Differential roles of metallothionein-like proteins in cadmium uptake and elimination by the scallop Chlamys nobilis.

Metallothionein-like proteins (MTLPs) are important inducible ligands for Cd detoxification, but whether MTLPs are involved in Cd bioaccumulation remains unclear. The present study investigated the potential interrelationships between MTLP turnover and Cd biokinetics in the scallop Chlamys nobilis with or without dissolved Cd pre-exposure. After Cd pre-exposure, increased synthesis rate and standing stock concentration of MTLPs as well as increased dissolved Cd uptake rate, were found in C. nobilis. However, the level of newly sequestrated Cd by MTLPs in the pre-exposed scallops was similar to that of the controls, indicating that MTLPs had no effect on Cd uptake from the dissolved phase. Instead, Cd uptake rate was positively related to the Cd binding rate of cellular debris and organelles. In addition, no significant difference was observed in the Cd dietary assimilation efficiency of the scallops, showing that MTLPs played a minimal role in Cd uptake from the dietary phase. Conversely, the Cd efflux rate constant decreased by 68 to 75% in the pre-exposed scallops in comparison with that in the control. Correspondingly, progressive elevations of MTLP-bound Cd and standing stock MTLP concentration were observed in the pre-exposed individuals during the Cd elimination period. In the pre-exposed scallops, the faster de novo MTLP synthesis could remobilize and recycle more Cd from other intracellular pools, effectively reducing the Cd elimination rate. The present study thus showed the differential roles of MTLPs in Cd uptake and elimination in the bivalves.