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

Electrical Nature and Coupling of the Exchange-Diffusion Reactions with Proton Translocation



  • 1The electrical nature of the malate-citrate, malate-Pi 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.
  • 2The study of the effect of ionophores and uncouplers on the exchange-diffusion reactions indicates that the citratein-malateout and the malatein-Pi,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 malatein-citrateout exchange appears to be inhibited by electroneutral influx of H+.
  • 3Measurement 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 Pi-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.
  • 4It is demonstrated that the H+: citrate and the H+: Pi ratios determined experimentally for the malate-citrate and malate-Pi exchanges practically coincide with the theoretical ratios expected for electroneutral malate2-citrate2- and malate2-HPO:2-4 exchanges.
  • 5The malate-2-oxoglutarate exchange is not affected by ionophores and/or uncouplers neither it is accompanied by proton translocation across the mitochondrial membrane.
  • 6It is concluded that the malate-citrate, malate-Pi and malate-2-oxoglutarate exchange diffusion reactions are electroneutral and that the proton translocation accompanying the malate-citrate and malate-Pi exchanges is determined by the ionization state of malic, citric and phosphoric acid.