Inhibition by oxalomalate of rat liver mitochondrial and extramitochondrial aconitate hydratase

Abstract

1. The effect of oxalomalate on the oxidation of citrate and cis-aconitate in rat liver mitochondria, and on the activity of mitochondrial and cytoplasmic aconitate hydratase, has been investigated. 2. Oxalomalate that was added to intact rat liver mitochondria at high concentrations (2mm) produced complete inhibition of citrate and cis-aconitate oxidation, but lower concentrations (0.1–0.25mm) inhibited oxidation of citrate more than that of cis-aconitate. 3. Aconitate hydratase that was either extracted from mitochondria or soluble in the cytoplasm, was strongly inhibited by low concentrations of oxalomalate (0.01–0.2mm), the mitochondrial enzyme being more sensitive than the soluble one. 4. Oxalomalate, when added together with citrate, produced competitive inhibition; the Ki values calculated were 1×10−6m for the mitochondrial and 2.5×10−6m for the cytoplasmic enzyme. 5. With both the enzymic preparations oxalomalate added together with the substrates inhibited the initial rate of the reaction citrate→cis-aconitate more than that of the reaction isocitrate→cis-aconitate. 6. After 2min of preincubation of the inhibitor with either of the enzymic preparations the inhibition increased tenfold and became irreversible; under these conditions both the reactions were inhibited to the same extent. 7. The inhibition by oxalomalate of aconitate hydratase appeared to be similar in many respects to that produced by fluorocitrate on the same enzyme.