Effect of Photosynthetic Intermediates on the Magnesium Inhibition of Oxygen Evolution by Barley Chloroplasts 1

AUTOR(ES)

Huber, Steven C.

RESUMO

Millimolar concentrations of Mg2+ inhibited CO2-dependent O2 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 O2 evolution by Mg2+ and reduced the Mg2+ inhibition of enzyme activation by light. Fructose 1,6-diphosphate and ribulose 5-phosphate also prevented the inhibition of O2 evolution by Mg2+ 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 Mg2+ inhibition of O2 evolution shortened the lag phase of CO2-dependent O2 evolution in the absence of M2+. Loading chloroplasts in the dark with 3-phosphoglycerate reduced both the lag phase of O2 evolution and the inhibition of O2 evolution by Mg2+. The results suggested that Mg2+ 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.

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