Effect of insulin on the metabolic distribution of carbons 1, 2, and 3 of pyruvate

It has long been known that the carbons of pyruvate are converted to CO2 at different points in the metabolic process. This report deals with the observation that insulin affects the oxidation of carbons 2 and 3 primarily and has little effect on the oxidation of the carboxyl carbon. Oxidation of different carbons of pyruvate and their incorporation into various metabolic components was studied in isolated rat hepatocytes. Insulin stimulated the 14CO2 production from 2-14C]- and 3-14C]pyruvate and from U-14C]alanine. However, it had little or no effect on the activity of the pyruvate dehydrogenase complex as measured by the evolution of 14CO2 from 1-14C]pyruvate or 1-14C] alanine. Insulin also stimulated the incorporation of carbons 2 and 3 of pyruvate into protein but had no effect on the incorporation of carbon 1. Incorporation of 1-14C]- and U-14C]alanine into protein was differentially enhanced by insulin in a manner similar to that of the pyruvate carbons. The fact that insulin stimulates the incorporation of 1-14C]alanine into protein but not 1-14C]pyruvate suggests the possibility of a compartmentation of pyruvate metabolism in the isolated hepatocytes. These studies show that the stimulation of 2-14C]- and 3-14C]pyruvate incorporation into protein involves the stimulatory effect of insulin on the activity of the Krebs cycle which is evident from the fact that insulin did not stimulate the pyruvate carbons to enter protein via alanine but the incorporation via glutamate was increased by about 40%.