Occurrence of two 3-hydroxyacyl-CoA dehydrogenases in rat liver.

3-Hydroxyacyl-CoA dehydrogenase was assayed for acetoacetyl pantetheine-reducing and acetoacetyl-CoA reducing activities in rat liver homegenates. Two isoenzymes of the enzyme, types I and II, were distinguished by the following procedures: trypsin treatment, heat treatment, CM-cellulose chromatography, antibody titration, and sucrose density gradient centrifugation of the light mitochondrial fraction. Type I enzyme was localized in mitochondria, and catalyzed the reduction of both acetoacetyl pantetheine and acetoacetyl-CoA. Type II enzyme was found mainly in peroxisomes, accompanied by a low activity in mitochondria or some other organelles, and was active with acetoacetyl-CoA but not with aceto acetylpantetheine. Both isozymes were induced by the administration to the rats of di-(2-ethylhexyl)phthalate, which enhances the peroxisomal beta-oxidation activity, but the extent of the induction of type II enzyme was much higher than that of type I enzyme. The activity of the former was found only in diethylhexylphthalate-treated rats.