Cyanide-resistant Respiration of Sweet Potato Mitochondria

The oxidation of malate and succinate by sweet potato mitochondria (Ipomoea batatas [L.] Lam.) was blocked only partly by inhibitors of complexes III (2-heptyl-4-hydroxyquinoline-N-oxide) and IV (cyanide and azide). The respiration insensitive to inhibitors of complexes III and IV was inhibited by salicylhydroxamic acid. Essentially complete inhibition was obtained with inhibitors of complex I (rotenone, amytal, and thenoyltrifluoroacetone) and complex II (thenoyltrifluoroacetone). The observations indicated that electrons were transferred to the cyanide-resistant pathway from ubiquinone or from nonheme iron (iron-sulfur) proteins of complexes I and II before reaching the b cytochromes. In contrast, the oxidation of exogenous NADH did not involve the alternate pathway, as indicated by complete inhibition by inhibitors of complexes III and IV and the absence of an effect of inhibitors of complexes I and II. Hence, electrons from exogenous NADH appear to pass directly to complex III in sweet potato mitochondria.