Primary and secondary radicals in the radiation-induced inactivation of yeast alcoholdehydrogenase

SummaryRadiation inactivation of YADH in dilute aqueous solution that does not contain oxygen is due to H atoms and OH radicals, whose inactivating efficiencies are in the ratio 9 : 1. The same species are responsible for the transient absorption arising in pulse-irradiated YADH in the region of 300–320 nm, and in this case their relative efficiencies are 7·5 to 1. This resulted from steady-state and pulse radiolysis experiments using radical scavengers such as t-butanol, formate and N2O. It appears that oxygen, when present, may also contribute to the enzyme inactivation through its radiation-induced radicals. Secondary radicals originated by reaction of OH radicals with CNS−, Br− and I−, all inactivate the enzyme at a faster rate than their primary parent radical. Their relative inactivating efficiencies are of the same order as their rate constants for reaction with YADH.