Postinductive actinomycin D effects on the concentrations of cadmium thionein, zinc thionein, and copper chelatin in rat liver

Metallothionein in human retinal pigment epithelial cells: expression, induction and zinc uptake

The retinal pigment epithelium (RPE) plays several important roles in the continual support and renewal of photoreceptor outer segments. In the present study, we have demonstrated that RPE cells contain a low molecular weight protein with a high capacity for zinc binding that is dependent on available sulfhydryl groups. This protein is inducible by a 24 hour incubation of cultured RPE in medium supplemented with zinc, cadmium, or dexamethasone. The induction of this protein is correlated with an increased capacity for zinc-65 uptake into cultured RPE. Analysis with a cDNA probe specific for the human metallothionein II gene corroborated the existence and induction of metallothionein gene products in RPE cells. Based on these properties, we have identified this protein as metallothionein. The induction of metallothionein likely has a critical influence on the zinc economy of the RPE.

In vivo and ex vivo displacement of zinc from metallothionein by cadmium and by mercury

Divalent cadmium and mercury ions are capable in vitro of displacement of zinc from metallothionein. This process has now been studied in vivo and ex vivo, using the isolated perfused rat liver system, in order to determine if this process can occur in the intact cell. Rats with normal and elevated (via preinduction with zinc) levels of hepatic zinc thionein were studied. Cd(II) completely displaces zinc from normal levels of metallothionein and on a one-to-one basis from elevated levels of metallothionein, both in vivo and ex vivo. Hg(II) displaces zinc from metallothionein (normal or elevated) rather poorly, as compared with Cd(II), in vivo, probably due to the kidneys preference for absorbing this metal. Ex vivo Hg(II) displaces zinc from metallothionein (normal or elevated) on a one-to-one basis, with considerably more mercury being incorporated into the protein than in vivo. The results of double-label ex vivo experiments using metal and [35S]cysteine (+/- cycloheximide) were consistent with the above experiments, indicating that de novo thionein synthesis was not required for short term incorporation of cadmium and mercury into metallothionein. These data are supportive of the hypothesis that cadmium and mercury incorporation into rat hepatic metallothionein during the first few hours after exposure to these metals can occur primarily by displacement of zinc from preexisting zinc thionein by a process which does not require new protein synthesis.

Immunohistochemical demonstration of the cytoplasmic copper-associated protein in the liver in primary biliary cirrhosis: its identification as metallothionein

Metallothionein was isolated from the liver of rats pretreated with cadmium. Monospecific anti-metallothionein antibodies were produced in rabbits. Twenty-six primary biliary cirrhosis (PBC) liver samples and three control liver samples were stained for metallothionein by an indirect immunoperoxidase technique. Nineteen of the PBC samples and the control samples were also stained for copper by Timm's silver sulphide method. In all PBC samples the copper concentration was determined. In addition, the zinc, cadmium, and mercury concentrations were measured in some samples. Eighteen of the PBC samples stained for metallothionein showed marked diffuse cytoplasmic and nuclear staining of a varying number of periportal hepatocytes. In none of them was granular staining detected. None of the control samples showed staining. Fourteen of the PBC samples stained for copper showed diffuse cytoplasmic and granular staining of the periportal hepatocytes. In none of them was nuclear staining detected. None of the control samples showed staining. No elevated zinc, cadmium, or mercury concentrations were found but the copper concentration was increased in 21 of the 26 PBC samples. The number of cells staining for copper and metallothionein was assessed semiquantitatively. Significant correlations were found between the grade of metallothionein staining and the liver copper concentration; the grade of Timm's staining and the liver copper concentration; and the grades of metallothionein staining and Timm's staining. The data suggest that the accumulation of copper induces synthesis of cytoplasmic and nuclear metallothionein, and that the increased amount of cytoplasmic copper is bound to a similarly increased amount of metallothionein.

