Mitochondria isolated from grown with sucrose as the sole source of carbon and energy oxidized citrate and -aconitate at high rates, and showed good respiratory control and high ADP/O ratios. The oxidation of both substrates was inhibited strongly by rotenone but only slightly by malonate. In contrast, isocitrate was oxidized slowly, with poor respiratory control. Treatment with detergent showed that both NAD- and NADP-isocitrate dehydrogenases were situated within the mitochondrial matrix. It thus appeared that the rate of oxidation of exogenous isocitrate was limited by a slow rate of translocation across the inner mitochondrial membrane.

When was grown in an acetate-containing medium, the mitochondrial pellet, prepared by differential centrifugation, catalysed a rapid oxidation of isocitrate and contained a high activity of isocitrate lyase. The oxidation of isocitrate and succinate was strongly inhibited by malonate but only slightly by rotenone. Density-gradient centrifugation revealed that the apparent oxidation of isocitrate by mitochondrial pellets was due to contamination by glyoxysomes. Isocitrate was converted into glyoxylate and succinate in the glyoxysomes, then succinate was translocated across the inner mitochondrial membrane and oxidized by the respiratory chain.


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