INTERRELATIONSHIPS BETWEEN 3-HYDROXY-3-METHYLGLUTARYL-COA SYNTHASE, ACETOACETYL-COA AND KETOGENESIS

Kinetic and physical approaches were employed to investigate the binding of acetoacetyl-CoA to [avian liver] hydroxymethylglutaryl-CoA synthase. The enzyme has an apparent Km for acetoacetyl-CoA (0.35 .mu.M) which is more than an order of magnitude lower than the Ki (6-10 .mu.M) measured for substrate inhibition by this metabolite. Hepatic acetoacetyl-CoA concentration, as measured by a sensitive and highly specific radioactive assay, appears to be in the 1-10 .mu.M range; the concentration decreases during diabetic ketoacidosis. Total hepatic activity of hydroxymethylglutaryl-CoA synthase and levels of mitochondrial enzyme protein, determined by radioimmunoassay, are not appreciably different in livers from control or ketoacidotic rats. In contrast to the decrease in hepatic acetoacetyl-CoA concentration observed during ketoacidosis, myocardial acetoacetyl-CoA levels are increased by at least 10-fold when compared to controls. Elevated acetoacetyl-CoA levels may serve to inhibit fatty acid utilization by the heart. Thus, a consideration of the multiple interactions of acetoacetyl-CoA with the enzymes involved in ketone body production and utilization may be useful in evaluating the metabolic significance of this intermediate.