Reactivation in vitro of zinc-requiring apo-enzymes by rat liver zinc-thionein

The ability of rat liver zinc-thionein to donate its metal to the apo-enzymes of the zinc enzymes horse liver alcohol dehydrogenase, yeast aldolase, thermolysin, Escherichia coli alkaline phosphatase and bovine erythrocyte carbonic anhydrase was investigated. Zinc-thionein was as good as, or better than, ZnSO(4), Zn(CH(3)CO(2))(2) or Zn(NO(3))(2) in donating its zinc to these apo-enzymes. Apo-(alcohol dehydrogenase) could not be reactivated by zinc salts or by zinc-thionein. Incubation of the other apo-enzymes with near-saturating amounts of zinc as ZnSO(4), Zn(CH(3)CO(2))(2), Zn(NO(3))(2), or zinc-thionein resulted in reactivation of the apo-enzymes. With apo-aldolase zinc-thionein gave 100% reactivation within 30min. Reactivation by ZnSO(4) and Zn(CH(3)CO(2))(2) was complete and instantaneous. Zinc-thionein was somewhat better than Zn(NO(3))(2) in completely reactivating apo-thermolysin. With apo-(alkaline phosphatase) 43% reactivation was obtained with Zn(CH(3)CO(2))(2) and 18% with zinc-thionein. With apo-(carbonic anhydrase) zinc-thionein was better than ZnSO(4), Zn(CH(3)CO(2))(2) or Zn(NO(3))(2), with a maximal reactivation of 54%. That zinc was really being transferred from zinc-thionein to apo-(carbonic anhydrase) was shown by the fact that 2,6-pyridine dicarboxylic acid and 1,10-phenanthroline had minimal effects on the reactivation of apo-(carbonic anhydrase) when added after the incubation {[apo-(carbonic anhydrase)+zinc thionein]+chelator}, but inhibited reactivation when added before the incubation {apo-(carbonic anhydrase)+[zinc-thionein+chelator]}. These observations support the idea that zinc-thionein can function in zinc homeostasis as a reservoir of zinc, releasing the metal to zinc-requiring metalloenzymes according to need.

Cadmium accumulation and distribution in human lung fibroblasts

An investigation was conducted on cadmium accumulation and its molecular distribution in growing cultures of human fetal lung fibroblasts (IMR-90). For the first 24-48 h post-exposure, the amount of cadmium per cell remained low and relatively constant; more than 50% of the intracellular Cd was associated with molecular weight components less than 2000 daltons. The presence of a Cd-binding component with a molecular weight of 11,800 daltons was also detected during this initial period. Based upon its molecular size, sensitivity to trypsin, and ability to coincorporate 35S along with 115Cd, we have concluded that this component is a protein. Its similarity to metallothionein was suggested by its molecular size, low level in cells never exposed to Cd, spectral properties, and heat stability. During the late log phase of growth, accumulation of Cd by fibroblasts occurred at a near-linear rate and the intracellular Cd level was proportional to the exogenous concentration. There was a corresponding increase in the amount of the fibroblast Cd-binding protein present, accompanied by a reduction in the amount of Cd associated with low molecular weight components. Synthesis of the Cd-binding protein appeared to occur at a more rapid rate than accumulation suggesting that its presence may be necessary for Cd transport and/or accumulation, an interpretation strengthened by the finding that cells previously passaged in Cd exhibited no lag in accumulation and accumulated 8-10-fold more Cd. By 168 h post-exposure, a plateau occurred in the intracellular Cd level as well as the amount of Cd-binding protein present. After this period, a redistribution of Cd from the metallothionein-like protein to high molecular weight proteins occurred. It is possible that this redistribution might be the cellular event that triggers the pathological changes known to occur after Cd-grown cultures have reached confluency.

Zinc-induced synthesis of low molecular weight zinc-binding protein by human lymphocytes

Incubation of human peripheral blood lymphocytes with zinc transferrin (with or without phytohemagglutinin) induces the synthesis of protein that elutes from a Sephadex G-75 column at aV e/V o value corresponding to a molecular weight of 6600. Synthesis depends on the concentration of zinc transferrin in the medium and is sensitive to actinomycin D. Detectable synthesis occurs 5h after initiation of lymphocyte culture and plateaus at 24-30h. Polyacrylamide gel electrophoresis of the zinc-induced protein showed two closely moving bands, both of which show immunologic identity to rat liver metallothionein. Partial characterization of this protein yielded the following results: absorbance maximum at 220 nm; zinc content of 5.8 mol/6600 daltons; sulfhydryl content of 20.2 mol/6600 daltons. Additionally, synthesis of zinc-induced protein is altered in both chronic lymphocytic leukemic and acute lymphoblastic leukemic lymphocytes.