Thionein

Grill, E., Winnacker, E.-L. Zenk, M.H. (1987). Phytochelatins, a class of heavy-metal-binding peptides from plants, are functionally analogous to metallo-thioneins. Proceedings of the National Academy of Sciences, USA, 84, 439-43. 

One of the more sensitive indicators of cadmium exposure is the inhibition of non-thionein hepatic metal binding proteins inhibition was observed in juvenile bluegills (Lepomis macrochirus)... 

Bakka, A. and M. Webb. 1981. Metabolism of zinc and copper in the neonate changes in the concentrations and contents of thionein-bound Zn and Cu with age in the livers of the newborn of various mammalian species. Biochem. Pharmacol. 30 721-725. 

Thioneins are apoproteins that are exceptionally sulfur-rich (composed of greater 30 mol% cysteine). These proteins are found in high abundance in liver and kidney cytoplasm where they form metallothioneins (the holo-protein forms) upon complexation with metal ions. Thi-onein synthesis is induced by the presence of metals, especially zinc, copper, mercury, and cadmium. 

Copper thionein Copper homeostasis Vertebrates Weser and Hartmann (1984)... 

The power of XPS-spectroscopy must be seen in the fast and efficient control of the homogeneity of an isolated protein. Commercial samples are sometimes not homogeneous enough or tend to show age dependent deterioration. These can readily be seen by XPS. When rapid and thorough isolation of a protein can be accomplished, no oxidised sulphur species are seen. A good example proved to be Cd, Zn-thionein which had no active redox metals. 

The cysteine rich Zn, Cd-thionein displays one homogeneous thiolate sulphur signal at 161.7 eV. Virtually no higher oxidation state of the sulphur is seen. Upon treatment with excessive H2O2, sulphonic acid is the exclusive detectable sulphur species at 167.4 eV (Fig. 4). 

Unlike the Cd, Zn-thioneins, the respective copper-thiolate proteins were much more susceptible to oxidation. The binding energy value of the S2p core electrons of yeast copper thionein lies at 162.0 eV (Fig. 5). Ageing of protein samples or the addition of substoichiometric concentrations of H2O2 give rise to the appearence to transient oxidation states including RSSR, RSO" and sulphonic acid. The copper thiolate... 

Fig. 5. S2p binding energy values of fetal bovine liver Cu-thionein (—) native Cu-thionein (...) after 24 h (—) after 36 h. (with permission of Biochim. Biophys. Acta. 491, 219 (1977))...

If animals ingest excessive amounts of Zn(n), Cd (II), Hg(II), or Cu(I) their livers and kidneys accumulate these metals as complexes of proteins called metallo-thioneins/1 e In mammals at least three related genes encode these metal-binding proteins. The best known, metallothionein II, has a highly conserved 61-residue sequence containing 20 cysteine residues and no aromatic residues. 

Resistance Sensitivity to these agents is decreased if cells have elevated glutathione levels or increased DNA repair, or if metallo-thionein (a protein rich in SH groups) is induced. 

The half-life of M-MT is dependent on the binding affinity of thionein for different metal ions. For instance, upon oxidation, Cu-MT forms insoluble polymers which are biologically unavailable and are eventually eliminated via biliary secretion. In contrast, thionein has lower affinity for Zn, making it more easily released from the protein and rendering the ion available for cellular processes. Furthermore, the rate of degradation may be influenced by differences in metal distribution between MT isoforms. It has been determined that MT degradation can occur in lysosomal and nonlysosomal (cytosolic) compartments. 

Zeng J, Heuchel R, Schaflfner W, Kagi JH (1991) Thionein (apometallothionein) can moduleite DNA binding and transcription activation by zinc finger containing factor Spl. FEES Lett... 

The bound Zn and/or Cd ions to MT are readily released from the protein by acidification (pH 2) yielding the metal-free protein designated as thionein or apoMT. The release of tightly bound Cu from the MT structure requires more harsh conditions such as 0.5 M HCl or treatment with specific copper chelators. From H NMR and far-UV CD studies, apoMT was estimated to contain a predominantly disordered structure. From this protein form, a number of various metal derivatives have been prepared through the method of metal reconstitution (see Section 6.1). ... 

Complete removal of zinc and thus inactivation of the enzyme can be accomplished in these systems at low D-PEN concentrations if a secondary scavenger chelator is added to the system. Such chelators bind metal that has been released from the enzyme but do not participate in the release.In the case of carboxypeptidase A, aM thionein (apo-metallothionen see Metallothioneins) inhibits catalysis by only about 10% over a 15-min period consistent with its action as a secondary chelator. However, in the presence of 250 aM D-PEN and aM thionein total inhibition is achieved in less than 15 min. D-PEN accelerates zinc equilibration between carboxypeptidase A and thionein (Scheme 1). This is accomplished by D-PEN catalyzing the release of Zn from the enzyme. Since D-PEN is in vast excess over both the enzyme and thionein, the enzyme-released zinc would be expected to bind to D-PEN first. However, since thionein binds zinc more tightly than D-penicillamine and can accept 7 moles of zinc per mole of thionein, it should be the ultimate acceptor of the released zinc. 

Zinc also inhibits a variety of nonmetalloenzymes. Inhibition constants in the nM range are observed in some cases. In these situations it is possible that the zinc could be involved in regulating the activity of the enzyme. Thionein has been demonstrated to reverse the inhibition glyceraldehyde-3-phosphate dehydrogenase by zinc ions, suggesting the possibility that thionein (apo-metaUothionien see Metallothioneins) can be involved in a regulatory capacity with zinc as its partner. ... 

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