In vitro formation of glucose 6-phosphate from 3-phosphoglycerate by rat liver cytosol

Liver cytosol preparations from 48 h starved rats, supplemented with 3 mM ATP, 0.5 mM sn-glycerol 3-phosphate, 10mM MgSO4 and a NADH-regenerating system showed a net glucose 6-phosphate formation from 2mM 3-phosphoglycerate of 0.72 micromol x min-1 x (g liver)-1. Glycerol 3-phosphate slightly increased when added at 0.5 mM; in its absence, dihydroxyacetone-phosphate formed from 3-phosphoglycerate was mainly reduced to glycerol 3-phosphate at the expense of fructose 1,6-bisphosphate and glucose 6-phosphate synthesis; when glycerol 3-phosphate was added at higher concentrations, net utilization occurred. The rate of glucose 6-phosphate formation positively correlated with the concentration of 3-phosphoglycerate and with the ratio of ATP/ADP. Increasing NADH concentrations stimulated the flow from 3-phosphoglycerate to glyceraldehyde 3-phosphate but also enhanced the reduction of dihydroxyacetone phosphate to glycerol 3-phosphate; as a result, the concentration of free NADH had little effect on the rate of glucose 6-phosphate formation. Lowering the concentration of magnesium from 10mM to 4mM, corresponding to free Mg2 concentrations of 7 and 0.6 mM respectively, resulted in a 30% increase of the rate of glucose 6-phosphate formation. The physiological relevance of the results presented is discussed.