Glutamate dehydrogenase from beef liver was irradiated with X-rays in solution in the presence of air. The ability of the enzyme to catalyze the oxidative deamination of glutamate and the reductive amination of pyruvate was inactivated with the same yield (G = 0.3 per protomer). The radiation inactivation of the modifying effects of ADP, GTP, sodium chloride, sulfhydryl blocking and substrate inhibition was measured. When assayed with glutamate as substrate, the different allosteric functions had the same or higher radiosensitivity than the catalytic activity. The most radiosensitive function was the substrate inhibition which was destroyed at a yield 3-4 times that of the catalytic activity. When measured with pyruvate as substrate only the GTP inhibition was more sensitive than the catalytic activity. The ADP and the sodium chloride inhibition was less radiosensitive. Thus, the allosteric effect of each particular effector showed widely different radiosensitivity when the enzyme was assayed with the two different substrates. The allosteric effects of ADP and GTP were affected in a similar way by X-rays and sodium chloride, both with glutamate and pyruvate as substrate, while sulfhydryl blocking had a different effect. The results indicate that destruction of sulfhydryl groups by X-rays is of minor importance for the radiation induced inactivation of the catalytic and allosteric activity of the enzyme.

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