Sulfide stabilization of the cadmium-gamma-glutamyl peptide complex of Schizosaccharomyces pombe.

Addition of cadmium salts to the growth medium of Schizosaccharomyces pombe leads to synthesis of a Cd.gamma-Glu peptide complex and an enhanced generation of sulfide ions. The gamma-Glu peptide complex functions in the detoxification of heavy metal ions. Native Cd.gamma-Glu peptide complexes contain acid-labile sulfide in the metal-thiolate cluster. Two forms of the complex exist differing primarily in their sulfide content. Sulfide concentrations up to 0.2 and 1.2 mol/mol of peptide were observed in native isolates of forms I and II, respectively. Addition of sulfide to the low sulfide form I converted it to a complex similar to form II. Properties of the Cd.gamma-Glu peptide complex were altered by the incorporation of sulfide ions. Sulfide-dependent electronic transitions in the ultraviolet were evident, and the absorbance maximum of the transition was related to the sulfide content and the bound metal ion. High sulfide forms of the Cd and Zn complexes exhibited absorbance peaks at 318 nm and 255 nm, respectively. Incorporation of sulfide into the Cd.gamma-Glu peptide complex imparted greater thermodynamic stability to the complex, an increased Stokes radius, and an enhanced Cd(II) binding capacity. Sulfide generation may be a cellular response in part to enhance the effectiveness of the gamma-Glu peptide system for Cd(II) detoxification.