PRELIMINARY EVIDENCE FOR A METALLOTHIONEIN-LIKE HEAVY METAL-BINDING PROTEIN IN THE TROPICAL MARINE BIVALVE TRIDACNA-CROCEA

The clam Tridacna crocea was exposed to 0.20 mg Cd/l for 20 days, The Cd levels in the different organs were as follows: kidney > mantle > gills. Gel filtration analyses of cytosols of mantle and gills from Cd-exposed clams showed that Cd was bound to proteins of similar molecular weight (MW) to rabbit metallothionein (MT). Renal cytosolic metals from untreated and Cd-treated clams were mainly associated with a compound of apparent MW < 10 kDa, SDS-PAGE electrophoresis revealed a MW similar to rabbit MT. The renal metal-binding protein was characterized using procedures which included heat treatment, sulfhydryl compound composition, reversed-phase high performance liquid chromatography and amino acid analysis, The amino acid composition of the metal-binding protein showed high levels of cysteine and glycine and absence of aromatic amino acids, consistent with the molecular properties of a family of metallothionein-like proteins.