Competition for ADP between pyruvate kinase and mitochondrial oxidative phosphorylation as a control mechanism in glycolysis.

Abstract

To assess a possible role of pyruvate kinase as a site for respiratory and glycolytic interaction, competition for ADP between pyruvate kinase and respiratory phosphorylation was measured in a model system consisting of rat liver mitochondria respiring in the presence of pyruvate kinase, phosphoenolpyruvate, ATP, and an ADP‐regenerating system consisting of glucose and purified yeast hexokinase. This system allowed determination of total ATP production, equivalent to ADP utilization, by measuring glucose 6‐phosphate formation; ADP utilization by pyruvate kinase by pyruvate formation; and respiratory ADP utilization by Pi uptake. O2 uptake was measured by means of an oxygen electrode. In the presence of a respiratory substrate such as succinate or glutamate‐malate, the addition of pyruvate kinase and phosphoenolpyruvate reduced O2 uptake as well as oxidative phosphorylation about 80%. Respiration was increasingly inhibited with increasing pyruvate kinase, and this inhibition was decreased with increased hexokinase or ATP. Mitochondrial respiratory inhibition by pyruvate kinase and phosphoenolpyruvate was accompanied by an increase in the ATP/ADP ratio from 0.3 to 32. These inhibitory effects of pyruvate kinase on respiration were abolished by addition of 2,4‐dinitrophenol. These results are in acccord with a role of pyruvate kinase as a determinant of glycolytic activity by competing with oxidative phosphorylation for the available ADP; and provide additional support for our previous suggestion that high glycolysis of certain tumors may be attributable to their extraordinarily high pyruvate kinase activity.