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 cultured cells. Although the third isoform has been suggested to be similar and 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 ionization - time of flight mass spectrometry (MALDI-TOF MS) (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-translational deacetylation event (i.e., an isoform of MT-II not co-translationally acetylated, based on the change in its composition relative to MT-II and MT-I after induction of MT expression). Although MT-I' was not separated under the present conditions, the MT-I fraction gave a mass peak corresponding to a species 42 Da smaller than MT-I. This suggested that non-acetylated isoforms of both MT-1 and MT-II were present.