The repair, protection and sensitization of papain with respect to inactivation by H2O2 and OH: effects of dithiothreitol, penicillamine, cystine and penicillamine disulphide.

While dithiothreitol repairs the peroxide-produced sulphenic acid derivative of papain in a fast reaction involving only one dithiothreitol molecule, penicillamine reacts with it to form papainCys25SSPen. Disulphide is also formed in the absence of peroxide (see article) by reactions of papainCys25S- AND PenS- radicals derived from -OH reactions in penicillamine-papain mixtures. A similar formation of papainCys25SSCys occurs in mixtures of cysteine and papain. However, unlike papainCys25SSCys, papainCys25SSPen cannot easily be restored to the active form of papain by the exchange reaction with CysSH, and this may have significance for an understanding of the sensitizing action of penicillamine observed in some in vivo systems. Under the action of OH radicals dithiothreitol has less of a tendency to form mixed disulphides and is more effective in repairing papain-OH intermediates than either cysteine or penicillamine. Due to secondary reactions of RSOH and other oxidized species the disulphides of cysteine and penicillamine are less effective than the sulphydryls in protecting papain against inactivation by -OH.