Purification and properties of plaice metallothionein, a cadmium-binding protein from the liver of the plaice (Pleuronectes platessa)

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Abstract
A low-molecular-weight protein induced in the liver of the plaice (Pleuronectes platessa) by exposure to cadmium was purified and characterized. It is closely similar to mammalian metallothioneins in all of its properties in that it is a single-chain cadmium-binding protein of approx. 7000 mol.wt. with a high cysteine content (31 mol%) and no aromatic amino acid residues. The thiol groups of the cysteine residues complex with the cadmium in a SH/Cd molar ratio of 3:1 and produce a characteristic absorption maximum at 250 nm. Unlike the mammalian metallothioneins, however, metal analyses reveal only traces of zinc and copper in addition to cadmium. The presence of carbohydrate previously assumed from a positive reaction with periodic acid/Schiff reagent has now been disproved, and the positive reaction attributed to interaction with the thiol groups in the protein.