Inhibition of photosynthetic carbon metabolism in isolated chloroplasts by iodoacetol phosphate.

Carbon dioxide-dependent and 3-phosphoglycerate (PGA)-dependent O(2) evolution by isolated chloroplasts of wheat 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 micromolar IAP the half-time is about 0.5 minutes at 1 millimolar PGA compared to 1.5 minutes at 10 millimolar PGA). A marked inhibition of NADP glyceraldehyde-3-P dehydrogenase 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 glyceraldehyde-3-P dehydrogenase and the half-time for inactivation at 5 nanomolar IAP was about 1 minute. 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 about three orders of magnitude higher than that with glyceraldehyde-3-P dehydrogenase. Micromolar levels of IAP had no effect on ribulose-1,5-bisphosphate carboxylase. Inhibition of chloroplast photosynthesis and of glyceraldehyde-3-P dehydrogenase in protoplast extracts with IAP follows pseudo first-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 millimolar) competitively inhibits uptake of (32)Pi. Iodoacetol phosphate had no effect on ferricyanide-dependent O(2) evolution with isolated chloroplasts. Also, IAP had no effect on light-dependent fixation of CO(2) through the carboxylation phase of the C(4) pathway with protoplast extracts of crabgrass mesophyll cells. The site of IAP inhibition of photosynthesis with wheat chloroplasts is suggested to be through the inactivation of glyceraldehyde-3-P dehydrogenase.