Effect of Phosphorylated Compounds and Inhibitors on CO2 Fixation by Intact Spinach Chloroplasts

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Abstract
The effect of some phosphorylated compounds principally sugar phosphates and the inhibitors, arsenite and iodoacetamide, upon CO2 fixation by intact spinach chloroplasts under anaerobic conditions was studied. Ribulose-1,5-diphosphate, fructose-1,6-diphosphate, ribose-5-phosphate, gly-ceraldehyde-3-phosphate, dihydroxyacetone phosphate, and glycerate-3-phosphate enhanced the rate; sedoheptulose-7-phosphate and -1,7-diphosphate inhibited; erythrose-4-phosphate, glycolaldehyde-phosphate, L-[alpha]-glycerol phosphate and, phosphoenolpyruvate had no effect; and fructose-6-phosphate and glucose-6-phosphate yielded inconclusive results. A lag in CO2 fixation with respect to time was overcome by all the enhancing compounds. Follwoing the lag period the rates of fixation by the control and that of the preparation containing the phosphorylated compound were similar. The addition of fructose-l,6-di-phosphate did not cause a marked change in the distribution of C14 among the products. Degradation of labeled glycerate-3-phosphate established that addition of phosphorylated intermediates did not impair the C cycle. The phosphorylated compounds affected CO2 fixation by filling up the pools of the photosynthetic C cycle depleted during isolation of the chloroplast. Linearity of CO2 fixation with respect to light intensity was obtained only in the presence of fructose-1,6-diphosphate. Inhibition of CO2 fixation by iodoacetamide and arsenite was relieved by the compounds which enhance fixation. Light also protected against inhibition of CO2 fixation by iodoacetamide and arsenite.

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