The Radiation-induced Inactivation of Lysozyme

Abstract

Lysozyme was irradiated in dilute aqueous solutions with ⁶⁰Co γ-rays. Variation of pH, the presence or absence of oxygen and nitrous oxide and other additives has made it possible to investigate the separate roles of hydroxyl radicals, solvated electrons, and hydrogen atoms as inactivating species. Hydrogen atoms inactivate the enzyme with an effectiveness that increases at low pH values; electrons are not particularly effective. The hydroxyl radical is the species responsible for about 90 per cent of the inactivation at intermediate pH. Inactivation measurements on solutions irradiated at different pH values show that the sensitivity to inactivation by hydroxyl radicals and hydrogen atoms depends on the configuration of the protein. The mechanism of the reaction with the hydroxyl radical in the presence and absence of oxygen has been studied by u.v.-absorption changes and probably involves oxindolylalamine or kynurenine structures. Changes in aggregation and molecular conformation have been investigated by gel filtration and optical rotatory dispersion. The evidence suggests that reaction of the hydroxyl radical with tryptophan residues is largely responsible for the radiation-induced inactivation.