113Cd nmr study of the metal cluster structure of human liver metallothionein

Cadmium-113 nuclear magnetic resonance (¹¹³Cd nmr) was used to elucidate the structural properties of the cadmium binding sites in human liver metallothionein. The isotopically labeled ¹¹³Cd-metallothionein was prepared by the in vitro exchange of the native metals (greater than 94% zinc) for ¹¹³CdCl₂ during isolation. The two isoproteins, MT-1 and MT-2, showed ¹¹³Cd nmr resonances in the chemical shift range 610–670 ppm. The multiplet structure of the resonances is due to two bond scalar interactions between adjacent ¹¹³Cd ions linked by cysteine thiolate ligands. Homonuclear ¹¹³Cd decoupling experiments allowed the determination of the metal cluster structure, which, similar to the rabbit liver metallothionein, consists of a four- and a three-metal cluster designated cluster A and cluster B, respectively. Chemical shift similarities in the ¹¹³Cd nmr spectra of the human, rabbit and calf liver MT-1 and MT-2 are observed, especially for cluster A. Small variations in chemical shifts are explained in terms of differences in the primary structure between the two human isoproteins.