Effects of grisorixin on glutamate transport and oxidation in rat liver mitochondria: Relationships between transport and oxidation

Abstract

Summary o 1. The effect of grisorixin (10−7 M), an ionophore of the nigericin group, on phosphate and glutamate uptake, in rat liver mitochondria, has been studied. Kinetic data were obtained by two methods : translocation of 32Pi and [3H] glutamate and facilitated diffusion of glutamate induced by the valinomycin-K+ system or by incubation of mitochondria in 100 mM K+ glutamate. Grisorixin inhibited glutamate and phosphate uptake while valinomycin increased them. ClCCP inhibited both by 50 per cent. Rotenone had but little inhibitory effect on the transport of either. 2. The effect of grisorixin on glutamate and glutamate + malate oxidation in rat liver mitochondria was investigated under conditions in which grisorixin does not uncouple : ([K+]ext = 10 mM, [grisorixin] = 10−7 M). Grisorixin inhibited both glutamate and glutamate + malate oxidation. Mitochondrial respiration in the presence of glutamate was partially restored when malate was added in the presence of ClCCP. Malate alone or ClCCP alone were not effective. The ClCCP effect on glutamate oxidation was studied under two conditions. When mitochondria were previously incubated with ClCCP, glutamate oxidation was strongly inhibited. On the contrary, when glutamate was added before ClCCP, normal oxidation uncoupling was observed. Phosphate always stimulated oxidation uncoupling of glutamate. 3. All the results are interpreted in terms of the relationships between transport and oxidation. It is clear that grisorixin or ClCCP can compete with anion translocation (phosphate or glutamate) for driving proton uptake by the mitochondrial internal membrane. Under these conditions, inhibition of transport results in inhibition of oxidation. In contrast valinomycin increased oxidation by stimulation of anion transport. Furthermore, results concerning glutamate transport and oxidation must be interpreted according to the glutamate/H+ co-transport system (I) and the glutamate-aspartate exchange system (II). It is postulated that system I would be directly linked to K+ translocation while system II might work even when K+ translocation is inhibited.