Cadmium-thionein in Tetrahymena thermophila and Tetrahymena pyriformis

The treatment of Tetrahymena thermophila with cadmium causes a reduction in growth rate according to dose; almost all the metal is accumulated in the cytosol where the Zn content is also increased threefold. Bio-Gel and Water 160 (HPLC) column chromatography show that Cd and Zn are bound to a protein with an ultraviolet (UV) spectrum that appears to be similar to that of Cd-metallothioneins isolated by higher organisms, but its molecular weight is greater: about 28 000 D, comparable to that of metallothionein isolated from Tetrahymena pyriformis. Further purification of these proteins by ion exchange chromatography revealed the presence of two peaks, considered as two isoforms of the metallothioneins present in both T. thermophila and T. pyriformis (MT 1 and MT 2). Their amino acid analyses confirmed that they are different isometallothioneins, MT 1 and MT 2, with about 30% cysteine, and aspartic acid, glycine and lysine as major amino acids. From our analyses we may conclude that Tetrahymena pyriformis MTs are similar to those present in invertebrates and vertebrates, while Tetrahymena thermophila MTs are peculiar in that they have cyclic amino acid histidine in both MT 1 and MT 2; furthermore, aromatic amino acid phenylalanine is also present in MT 2.