Abstract
Millimolar concentrations of Mg(2+) inhibited CO(2)-dependent O(2) evolution by barley (Hordeum vulgare L.) chloroplasts and also prevented the activation of NADP-glyceraldehyde-3-phosphate dehydrogenase, ribulose-5-phosphate kinase, and fructose-1,6-diphosphatase by light in intact chloroplasts. When added in the dark, 3-phosphoglycerate prevented the inhibition of O(2) evolution by Mg(2+) and reduced the Mg(2+) inhibition of enzyme activation by light. Fructose 1,6-diphosphate and ribulose 5-phosphate also prevented the inhibition of O(2) evolution by Mg(2+) whereas glucose 1-phosphate, glucose 6-phosphate, ribulose 1,5-diphosphate, and citrate had no effect. Phosphoenolpyruvate gave an intermediate response. Metabolites that prevented the Mg(2+) inhibition of O(2) evolution shortened the lag phase of CO(2)-dependent O(2) evolution in the absence of M(2+). Loading chloroplasts in the dark with 3-phosphoglycerate reduced both the lag phase of O(2) evolution and the inhibition of O(2) evolution by Mg(2+). The results suggested that Mg(2+) inhibition was lessened either by external metabolites that compete with inorganic phosphate for transport into the chloroplast or by a high concentration of internal metabolites.