Euglena gracilis cadmium-binding protein-II contains sulfide ion

Sulfide ions are a constituent of the cadmium-binding protein-II in the alga Euglena gracilis. Their presence was demonstrated by the methylene blue assay, by acid labilization induced reductions in the Cd-S charge transfer band at 254 nm and by reactions with the thiol reagent, 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB). Direct reduction of DTNB by sulfide and precipitation of CdS yield a complex stoichiometry for the DTNB reaction. The S2-/Cd2+ ratios determined, 1.25 +/- 0.10 (methylene blue) and 1.37 +/- 0.16 (DTNB), are in good agreement.