Structure of the metal clusters in rabbit liver metallothionein

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Abstract
Cd-113 NMR was used to determine the structures of the multiple Cd binding sites in the 2 major isoproteins of rabbit liver metallothionein. The isotopically 113Cd-labeled metallothionein used in these studies was isolated from the livers of rabbits that had been subjected to repeated injections of 113CdCl2. The native protein isolated from these livers contains an appreciable amount of Zn in addition to Cd, ranging from 2-3 mol/mol of protein out of a total metal content of 7 mol/mol of protein. The 113Cd NMR spectrum of Cd, Zn-containing metallothionein is complex, reflecting the fact that the native protein is a heterogeneous mixture of species containing different relative amounts of Zn and Cd. Replacement of the native Zn with 113Cd in vitro gave a protein whose 113Cd NMR spectrum was much simpler, containing 8 distinct multiplets with chemical shifts ranging from 611-670 ppm. The origin of the multiplet structures was 113Cd-113Cd scalar coupling arising from 2-bond interactions between 113Cd ions linked to one another by bridging cysteine thiolate ligands. The size and structures of the metal clusters in the protein were determined by the application of selective homonuclear 113Cd decoupling techniques. Analysis of these data showed that rabbit liver metallothionein contains 2 separate metal clusters, 1 containing 4 Cd2+ ions and the other containing 3. These 2 clusters, whose structures are the same in both isoproteins, were designated cluster A and cluster B, respectively. Structures for the clusters are proposed that account for the 113Cd spin coupling data and the participation of all 20 of the cysteine residues in metal ligation, 11 in cluster A and 9 in cluster B. The appearance in the spectrum of eight multiplets rather than the 7 that would be expected on the basis of the number of metal binding sites in the protein is an indication of some residual heterogeneity in the 113Cd-labeled metallothionein sample. The origin of this heterogeneity is suggested to be the presence of a protein species that lacks metal ions at its cluster B binding sites.