Cauliflower mitochondria were irradiated with 50 kV of unfiltered X rays. The capacity of the organelles to utilize oxygen and to phosphorylate ADP were then determined by standard assay procedures. The ability to phosphorylate decayed exponentially with dose, with a target volume of <tex-math>$1.02\times 10^{6}$</tex-math> daltons. Oxidation was considerably less sensitive to irradiation. The sensitive volume for phosphorylation is consistent with the target being the entire mitochrondrial ATP synthetase complex. The enzymatic centers for oxidation and phosphorylation appear to act independently.

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