Modulation of rat-liver mitochondrial acetyl-CoA acetyltransferase activity by a reversible chemical modification with coenzyme A.

The mitochondrial acetyl-CoA acetyltransferase (acetoacetyl-CoA thiolase, EC 2.3.1.9) is involved in ketone body biosynthesis. In its unmodified state, referred to as transferase B in former publications (Huth, W. (1981) Eur. J. Biochem. 120, 557-562), the enzyme is characterized by the highest specific activity of 21.65 mumol/min per mg protein (direction of acetoacetyl-CoA synthesis); several forms of the enzyme with lower specific activities result from chemical modification by an apparent covalent binding of CoASH. The chemical modification results in an inactivation of the enzyme: a 2 h incubation with 0.2 mM CoASH at pH 8.1 at 30 degrees C inactivates up to 95%. Both processes, the CoASH-binding and the resulting inactivation, can be simultaneously reversed by treatment with glutathione. The specificity of inactivation is limited to CoASH and the intact sulfhydryl group is a prerequisite for this process. The enzyme exhibits a limited number (n = 3.2) of high-affinity (Ka = 26.7 microM) specific binding sites for CoASH. The inactivation-reactivation cycle of acetyl-CoA acetyltransferase by CoASH and glutathione may involve a protein disulfide-thiol exchange and represents a mode of control in modulating the amount of active enzyme.