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Properties of an Oligomycin-sensitive Adenosine Diphosphate-Adenosine Triphosphate Exchange Reaction in Intact Beef Heart Mitochondria

From the ¹ From the Department of Physiological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

Intact beef heart mitochondria having high acceptor control ratios catalyze an oligomycin-sensitive adenosine diphosphate-ATP exchange reaction in the presence of 6.0 mm Mg⁺⁺ at a rate of about 400 to 600 mµmoles per min per mg of mitochondrial protein, or somewhat higher than the rate of oxidative phosphorylation in beef heart mitochondria. The oligomycin-sensitive component of the total ADP-ATP exchange activity is saturated with Mg⁺⁺ at much lower concentrations (1.0 mm) than the oligomycin-insensitive component (8.0 mm). Gramicidin, valinomycin, aurovertin, azide, arsenate, and atractyloside inhibit the ADP-ATP exchange, to an extent not exceeding the inhibition by oligomycin, whereas the adenylate kinase activity is insensitive to these agents.

Beef heart mitochondria also catalyze uridine diphosphate-UTP, cytidine diphosphate-CTP, guanosine diphosphate-GTP, and ADP-dCTP exchanges, but these are insensitive to oligomycin and occur at less than 10% of the rate of the ADP-ATP exchange. Freezing and thawing of beef heart mitochondria, which has been reported not to cause loss of oxidative phosphorylation as measured by the P:O ratio, caused loss of the oligomycin-sensitive ADP-ATP exchange, loss of acceptor control of respiration, and gain of ATPase activity. It is suggested that the loss of oligomycin-sensitive ADP-ATP exchange activity during preparation of phosphorylating submitochondrial particles is caused by a molecular transformation of the ATP-synthesizing system so that the affinity for ADP is decreased, with a resulting gain of ATPase activity.

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