The inhibition of L-3-hydroxyacyl-CoA dehydrogenase by acetoacetyl-CoA and the possible effect of this inhibitor on fatty acid oxidation

Peroxisome-generated succinate induces lipid accumulation and oxidative stress in the kidneys of diabetic mice

Diabetes normally causes lipid accumulation and oxidative stress in the kidneys, which plays a critical role in the onset of diabetic nephropathy; however, the mechanism by which dysregulated fatty acid metabolism increases lipid and reactive oxygen species (ROS) formation in the diabetic kidney is not clear. As succinate is remarkably increased in the diabetic kidney, and accumulation of succinate suppresses mitochondrial fatty acid oxidation and increases ROS formation, we hypothesized that succinate might play a role in inducing lipid and ROS accumulation in the diabetic kidney. Here we demonstrate a novel mechanism by which diabetes induces lipid and ROS accumulation in the kidney of diabetic animals. We show that enhanced oxidation of dicarboxylic acids by peroxisomes leads to lipid and ROS accumulation in the kidney of diabetic mice via the metabolite succinate. Furthermore, specific suppression of peroxisomal β-oxidation improved diabetes-induced nephropathy by reducing succinate generation and attenuating lipid and ROS accumulation in the kidneys of the diabetic mice. We suggest that peroxisome-generated succinate acts as a pathological molecule inducing lipid and ROS accumulation in kidney, and that specifically targeting peroxisomal β-oxidation might be an effective strategy in treating diabetic nephropathy and related metabolic disorders.

Cloning, expression, and characterization of a novel diketoreductase from Acinetobacter baylyi

Reductions of carbonyl groups catalyzed by oxidoreductases are involved in all biological processes and are often a class of important biocatalyst. In this article, we report a novel enzyme designated as diketoreductase (DKR) that was able to reduce two carbonyl groups in a diketo ester to corresponding dihydroxy ester with excellent stereoselectivity. The DKR was cloned from Acinetobacter baylyi by reverse genetic method, heterogeneously expressed in Escherichia coli, and purified to homogeneity by two chromatographic steps. This novel enzyme exhibited dual cofactor specificity, with a preference of NADH over NADPH. The dihydroxy ester product catalyzed by the DKR was only 3R,5S-stereoisomer with both diastereomeric excess and enantiomeric excess values more than 99.5%. In addition, some biochemical properties of the enzyme, such as the optimal pH and temperature, were also characterized. Furthermore, sequence analysis indicated that this new enzyme was homologous to bacterial 3-hydroxyacyl coenzyme-A dehydrogenase. More importantly, based on the unique catalytic activity and excellent stereoselectivity, the DKR could be utilized in the synthesis of valuable chiral drug intermediates, such as Lipitor.

A proteomics approach to identify protein expression changes in rat liver following administration of 3,5,3'-triiodo-L-thyronine

We analyzed whole cell protein content of rat liver following T3 administration. Fourteen differentially expressed proteins were unambiguously identified and were involved in substrates and lipid metabolism, energy metabolism, detoxification of cytotoxic products, calcium homeostasis, amino acid catabolism, and the urea cycle. This study represents the first systematic identification of T3-induced changes in liver protein expression profile and provides novel information at the molecular, cellular, and tissue level of T3 action.

Mitochondrial beta-oxidation

Mitochondrial beta-oxidation is a complex pathway involving, in the case of saturated straight chain fatty acids of even carbon number, at least 16 proteins which are organized into two functional subdomains; one associated with the inner face of the inner mitochondrial membrane and the other in the matrix. Overall, the pathway is subject to intramitochondrial control at multiple sites. However, at least in the liver, carnitine palmitoyl transferase I exerts approximately 80% of control over pathway flux under normal conditions. Clearly, when one or more enzyme activities are attenuated because of a mutation, the major site of flux control will change.

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

Studies on the inactivation of bovine liver enoyl-CoA hydratase by (methylenecyclopropyl)formyl-CoA: elucidation of the inactivation mechanism and identification of cysteine-114 as the entrapped nucleophile

