Interprotein metal exchange reactions of metallothionein.

Different methods of preparing rabbit liver 113Cd,ZnMT in vitro were investigated in an attempt to reproduce the distinctive and reproducible manner in which the metals are distributed between the 2 clusters in vivo in protein produced in response to Cd administration. 113Cd NMR was used to demonstrate that the native metal distribution is not produced by Cd displacement of Zn from ZnMT, but rather by a direct metal interchange reaction between CdMT and ZnMT. Kinetic studies of this heretofore unknown reaction indicate that even at low physiological concentrations, CdMT and ZnMT cannot coexist without rapidly exchanging metals to produce mixed-metal protein species containing Cd and Zn located preferentially in the 4- and 3-metal clusters, respectively. The self-exchange reaction of Cd among the 7 binding sites of CdMT was also investigated and it was found that exchange of Cd among sites in the 3-metal cluster was very rapid (time-scale of seconds) while among sites of the 4-metal cluster or between clusters it was several orders of magnitude slower.