A survey of enzymes which generate or use acetoacetyl thioesters in rat liver

Reactions that generate and remove acetoacetyl-CoA and acetoacetate were measured in mitochondria and cytosol of rat liver. The activities surveyed include acetoacetyl-CoA hydrolase, acetoacetyl-glutathione hydrolase, acetoacetyl-CoA:glutathione acyl transferase, 3-ketothiolases I and II, 3-hydroxy-3-methylglutaryl-CoA lyase and synthase, and acetoacetyl-CoA synthetase. Phosphocellulose chromatography shows that cytosol contains at least four acetoacetyl-CoA hydrolase activities, two of which do not coincide with 3-ketothiolases or 3-hydroxy-3-methylglutaryl-CoA lyase, while mitochondria contain at least three acetoacetyl-CoA hydrolase activities that overlap partially or completely with 3-ketothiolases and 3-hydroxy-3-methyl-glutaryl-CoA lyase. Two of the mitochondrial acetoacetyl-CoA hydrolase activities are not found in cytosol. Cytosol contains at least two and mitochondrial extracts at least six acetoacetyl-glutathione hydrolase activities. Mitochondria and cytosol both contain two isozymes of 3-ketoacyl-CoA thiolase (thiolases Ia and Ib). Chain length specificities show that the mitochondrial and cytosolic forms of thiolase Ia differ from each other. We report a new isozyme of 3-ketoacyl-CoA thiolase (thiolase I) in rat liver cytosol.