CHARACTERIZATION OF CADMIUM BINDING-PROTEIN FROM SCENEDESMUS-QUADRICAUDA AND CD TOXICITY REVERSAL BY PHYTOCHELATIN CONSTITUTING AMINO-ACIDS AND CITRATE

Cadmium binding protein was isolated from a fresh water green alga, Scenedesmus quadricauda, exposed to cadmium chloride. Cadmium binding protein was purified by DEAE-cellulose chromatography followed by gel filtration and an apparent molecular weight of 3800 was determined by gel filtration on Sephadex G-50. This protein contained 9+/-1 % of its sulfur as sulphide ions and A254 was lost upon acidification to pH 1.5. [S-35]Cystine labelling showed that this protein contained cysteine. Glutathione content in cells decreased during cadmium exposure. Buthionine sulfoximine, an inhibitor of glutathione synthesis, was found to be not toxic to cells not exposed to Cd, but when exposed, significant growth reduction was observed when compared with cultures treated with Cd alone. Glutathione addition to the medium restored the growth of BSO treated cells. These findings confirmed that Cd-BP is nothing but a phytochelatin. Addition of phytochelatin constituting amino acids - cysteine, glutamate, glycine and citrate - to the medium restored the growth of Cd treated cells. Among calcium and magnesium salts, calcium chloride and sulphate salts of both of these ions also restored growth. It is assumed that sulphate and amino acid supplementation to the medium increased Cd-BP synthesis in cells; hence, growth recovery was observed.