Inducible Cadmium Binding Complexes of Cabbage and Tobacco

Cadmium complexes with apparent molecular weights of 10,000 were observed in aqueous extracts of Cd-treated cabbage (Brassica capitata L., cv. red danish) and tobacco (hybrid of Nicotiana glauca and N. langsdorffii) plants. The amount of complex (as Cd) recovered was found to be dependent on the concentration of the metal in the growth medium and the total time of exposure of plants to the metal. Induction of the complex at moderate levels of (112)Cd exposure was monitored after labeling the complex with (109)Cd in vitro. The constitutive nature of the ligand of the complex in cabbage and tobacco leaves was suggested when control plant extracts were exposed to (109)Cd. Such extracts contained (109)Cd, which eluted from Sephadex G-50 in the region of Cd complex. Simultaneous labeling with (112)Cd and (35)S or (32)P indicated that the complex contained sulfur but probably not phosphorus. The amount of (35)S which eluted coincident with (112)Cd complex increased during complex induction. No evidence was found for the presence of 10,000 molecular weight Cd complex in stem exudates (vascular sap) of Cd-treated plants.The results obtained are consistent with the presence in these tissues of a ligand which is both inducible and constitutive and binds Cd in mercaptide bonds. All of these properties, and others reported earlier, are characteristic of Cd-metallothionein formed in animals.