Oxidation of active site residues (His and Trp), following catalytic inactivation of human carbonic anhydrase I by a copper-ATCUN conjugate of sulfanilimide, is evidenced by mass spectrometric analysis of tryptic and chymotryptic digest of the modified CA-I. Accordingly, residue oxidation rather than protein cleavage is the demonstrated mods of inactivation. An apparent second-order rate constant, k(2) similar to 7600 M(-1)min(-1), has been determined for catalytic inactivation of CA-I.