Glyoxylate and Glutamate Effects on Photosynthetic Carbon Metabolism in Isolated Chloroplasts and Mesophyll Cells of Spinach

Abstract

Addition of .mu.M sodium glyoxylate to spinach chloroplasts was inhibitory to photosynthetic incorporation of 14CO2 under conditions of both low (0.2 mM or air levels) and high (9 mM) CO2 concentrations. Incorporation of 14C into most metabolites decreased. Labeling of 6-P[phosphate]-gluconate and fructose-1,6-bis-P increased. This suggested that glyoxylate inhibited photosynthetic Ca metabolism indirectly by decreasing the reducing potential of chloroplasts through reduction of glyoxylate to glycolate. This hypothesis was supported by measuring the reduction of [14C]glyoxylate by chloroplasts. Incubation of isolated mesophyll cells with glyoxylate had no effect on net photosynthetic CO2 uptake, but increased labeling was observed in 6-P-gluconate, a key indicator of decreased reducing potential. The possibility that glyoxylate was affecting photosynthetic metabolism by decreasing chloroplast pH cannot be excluded. Increased 14Clabeling of ribulose-1,5-bis-P and decreased 3-P-glyceric acid and glycolate labeling upon addition of glyoxylate to chloroplasts suggested that ribulose-bis-P carboxylase and oxygenase might be inhibited either indirectly or directly by glyoxylate. Glyoxylate addition decreased 14CO2 labeling into glycolate and glycine by isolated mesophyll cells but had no effect on net 14CO2 fixation. Glutamate had little effect on net photosynthetic metabolism in chloroplast preparations but it increased 14CO2 incorporation by 15% in isolated mesophyll cells under air levels of CO2.