Inhibition of Glycolysis by Pyruvate in Relation to the Accumulation of Citric Acid Cycle Intermediates in the Perfused Rat Heart

Abstract

RECENT investigations have established that the addition of metabolic fuels such as acetoacetate, fatty acids, lactate or pyruvate to hearts perfused with glucose and insulin inhibits the rate of glucose uptake and oxidation^1–4. It has been proposed that during respiration of these substrates, glycolysis is inhibited by a common mechanism; namely, an indirect inhibition of phosphofructokinase as indicated by increased levels of hexose monophosphates and decreased levels of fructose-1,6-diphosphate⁵. Furthermore, phosphofructokinase activity has been shown to be depressed in hearts from diabetic rats which also have a decreased rate of glucose utilization⁶. It has recently been reported that citrate not only inhibits phosphofructokinase activity in cell-free system, but also accumulates in hearts from diabetic rats, and in isolated hearts perfused with long- and short-chain fatty acids^7–9. These results have led to the suggestion that the rate of glycolysis may in some measure be controlled by the activity of the citric acid cycle. Williamson¹⁰ has reported that the rate-limiting step of glycolysis in perfused hearts with pyruvate as substrate occurred not at the phosphofructokinase step, as proposed by Newsholme et al.⁵, but at a site located between glyceraldehyde-3-phosphate and pyruvate. It was therefore considered of interest to examine the kinetics of the changes of the glycolytic intermediates after addition of pyruvate, in conjunction with the levels of certain citric acid cycle intermediates in both the intact heart and isolated heart mitochondria.