Kinetics of Enzyme Inactivation by Ionizing Radiation

Abstract

A general kinetic analysis of enzyme inactivation in solution by ionizing radiation has been carried out. In the treatment the possibility is included that active and inactive enzyme molecules may deactivate radiation- induced radicals at different rates. The analysis has proved capable of explaining several experimental observations which were difficult to reconcile with previous treatments. The treatment reveals that, if the active enzyme molecules remove the agents formed from water more rapidly than do the radiation- inactivated ones, deviations from exponential dose-inactivation curves will occur. This may explain such deviations found in the literature. Significant deviations will ordinarily be expected only when the yield of inactivation is high. The almost linear dose-inactivation relationship previously found for papain can be adequately explained on this basis. Special circumstances are responsible for the finding that alcohol dehydrogenase from yeast and rabbit muscle glyceraldehyde-3-phosphate dehydrogenase exhibit nonexponertial dose-inactivation curves, in spite of the fact that their over-all yields of inactivation are low. A procedure is given for determining experimentally the true yield of inactivation when contaminating and protective substances are present. It follows from the analysis that the presence of inactive enzyme molecules in the preparation prior to the exposure will affect the yieldmore » of inactivation only when the active and inactive enzyme molecules differ in their ability to deactivate radicals. The very high observed yield of inactivation previously found in the case of papain can in part be ascribed to the presence in the preparation of inactive enzyme molecules. For enzymes which are inactivated at low yields, the true yield of inactivation will be nearly equal to the observed yield at high enzyme concentration. (auth)« less