Studies on the appearance of a hepatic copper-binding protein in normal and zinc-deficient rats

Abstract

I . A study has been made by gel-filtration techniques of the soluble copper-and zincbinding proteins in rat liver after both intraperitoneal injection of Cu and dietary Cu supplementation.2. Liver Cu and Zn concentrations increased after injection of Cu, both metals accumulating in the cytosol, mainly in a fraction with an apparent molecular weight of about IZOOO.3. When Zn-deficient rats were injected with Cu, there was little change in liver Zn concentration and the occurrence of Cu in the low-molecular-weight form (about 12 000) was more transient. At most periods after injection, Cu accumulated mainly in a fraction with a molecular weight greater than 65 000.4. When the rats were Cu-loaded by dietary supplementation, virtually no Cu or Zn was found in the low-molecular-weight form in Zn-deficient rats, although they were found in the Zn-supplemented animals.5. The results suggest that Zn is essential for the accumulation of Cu in this form, but not for Cu to stimulate production of the metal-binding protein by a process requiring active protein synthesis.

Effect of zinc status of rats on the synthesis and degradation of copper-induced metallothioneins

“…In contrast, the disappearance of metal from the copper-thioneins, as from the Zn2+-induced protein (Feldman & Cousins, 1976), proceeded at the same rate as the degradation of the protein. This suggests that these processes occurred concomitantly and therefore supports the claim (Bremner & Davies, 1976) that the failure of Zn2+-deficient animals to accumulate significant amounts of copper-thionein in their liver, regardless of their Cu2+ status, is a consequence of the decreased stability of the copper-thionein in these animals. This could be related to the decreased Zn2+ content of the Cu2+-induced metallothionein in these animals.…”

“…Injection of Cu2+ into rats has been shown to stimulate the incorporation of both copper and Zn2+ into a low-molecular-weight hepatic protein (Bremner & Davies, 1976), which was unequivocally identified as metallothionein (Bremner & Young, 1976b). The inhibitory effect of cycloheximide on this process indicated that active protein synthesis was involved (Bremner & Davies, 1976), although Bloomer & Sourkes (1973) suggested that the copper-protein was constitutive.…”

“…This could be related to the decreased Zn2+ content of the Cu2+-induced metallothionein in these animals. It is perhaps significant that in previous studies (Bremner & Davies, 1976), where a shorter half-life (10h) was found for the disappearance of copper from the protein, even less Zn2+ was present in the copper-thionein.…”

“…The inhibitory effect of cycloheximide on this process indicated that active protein synthesis was involved (Bremner & Davies, 1976), although Bloomer & Sourkes (1973) suggested that the copper-protein was constitutive. The finding in these studies of increased incorporation of 35S from cysteine into the protein after Cu2+ injection strongly supports the view that Cu2+ induces synthesis of metallothionein de novo.…”

Hepatic Metallothionein as a Source of Zinc and Cysteine during the First Year of Life

“…Notable in these studies was the observation that zinc uptake was prevented when rats had received prior administration of actinomycon D, a known inhibitor of mRNA synthesis. Although tissue levels of metallothionein are low in adults, its synthesis can be induced by different metals such as cadmium, zinc, copper, and mercury thus explaining their putative role as protective and storage proteins (5)(6)(7)(8). For example, synthesis of the thioneins takes place in liver, kidney, and spleen when animals are treated with sublethal doses of cadmium and certain other metallic salts (9).…”