Inhibition of very long chain acyl-CoA dehydrogenase during cardiac ischemia

The heart utilizes primarily fatty acids for energy production. During ischemia, however, diminished oxygen supply necessitates a switch from beta-oxidation of fatty acids to glucose utilization and glycolysis. Molecular mechanisms responsible for these alterations in metabolism are not fully understood. Mitochondrial acyl-CoA dehydrogenase catalyzes the first committed step in the beta-oxidation of fatty acids. In the current study, an in vivo rat model of myocardial ischemia was utilized to determine whether specific acyl-CoA dehydrogenases exhibit ischemia-induced alterations in activity, identify mechanisms responsible for changes in enzyme function, and assess the effects on mitochondrial respiration. Very long chain acyl-CoA dehydrogenase (VLCAD) activity declined 34% during 30 min of ischemia. Loss in activity appeared specific to VLCAD as medium chain acyl-CoA dehydrogenase activity remained constant. Loss in VLCAD activity during ischemia was not due to loss in protein content. In addition, activity was restored in the presence of the detergent Triton X-100, suggesting that changes in the interaction between the protein and inner mitochondrial membrane are responsible for ischemia-induced loss in activity. Palmitoyl-carnitine supported ADP-dependent state 3 respiration declined as a result of ischemia. When octanoyl-carnitine was utilized state 3 respiration remained unchanged. State 4 respiration increased during ischemia, an increase that appears specific to fatty acid utilization. Thus, VLCAD represents a likely site for the modulation of substrate utilization during myocardial ischemia. However, the dramatic increase in mitochondrial state 4 respiration would be predicted to accentuate the imbalance between energy production and utilization.

Novel methylenecyclopropyl-based acyl-CoA dehydrogenase inhibitor

A novel hexyl-substituted methylenecyclopropyl acetyl-CoA was tested as an enzyme-specific acyl-CoA dehydrogenase inhibitor. Its CoA ester generated in situ from the carboxylic acid and CoASH, displayed marked differences in inhibition specificity as compared to methylenecyclopropyl acetyl-CoA, consistent with the substrate specificities of the target enzymes. Thus methylenecyclopropyl acetyl-CoA inactivated short-chain-specific acyl-CoA dehydrogenase rapidly, medium-chain-specific acyl-CoA dehydrogenase much more slowly and had no effect on long-chain- or very long-chain-specific acyl-CoA dehydrogenases. The hexyl-substituent on the methylenecyclopropyl ring gave an inhibitor which rapidly inactivated MCAD and LCAD whilst VLCAD was inhibited more slowly and SCAD was essentially unaffected. In some cases (e.g. SCAD and MCPA-CoA) inhibition was accompanied by flavin bleaching. In other cases (e.g. LCAD and C6MCPA) less pronounced bleaching suggests a different chemistry of inhibition.

Mechanism-based inhibitor discrimination in the acyl-CoA dehydrogenases

The catalytically essential glutamate base in the acyl-CoA dehydrogenase family is found either on the loop between J and K helices (e.g., in short-chain, medium-chain, and glutaryl-CoA dehydrogenases) or on the G helix (long-chain and isovaleryl-CoA dehydrogenases). While active-site bases at either position are functionally equivalent with respect to alpha-proton abstraction, reactions that require removal of a gamma-proton show marked differences between the two enzyme classes. Thus short-chain, medium-chain, and glutaryl-CoA dehydrogenase are rapidly inactivated by 2-pentynoyl-CoA with abstraction of a gamma-proton, whereas isovaleryl-CoA dehydrogenase is not significantly inhibited. This resistance is not due to weak binding: the complex between isovaleryl-CoA dehydrogenase and 2-pentynoyl-CoA shows a Kd of 1.8 microM at pH 7.6. Migration of the catalytic base to the loop between J and K helices (using the Glu254Gly/Ala375Glu double mutant) makes isovaleryl-CoA dehydrogenase sensitive to irreversible inhibition by 2-pentynoyl-CoA. Molecular modeling suggests that this mutation brings the catalytic base close enough to abstract a gamma-proton from the bound inhibitor. Experiments with two mechanism-based inactivators that target the FAD of the medium- and short-chain acyl-CoA dehydrogenases support this conclusion. 3-Methyl-3-butenoyl-CoA requires activation by alpha-proton abstraction and rapidly yields a reduced flavin adduct with wild-type isovaleryl-CoA dehydrogenase. In contrast, the isomeric 3-methyl-2-butenoyl-CoA is inert toward this enzyme because it cannot be activated by gamma-proton abstraction. Molecular modeling supports these observations. This unusual selectivity toward mechanism-based inactivators provides additional discrimination between members of the acyl-CoA dehydrogenase family.

