Human hepatic metallothionein (MT) was separated into six isoforms by using reversed-phase h.p.l.c. at the analytical and preparative levels. By comparison with the h.p.l.c. elution profiles of the charge-separable species MT-1 and MT-2 isolated by the procedure of Bühler & Kägi [(1974) FEBS Lett. 39, 229-234], five of these isoproteins are identified as hitherto unresolved subforms of MT-1, and one is identical with MT-2. The six isoforms have distinct and reproducible retention times at neutral pH, where the metal remains bound to the protein, and at low pH, where the metal is removed. Their amino acid compositions display the high cysteine content and the lack of aromatic amino acids and of histidine typical of mammalian metallothioneins, but they differ significantly with respect to all other amino acids. A survey of autopsy material indicates that in adult human liver all six isoforms are usually expressed, albeit in somewhat variable relative proportions.

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