Metallothionein: a cadmium- and zinc-containing protein from equine renal cortex.

The initial purification of metallothionein from equine renal cortex yielded preparations that were electrophoretically heterogeneous and contained 2.9% cadmium, 0.6% zinc, and 4.1% sulfur (1). By diethylaminoethyl cellulose column chromatography, we have now obtained electrophoretically and ultracentrifugally homogeneous metallothionein containing 5.9 ‘% cadmium, 2.2% zinc, and 8.5% sulfur. Most of the sulfur in metallothionein is accounted for by its cysteine content. The molecular weight of the protein is 10,000 =t 260. The specific absorption of metallothionein at 250 rnp is shown to be a convenient monitor for its isolation. This absorption is characteristic of cadmium mercaptides, as indicated by the study of metallothionein and of cadmium complexes of monoand dimercaptans. These spectral properties have been utilized in the examination of cadmium and zinc binding to thionein, the metal-free protein. A preliminary report has been given (2).