The regulation of pyruvate dehydrogenase in the isolated perfused rat heart.

The regulation of the pyruvate dehydrogenase mul- tienzyme complex was studied in the isolated perfused rat heart. Because 95% of the “CO2 produced as a result of the decarboxylation of [l-‘4C]pyruvate occurs via pyruvate dehydrogenase in the heart, the rate of 14COz production from infused [l-‘4C]pyruvate was utilized as the monitor of the metabolic flux through this enzyme complex. In contrast to results obtained in the perfused liver, infusion of fatty acids of varying chain length into the heart resulted in the inhibition of flux through pyruvate dehydrogenase under all metabolic condi- tions tested. Both acetoacetate and @-hydroxybutyrate infusion caused nearly identical degrees of inhibition of pyruvate dehydrogenase flux (i.e. 85%). However, acetoacetate and &hydroxybutyrate caused changes in the percentage of pyruvate dehydrogenase in the active form from 56 to 43 and 33%. respectively. On the basis of measurements of various nucleotide species in the heart tissue extracts under the different conditions of perfusion, it is suggested that the primary effector of the inhibition/inactivation of pyruvate dehydrogenase by fatty acids or ketone bodies, or both, was the acetyl- CoA/CoASH ratio. Further, it was shown that the ki- netics of fatty acid-mediated inactivation of pyruvate dehydrogenase are certainly no slower than the overall negative change in the metabolic flux through the en- zyme complex upon initiation of the fatty acid infusion. It is concluded that a combination of measurements of (a) metabolic flux rates, (b) the active/total pyruvate dehydrogenase activities, and (c) the tissue levels of various effector molecules of this regulatory system represents an effective approach to a precise determi- nation of the responses of a very complex regulatory system for this multienzyme complex during physiolog- ically meaningful metabolic state changes in cardiac tissue. Most of our present notions concerning the regulation of the pyruvate dehydrogenase multienzyme complex have been based primarily upon experiments performed using the puri- lied enzyme complex or mitochondrial preparations from var- ious tissues and species. What has emerged from an intense experimental consideration in numerous laboratories is a rather complex system of control based upon two different types of regulatory mechanisms. Pyruvate dehydrogenase