Suspensions of pea chloroplasts, prepared by differential centrifugation, catalyzed oxaloacetate-dependent O2 evolution (mean rate of 29 determinations 10.9 micromoles per milligram of chlorophyll per hour, sd 3.2) with the concomitant production of malate. At optimum concentrations of oxaloacetate, both reactions were light-dependent, inhibited by 3-(3,4- dichlorophenyl)-1, 1-dimethylurea and oxalate, and enhanced 2.5- to 4-fold by 10 millimolar NH4Cl. At concentrations of oxaloacetate (<50 micromolar), 10 millimolar NH4Cl was inhibitory. The ratio of O2 evolved to malate produced was 0.39 to 0.58. The ratio of O2 evolved to oxaloacetate supplied was commensurate with the theoretical value of 0.5.Chloroplast suspensions contained both NAD- and NADP-malate dehydrogenase activities. It was concluded from oxalate inhibition studies and the promotion of oxaloacetate-dependent O2 evolution by shocked chloroplasts by NADPH (but not NADH) that the reaction was mediated via the NADP enzyme.
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