Electron Transport and Oxidative Phosphorylation inArumSpadix Mitochondria

Abstract

Arum spadix mitochondria exhibited a rapid cyanide-resistant oxygen uptake when oxidizing malate, NADH2 or succinate, and a slower, cyanide-sensitive oxygen uptake when oxidizing ascorbate+tetramethylphenylenediamine (TMPD). Cytochrome oxidase does not therefore appear to function as the terminal oxidase in the presence of cyanide, and the rather low cytochrome c oxidase activity obtained using ascorbate+TMPD may exclude it from possessing a major role even in the absence of cyanide. ATP synthesis has been shown to accompany substrate oxidation. In the presence of antimycin A the P: O ratio accompanying malate oxidation was reduced by half, while phosphorylation accompanying NADH2 or succinate oxidation was almost completely abolished. It is proposed that electrons from exogenous NADH2 enter the electron transport chain at a site after that where endogenous NADH2 donates electrons and that electrons from exogenous NADH2 are not coupled to ATP synthesis at site 1. The cyanide-resistant, non-phosphorylating electron-transport pathway may function in the absence of cyanide and account for the low efficiency of energy conservation observed in this tissue.