Metal-induced formation of metallothionein in rat liver

Abstract

Metallothionein, which accumulates in livers of rats which have received subcutaneous injections of cadmium, has been purified to homogeneity by the use of Sephadex chromatography, acetone fractionation and chromatography on diethyl aminoethyl (DEAE)-cellulose. Anion exchange chromatography of Cd-thionein yielded two components differing in amino acid composition. Both forms of purified Cd-thionein displayed such characteristic features as high cysteine content and lack of aromatic amino acids. A molecular weight of 10,200 and a typical metal content of five atoms of cadmium and two atoms of zinc per protein molecule were common features of the two forms. The low molecular weight proteins which are induced in rats exposed to zinc, mercury or silver have been purified by the same procedure as was used for Cd-thionein. In each case the thionein was resolved into what appeared to be the same two fractions on DEAE-cellulose. The two forms of each metalloprotein exhibited mobilities identical to those of the corresponding Cd-thionein on polyacrylamide-gel electrophoresis. The amino acid compositions of the more anionic forms of Hg-thionein and Zn-thionein were quite similar to that of the corresponding Cd-thionein. Thus, the identity of the proteins induced in rats by zinc, mercury and silver with the previously known metallothionein induced by cadmium has been established.