The Transport of Citric‐Acid‐Cycle Intermediates in Rat‐Liver Mitochondria

Abstract

The electrical nature of the malate‐citrate, malate‐P i and malate‐2‐oxoglutarate exchange diffusion reactions in rat liver mitochondria has been studied. The exchanges have been followed in the presence of respiratory inhibitors and oligomycin to suppress energy supply. The study of the effect of ionophores and uncouplers on the exchange‐diffusion reactions indicates that the citrate in ‐malate out and the malate in ‐P i,out exchanges are stimulated by inducing with nigericin or with valinomycin plus carbonyl cyanide p ‐trifluoromethoxy phenylhydrazone (FCCP) electroneutral influx of H + . These exchanges are unaffected by inducing electrogenic efflux of K + from mitochondria with valinomycin. The malate in ‐citrate out exchange appears to be inhibited by electroneutral influx of H + . Measurement of the extra‐ and intramitochondrial pH shows that the malate‐citrate exchange diffusion is accompanied by translocation of protons, across the mitochondrial membrane, in the same direction as citrate and the P i ‐malate exchange by proton translocation in the direction of malate. Evidence is presented that the proton translocation is a direct consequence of the anion exchange. It is demonstrated that the H + : citrate and the H + : P i ratios determined experimentally for the malate‐citrate and malate‐P i exchanges practically coincide with the theoretical ratios expected for electroneutral malate 2 ‐citrate 2 ‐ and malate 2 ‐HPO: 2‐ 4 exchanges. The malate‐2‐oxoglutarate exchange is not affected by ionophores and/or uncouplers neither it is accompanied by proton translocation across the mitochondrial membrane. It is concluded that the malate‐citrate, malate‐P i and malate‐2‐oxoglutarate exchange diffusion reactions are electroneutral and that the proton translocation accompanying the malate‐citrate and malate‐P i exchanges is determined by the ionization state of malic, citric and phosphoric acid.