Determination of the association constant of platinum(II) to metallothionein

The association constant for platinum(II) binding to rabbit liver metallothionein (MT) has been determined for the first time by two competition schemes. In the competition of hydrogen ions for the metal binding sites, the specific absorbancy was measured as a function of pH. Assuming binding of each metal atom to 2.86 SH groups with a pK(a) of 8.9, the association constants for platinum, cadimum, and zinc were calculated from the hydrogen concentration required for 50% loss of absorbancy. In another competition of platinum for cadmium binding sites, the displacement of cadmium present in Cd7MT2 by platinum was monitored by determining the concentration of free cadmium via cadmium selective electrode analysis. The association constant of platinum to MT, K-PtMT = 2.3 X 10(23), was obtained and is in very good agreement with the result derived from the hydrogen-platinum competition. The affinity of platinum to MT was demonstrated to be approximately 10(7) and 30 times more firm than zinc and cadmium, respectively. On this basis, the role of MT is discussed in relation to its regulation for cis-DDP metabolism in vivo. (C) 1997 Elsevier Science Inc.