Oxidative burst and metallothionein as a scavenger in macrophages

The role of metallothionein (MT) in the scavenging of superoxide radicals (.O-2(-)) generated by macrophages has been examined. The present work has focused on the effects of added cadmium, a known inducer of MT biosynthesis, on determined amounts of superoxide radicals produced by in vitro cultured rat peritoneal macrophages on their stimulation with phorbol-12-myristate-13-acetate (PMA). The levels of superoxide radicals (.O-2(-)) have been found to decrease when cadmium was added to cells exposed to PMA, However, substantially lower levels of MT have been determined in this case compared to cells untreated with PMA. This effect could be reversed by incubation of the PMA and cadmium-treated cells with a reducing agent, 2-mercaptoethanol (2-ME). Results suggest that .O-2(-) caused thiolate oxidation and subsequent metal loss, thus reducing the cellular MT content as quantified by the silver saturation method. This conclusion is supported by cell-free experiments in which the oxidation of rabbit MT-I by a xanthine/xanthine-oxidase system could be reversed by its subsequent reduction with 2-ME. The data presented provide direct evidence of the involvement of MT in scavenging superoxide radicals in living cells.