Cadmium-induced metallothioneins from the common sea mussel, Mytilus edulis, were shown to comprise of two groups of isoforms having apparent molecular masses of 10kDa and 20kDa. The 1 0-kDa group was resolved by anion-exchange chromatography into four fractions while the 20-kDa group was resolved into three fractions using this method. After metal removal and Smethylation of the cysteine residues using methyl-p-nitrobenzenesulphonate the complete amino acid sequences were determined. Five isoforms of the 20-kDa group were shown to possess monomeric units consisting of 71 amino acids. These proteins were distinct from the four 72-amino-acid proteins of the 10-kDa group. The FASTA algorithm has been used to compare the degree of similarity between the mussel metallothionein MT-10-IV isoform and other metallothioneins. The mussel MT-10-IV isoform exhibited substantial similarity to other molluscan metallothioneins. Moreover, the mussel metallothionein exhibited more similarity to vertebrate metallothioneins than to those of non-molluscan invertebrates, thus suggesting that the mussel metallothioneins are class I metallothioneins.Despite the discovery of the small cysteine-rich metalbinding-protein metallothionein (MT) more than 30 years ago in equine kidney cortex, the function of this protein is still not fully understood. MT is known to bind metals such as Zn and Cu (which are essential for cell growth and development) and Cd and Hg (which are toxic), which suggests that MT might have more than one function.The search for putative functions for these proteins is exacerbated by the existence of numerous inducers and isoforms. The range of factors which regulate MT synthesis and degradation together with the ubiquity and sequence conservation of MT perhaps suggests that these proteins have a central role in cell metabolism. The proposed functions for MT include the detoxification of heavy metals and the metabolism of essential metals. Zeng et al. (1991) have suggested that thionein may be the active form of the protein and it may be involved in the control of gene expression by virtue of its ability to remove Zn from Zn-finger proteins. Moreover, a tissue-specific isoform of MT in astrocytes (also called growth-inhibitory factor) which inhibits the survival Correspondence to

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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.

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