X-Ray Inactivation of Papain in Solution

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Abstract
Crystalline papain is the most radiosensitive enzyme so far studied. Dose inactivation is nonexponential, and for low dose rates it approachs a linear function. The enzyme inactivation could be accounted for almost entirely by a radiochemical oxidation of its single essential SH group. The enzyme could be partly reactivated by cysteine addition after the exposure. In part, inter molecular protein disulfides were formed which were rather resistant to further radiochemical oxidation. The observed ionic yield decreased strongly with increasing dose rate, and the inactivation showed a pronounced aftereffect. These effects were due to the action of radiation-induced hydrogen peroxide. In the presence of catalase the yield of inactivation was the same in air as in nitrogen (G = 1.2). Blocking of the active SH group with a cysteamine residue reduced the enzyme inactivation by X-rays by factors of 8.2 and 5.5, in the absence and in the presence of catalase, respectively.