Cyanylation of rhodanese by 2-nitro-5-thiocyanobenzoic acid.

Cyanylation of rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) with a nearly stoichiometric amount of 2-nitro-5-thiocyanobenzoic acid produces a modification of the essential sulfhydryl group. Different S-cyano derivatives are obtained with the enzyme intermediate bearing transferable sulfur bound as a persulfide group (sulfur-rhodanese: E-S-SH) and with the sulfur-free rhodanese (E-SH). The interaction of a neighboring sulfhydryl group splits thiocyanate from the sulfur-rhodanese derivative and cyanide from the sulfur-free rhodanese derivative. In both cases an intramolecular disulfide bond is formed. Iodoacetate is effective on the modified enzyme only after cyanide addition which splits the disulfide so that the essential sulfhydryl group can be alkylated.