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© 1979 Oxford University Press

RESEARCH-ARTICLE

The Oxidation of Malate by Isolated Turnip (Brassica napus L.) Mitochondria

III. THE EFFECTS OF INHIBITORS

HARRIS GOONEWARDENA 1 and S. BRIAN WILSON

Department of Biochemistry, University of Aberdeen Marischal College, Aberdeen AB9 IAS

Oxaloacetic acid (OAA) and fluoro-oxaloacetic acid (FOAA) are both specific inhibitors of malate dehydrogenases. Exogenous OAA penetrates the mitochondria rapidly, being converted to pyruvate, citrate, or malate depending on the reaction conditions, with the relief of the inhibition, whereas inhibition by FOAA is permanent Exogenous OAA removal has been used as a model for endogenous OAA removal by measuring the time taken for the inhibition to be relieved in the presence of various inhibitors and substrates. Hydroxymalonate behaves primarily as a malic enzyme inhibitor while butylmalonate acts as an inhibitor of malate transport in intact mitochondria, although at high concentrations both compounds inhibit the malate dehydrogenase.

The NAD associated with the various enzymes behaves in a complex compartmented manner. It is concluded that the oxidation of malate takes place mainly via the membrane-bound malate dehydrogenase, linked through all three phosphorylation sites. The OAA produced is removed by the combined action of the soluble malate dehydrogenase and malic enzyme of the matrix. Reducing equivalents from the malic enzyme can also gain access to the full respiratory chain and utilize all three phosphorylation sites.

Evidence is presented for malic enzyme activity on the outside of the inner membrane and in the matrix compartment


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