Involvement of metallothionein in Zn accumulation and elimination strategies in Ruditapes decussatus.

The aim of this study was to identify the involvement of metallothionein (MT) in Zn accumulation and elimination strategies in different tissues (gills, digestive gland, and remaining tissues) of Ruditapes decussatus. Clams were exposed to two Zn concentrations (100 and 1000 microg/L) for 40 days, followed by a depuration period of 50 days. The accumulation and elimination processes were complemented by subcellular Zn distribution, to determine the affinity of this element for the different cellular partitions. Subsequently, the involvement of MT was followed. Zinc concentrations in the tissues of R. decussatus exposed to 100 microg/L was partially regulated, whereas in those exposed to 1000 microg/L, Zn was accumulated (exponentially and linearly) throughout the exposure period. The greatest amount of Zn is associated to the insoluble fraction; however, in the highest Zn exposure, approximately 30% is in the thermostable fraction, where MT occurs. Gel filtration chromatography confirmed that the Zn ions accumulated in this fraction were bound to MT. This protein is actively involved in the elimination of this metal, through the rapid degradation of the Zn-MT complex because MT and Zn are turning over simultaneously. Therefore, MT participates in Zn homeostasis in R. decussatus when Zn exposure is low. When Zn accumulation increases, the role of this protein changes from a constitutive function to the detoxification of the excess of Zn ions.