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Articles

Reduction of Nitrate via a Dicarboxylate Shuttle in a Reconstituted System of Supernatant and Mitochondria from Spinach Leaves ¹

Department of Biology, Queen's University, Kingston, Ontario K7L 3N6 Canada

Substantial rates of nitrate reduction could be achieved with a reconstituted system from spinach leaves containing supernatant, mitochondria, NAD⁺, oxaloacetate (OAA), and an oxidizable substrate. Appropriate substrates were glycine, pyruvate, citrate, isocitrate, fumarate, or glutamate. The reduction of NO₃^– with any of the substrates could be inhibited by n-butyl malonate, showing that the transfer of reducing power from the mitochondria to the supernatant involved the malate exchange carrier. The addition of ADP to the reconstituted system decreased NO₃^– reduction and this decrease could be reversed by the addition of rotenone or antimycin A. The operation of the OAA/malate shuttle was achieved most quickly in the system when low concentrations (0.1 millimolar) of OAA were added. A corresponding increase in the lag time for the operation of the OAA/malate shuttle was observed when the OAA concentration was increased. Concentrations for half-maximal activity of OAA, glycine, NAD⁺, and NO₃^– in the reconstituted system were 42 micromolar, 0.5 millimolar, 0.25 millimolar, and 26 micromolar, respectively. The transfer of reducing power from the mitochondria to the soluble phase via the OAA/malate shuttle can not only provide NADH for cytoplasmic reduction but can also sustain oxidation of tricarboxylic cycle acids and the generation of -ketoglutarate independently of the respiratory electron transport chain.

¹ This work was supported in part by the National Research Council of Canada.

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