The oxidation of rabbit liver metallothionein-II by 5,5'-dithiobis(2-nitrobenzoic acid) and glutathione disulfide.

Because metallothionein (MT) may undergo thiol-disulfide or other redox reactions under certain cellular conditions, the partially and completely oxidized products of the reactions of Cd7-MT-II with the electrophile 5,5'-dithiobis(2-nitrobenzoic acid), ESSE, and oxidized glutathione, GSSG, were characterized. Reaction with the stoichiometric quantity of ESSE (1 ESSE per MT thiolate) generates monomeric and polymeric MTs with three types of disulfide bonds: intra- and intermolecular CyS-SCy linkages and a small number (2-3/MT) of mixed disulfides, CyS-SE, involving thionitrobenzoate (ES-). Reaction with substoichiometric quantities of ESSE (0.02 or 0.1 per MT thiolate) causes the formation of intra- and intermolecular CyS-SCy disulfides, but no mixed disulfides. In the latter reactions, two equivalents of ES- are released per mole of ESSE, but the release is described by a single first-order rate constant (k = 3.0 +/- 0.5 sec-1). Substantial amounts of cadmium remained bound to the MT monomers and polymers after reaction with the substoichiometric quantities. Despite the Cd bound to the MT after reaction with 0.1 ESSE per MT thiolate, no 111Cd NMR signals were detected, indicating rapid equilibration of the remaining metal ions among the disrupted binding sites. Large excesses of the endogenous aliphatic disulfide, GSSG, displace Zn+2 from Zn7-MT slowly. The reaction is complete after 24 hours with 5000 microM GSSG, but only 25% complete after 72 hours with 250 microM GSSG. Approximately one Cd+2 is displaced rapidly from Cd7MT by 5000 microM GSSG and half as much by 250 microM GSSG, but no further reaction occurs. It is unlikely that GSSG oxidation of MTs would be physiologically significant.