A pulse radiolytic study on the reaction of hydroxyl and superoxide radicals with yeast Cu(I)-thionein

Coordination of three and four Cu(I) to the alpha- and beta-domain of vertebrate Zn-metallothionein-1, respectively, induces significant structural changes

Vertebrate metallothioneins are found to contain Zn(II) and variable amounts of Cu(I), in vivo, and are believed to be important for d10-metal control. To date, structural information is available for the Zn(II) and Cd(II) forms, but not for the Cu(I) or mixed metal forms. Cu(I) binding to metallothionein-1 has been investigated by circular dichroism, luminescence and 1H NMR using two synthetic fragments representing the alpha- and the beta-domain. The 1H NMR data and thus the structures of Zn4alpha metallothionein (MT)-1 and Zn3betaMT-1 were essentially the same as those already published for the corresponding domains of native Cd7MT-1. Cu(I) titration of the Zn(II)-reconstituted domains provided clear evidence of stable polypeptide folds of the three Cu(I)-containing alpha- and the four Cu(I)-containing beta-domains. The solution structures of these two species are grossly different from the structures of the starting Zn(II) complexes. Further addition of Cu(I) to the two single domains led to the loss of defined domain structures. Upon mixing of the separately prepared aqueous three and four Cu(I) loaded alpha- and beta-domains, no interaction was seen between the two species. There was neither any indication for a net transfer of Cu(I) between the two domains nor for the formation of one large single Cu(I) cluster involving both domains.

Redox silencing of copper in metal-linked neurodegenerative disorders: reaction of Zn7metallothionein-3 with Cu2+ ions

Dysregulation of copper and zinc homeostasis in the brain plays a critical role in Alzheimer disease (AD). Copper binding to amyloid-beta peptide (Abeta) is linked with the neurotoxicity of Abeta and free radical damage. Metallothionein-3 (MT-3) is a small cysteine- and metal-rich protein expressed in the brain and found down-regulated in AD. This protein occurs intra- and extracellularly, and it plays an important role in the metabolism of zinc and copper. In cell cultures Zn7MT-3, by an unknown mechanism, protects neurons from the toxicity of Abeta. We have, therefore, used a range of complementary spectroscopic and biochemical methods to characterize the interaction of Zn7MT-3 with free Cu2+ ions. We show that Zn7MT-3 scavenges free Cu2+ ions through their reduction to Cu+ and binding to the protein. In this reaction thiolate ligands are oxidized to disulfides concomitant with Zn2+ release. The binding of the first four Cu2+ is cooperative forming a Cu(I)4-thiolate cluster in the N-terminal domain of Cu4,Zn4MT-3 together with two disulfides bonds. The Cu4-thiolate cluster exhibits an unusual stability toward air oxygen. The results of UV-visible, CD, and Cu(I) phosphorescence at 77 K suggest the existence of metal-metal interactions in this cluster. We have demonstrated that Zn7MT-3 in the presence of ascorbate completely quenches the copper-catalyzed hydroxyl radical (OH.) production. Thus, zinc-thiolate clusters in Zn7MT-3 can efficiently silence the redox-active free Cu2+ ions. The biological implication of our studies as to the protective role of Zn7MT-3 from the Cu2+ toxicity in AD and other neurodegenerative disorders is discussed.

Effect of overexpression of LAS17 on stress tolerance and the stability of extrachromosomal DNA in Saccharomyces cerevisiae

The effects of the overexpression of LAS17/BEE1, which encodes a yeast protein exhibiting sequence homology to the Wiscott-Aldrich syndrome protein, on the cell growth of Saccharomyces cerevisiae were examined. Sake yeast strain UT-1 grows at a faster rate as a result of the overexpression of LAS17 than control cultures under various stresses such as high temperature, high ethanol concentration, and oxidative stress, and the tolerance to these stresses was increased compared with the control. Moreover, a high cell survival rate was attained with overexpression of LAS17, when cells in the stationary phase of the growth cycle were subjected to heat killing (48 degrees C) or ethanol killing (20% v/v). In addition, the rate of induction of rho- was markedly reduced by overexpression of LAS17 when serine, tyrosine, and aspartic acid were used as N sources and the yeast was cultured at 35 degrees C, while rho- strains in control cultures were induced at a high frequency. After the incubation of cells harboring a multicopy vector in YPD or synthetic complete medium, almost all of the cells inherited the vector at about 15 copies per cell as a result of the overexpression of LAS17, whereas the cells harboring the control vector accounted for only 15% of the total number of cells. These results suggest that Las17p might be a multifunctional protein involved in cell growth regulation, extrachromosomal DNA transportation and stress responses.

