Identification of non-acetylated metallothioneins induced in rat liver by zinc

In addition to the usual two rat metallothionein (MT) isoforms (MT-I and MT -II), a third isoform of metallothionein (MT-II') is known to be induced by zinc in the liver of rats and mice, and by epidermal growth factors in cul tured cells. Although the third isoform has been suggested to be similar an d related to MT-II based on its behavior on size-exclusion and ion exchange HPLC columns, further characterization has not been performed. MT-II' was identified in the present study as the unacetylated isoform of MT-II based on mass spectrometric data obtained by matrix assisted laser desorption ion ization - time of flight mass spectrometry (MALDI-TOFMS) (i.e., MT-II' was 42 Da smaller than MT-II in its molecular mass). Chromatographic properties of MT-II' were consistent with this species being an unacetylated isoform of MT-II arising as a result on lack of acetylation rather than a post-tran slational deacetylation event (i.e., an isoform of MT-II not co-translation ally acetylated, based on the change in its composition relative to MT-II a nd MT-1 after induction of MT expression). Although MT-I' was not separated under the present conditions, the MT-I fraction gave a mass peak correspon ding to a species 42 Da smaller than MT-I. This suggested that non-acetylat ed isoforms of both MT-I and MT-II were present.