Oxidative phosphorylation

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Abstract
The rate of incorporation of KH2P32O4 into adenosinetriphosphate (ATP) in respiring tissue suspensions was measured under conditions where a steady state existed with respect to the concns. of inorganic phosphate and ATP. From these measurements the rate at which the pyrophosphate bonds of ATP are formed was calculated. The theoretical and practical aspects of the procedure are discussed. From the rate of formation of pyrophosphate bonds and of the simultaneous O2 uptake the ratio equivalents of organic phosphate formed atoms of oxygen consumed, the "phosphorylation quotient", was calculated. Its value was found to be near 2 when the main oxidative process was the oxidation by oxygen of succinate to fumarate and malate and near 3 when the main oxidative process was the oxidation of alpha-ketoglutarate to fumarate and malate. The value of the ratio for the oxidation of ketoglutarate to succinate must, therefore, be 4. These results confirm the highest values obtained by previous authors with entirely different procedures. The thermodynamic efficiency of the oxidative phosphorylation was calculated to be in the order of 80% when succinate or ketoglutarate was the main substrate of oxidation. Efficiencies of this order are possible only if the free energy of the reactions between oxygen and the cytochromes is utilized. In liver suspensions 2 phosphate groups of ATP reacted with almost identical speeds. In pigeon-breast muscle suspensions the reactivity of the 2 phosphate group was under some conditions much slower than that of the terminal group. The slow reactivity of the 2 phosphate group in muscle tissue appears to be due to the presence of a specific inhibitor of one of the enzymes concerned.