Copper- and zinc-binding proteins in sheep liver and intestine: effects of dietary levels of the metals

Abstract

Liver cytosol from sheep fed diets containing 2.2, 11.3 or 47 microgram Cu/g diet with or without supplemental zinc (543 or 46 microgram Zn/g diet), fractionated on Sephadex G-100, yielded three main copper- and zinc-containing proteins with approximate molecular weights of greater than 150,000, 27,000 and 10,000. Amino acid analysis of the 10,000-molecular-weight proteins were of the metallothionein type. Copper-chelatin was not present in sheep liver cytosol. Copper concentration of the metallothionein fraction increased (P less than 0.01) as dietary copper increased from 2.2 to 11.3 microgram Cu/g, but did not increase further when dietary copper increased to 47 microgram Cu/g in unsupplemented sheep. A low-molecular-weight (approximately 3,500) copper-, but not zinc-containing fraction appeared at this highest level of copper. Zinc supplementation of the diet increased not only the zinc content of the metallothionein fraction but also its copper content, most dramatically in sheep fed the highest copper level. In intestinal mucosal cytosol, no copper and little zinc was associated with the metallothionetin fraction which was not affected by dietary treatment. Evidence from this study suggests that sheep have limited capacity to synthesize metallothionein in response to increased dietary copper.