ACBD6, dependent on the length, the degree of unsaturation, and the stereoisomeric conformation of their aliphatic chain. ACBD1 and ACBD6 negatively affected the formation of phosphatidylcholine (PC) and phosphatidylethanolamine in the red blood cell membrane. The acylation rate of lysophosphatidylcholine into PC catalyzed by the red cell lysophosphatidylcholine-acyltransferase 1 protein was limited by the transfer of the acyl-CoA substrate from ACBD6 to the acyltransferase enzyme. These findings provide evidence that the binding properties of ACBD6 are adapted to prevent its constant saturation by the very abundant C16:0-CoA and protect membrane systems from the detergent nature of free acyl-CoAs by controlling their release to acyl-CoA-utilizing enzymes.

Partial characterization of a malonyl-CoA-sensitive carnitine O-palmitoyltransferase I from Macrobrachium borellii (Crustacea: Palaemonidae)

The shuttle system that mediates the transport of fatty acids across the mitochondrial membrane in invertebrates has received little attention. Carnitine O-palmitoyltransferase I (EC 2.3.1.21; CPT I) is a key component of this system that in vertebrates controls long-chain fatty acid beta-oxidation. To gain knowledge on the acyltransferases in aquatic arthropods, physical, kinetic, regulatory and immunological properties of CPT of the midgut gland mitochondria of Macrobrachium borellii were assayed. CPT I optimum conditions were 34 degrees C and pH=8.0. Kinetic analysis revealed a Km for carnitine of 2180+/-281 microM and a Km for palmitoyl-CoA of 98.9+/-8.9 microM, while V(max) were 56.5+/-6.6 and 36.7+/-4.8 nmol min(-1) mg protein(-1), respectively. A Hill coefficient, n~1, indicate a Michaelis-Menten behavior. The CPT I activity was sensitive to regulation by malonyl-CoA, with an IC(50) of 25.2 microM. Electrophoretic and immunological analyses showed that a 66 kDa protein with an isoelectric point of 5.1 cross-reacted with both rat liver and muscle-liver anti CPT I polyclonal antibodies, suggesting antigenic similarity with the rat enzymes. Although CPT I displayed kinetic differences with insect and vertebrates, prawn showed a high capacity for energy generation through beta-oxidation of long-chain fatty acids.

Thioredoxin-linked lipid hydroperoxide peroxidase activity of human serum albumin in the presence of palmitoyl coenzyme A

Human Serum Albumin (HSA) exerted a significant lipid peroxidase activity with the use of a thiol-reducing equivalent such as dithiothreitol (DTT). Carboxyl group-modified HSA (CM-HSA) showed a 10-fold stronger lipid peroxidase activity (1.6 nmol/min/mg) than that of HSA (0.17 nmol/min/mg). Instead of DTT, thioredoxin (Trx) also supported reducing equivalent to the reduction of lipid hydroperoxide by CM-HSA. Contrast to CM-HSA, HSA did not reduce lipid peroxide with the use of Trx. In the presence of palmitoyl coenzyme A (palmitoyl-CoA) however, HSA used Trx as an electron donor to the reduction of lipid hydroperoxide. The Trx-linked peroxidase activity of HSA sharply increased with elongation in the carbon chain of the acyl moiety of acyl-CoA, showing an optimum activity in the presence of palmitoyl-CoA. Fluorescence study indicates the conformational changes of HSA induced by palmitoyl-CoA. Together, these data suggest that palmitoyl-CoA-bound HSA has a capability to remove lipid peroxide with the use of electrons given by Trx system.

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.

Microsomal long chain fatty acyl-CoA transacylation: differential effect of sterol carrier protein-2

The recent discovery that sterol carrier protein-2 (SCP-2) binds long chain++ (LCFA-CoA) with high affinity (A. Frolov et al., J. Biol. Chem. 271 (1997) 31878-31884) suggests new possible functions of this protein in LCFA-CoA metabolism. The purpose of the present investigation was to determine whether SCP-2 differentially modulated microsomal LCFA-CoA transacylation to cholesteryl esters, triacylglycerols, and phospholipids in vitro. Microsomal acyl-CoA:cholesterol acyltransferase (ACAT) activity measured with liposomal membrane cholesterol donors depended on substrate LCFA-CoA level, mol% cholesterol in the liposomal membrane, and total amount of liposomal cholesterol. As compared to basal activity without liposomes, microsomal ACAT was inhibited 30-50% in the presence of cholesterol poor (1.4 mol%) liposomes. In contrast, cholesterol rich (>25 mol%) liposomes stimulated ACAT up to 6.4-fold compared to basal activity without liposomes and nearly 10-fold as compared to cholesterol pool (1.4 mol%) liposomes. Increasing oleoyl-CoA reversed the inhibition of microsomal ACAT by cholesterol poor (1.4 mol%) liposomes, but did not further stimulate ACAT in the presence of cholesterol rich (35 mol%) liposomes. In contrast, high (100 microM) oleoyl-CoA inhibited ACAT nearly 3-fold. This inhibition was reversed by LCFA-CoA binding proteins, bovine serum albumin (BSA) and SCP-2. SCP-2 was 10-fold more effective (mole for mole) than BSA in reversing LCFA-CoA inhibited microsomal ACAT. Concomitantly, under conditions in which SCP-2 stimulated ACAT it equally enhanced transacylation of oleoyl-CoA into phospholipids, and 5.2-fold enhanced oleoyl-CoA transacylation to triacylglycerols. In summary, SCP-2 appeared to exert its greatest effects on microsomal transacylation in vitro by reversing LCFA-CoA inhibition of ACAT and by differentially targeting LCFA-CoA to triacylglycerols. These data suggest that the high affinity interaction of SCP-2s with LCFA-CoA may be physiologically important in microsomal transacylation reactions.

