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Stoichiometry of Proton Translocation Coupled to Substrate Oxidation in Plant Mitochondria

Laboratoire de Biologie Végétale IV, Centre National de la Recherche Scientifique (UA 578), Université Pierre et Marie Curie, 12, rue Cuvier, 75005 Paris, France

The proton translocation coupled to the electron flux from succinate, exogenous NADH, and NAD⁺-linked substrates (malate and isocitrate) to cytochrome c and to oxygen was studied in purified potato (Solanum tuberosum) mitochondria using oxygen and ferricyanide pulse techniques. In the presence of valinomycin plus K⁺ (used as a charge compensating cation), optimum values of H⁺/2 e^– were obtained when low amounts of electron acceptors (oxygen or ferricyanide) were added to the mitochondria (1-2 nanogram [2 e^–] equivalents per milligram protein). The stoichiometry of proton translocation to electron flux was unaffected in the presence of N-ethylmaleimide, an inhibitor of the Pi/H⁺ symport. With succinate as substrate, H⁺/2 e^– ratios were 4.0 ± 0.2 and 3.7 ± 0.3 with oxygen and ferricyanide as electron acceptors, respectively. With exogenous NADH, H⁺/2e^– ratios were 4.1 ± 0.9 and 3.4 ± 0.2, respectively. The proton translocation coupled to the oxidation of NAD⁺-linked substrates (malate, isocitrate) was dependent upon the presence of adenylates (ADP, AMP, or ATP). For malate (+ glutamate) oxidation the observed H⁺/2 e^– ratios were increased from 3.6 ± 2.2 to 6.5 ± 0.5 in the presence of 20 micromolar ADP.