Acceptor substrate-potentiated inactivation of bovine liver rhodanese

The interaction of bovine liver rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) with the acceptor substrates, dithiothreitol or cyanide, was studied. When incubated in the presence of cyanide or dithiothreitol, rhodanese was inactivated in a time-dependent process. This inactivation was detectable only at low enzyme concentrations; the rate and degree of inactivation could be modulated by varying the substrate concentration or the system pH. Activity measurements and fluorescence spectroscopy techniques were used in examining the inactivation phenomenon. Sulfur transfer to dithiothreitol was measured by direct assay and was shown to involve the dequenching of enzymic intrinsic fluorescence that had been previously observed only with cyanide as the acceptor substrate. Substrate-potentiated inactivation of rhodanese (with cyanide) has been reported before, but the cause and nature of this interaction were unexplained. The results presented here are consistent with an explanation invoking oxidation of rhodanese in the course of inactivation.