Hepatic carnitine palmitoyltransferase activity in cattle

A fluorometric assay for the determination of hepatic carnitine palmitoyltransferase (CPT) activity was slightly modified for use with cattle samples. With this assay, the Km value was 0.56 +/- 0.10 mM with respect to L-carnitine (mean +/- SD, n = 4) and was 9.6 +/- 2.2 microM (n = 3) with respect to palmitoyl-CoA. The average hepatic CPT activity was 33.6 +/- 2.0 mumol CoASH/min/g protein in 38 healthy cattle and was similar in both sexes and among breeds. Hepatic CPT activity showed no correlation with serum phospholipid, free fatty acid, triglyceride or total cholesterol concentrations.

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.

Biochemical evidence for heterozygosity in muscular carnitine palmitoyltransferase deficiency

Carnitine palmitoyltransferase (CPT) was studied in muscle homogenates of four patients with recurrent attacks of rhabdomyolysis due to muscular CPT deficiency and in those of the clinically asymptomatic father and mother of two patients. In controls CPT II was readily solubilized by the addition of Triton X-100 and 1% Tween 20. In contrast, CPT I was inactivated by Triton X-100 but remained catalytically active and membrane bound in the presence of 1% Tween 20. Total CPT activity was normal in patients and in both parents when measured under optimal assay conditions. After addition of 1% Tween 20 the insoluble CPT activity was also normal in patients and in both parents. The soluble CPT activity, however, was almost completely lost in patients but was only partially decreased in both parents. The data indicate that in patients an enzymatically active CPT II exists which is abnormally sensitive to inhibition by Tween 20, and that CPT I activity is not compensatorily increased in patients. A partial CPT II deficiency can be identified in heterozygotes most sensitively by the separate determination of soluble and insoluble CPT activities in the presence of 1% Tween 20.

Inhibition of carnitine palmitoyltransferase in normal human skeletal muscle and in muscle of patients with carnitine palmitoyltransferase deficiency by long- and short-chain acylcarnitine and acyl-coenzyme A

The inhibition of total carnitine palmitoyltransferase (CPT) by short- and long-chain acylcarnitine and acyl-coenzyme A (acyl-CoA) was studied in muscle homogenates of normal controls and of five new patients with CPT deficiency using the isotope forward assay. Acetylcarnitine inhibited neither normal CPT activity nor the CPT of patients. D,L-Palmitoylcarnitine almost completely inhibited CPT in patients but only 55% of normal activity. In controls the CPT fraction sensitive to inhibition by palmitoylcarnitine appeared to be identical with the fraction sensitive to inhibition by malonyl-CoA and succinyl-CoA, which probably represents CPT II. The abnormal inhibition of CPT by palmitoylcarnitine was more likely due to product inhibition than to a detergent effect. Acetyl-CoA concentrations up to 0.4 mM and palmitoyl-CoA above optimal substrate concentrations up to 0.3 mM both inhibited normal CPT by about 25%, whereas the CPT of patients was significantly more inhibited by both substances than was normal CPT. The inhibition by acetyl-CoA was probably due to the structural relationship with malonyl-CoA and succinyl-CoA. The abnormal inhibition of CPT in patients by palmitoyl-CoA was due either to an abnormal substrate inhibition or to a detergent effect on CPT II similar to that of Triton X-100. The data indicate that in CPT deficiency total CPT activity is normal under optimal assay conditions. CPT II, however, is abnormally inhibited by fatty acid metabolites that accumulate during fasting.

Interaction of acyl-CoA binding protein (ACBP) on processes for which acyl-CoA is a substrate, product or inhibitor

It is shown that acyl-CoA binding protein (ACBP), in contrast with fatty acid binding protein (FABP), stimulates the synthesis of long-chain acyl-CoA esters by mitochondria. ACBP effectively opposes the product feedback inhibition of the long-chain acyl-CoA synthetase by sequestration of the synthesized acyl-CoA esters. Feedback inhibition of microsomal long-chain acyl-CoA synthesis could not be observed, due to the formation of small acyl-CoA binding vesicles during preparation and/or incubation. Microsomal membrane preparations are therefore unsuitable for studying feedback inhibition of long-chain acyl-CoA synthesis. ACBP was found to have a strong attenuating effect on the long-chain acyl-CoA inhibition of both acetyl-CoA carboxylase and mitochondrial adenine nucleotide translocase. Both processes were unaffected by the presence of long-chain acyl-CoA esters when the ratio of long-chain acyl-CoA to ACBP was below 1, independent of the acyl-CoA concentration used. It is therefore not the acyl-CoA concentration as such which is important from a regulatory point of view, but the ratio of acyl-CoA to ACBP. The cytosolic ratio of long-chain acyl-CoA to ACBP was shown to be well below 1 in the liver of fed rats. ACBP could compete with the triacylglycerol-synthesizing pathway, but not with the phospholipid-synthesizing enzymes, for acyl-CoA esters. Furthermore, in contrast with FABP, ACBP was able to protect long-chain acyl-CoA esters against hydrolysis by microsomal acyl-CoA hydrolases. The results suggest that long-chain acyl-CoA esters synthesized for either triacylglycerol synthesis or beta-oxidation have to pass through the acyl-CoA/ACBP pool before utilization. This means that acyl-CoA synthesized by microsomal or mitochondrial synthetases is uniformly available in the cell. It is suggested that ACBP has a duel function in (1) creating a cytosolic pool of acyl-CoA protected against acyl-CoA hydrolases, and (2) protecting vital cellular processes from being affected by long-chain acyl-CoA esters.

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.

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.