Orthophosphate Control of Glucose-6-Phosphate Dehydrogenase Light Modulation in Relation to the Induction Phase of Chloroplast Photosynthesis

Abstract

High concentrations of orthophosphate (Pi) inhibited CO(2)-dependent O(2) evolution and prevented the inactivation of glucose-6-P dehydrogenase by light in intact spinach and barley chloroplasts. Addition of glycerate-3-P to chloroplasts inhibited by Pi in the light, induced O(2) evolution and caused rapid inactivation of glucose-6-P dehydrogenase. The activity of phosphofructokinase detected in chloroplast preparations was not affected by light or by Pi.Dihydroxyacetone-P was a major product of chloroplast photosynthesis when optimum concentrations of Pi were used. Chloroplasts continued to form dihydroxyacetone-P at a slow rate in the presence of Pi at concentrations (2 to 4 millimolar) that gave complete inhibition of CO(2)-dependent O(2) evolution. Formation of dihydroxyacetone-P in the presence of 4 millimolar Pi was stimulated by light and either O(2) (150 micromolar) or sparker amounts of oxaloacetate.Conditions that favored dihydroxyacetone-P formation (high O(2) or low O(2) plus oxaloacetate) increased the optimum Pi concentration for CO(2)-dependent O(2) evolution and stimulated O(2) evolution at high concentrations of Pi. The stimulation of O(2) evolution at superoptimal concentrations of Pi by O(2) or oxaloacetate was prevented by dithiothreitol.The results suggested that formation of pentose-P pathway intermediates via the oxidative pentose-P pathway may be limited by availability of NADP in the light but may occur at significant rates and thereby contribute to termination of the induction phase of O(2) evolution.