Metal-Binding Proteins of the Syrian Hamster Ovaries: Apparent Deficiency of Metallothionein

Open Access
Publisher Website
Google Scholar
Abstract
A deficiency of metallothionein, a high-affinity metal-binding protein thought to detoxify cadmium, has been observed in rat and mouse testes, tissues that are highly susceptible to the necrotizing and carcinogenic effects of cadmium. Like the testes, the ovaries undergo a hemorrhagic necrosis when exposed to cadmium, and female Syrian hamsters have recently been shown to be highly susceptible to cadmium. However, the nature of cadmium-binding proteins in the ovary is unknown; thus, this study was undertaken to define the nature of any such proteins in the Syrian hamster ovary. A low molecular weight (Mr) zinc- and cadmium-binding protein was detected in cytosol derived from the ovaries after gel filtration that eluted with a relative elution volume similar to authentic metallothionein. This protein was extractable by heat-treatment and sequential acetone precipitation. When such extracts were further purified with a reverse phase high performance liquid chromatography (HPLC) technique developed for the isolation of metallothionein isoforms, two forms were separated. However, neither of these could be classified as metallothionein on the basis of amino acid composition, since both were particularly low in cysteine, a very common amino acid in metallothionein. The ovarian protein also contained significant amounts of aromatic amino acids, unlike metallothionein-which is devoid of aromatics, and contained much more glutamate than metallothionein. Hamsters were also made resistant to cadmium-induced ovarian necrosis by zinc treatment. Such zinc treatment, however, did not alter levels of this protein, yet caused a marked induction of hepatic metallothionein. Likewise, cadmium treatment did not increase the levels of the ovarian metal-binding protein yet markedly induced hepatic metallothionein. These results indicate that the low Mr cadmium- and zinc-binding protein(s) present in the ovaries of Syrian hamsters is not metallothionein and provides a further correlation between metallothionein deficiency and high sensitivity to the effects of cadmium.

This publication has 22 references indexed in Scilit: