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

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Abstract
High concentrations of Pi inhibited CO2-dependent O2 evolution and prevented the inactivation of glucose-6-phosphate [G-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 O2 evolution and caused rapid inactivation of G-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 (2-4 mM) that gave complete inhibition of CO2-dependent O2 evolution. Formation of dihydroxyacetone-P in the presence of 4 mM Pi was stimulated by light and either O2 (150 .mu.M) or sparker amounts of oxaloacetate. Conditions that favored dihydroxyacetone-P formation (high O2 or low O2 plus oxaloacetate) increased the optimum Pi concentration for CO2-dependent O2 evolution and stimulated O2 evolution at high concentrations of Pi. The stimulation of O2 evolution at superoptimal concentrations of Pi by O2 or oxaloacetate was prevented by dithiothreitol. 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 O2 evolution.

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