Effect of Sulfur Binding on Rhodanese Fluorescence

Abstract

Intrinsic fluorescences of the sulfur‐transferring protein rhodanese (or thiosulfate: cyanide sulfurtransferase) and of artificially persulfurated bovine serum albumin were studied. Both are attributable mainly to tryptophan side‐chains. Addition of cyanide or sulfite to these proteins causes a 20–25% increase in fluorescence. Both proteins exhibit a low intensity absorption band at about 330 nm which disappears on addition of cyanide or sulfite. This absorption can be restored in rhodanese, after the treatment with cyanide, by excess thiosulfate; in the same time the fluorescence decreases to that of sulfur‐containing enzyme. A change in solvent viscosity, obtained by addition of glycerol, reduces the quenching effect in both proteins. On the basis of these data a long‐range energy‐transfer mechanism between the donor (tryptophan) and an acceptor is proposed. The acceptor involved seems to be one or more persulfide groups (R‐SSH) in both proteins. The presence of a persulfide group into rhodanese fits very well with the current knowledge of the mechanism of this enzyme.