Chemical modification of rhodanese with sulphite

The essential sulphydryl group of bovine liver rhodanese (thiosulphate: cyanide sulphurtrasferase, E.C. 2.8.1.1.) is modified by sulphite produced during the enzymatic reaction or added to the fully active enzyme. The enzyme treated with labelled reagent incorporates 1 equivalent of SO3(2-) and loses one -SH group with the formation of a S-sulphonate group at the active site. Mercaptoethanol is effective in both restoring enzyme activity and removing bound sulphite from protein. The inactivation process is dependent on the presence of oxygen and is antagonized by chelation of metal ions, that catalyze sulphite autoxidation, or by scavenging free radicals with mannitol or benzoate. Since the presence of superoxide dismutase and/or catalase protects the enzyme only to a small extent, the inactivation process should be attributed to sulphite radicals rather than intermediates of oxygen reduction.