Effect of pressure on pulse radiolysis reduction of proteins

Pulse radiolysis experiments were performed on proteins under pressure. Whereas many spectroscopic techniques have shown protein modifications at different pressure ranges, the present measurements performed using the water radiolysis allowed to generate radical species and to study the mechanisms implied in their reactions with proteins. This work gives the first results obtained on the effects of pressure on the rate constants of the proteins reduction by the hydrated electron at pressures up to 100 MPa. The reaction with the hydrated electron was investigated on two classes of protein: the horse myoglobin and the mussel metallothioneins. We have successively studied the influence of the pH value of metmyoglobin solutions (pH 6, 7 and 8) and the influence of the metals nature (Zn,Cu,Cd) bound to metallothioneins. For both protein, whatever the experimental conditions, the pressure does not influence the value of the reduction rate constant in the investigated range (0.1-100 MPa).

Advances in the structure and chemistry of metallothioneins

A low molecular weight (6-7 kDa) class of metalloproteins, designated as metallothioneins (MTs), exhibit repeated sequence motifs of either CxC or CxxC through which mono or divalent d(10) metal ions are bound in polymetallic-thiolate clusters. The preservation of metal-thiolate clusters in an increasing number of three-dimensional structures of these proteins signifies the importance of this structural motif. This review focuses on the recent developments regarding the versatile and striking chemical reactivity of MTs as well as on the existence of conformational/configurational dynamics within their structure. Both properties and their interplay are likely to be essential for the still elusive biological function of these proteins.

Cu-metallothioneins (Cu(I)8-MTs) in LEC rat livers 13 weeks after birth still act as antioxidants

Redox properties of metallothioneins (MTs) and Cu in the cytosol from Long-Evans Cinnamon (LEC) rat livers 13 weeks after birth were investigated. MTs from LEC rat livers contain 8 g atoms of Cu and 1 g atom of Zn per mole of protein (Cu(I)8-MTs). Titration of Cu(I)8-MTs with CuCl2 indicates that Cu(I)8-MTs were able to reduce further 2-g atoms of cupric ions per mole MTs as bound form. Hg2+-induced hydroxyl radical generation from Cu(I)8-MTs was demonstrated by ESR using the spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The intensity of DMPO-OH signal from Cu-loaded MTs was increased with the increasing number of Cu in MTs. The used cytosol fraction contained 1.37 mM total Cu and 5 mM DTNB titrable-SH groups has a potential to reduce 2 mM CuCl2. No ESR signal due to Cu2+ was also detected with LEC rat liver cytosol, whereas strong Cu2+ signal appeared by the addition of HgCl2. The rate constants for the reaction of Cu(I)8-MTs with superoxide and hydroxyl radicals were estimated to be 2 x 10(6) and > or = 10(12) M(-1)s(-1), respectively, from competition kinetics. Cu2+-catalyzed oxidation of DNA was strongly inhibited both in the presence of Cu-unsaturated MTs and GSH. The results suggest that Cu(I)8-MTs from LEC rat livers just before hepatitis still act as antioxidants.

Metal-induced hydroxyl radical generation by Cu(+)-metallothioneins from LEC rat liver

Reactions of LEC (Long-Evans rats with a cinnamonlike coat color) rat liver Cu(I)-metallothioneins (MTs) with HgCl2 or K3Fe(CN)6 were investigated by ESR spectroscopy and generation of hydroxyl radicals was demonstrated using the ESR spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO). When Cu(I)-MTs were incubated with more than one equivalent mole HgCl2 or K3Fe(CN)6 to Cu+ bound to MTs, strong signals due to Cu2+ appeared. ESR spectra, which were a combination of the DMPO-OH adduct signal and a six-line signal, were observed in the reaction of Cu(I)-MTs with HgCl2, whereas no oxygen radical signal was seen with K3Fe(CN)6. The DMPO-OH signal intensity was greater in the presence of SOD while the signal disappeared in the presence of catalase. The results suggest that addition of HgCl2 causes the liberation of cuprous ions from MTs followed by a reaction with oxygen, leading to hydroxyl radical formation through a Fenton-type Haber-Weiss reaction.

