The Regulatory Role for Magnesium in Glycolytic Flux of the Human Erythrocyte

Abstract

31P NMR was used to measure the intracellular free magnesium concentration ([Mg2+]i) in human erythrocytes while [Mg2+]i was changed between 0.01 and 1.2 mM using the divalent cationophore A23187. 13C NMR and [2-13C]glucose were used to determine the kinetic effects of [Mg2+]i by measuring the flux through several parts of the glucose pathway. Glucose utilization was strongly dependent on [Mg2+]i, with half-maximal flux occurring at 0.03 mM. The rate-limiting step was most likely at phosphofructokinase, which has a Km(Mg2+) of 0.025 mM in the purified enzyme. Phosphorylated glycolytic intermediate concentration was also strongly dependent on [Mg2+]i and [MgATP], and glucose transport plus hexokinase may have been partially rate-determining at [Mg2+]i below ∼0.1 mM. The pentose phosphate shunt activity was too low to determine the dependence on [Mg2+]i. Phosphoglycerate kinase and 2,3-diphosphoglycerate mutase fluxes were also measured, but were not rate-limiting for glycolysis and showed no Mg2+ dependence. Human erythrocyte [Mg2+]i varies between 0.2 mM (oxygenated) and 0.6 mM (deoxygenated), well above the measured [Mg2+]i(1/2). It is unlikely, then, that [Mg2+]i plays a regulatory role in normal erythrocyte glycolysis. 31P NMR was used to measure the intracellular free magnesium concentration ([Mg2+]i) in human erythrocytes while [Mg2+]i was changed between 0.01 and 1.2 mM using the divalent cationophore A23187. 13C NMR and [2-13C]glucose were used to determine the kinetic effects of [Mg2+]i by measuring the flux through several parts of the glucose pathway. Glucose utilization was strongly dependent on [Mg2+]i, with half-maximal flux occurring at 0.03 mM. The rate-limiting step was most likely at phosphofructokinase, which has a Km(Mg2+) of 0.025 mM in the purified enzyme. Phosphorylated glycolytic intermediate concentration was also strongly dependent on [Mg2+]i and [MgATP], and glucose transport plus hexokinase may have been partially rate-determining at [Mg2+]i below ∼0.1 mM. The pentose phosphate shunt activity was too low to determine the dependence on [Mg2+]i. Phosphoglycerate kinase and 2,3-diphosphoglycerate mutase fluxes were also measured, but were not rate-limiting for glycolysis and showed no Mg2+ dependence. Human erythrocyte [Mg2+]i varies between 0.2 mM (oxygenated) and 0.6 mM (deoxygenated), well above the measured [Mg2+]i(1/2). It is unlikely, then, that [Mg2+]i plays a regulatory role in normal erythrocyte glycolysis.