The determination of the proton-motive force during cyanide-insensitive respiration in plant mitochondria

Abstract

The magnitude of the components of the proton-motive force (Δ p) generated in the presence of antimycin A has been determined for potato, mung bean, skunk cabbage, and Arum spadix mitochondria, Δ p was calculated from the distribution of rubidium, methylamine, and 5,5′-dimethyl-2,4-oxazolodine-dione. In the presence of antimycin A, the oxidation of succinate generates a Δ p of 40–50 mV, and this value is independent of the degree of antimycin A insensitivity of the various mitochondria. Under such conditions, the addition of ADP failed to either stimulate the respiratory rate or reduce Δ p. Although oxygen consumption via the alternative pathway was sensitive to hydroxamic acids, no change in the components of the proton motive force was detected. The addition of an uncoupler in the presence of antimycin A and succinate reduced Δ p to zero while respiration remained unaltered. The oxidation of malate in the presence of antimycin A generates a Δ p of 150 mV, which was reduced to 144 mV under State 3 conditions. The addition of salicylhydroxamic acid inhibited oxygen uptake and reduced Δ p to 40 mV. It is concluded that the oxidation of succinate by the alternative respiratory pathway does not generate a proton-motive force and is not coupled to ATP synthesis. The oxidation of malate by the alternative pathway, however, can conserve energy as ATP presumably via coupling Site I of the main respiratory chain.