1. N-Ethylmaleimide inhibited the influx and efflux of Pi in rat liver mitochondria. 2. The efflux was stimulated by either succinate or malate in the presence of N-ethylmaleimide, and this stimulation was reversed by 2-n-butylmalonate. 2-Oxoglutarate and citrate, even in the presence of low concentrations of malate, were relatively ineffective in stimulating efflux of Pi under these conditions, as was glutamate. 3. By using radioactively labelled Pi and dicarboxylate ions an exchange was demonstrated, the stoicheiometry of which was 1.3±0.5 dicarboxylate ions:1 Pi (n=10). 4. An exchange between unlabelled and labelled Pi in the presence of N-ethylmaleimide was found which was sensitive to 2-n-butylmalonate. 5. It is concluded that the mitochondrial dicarboxylate carrier can transport phosphate by an exchange diffusion with certain penetrant dicarboxylic acids or with phosphate itself. The exchange mechanism is sensitive to 2-n-butylmalonate but is unaffected by N-ethylmaleimide; the action of mersalyl in this context is commented on.

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