3-Hydroxy-3-methylglutaryl-CoA lyase: catalysis of acetyl coenzyme A enolization

3-Hydroxy-3-methylglutaryl-CoA lyase, which performs the cleavage of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to acetoacetate and acetyl-CoA by a Claisen-type reaction, also catalyzes enolization of acetyl-CoA. The rate of detritiation of methyl-labeled acetyl-CoA is proportional to enzyme concentration and is diminished by an antiserum that also inhibits the cleavage reaction. The tritium-exchange reaction requires both divalent cation and acetoacetate. An analogue of HMG-CoA, 3-hydroxyglutaryl-CoA, was prepared by reaction of acetonedicarboxylic anhydride with CoASH and reduction of the ketoacyl-CoA product with cyanohydridoborate. While 3-hydroxyglutaryl-CoA does not appear to be a substrate for HMG-CoA lyase, it competitively inhibits both the cleavage reaction (Ki = 50 microM) and the tritium exchange from acetyl-CoA (Ki = 95 microM). Agreement between the Ki values measured for cleavage and for tritium exchange supports the hypothesis that the slow tritium exchange is a lyase-dependent reaction. Initial attempts to demonstrate complete reversibility of the cleavage reaction have not been successful. However, the data suggest that the cleavage of HMG-CoA is at least partially reversible and indicate that enolization of acetyl-CoA may be dependent upon a conformational change of HMG-CoA lyase, induced by binding of acetoacetate, in a manner analogous to the keto acid dependent tritium exchange catalyzed by malate synthase and citrate synthase.