The inhibitory properties of (methylenecyclopropyl)formyl-CoA (MCPF-CoA), a metabolite derived from a natural amino acid, (methylenecyclopropyl)glycine, against bovine liver enoyl-CoA hydratase (ECH) were characterized. We have previously demonstrated that MCPF-CoA specifically targets ECHs, which catalyze the reversible hydration of alpha,beta-unsaturated enoyl-CoA substrates to the corresponding beta-hydroxyacyl-CoA products. Here, we synthesized (R)- and (S)-diastereomers of MCPF-CoA to examine the stereoselectivity of this inactivation. Both compounds were shown to be competent inhibitors for bovine liver ECH with nearly identical second-order inactivation rate constants (k(inact)/K(I)) and partition ratios (k(cat)/k(inact)), indicating that the inactivation is nonstereospecific with respect to ring cleavage. The inhibitor, upon incubation with bovine liver ECH, labels a tryptic peptide, ALGGGXEL, near the active site of the protein, where X is the amino acid that is covalently modified. Cloning and sequence analysis of bovine liver ECH gene revealed the identity of the amino acid residue entrapped by MCPF-CoA as Cys-114 (mature sequence numbering). On the basis of gHMQC (gradient heteronuclear multiple quantum coherence) analysis with [3-(13)C]-labeled MCPF-CoA, the ring cleavage is most likely induced by the nucleophilic attack at the terminal carbon of the exomethylene group (C(2)'). We propose a plausible inactivation mechanism that involves relief of ring strain and is consistent with examples found in the literature. In addition, these studies provide important clues for future design of more efficient and selective inhibitors to control and/or regulate fatty acid metabolism.

Control of mitochondrial beta-oxidation: sensitivity of the trifunctional protein to [NAD+]/[NADH] and [acetyl-CoA]/[CoA]

Isolated human mitochondrial trifunctional protein was incubated with 2-hexadecenoyl-CoA, CoA and NAD+ and the resultant CoA esters measured. Steady state with respect to the concentrations of the intermediates 3-hydroxyhexadecanoyl-CoA and 3-ketohexadecanoyl-CoA and the rate of formation of the product tetradecanoyl-CoA was reached within 4 min. Flux was greatly enhanced by the addition of Tween 20 (0.2% v/v) which stimulated 3-ketoacyl-CoA thiolase activity by over 7-fold. When 3-ketoacyl-CoA thiolase was not stimulated, 3-hydroxyhexadecanoyl-CoA was the prominent CoA ester accumulated, presumably due to inhibition of 3-hydroxyacyl-CoA dehydrogenase activity by accumulated 3-ketoacyl-CoA, analogous to the inhibition of short-chain 3-hydroxyacyl-CoA dehydrogenase by 3-ketoacyl-CoA. When [NAD+]/[NADH] was varied at a fixed total [NAD++NADH], the overall flux was only inhibited by [NAD+]/[NADH] less than 1. In contrast, when [acetyl-CoA]/[CoA] was varied at a fixed total [CoA], much greater sensitivity was observed.

Mammalian mitochondrial beta-oxidation

The enzymic stages of mammalian mitochondrial beta-oxidation were elucidated some 30-40 years ago. However, the discovery of a membrane-associated multifunctional enzyme of beta-oxidation, a membrane-associated acyl-CoA dehydrogenase and characterization of the carnitine palmitoyl transferase system at the protein and at the genetic level has demonstrated that the enzymes of the system itself are incompletely understood. Deficiencies of many of the enzymes have been recognized as important causes of disease. In addition, the study of these disorders has led to a greater understanding of the molecular mechanism of beta-oxidation and the import, processing and assembly of the beta-oxidation enzymes within the mitochondrion. The tissue-specific regulation, intramitochondrial control and supramolecular organization of the pathway is becoming better understood as sensitive analytical and molecular techniques are applied. This review aims to cover enzymological and organizational aspects of mitochondrial beta-oxidation together with the biochemical aspects of inherited disorders of beta-oxidation and the intrinsic control of beta-oxidation.

Intermediate channeling on the trifunctional beta-oxidation complex from pig heart mitochondria

The kinetic properties of the purified trifunctional beta-oxidation complex (TOC) from pig heart mitochondria were analyzed with the aim of elucidating the functional consequence of having three sequentially acting enzymes of beta-oxidation associated in one complex. The kinetic parameters of TOC and of the component enzymes of TOC, long-chain enoyl-CoA hydratase, long-chain 3-hydroxyacyl-CoA dehydrogenase, and long-chain 3-ketoacyl-CoA thiolase, were determined with substrates having acyl chains with 16 carbon atoms. Quantification by high performance liquid chromatography of intermediates formed during the degradation of 2-trans-hexadecanoyl-CoA to myristoyl-CoA and acetyl-CoA by TOC revealed the accumulation of 3-hydroxyhexadecanoyl-CoA, whereas 3-ketohexadecanoyl-CoA was undetectable. The observed rates of NADH and acetyl-CoA formation were higher than the theoretical rates calculated by use of the kinetic parameters and measured concentrations of intermediates. When the sequence of reactions catalyzed by TOC was inhibited by acetyl-CoA, the steady-state concentration of the 3-hydroxyacyl-CoA intermediate was not affected, whereas a small amount of 3-ketohexadecanoyl-CoA was detected. The differences between observed and predicted reaction rates and between measured and expected concentrations of intermediates are best explained by the operation of a channeling mechanism. As a consequence of intermediate channeling between the active sites on the complex, more coenzyme A is available in the mitochondrial matrix and metabolites like 3-ketoacyl-CoA thioesters, which are strong inhibitors of several beta-oxidation enzymes, do not accumulate.

