Reduced thioredoxin as a sulfur-acceptor substrate for rhodanese

Mammalian rhodanese (thiosulfate: cyanide sulfurtransferase, EC 2.8.1.1) catalyzes the transfer of sulfane sulfur from donors such as S2O2-(3) and organic thiosulfonate anions (RS(O)2S-) to nucleophilic acceptors such as CN- and dithiols. The work reported here used an NADPH-coupled assay with thioredoxin reductase to show that reduced thioredoxin at micromolar concentrations is also an effective sulfur-acceptor substrate for rhodanese under conditions where millimolar concentrations of lipoate or dithiothreitol would be required. At near K(m) concentrations of the other substrate, apparent K(m) values for thioredoxin and methane thiosulfonate were 18.5 +/- 1.8 microM and 20 +/- 4 mM, respectively. The physiological compound alanine thiosulfonate also could serve as a donor substrate. In these systems, after a brief lag, inorganic sulfide accumulated as a final product. A formal mechanism in accord with all the results is proposed.