Metallothionein, a class of low molecular weight, metal- and cysteine-rich Proteins, are postulated to play a central role in metal-related biological processes such as the detoxification of Cd2+and Pb2+. The thermal Properties of the interactions of rabbit liver Zn7MT2 and apo-MT2 with Cd2+ and Pb2+ were studied using isothermal titration calorimeter. The same product Cd7MT formed in the reactions of Cd2+ with Zn7MT2 and apo-MT2, while the reactions of Ph2+ with Zn7MT2 and apo-MT2 Produced two MT species Ph7MT and Ph7MT’. The thermodynamic parameters （△H、 △G、 △S, etc. ） for the reactions of MT with 1 mol metal ions were measured at 293. 15 K, pH 4.70. All of these reachons are spontaneous, exothermic processes. The values of △G and K show that Cd2+ is more affinitive to MT than Ph2+. The equilibrium constants K of the replacement reactions of Zn7MT2 by 1 mol Cd2+ and Pb2+ are 2. 7×105 and 1. 3×105 respectively, that means the concentrations of free Cd2+ and Pb2+ are depressed to about 1. 0×10-5 after the reactions with Zn7MT2. These data can be used to evaluate quantitatively the ability of MT in Cd2+ and Ph2+ detoxificatio.