Assay of L-3-hydroxyacyl-coenzyme A dehydrogenase with substrates of different chain lengths

A method for assaying L-3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) which permits rate measurements with L-3-hydroxyacyl-CoA substrates of various chain lengths at physiological pH is described. The method is based on a coupled assay system in which 3-ketoacyl-CoA compounds formed by the dehydrogenase are cleaved by 3-ketoacyl-CoA thiolase (EC 2.3.1.16) in the presence of CoASH. The advantages of this assay method are its irreversibility and elimination of product inhibition. The assay procedure was used to determine the kinetic parameters (Km, Vmax) of pig heart L-3-hydroxyacyl-CoA dehydrogenase with several substrates of various chain lengths. The data obtained show the enzyme to be most active with medium-chain substrates whereas Km values for medium-chain and long-chain substrates are almost equal but much lower than those previously reported.

Computer simulation of metabolism in palmitate-perfused rat heart. I. Palmitate oxidation

A computer model of the fatty acid oxidation pathway in perfused rat heart was constructed. It includes uptake, activation, and beta-oxidation of fatty acids, triglyceride synthesis and hydrolysis, and carnitine-dependent transport of acyl groups across the mitochondrial membrane under pseudosteady state conditions. Fatty acid utilization may be limited by beta-oxidation in hypoxia or ischemia but probably not in aerobic conditions. Nonesterified fatty acids bound to proteins are found to be metabolically available. The model predicts that stearate, but not palmitate, can support the highest observed respiration rate for perfused rat heart without supplementation by other substrates. Fatty acids are preferentially oxidized rather than being stored as triglycerides because the cystosolic acyl CoA level is lower than the Km for triglyceride synthesis. It is suggested that feedback inhibition of triglyceride lipase regulates utilization of triglycerides as fuel in aerobic hearts.

Interrelationships between 3-hydroxy-3-methylglutaryl-CoA synthase, acetoacetyl-CoA and ketogenesis

Kinetic and physical approaches have been employed to investigate the binding of acetoacetyl-CoA to hydroxymethylglutaryl-CoA synthase. The enzyme has an apparent Km for acetoacetyl-CoA (0.35 microM) which is more than an order of magnitude lower than the Ki (6--10 microM) measured for substrate inhibition by this metabolite. Hepatic acetoacetyl-CoA concentration, as measured by a sensitive and highly specific radioactive assay appears to be in the 1--10 microM range; the concentration decreases during diabetic ketoacidosis. Total hepatic activity of hydroxymethylglutaryl-CoA synthase and levels of mitochondrial enzyme protein, determined by radioimmunoassay, are not appreciably different in livers from control or ketoacidotic animals. In contrast to the decrease in hepatic acetoacetyl-CoA concentration observed during ketoacidosis, myocardial acetoacetyl-CoA levels are increased by at least tenfold when compared to controls. Elevated acetoacetyl-CoA levels may serve to inhibit fatty acid utilization by the heart. Thus, a consideration of the multiple interactions of acetoacetyl-CoA with the enzymes involved in ketone body production and utilization may be useful in evaluating the metabolic significance of this intermediate.

beta-Oxidation of the coenzyme A esters of elaidic, oleic, and stearic acids and their full-cycle intermediates by rat heart mitochondria

beta-Oxidation rates for the CoA esters of elaidic, oleic and stearic acids and their full-cycle beta-oxidation intermediates and for the carnitine esters of oleic and elaidic acids were compared over a wide range of substrate and albumin concentrations in rat heart mitochondria. The esters of elaidic acid were oxidized at about half the rate of the oleic acid esters, while stearoyl-CoA was oxidized equally as rapid as oleoyl-CoA. The full-cycle beta-oxidation intermediates of elaidoyl-CoA (trans-16 : 1 delta 7, -14 : 1 delta 5, and -12 : 1 delta 3) were found to be oxidized at rates nearly equal to those for the corresponding intermediates of oleoyl-CoA. Therefore, after the first cycle of beta-oxidation, oleoyl-CoA and elaidoyl-CoA are oxidized at nearly equal rates. The activity of fatty acyl-CoA dehydrogenase was higher with elaidoyl-CoA and its full-cycle intermediates as substrates than with the corresponding cisisomers. It was concluded that the slower oxidation rate of elaidic acid is not due to slower oxidation of any of its full-cycle beta-oxidation intermediates, nor to slower activity of fatty acyl-CoA dehydrogenase, nor to outer mitochondrial carnitine acyltransferase. Possible explanations to account for the slower oxidation rate of elaidic acid are discussed.