Low molecular weight cadmium and selenium containing proteins unlike metallothionein in animals

Since the assumed metallothionein (MT) fractions from testis did not respond to cadmium and zinc exposure like MT in other tissues, they were purified using gel filtration and ion exchange resins to determine if they were indeed MT or other proteins. The major amino acids were found to be aspartate, glutamate and glycine with a low cysteine (less than 3%) content. The amino acid content is remarkably similar to a low molecular weight (MW) cadmium-binding protein isolated from oysters with properties unlike MT. Selenium has been shown to be present in a low MW protein called the G protein in various tissues including the testis. The major amino acids in a partially purified preparation were found to be aspartate, glutamate, glycine and lysine with a low methionine (about 2.5%) but a very low cysteine (less than 0.3%) content. The selenium is present in this protein as selenocysteine. The metabolic significance of this selenoprotein is not known, but it should be indicated that selenium will alter the binding of cadmium in testicular proteins.