Metallothionein and trace element metabolism in sheep tissues in response to high and sustained zinc dosages. 1. Characterization and turnover of metallothionein isoforms

Abstract

Romney wether sheep were given ZnO on a regular basis, as required in protocols for the treatment of facial eczema, at either 12 g ZnO fortnight-1 or 12 g ZnO week-1, and the effect of Zn on the synthesis of different metallothionein (MT) isoforms isolated from liver tissue characterized and quantified. Metallothionein isoforms in liver of dosed and non-dosed sheep were quantitatively determined by reverse phase HPLC, with markedly increased concentrations of total MT-Ia protein found in the liver tissue of Zn-dosed groups (634 pg g-l fresh weight for the weekly dosed group) compared with 49 pg g-1 fresh weight in non-dosed sheep. The concentration of MT-II protein was not significantly increased. Individual sheep varied widely in the relative amounts of each MT isoform expressed with increased amounts of types of MT-Ia especially, and MT-I1 in some animals, which bound predominantly Cu. The half-lives for [35S]-cysteine and 67Zn in MT isoforms from weekly dosed sheep were found to be 2.5-3.5 days and 5-24 h respectively, indicating relatively fast exchange of Zn with MT compared with synthesis. Rate of synthesis for MT-Ia was twice that for MT-11. Data reflecting relationships between MT isoforms, Cu and Zn concentrations in liver are presented and discussed.

The accumulation of potentially-toxic metals by grazing ruminants

“…Cd absorption is most commonly affected by the Zn status of animals, and Cd has been demonstrated to increase Zn accumulation in sheep, perhaps by the stimulation of MT production (Doyle & Pfander, 1975;Grace et al 1993;Lee et al 1994). Conversely, Chiy et al (1998) found that a high Zn intake reduces Cd absorption in sheep, suggesting competition between the two elements.…”

“…The characteristic of Class B elements to form ligands with N and/or S, includes metallothioneins (MT; Cousins, 1985;Lee et al 1994), phytochelatin (Marschner, 1995), complexes with globular proteins (caeruloplasmin; Underwood & Suttle, 1999) and excretion in bile (e.g. Cd; Kiyozumi & Kojima, 1978).…”