Lactate dehydrogenase was irradiated with60 Co and the chemical and physical properties determined. Degradation of some amino acids occurred, principally alanine, cysteine, methionine, serine, tryptophan, tyrosine, and valine. The sedimentation coefficient increased and the diffusion constant decreased after exposure to ionizing radiation. Both the sedimentation and molecular weight data indicated aggregation of the irradiated lactate dehydrogenase. The Michaelis constant increased for pyruvate but remained unchanged for NADH. Readily titratable sulfhydryl groups increased after irradiation, suggesting unfolding of the tertiary structure. Two moles of PCMB could react with the irradiated lactate dehydrogenase without additional loss of biological activity. It is suggested that radiation damage to lactate dehydrogenase is initiated by the breaking of bonds responsible for the tertiary structure.

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