In order to minimize problems associated with ketoacidosis, which result from an imbalance in ketone body synthesis and utilization, it is necessary to understand the mechanisms controlling ketogenesis. While ketogenic regulation probably involves several control points, preliminary results support the hypothesis that a control site is located within the hydroxymethylglutaryl-coenzyme A (HMG-CoA) cycle, which is the actual site of ketone body synthesis in liver. The precise location of the site of control within this cycle is unclear, as is the significance of control within the HMG-CoA in comparison with that exerted at other regulatory sites. Purification of HMG-CoA synthase and HMG-CoA lyase will permit detailed study of the catalytic mechanisms of these enzymes and also an investigation of how interaction of certain mitochondrial metabolites with these enzymes may alter HMG-CoA cycle activity. Study of the properties of HMG-CoA cycle enzymes will facilitate the design of experiments which will unambiguously and quantitatively determine changes in enzyme activity which occur upon transition from the normal to the ketotic state. Enzymes which display a change in activity that explains the elevated ketogenic capacity characteristic of the diabetic state will be studied in detail. The question of whether enzyme activation is due to an increase in the number of enzyme molecules (long term control) or due to modification of pre-existing enzyme (short term control) will be addressed. Evaluation of the significance of an HMG-CoA cycle control point in overall regulation of ketogenesis requires an estimate of the relative contributions of other postulated control schemes. Measurement of key mitochondrial metabolites, e.g., Coenzyme A and its acylderivatives, under conditions identical to those used in producing fluctuation in HMG-CoA cycle activity, will test how the concentrations or the ratio of concentrations of these compounds affect ketogenesis and will permit a determination of the importance of these factors in ketogenic control in comparison to regulation exerted via fluctuation in activity of HMG-CoA cycle enzymes.