Abstract
Mitochondria were isolated from rat liver and exposed to gamma radiation. Cytochrome oxidase activity was depressed by doses of radiation ranging from 2,500 r up. Phosphorylation resulting from the oxidation of succinate was depressed by doses ranging from 2500 r. The phosphorylation mechanism is much more sensitive to gamma radiation than cytochrome oxidase. It is suggested that the utilization of substrates by enzymes within the particulate may protect these enzymes against ionizing radiation.