Cadmium-binding proteins from a tunicate, Pyura stolonifera

The determination of cadmium levels in tissues of the tunicate Pyura stolonifera collected from uncontaminated sites revealed highest concentrations in the liver. After keeping P. stolonifera under laboratory conditions in Cd-containing water for 15 days, cadmium accumulated most markedly in liver tissue. Liver tissue of Cd-exposed specimens was used for the isolation of Cd-binding proteins. Five Cd-binding proteins, which differ in their chromatographic properties, could be purified. At least four of these Cd-binding proteins are heat stable and cysteine-rich. The N-terminal sequence (Met-Asp-Pro-Cys-Asn-Cys-Ala-Glu...) of at least two of these peptides resembles fish (plaice) metallothionein. Unlike vertebrate metallothioneins, P. stolonifera Cd-binding proteins are not N-terminally blocked by acetylation of methionine.