The amino acid composition of the zinc-induced metallothionein isoforms in rat brain.

Recent investigation from this laboratory has identified in the rat brain a zinc-inducible and actinomycin D-inhibited metallothionein with an elution volume (Ve/Vo) of 2.08 and a molecular weight of smaller than 10,000 daltons. Furthermore, purification of the zinc-induced metallothionein by ion exchange chromatography on DEAE-Sephadex A-25 columns produced two isoforms, eluting, respectively, at 68 and 130 mM of Tris-acetate buffer, pH 7.5. In this paper, we report that zinc-induced metallothionein produces also two distinct isoforms on reverse phase high performance liquid chromatography that exhibit retention times of 17.23 and 18.53 minutes, respectively. Brain metallothionein was characterized further by studies showing that the zinc-induced metallothionein incorporated a large quantity of [35S]cysteine and that isoforms I and II contain 17 and 18 cysteine residues, respectively, while being devoid of any arginine, histidine, leucine, phenylalanine or tyrosine. The precise functions of the brain metallothionein isoforms, which may be related to the transport and homeostasis of essential elements such as zinc and copper, remain to be elucidated.