Biochemical defense mechanisms against copper-induced oxidative damage in the blue crab, Callinectes sapidus

The blue crab (Callinectes sapidus) has a very dynamic copper metabolism associated with the biosynthesis and degradation of its respiratory pigment hemocyanin. In this study we report on the cellular defense mechanisms used by the crab to protect itself from copper toxicity. Short-term copper-exposure studies, conducted by incubating hepatopancreas tissue explants in copper-containing medium, show that copper taken up by the cells during the first 60 min combines with low-molecular-weight copper complex(es), which include Cu(I)-glutathione. Thereafter, copper binds to newly synthesized metallothionein (MT), with a concomitant decrease in Cu(I)-glutathione. Copper does not displace zinc from the endogenous ZnMT pool. Long-term exposure by means of copper-rich diets results in the synthesis of two MT isoforms in the hepatopancreas: CuMT-I and CuMT-II (D. Schlenk and M. Brouwer, 1991, Aquat. Toxicol. 20, 25-34). Transfer of copper from Cu(I)-glutathione to apoMT-I and apoMT-II can be accomplished in vitro. Cu(I) binding by the two isoforms is very different. Cu(I) binds to apoMT-I in a strictly cooperative manner. No partially filled Cu(I)-thiolate clusters appear to be present. In contrast, the Cu(I)-thiolate clusters in MT-II are formed only after more than four Cu(I) ions are bound. Long-term copper exposure leads to increased activity of two antioxidant enzymes: glutathione peroxidase and manganese superoxide dismutase (SOD). No CuZnSOD is found. Activities of catalase and glutathione reductase and the intracellular levels of glutathione are unaffected by copper. The defense mechanisms are not entirely sufficient for preventing copper-induced oxidative damage. Levels of oxidized lipids are significantly higher in copper-exposed crabs, but oxidized protein levels are nearly the same.