Cadmium detoxification in protists

Studies on cadmium (Cd) detoxification mechanisms mostly concern multicellular organisms, while information on eukaryotic unicellular organisms such as protists is very scanty. This study focuses on these organisms, and deals with the response of some species of ciliates to a non-essential metal like Cd. The effect of accumulation and tolerance are reported for Tetrahymena pyriformis T. pigmentosa and T. thermophila, and for three species of Hypotrichida. Cd is bound to both particulate and soluble fractions. These two compartments, which play an important role in intracellular metal homeostasis, are different in the species considered. In Hypotrichida, the particulate compartment binds Cd very promptly, while it is still present often three days of treatment in the soluble fraction of Tetrahymena. Two Cd-Zn binding isothioneins were isolated from the soluble fraction of T. pyriformis and T. pigmentosa. The primary structure revealed that the equivalent proteins from the two species have identical sequences and that the two isoforms differ only in the presence or absence of a lysine residue at the N-terminus. These metallothioneins are unusually long and have a unique internal homology which suggests that the proteins originate by gene duplication. The chains contain 331 cysteine residues, 16 of which are arranged in unique repeating motifs. The similarities of protist metallothioneins with other eukaryotic metallothioneins are discussed.