Coupling mechanisms in anionic substrate transport across the inner membrane of rat-liver mitochondria

The translocation of P_i, malate, -oxoglutarate, and citrate across the inner membrane of rat-liver mitochondria has been studied. Investigation on the effect of pH on anionic substrate translocation across the mitochondrial membrane shows that their distribution across the inner membrane can be governed by transmembrane pH difference. However, evidence is presented that the translocation of P_i, but not that of malate, -oxoglutarate, or citrate can bedirectly coupled to an OH^– counterflux (H₂PO ₄ ^– –OH^– exchange-diffusion). and malate-tricarboxylate exchange-diffusion reactions is directly demonstrated. The study of the effect of uncouplers on the efflux from mitochondria of substrate anions, in the absence of counteranion, and on the anion exchange-diffusions shows that uncouplers act in at least two ways: they promote the efflux of P_i from mitochondria and inhibitdirectly the exchange-diffusion reactions. The kinetics of this inhibition are described. These results are discussed in the light of previous work on the effect of uncouplers on the distribution of substrate anions across the inner membrane of isolated mitochondria. Coupling mechanisms in substrate anion translocation and aspects of the energetics of anion translocation are discussed.