Biochemical characterization of purified, human recombinant Lys304-->Glu medium-chain acyl-CoA dehydrogenase containing the common disease-causing mutation and comparison with the normal enzyme

Recombinant, normal human medium-chain acyl-CoA dehydrogenase (MCADH) and the common, human disease-causing K304E mutant ([Glu304]MCADH) protein were expressed in Escherichia coli using an optimized system, and the enzymes were purified to apparent homogeneity. The crucial factor leading to the production of active [Glu304]MCADH protein is the expression in E. coli cells at reduced temperature (28 degrees C). Expression in the same system at 37 degrees C results in very low amounts of active mutant protein. Several catalytic and physicochemical parameters of these two proteins have been determined and were compared to those of purified pig kidney MCADH. Although [Glu304]MCADH has approximately the same rate of substrate reduction with dodecanoyl-CoA and the same V(max) as human MCADH with the best substrate for the latter, octanoyl-CoA, the K(m) in the mutant MCADH is fourfold higher, which generates a correspondingly lower catalytic efficiency. Importantly, V(max) obtained using the natural acceptor, electron transfer flavoprotein, is only a third that for human MCADH. The V(max)/K(m) versus chain-length profile of the mutant shows a maximum with dodecanoyl-CoA which differs markedly from that of human MCADH, which has maximal efficiency with octanoyl-CoA. The substrate specificity of the mutant is broader with a less pronounced activity peak resembling long-chain acyl-CoA dehydrogenase. The purified mutant enzyme exhibits a reduced thermal stability compared to human wild-type MCADH. The major difference between the two proteins expressed in E. coli is the more pronounced lability of the K304E mutant in crude extracts, which suggests a higher susceptibility to attack by endogenous proteases. Differences between tetrameric [Glu304]MCADH which survives the first step(s) of purification and corresponding MCADH are minor. The overall differences in properties of [Glu304]MCADH together with its impaired folding and tetramer assembly may contribute to the generation of the abnormalities observed in patients homozygous for the K304E mutation.

Fatty acid delta 5 desaturation in rat liver cell nuclei

Activity of one of the key enzymes involved in arachidonic acid (20:4 n-6) biosynthesis, the delta 5 desaturase, was found in rat liver cell nuclei. Up to now, it has been shown that the fatty acid desaturases are located exclusively in the endoplasmic reticulum. Similarly to what happens with microsomal enzyme the nuclear delta 5 desaturase enzyme was only fully active in the presence of a cytosolic factor. In this condition it reached a specific activity of 50 pmol 20:4 n-6 formed/min/mg of protein. This fact would imply that purified nuclei like purified microsomes lack a soluble cytosol factor necessary for the total desaturation reaction expression. Besides the nuclear delta 5 desaturase has an optimal pH of 7.6 and is inhibited by 1 or 10 mM KCN. Low long chain acyl-CoA synthetase activity that catalyzes the formation of 20:3 n-6-CoA, was also found in liver nuclei. This step would be essential in nuclear desaturation since when ATP and/or CoA (necessary for the acylation reaction) are omitted from the incubation mixture, the desaturation reaction does not take place.

Selective inactivation of various acyl-CoA dehydrogenases by (methylenecyclopropyl)acetyl-CoA

Inactivation of five distinct acyl-CoA dehydrogenases by (methylenecyclopropyl)acetyl-CoA (MCPA-CoA), the toxic metabolite of hypoglycin from unripe ackee fruit, was investigated using purified enzyme preparations. Short-chain acyl-CoA (SCADH), medium-chain acyl-CoA (MCADH) and isovaleryl-CoA (IVDH) dehydrogenases were severely and irreversibly inactivated by MCPA-CoA, while 2-methyl-branched chain acyl-CoA dehydrogenase (2-meBCADH) was only slowly and mildly inactivated. Long-chain acyl-CoA dehydrogenase (LCADH) was not significantly inactivated, even after prolonged incubation with MCPA-CoA. Inactivation of SCADH, MCADH and IVDH was effectively prevented by the addition of substrate. This mode of inactivation by MCPA-CoA explains the urinary metabolite profile in hypoglycin treated-rats, which includes large amounts of metabolites from fatty acids and leucine, and relatively small amounts of those from valine and isoleucine. Spectrophotometric titration of SCADH and MCADH with MCPA-CoA, together with the protective effects of substrate, indicates that MCPA-CoA is acted upon by, and exerts in turn irreversible inactivation of, SCADH and MCADH, confirming that MCPA-CoA is a suicide inhibitor (Wenz et al. (1981) J. Biol. Chem. 256, 9809-9812). Spectrophotometric titration data of LCADH and MCPA-CoA is typical of non-reacting CoA ester.

2-Mercaptoacetate administration depresses the beta-oxidation pathway through an inhibition of long-chain acyl-CoA dehydrogenase activity

To elucidate the mechanisms through which 2-mercaptoacetate administration inhibits fatty acid oxidation in the liver, the respiration rates induced by different substrates were studied polarographically in rat hepatic mitochondria isolated 3 h after 2-mercaptoacetate administration. Palmitoyl-L-carnitine oxidation was almost completely inhibited in either the absence or presence of malonate. Octanoate oxidation was also inhibited, and the intramitochondrial acyl-CoA content was markedly increased. The oxidation rate of pyruvate and 2-oxoglutarate on the one hand and of 3-hydroxybutyrate, succinate and glutamate on the other was either normal or only slightly decreased. In the presence of 2,4-dinitrophenol, the extent of the inhibition of palmitoyl-L-carnitine oxidation was unchanged. All these results are consistent with the hypothesis that the 2-mercaptoacetate inhibition of fatty acid oxidation is due to an inhibition of the beta-oxidation pathway itself. Finally, the mitochondrial defect responsible for this inhibition was shown to be an inhibition of palmitoyl-CoA dehydrogenase activity (EC 1.3.99.3).