Inactivation of Urease by X-rays

Summary

Urease, when irradiated by x-rays in dilute aqueous solutions, is partially inactivated. A dose of 5000 r caused almost complete destruction of the enzyme, and 500 r caused 22 per cent inhibition. Enzyme inactivation was small when measured immediately after the irradiation, but increased gradually during a period of 2 hours after the irradiation. Protection against irradiation could be accomplished in many ways. The enzyme was self-protective, a larger concentration of enzyme reducing the inactivation with a given dose, and other proteins, such as serum albumin, were also effective. Several buffer salts protected, citrate and phosphate being most effective. Amino acids protected, but not to an equal extent, and of those tested, cysteine and methionine were found to be the most, and alanine and tryptophan the least, effective. Addition of versene and the blocking of–SH groups of the enzyme also protected. It is considered likely that the enzyme inactivation is a result of the oxidation of urease —SH groups by free radicals.