Control of Red Cell Glycolysis

Fructose diphosphate and the triose phosphates are known to accumulate in red blood cells when they metabolize glucose at high rates due to the presence of orthophosphate. This has led to the suggestion that either glyceraldehyde phosphate dehydrogenase or glycerate phosphate kinase is rate-limiting under these conditions. However, the present work shows, by a study of isotope mixing rates and mass action ratios, that under conditions of triose phosphate accumulation both steps proceed much more rapidly than the limiting rate of lactate production. It is observed that the conditions of triose phosphate accumulation also result in net synthesis of glycerate 2,3-diphosphate and in the occurrence of extremely high ratios of lactate to pyruvate, i.e. DPNH to DPN+, thus explaining how the triose phosphates can be so elevated and yet be at equilibrium. The net synthesis of glycerate 2,3-diphosphate, which requires an unknown endogenous oxidative system, is presumed to occur because the glycerate 1,3-diphosphate level exceeds that of the cell in vivo. This results at high glycolytic rates because the glycerate 1,3-diphosphate steady state concentration is determined by equilibria with the steady state concentrations of phosphoenolpyruvate, ADP, and ATP, which increase, decrease, and increase, respectively, under high glycolytic rate conditions (ROSE, I. A., AND WARMS, J. V. B., J. Biol. Chem., 241, 4848 (1966)). Thus, these effects are not evidence of ratelimiting processes in the utilization of triose phosphates as previously supposed, but rather are consequences of the control of the rates of pyruvate kinase and glycerate diphosphate mutase by the concentration of their substrates and of the inability of the red cell, in vitro, to maintain the DPNH to DPN+ ratio at its in vivo level in the face of net synthesis of glycerate 2,3-diphosphate.