The role of iron-sulfur clusters in in vivo hydroxyl radical production

The in vivo production of HO- requires iron ions, H2O2 and O2- or other oxidants but probably does not occur through the Haber-Weiss reaction. Instead oxidants, such as O2-, increase free iron by releasing Fe(II) from the iron-sulfur clusters of dehydratases and by interfering with the iron-sulfur clusters reassembly. Fe(II) then reduces H2O2, and in turn Fe(III) and the oxidized cluster are re-reduced by cellular reductants such as NADPH and glutathione. In this way, SOD cooperates with cellular reductants in keeping the iron-sulfur clusters intact and the rate of HO. production to a minimum. O2- and other oxidants can release iron from Fe(II)-containing enzymes as well as copper from thionein. The released Fe(III) and Cu(II) are then reduced to Fe(II) and Cu(I) and can then participate in the Fenton reaction. In mammalian cells oxidants are able to convert cytosolic aconitase into active IRE-BP, which increases the "free" iron concentration intracellularly both by decreasing the biosynthesis of ferritin and increasing biosynthesis of transferrin receptors. The biological role of the soxRS regulon of Escherichia coli, which is involved in the adaptation toward oxidative stress, is presumably to counteract the oxidative inactivation of the iron clusters and the subsequent release of iron with consequent increased rate of production of HO.

The antioxidative activity of the mucoregulatory agents: ambroxol, bromhexine and N-acetyl-L-cysteine. A pulse radiolysis study

Ambroxol and bromhexine are shown to be scavengers of both superoxide and hydroxyl radicals as determined by pulse radiolysis experiments. The dismutation of superoxide was accelerated 3-fold by bromhexine and 2.5-fold by ambroxol over the rate of spontaneous dismutation. The reaction constants of hydroxyl radicals with bromhexine and ambroxol were determined by competition kinetics to be 1.58 +/- 0.15 x 10(10) M-1S-1 and 1.04 +/- 0.1 x 10(10) M-1S-1, respectively. N-acetyl-L-cysteine also reacted with hydroxyl radicals (1.28 +/- 0.14 x 10(10) M-1S-1) but not with superoxide radical. These effects may be clinically relevant in the treatment of oxidant-associated lung damage induced by inflammatory agents and/or environmental pollutants.

Oxidative stress induced heat shock factor phosphorylation and HSF-dependent activation of yeast metallothionein gene transcription

Metallothioneins (MTs) are a class of low-molecular-weight, cysteine- rich metal-binding proteins that function in metal detoxification and oxidative stress protection. We demonstrate that transcription of the Saccharomyces cerevisiae MT gene CUP1 is strongly activated by the superoxide anion generator menadione. This activation is exacerbated in a strain lacking the gene encoding Co, Zn superoxide dismutase (SOD1). CUP1 transcriptional activation by oxidative stress is dependent on a functional CUP1 promoter heat shock element (HSE) and the carboxy-terminal trans-activation domain of heat shock transcription factor (HSF). Furthermore, protection against oxidative stress conferred by CUP1 in a (sod1)delta strain requires HSF-mediated CUP1 transcription. Although in response to heat, HSF-mediated CUP1 transcription and HSF phosphorylation are transient, both CUP1 gene expression and HSF phosphorylation are sustained in response to oxidative stress. Moreover, the patterns of tryptic phosphopeptides resolved from HSF derived from cells subjected to heat shock or oxidative stress are distinct. These results demonstrate that transcription of the S. cerevisiae metallothionein gene under conditions of oxidative stress is mediated by HSF and that in response to distinct activation stimuli, HSF is differentially phosphorylated in a manner that parallels metallothionein gene transcription.

Transient thiyl radicals in yeast copper(I) thionein

In an EPR study employing yeast copper(I) thionein, GSH and Cu-GSH it was shown that thiyl radicals could be successfully generated from the thiolate sulfur via oxidation by photochemically formed superoxide at 77 K. The g-value was 2.036. Essentially no EPR detectable copper(II) was monitored under the experimental conditions, indicating that the oxidation reduction process is restricted to the thiolate sulfur. The Cu(I)-thiolate chromophores remained fully intact as deduced from chiroptical and luminescence measurements. Thus, copper thionein is supposed to be actively involved in the scavenging of oxygen free radicals by a reversible thiolate oxidation reduction cycle. The coordinated Cu(I) seems to serve as a prominent candidate to stabilize the transiently formed thiyl radical.