Oxidative inactivation of various key enzymes and alpha-1-proteinase inhibitor (alpha-1-PI) was studied by treatment with N-chloramines and the metal-catalyzed oxidation (MCO)-systems ascorbate/Fe(III) and ascorbate/Cu(II). Chlorinated amines completely inhibited alpha-1-PI, fructose-1,6-bis phosphatase (Fru-P2ase) and glyceraldehyde phosphate dehydrogenase (GAPD) at a low molar excess, and glucose-6-phosphate dehydrogenase (G6PD) at a high molar excess, but did not impair beta-N-acetylglucosaminidase (beta-NAG), alkaline phosphatase (AP) or lactate dehydrogenase (LDH). MCO-systems affected the activities of Fru-P2ase, GAPD, AP, LDH and G6PD, but not those of beta-NAG or alpha-1-PI. EDTA prevented inactivation of Fru-P2ase, G6PD and LDH by ascorbate/Cu(II) and of Fru-P2ase by ascorbate/Fe(III) suggesting a site-specific oxidation catalyzed by a protein-bound metal ion. In conclusion, N-chloramines and MCO-systems exhibited different properties with regard to oxidative inactivation, sulfhydryl-enzymes were susceptible to both systems, but other enzymes were only susceptible to one or neither system.