Metal-specific induction of metallothionein isoforms in the blue crab Callinectes sapidus in response to single- and mixed-metal exposure.

Metallothioneins (MTs) play an important role in the metabolism of copper and zinc during the molt cycle of the blue crab. In this study we examined the hypothesis that MT expression in crabs is metal specific. Anion-exchange chromatography showed one major ZnCuMT (ZnMT-I) in control crabs, two MT isoforms in cadmium-treated crabs (CdMT-I, CdMT-II), and three forms (CuMT-I, CuMT-II, CuMT-III) in copper-treated animals. Amino acid analysis of the carboxymethylated apo-MTs, purified by reversed-phase HPLC, showed minor differences between ZnMT-I, CdMT-I, CdMT-II, CuMT-I, and CuMT-II, while CuMT-III was unique. After combined exposure to cadmium and copper, four MTs with differing copper/cadmium ratios were observed, equivalent to CdMT-II and the three CuMTs. We conclude that the blue crab has four genes that encode different MTs. Transcription of the CdMT-I gene is induced by cadmium, but inhibited by copper. CuMT-I, CdMT-II, and ZnMT-I may be the products of a single gene responsive to copper, cadmium, and zinc. Expression of the CuMT-II and CuMT-III genes is initiated by copper and not by cadmium. We believe that CdMT-I and CuMT-III are important in detoxification, whereas Zn/CuMT-I and CuMT-II are involved in regulatory functions. These results show the importance of the use of mixed-metal exposures in the study of the molecular mechanisms of metal regulation and function.