Inhibition of Photosynthetic Carbon Metabolism in Isolated Chloroplasts by Iodoacetol Phosphate

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Abstract
CO2-dependent and 3-phosphoglycerate (PGA)-dependent O2 evolution by isolated chloroplasts of wheat [Triticum aestivum] is inhibited by micromolar levels of iodoacetol phosphate (IAP). Loss of the activity is time-dependent and a higher concentration of PGA increases the half-time for inhibition (e.g., at 40 .mu.M IAP the half-time is .apprx. 0.5 min at 1 mM PGA compared to 1.5 min at 10 mM PGA). A marked inhibition of NADP glyceraldehyde-3-phosphate dehydrogenase [GPDase] was observed when chloroplasts were pretreated with micromolar levels of IAP, osmotically shocked, and several stromal enzymes assayed. Extraction of enzymes from wheat protoplasts and treatment with IAP showed that nanomolar levels of the compound completely inhibited NAD and NADP GPDase and the half-time for inactivation at 5 mM IAP was .apprx. 1 min. The inhibitory effect of IAP was not reversed by passing the enzyme extract through a column of Sephadex G-25. The concentration of IAP required for inhibition of the chloroplastic triose phosphate isomerase is .apprx. 3 orders of magnitude higher than that with GPDase. Micromolar levels of IAP had no effect on ribulose-1,5-biosphosphate carboxylase. Inhibition of chloroplast photosynthesis and of GPDase in protoplast extracts with IAP follows pseudo 1st-order kinetics. Pretreatment of chloroplasts with IAP did not inactivate the phosphate translocator of the chloroplast envelope. Iodoacetol phosphate may enter the chloroplasts through the phosphate translocator since a high concentration of IAP (0.5 mM) competitively inhibits uptake of 32Pi. Iodoacetol phosphate had no effect on ferricyanide-dependent O2 evolution with isolated chloroplasts. IAP had no effect on light-dependent fixation of CO2 through the carboxylation phase of the C4 pathway with protoplast extracts of crabgrass [Digitaria sanguinalis] mesophyll cells. The site of IAP inhibition of photosynthesis with wheat chloroplasts is suggested to be through the inactivation of GPDase.