Functional Characterization of Δ³,Δ²-Enoyl-CoA Isomerases from Rat Liver*

The degradation of unsaturated fatty acids by β-oxidation involves Δ³,Δ²-enoyl-CoA isomerases (enoyl-CoA isomerases) that catalyze 3-cis → 2-trans and 3-trans → 2-trans isomerizations of enoyl-CoAs and the 2,5 → 3,5 isomerization of dienoyl-CoAs. An analysis of rat liver enoyl-CoA isomerases revealed the presence of a monofunctional enoyl-CoA isomerase (ECI) in addition to mitochondrial enoyl-CoA isomerase (MECI) in mitochondria, whereas peroxisomes contain ECI and multifunctional enzyme 1 (MFE1). Thus ECI, which previously had been described as peroxisomal enoyl-CoA isomerase, was found to be present in both peroxisomes and mitochondria. This enzyme seems to be identical with mitochondrial long-chain enoyl-CoA isomerase (Kilponen, J.M., Palosaari, P.M., and Hiltunen, J.K. 1990. Biochem. J. 269, 223–226). All three hepatic enoyl-CoA isomerases have broad chain length specificities but are distinguishable by their preferences for one of the three isomerization reactions. MECI is most active in catalyzing the 3-cis → 2-trans isomerization; ECI has a preference for the 3-trans → 2-trans isomerization, and MFE1 is the optimal isomerase for the 2,5 → 3,5 isomerization. A functional characterization based on substrate specificities and total enoyl-CoA isomerase activities in rat liver leads to the conclusion that the 3-cis → 2-trans and 2,5 → 3,5 isomerizations in mitochondria are catalyzed overwhelmingly by MECI, whereas ECI contributes significantly to the 3-trans → 2-trans isomerization. In peroxisomes, ECI is predicted to be the dominant enzyme for the 3-cis → 2-trans and 3-trans → 2-transisomerizations of long-chain intermediates, whereas MFE1 is the key enzyme in the 2,5 → 3,5 isomerization.