In vivo metallothionein and glutathione status in an acute response to cadmium in Mercenaria mercenaria brown cells

Abstract

Brown cells that are found in the red glands of Mercenaria mercenaria accumulate, detoxify and excrete cadmium. Brown cell involvement in metal detoxification was due in part to endogenous glutathione (GSH) and protein sulfhydryl. Metallothionein (MT) and GSH have been shown to play an important role in metal detoxification in bivalve molluscs. This study showed that the protein sulfhydryl in brown cells of Mercenaria was in fact MT, that brown cell GSH functioned in acute protection against Cd²⁺ toxicity, that GSH provided the initial defense against Cd²⁺ toxicity prior to MT induction and that MT variants were unequal in response to Cd²⁺. During treatment of Mercenaria with 0.5 and 1.0 ppm Cd²⁺, brown cells were analyzed for MT by capillary electrophoresis and GSH colorimetrically after 0.25, 1, 2, 3, and 4 days. The data indicated that the cadmium-binding protein was MT with an apparent molecular weight of 9 kDa determined by gel filtration or 6 kDa as indicated by capillary electrophoresis. Glutathione appeared to prevail in the brown cell acute response to 0.5 ppm Cd²⁺, whereas MT appeared to prevail in the acute response to 1.0 ppm Cd²⁺. Capillary electrophoresis can be used to monitor and quantify MT and its variants in brown cells without need for prior separation of cytosolic components by chromatography. The change in MT-II was greater relative to the change in MT-I in the brown cell acute response to 0.5 ppm Cd²⁺, whereas the change in MT-I was greater relative to the change in MT-II in the acute response to 1.0 ppm Cd²⁺. The variants of brown cell MT appeared to respond differentially to Cd²⁺ depending upon the Cd²⁺ treatment concentration.