The Inactivation of Alcohol Dehydrogenase by X-Rays and the Dose-Modifying Effect of Some Added Substances

Abstract

The inactivation yield of yeast alcohol dehydrogenase in 0.05 M phosphate buffer, pH 7, was found to be 0.14 molecule/100 eV. The apparent yield was constant in the range 0.1 to 1 mg/ml, showing that adventitous impurities had no appreciable effect. The yield was little affected by changing the pH to 6 or 8, or by omitting the buffer. Possible reasons for discrepancies in earlier measurements are discussed. Many reactions with radiation-produced intermediates are necessary to inactivate 1 enzyme molecule. In 50-fold molar excess, cysteine, cysteamine, and cystine exerted a limited protective action (dose-reduction factor 2 or less); NaI and 3-iodopropionicacid had a substantial sensitizing effect, cystamine a slight effect, and methanesulfonate ion and ethyl methanesulfonate negligible effects.