Characterization of metallothionein‐like proteins from zebra mussels (Dreissena polymorpha)

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Abstract
Zebra mussels (Dreissena polymorpha) are freshwater mollusks that have recently infested the Great Lakes ecosystem. Possessing a large capacity for filtration, these mussel populations act as bioconcentrators for contaminants, such as heavy metals, found in the Great Lakes ecosystem. Metallothionein is a low‐molecular‐weight, heavy metal‐binding protein found in most living organisms. Characterization and partial purification of metallothionein‐like Cd‐binding proteins from zebra mussels were performed. Zebra mussels were exposed to 500 μg/L Cd for 14 d. During the exposure period, two mussels were removed on alternate days for analysis of Cd‐binding proteins. Gel‐filtration high‐performance liquid chromatography‐microatomization‐atomic absorption spectrophotometry results showed a single Cd‐binding molecular weight protein fraction after 2 d of Cd exposure. After 10 d of Cd exposure, however, mussels exhibited an additional, higher molecular weight, Cd‐binding protein fraction. The lower molecular weight metallothionein‐like Cd‐binding protein was further isolated and purified by acetone fractionation, Sephadex G75, and diethylaminoethyl anion‐exchange chromatography. The quantities of Zn, Cu, and Cd in the anion‐exchange metallothionein‐like protein isoforms were determined by inductively coupled plasma‐mass spectrometry. The ability to bioconcentrate heavy metals in a metallothionein‐like form coupled with their large population in the Great Lakes make zebra mussels suitable for use in a freshwater biomonitoring program for aquatic metal contamination.

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