Carboxypeptidase A and the apoenzyme resulting from removal of its zinc atom were irradiated with X-rays in the presence and in the absence of the chemical protector cysteine. Zinc was then replaced in the apoenzyme, and the resulting loss of peptidase activity of holo- and apoenzyme was determined. Under the conditions of these experiments cysteine was found to give complete protection to the enzyme, but very little to the apoenzyme. Furthermore, after the apoenzyme had been irradiated, its loss of ability to regain peptidase activity on the replacement of zinc ions was found to correspond closely with its loss of ability to bind zinc ions. The results indicate that the zinc-binding site is the crucial one involved in radiation damage to the enzyme, and that the principal mode of action of cysteine as a radiation protector is its chelation to zinc at this sensitive site.

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