The inhibition by fluorocitrate of rat liver mitochondrial and extramitochondrial aconitate hydratase

Abstract

1. The effects of synthetic fluorocitrate were studied on: (a) the oxidation of citrate and cis-aconitate by rat liver mitochondria; (b) the activity of the aconitate hydratase found in the liver cell sap; (c) the activity of the aconitate hydratase solubilized from liver mitochondria. 2. Fluorocitrate was found to be a potent inhibitor of oxidation of citrate but only a weak inhibitor of oxidation of cis-aconitate: 6·7μm-fluorocitrate (containing 4% of the inhibitory isomer) caused 94% inhibition of the oxidation of citrate (2mm) whereas 1·0mm-fluorocitrate was necessary to provoke the same inhibition when cis-aconitate (2mm) was the substrate. The degree of inhibition varied in relation to the respiratory state of mitochondria when fluorocitrate was added. The inhibition could be partially reversed by cis-aconitate. 3. The aconitate hydratase extracted from the mitochondria was much less inhibited by fluorocitrate than was the mitochondria-bound enzyme, and the aconitate hydratase found in the cell sap was even less sensitive. 0·3mm-Fluorocitrate was required to cause 50% inhibition of the reaction citrate→cis-aconitate, catalysed by the aconitate hydratase extracted from the mitochondria, and 1·2m-fluorocitrate for the extramitochondrial enzyme. For both enzymes the reaction citrate→cis-aconitate was 2–3 times more sensitive to fluorocitrate than was the reaction isocitrate→cis-aconitate. The inhibition was of the competitive type for both reactions.