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Metallothionein (MT) is a sulfhydryl-rich protein involved mainly in heavy metal homeostasis and detoxification. In this study, the use of the mussel as an experimental model allowed us to test MT antioxidant properties at the molecular, cellular, and organism level. MT induction was achieved by mussel exposure to Cd (200 microg/l) in aquaria for 7 days followed by detoxification in the sea for 28 days. Cd-preexposed and nonexposed mussels were then treated with Fe (300-600 microg/l) in aquaria for 3 days. Biochemical assays on digestive gland tissue showed that treatment with Fe led to a significant increase in oxyradical production and malondialdehyde level only in mussels not preexposed to Cd. The Cd-dependent resistance to oxidative stress was ascribed to MT induction, as Cd produced no significant variation of reduced glutathione and major antioxidant enzymes. Digital imaging of isolated digestive gland cells showed lower oxyradical rise and higher viability in cells from Cd-preexposed mussels after treatments with 0.5-5 mM H2O2. Analyses on whole organisms showed that anoxic survival was lowered in mussels that had been treated with Fe, but such an effect was less pronounced in Cd-preexposed mussels compared with nonpreexposed ones. In conclusion, data suggest an antioxidant role for MT, which seems to occur through oxyradical scavenging and is able to protect both isolated cells and the entire organism from oxidative stress.

Cu and Zn concentrations in the gills of the Antarctic scallop Adamussiurn colbeck! (Smith, 1902) were found to be significantly lower than in the gills of mussel Mytilus galloprovincialis (Lam ) and scallop Pecten jacobaeus (L.); however, the Cd concentration was significantly higher. In digestive gland, copper concentrations were not significantly different for all 3 species, but A. Colbecki had a lower Zn concentration and an extremely high Cd concentration (ca 27 pg g-' wet wt). In A. colbeckidigestive gland, about 70 % of the total Cd was associated w t h the particulate fraction, and the rest was mostly bound to a metallothionein-type protein. In P. jacobaeus a major fraction (about 60 %) of the Cd present in the digestive gland was bound to cytosolic metallothioneins. Aging was found not to affect the concentration of Cd bound to metallothioneins in A. colbecki digestive gland, although the Cd concentration is slightly reduced in scallops > 10 yr old compared to younger (3 to 7 yr old) scallops.

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