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Polarographic Study of Dicarboxylic-Acid-dependent Export of Reducing Equivalents from Illuminated Chloroplasts

Botany Department, La Trobe University, Bundoora, Victoria 3083 Australia

Isolated pea chloroplasts, prepared by differential centrifugation, catalyzed O₂ evolution in the light in the presence of 0.03 to 3 millimolar malate, 0.12 to 1.2 millimolar NAD, 4 millimolar pyruvate and exogenous NAD-malate dehydrogenase and lactate dehydrogenase. The reaction, which did not occur in the absence of any one of these factors, was accompanied by the consumption of pyruvate; the ratio of O₂ evolved to pyruvate consumed was <0.5. When 0.1 millimolar [¹⁴C]malate was supplied most of the ¹⁴C label was recovered as malate. At low concentrations of malate (<0.1 millimolar), the ratio of O₂ evolved to malate supplied was greater than 0.5.

Chloroplasts catalyzed oxaloacetate-dependent O₂ evolution in approximate agreement with the theoretical stoichiometry for the light-coupled reduction of oxaloacetate to malate. Subsequent addition of NAD, pyruvate, NAD-malate dehydrogenase, and lactate dehydrogenase caused immediate resumption of O₂ evolution with the concomitant reduction of pyruvate to lactate externally to the chloroplasts. Resumption of O₂ evolution did not occur in the absence of oxaloacetate. The results are consistent with a mechanism in which oxaloacetate/malate cycles continuously between the chloroplasts and the external solution and serves as a carrier for the transfer of light-generated reducing equivalents for pyruvate reduction outside the chloroplast.