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971.
Chemical investigation of hassium (element 108) 总被引:5,自引:0,他引:5
Düllmann ChE Brüchle W Dressler R Eberhardt K Eichler B Eichler R Gäggeler HW Ginter TN Glaus F Gregorich KE Hoffman DC Jäger E Jost DT Kirbach UW Lee DM Nitsche H Patin JB Pershina V Piguet D Qin Z Schädel M Schausten B Schimpf E Schött HJ Soverna S Sudowe R Thörle P Timokhin SN Trautmann N Türler A Vahle A Wirth G Yakushev AB Zielinski PM 《Nature》2002,418(6900):859-862
The periodic table provides a classification of the chemical properties of the elements. But for the heaviest elements, the transactinides, this role of the periodic table reaches its limits because increasingly strong relativistic effects on the valence electron shells can induce deviations from known trends in chemical properties. In the case of the first two transactinides, elements 104 and 105, relativistic effects do indeed influence their chemical properties, whereas elements 106 and 107 both behave as expected from their position within the periodic table. Here we report the chemical separation and characterization of only seven detected atoms of element 108 (hassium, Hs), which were generated as isotopes (269)Hs (refs 8, 9) and (270)Hs (ref. 10) in the fusion reaction between (26)Mg and (248)Cm. The hassium atoms are immediately oxidized to a highly volatile oxide, presumably HsO(4), for which we determine an enthalpy of adsorption on our detector surface that is comparable to the adsorption enthalpy determined under identical conditions for the osmium oxide OsO(4). These results provide evidence that the chemical properties of hassium and its lighter homologue osmium are similar, thus confirming that hassium exhibits properties as expected from its position in group 8 of the periodic table. 相似文献
972.
The bacterial AlkB protein is known to be involved in cellular recovery from alkylation damage; however, the function of this protein remains unknown. AlkB homologues have been identified in several organisms, including humans, and a recent sequence alignment study has suggested that these proteins may belong to a superfamily of 2-oxoglutarate-dependent and iron-dependent oxygenases (2OG-Fe(ii)-oxygenases). Here we show that AlkB from Escherichia coli is indeed a 2-oxoglutarate-dependent and iron-dependent DNA repair enzyme that releases replication blocks in alkylated DNA by a mechanism involving oxidative demethylation of 1-methyladenine residues. This mechanism represents a new pathway for DNA repair and the third type of DNA damage reversal mechanism so far discovered. 相似文献
973.
The harlequin mouse mutation downregulates apoptosis-inducing factor 总被引:35,自引:0,他引:35
Klein JA Longo-Guess CM Rossmann MP Seburn KL Hurd RE Frankel WN Bronson RT Ackerman SL 《Nature》2002,419(6905):367-374
Harlequin (Hq) mutant mice have progressive degeneration of terminally differentiated cerebellar and retinal neurons. We have identified the Hq mutation as a proviral insertion in the apoptosis-inducing factor (Aif) gene, causing about an 80% reduction in AIF expression. Mutant cerebellar granule cells are susceptible to exogenous and endogenous peroxide-mediated apoptosis, but can be rescued by AIF expression. Overexpression of AIF in wild-type granule cells further decreases peroxide-mediated cell death, suggesting that AIF serves as a free radical scavenger. In agreement, dying neurons in aged Hq mutant mice show oxidative stress. In addition, neurons damaged by oxidative stress in both the cerebellum and retina of Hq mutant mice re-enter the cell cycle before undergoing apoptosis. Our results provide a genetic model of oxidative stress-mediated neurodegeneration and demonstrate a direct connection between cell cycle re-entry and oxidative stress in the ageing central nervous system. 相似文献
974.
Coulomb blockade and the Kondo effect in single-atom transistors 总被引:7,自引:0,他引:7
Park J Pasupathy AN Goldsmith JI Chang C Yaish Y Petta JR Rinkoski M Sethna JP Abruña HD McEuen PL Ralph DC 《Nature》2002,417(6890):722-725
Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems. Experiments to date have examined a multitude of molecules conducting in parallel, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions or scanning probes as well as three-terminal single-molecule transistors made from carbon nanotubes, C(60) molecules, and conjugated molecules diluted in a less-conducting molecular layer. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect. 相似文献
975.
VEGF regulates haematopoietic stem cell survival by an internal autocrine loop mechanism 总被引:49,自引:0,他引:49
Gerber HP Malik AK Solar GP Sherman D Liang XH Meng G Hong K Marsters JC Ferrara N 《Nature》2002,417(6892):954-958
Vascular endothelial growth factor (VEGF) is a principal regulator of blood vessel formation and haematopoiesis, but the mechanisms by which VEGF differentially regulates these processes have been elusive. Here we describe a regulatory loop by which VEGF controls survival of haematopoietic stem cells (HSCs). We observed a reduction in survival, colony formation and in vivo repopulation rates of HSCs after ablation of the VEGF gene in mice. Intracellularly acting small-molecule inhibitors of VEGF receptor (VEGFR) tyrosine kinase dramatically reduced colony formation of HSCs, thus mimicking deletion of the VEGF gene. However, blocking VEGF by administering a soluble VEGFR-1, which acts extracellularly, induced only minor effects. These findings support the involvement in HSC survival of a VEGF-dependent internal autocrine loop mechanism (that is, the mechanism is resistant to inhibitors that fail to penetrate the intracellular compartment). Not only ligands selective for VEGF and VEGFR-2 but also VEGFR-1 agonists rescued survival and repopulation of VEGF-deficient HSCs, revealing a function for VEGFR-1 signalling during haematopoiesis. 相似文献
976.
Lincoln AJ Wickramasinghe D Stein P Schultz RM Palko ME De Miguel MP Tessarollo L Donovan PJ 《Nature genetics》2002,30(4):446-449
In a wide variety of animal species, oocyte maturation is arrested temporarily at prophase of meiosis I (ref. 1). Resumption of meiosis requires activation of cyclin-dependent kinase-1 (CDK1, p34cdc2), one component of maturation-promoting factor (MPF). The dual specificity phosphatases Cdc25a, Cdc25b and Cdc25c are activators of cyclin-dependent kinases; consequently, they are postulated to regulate cell-cycle progression in meiosis and mitosis as well as the DNA-damage response. We generated Cdc25b-deficient (Cdc25b-/-) mice and found that they are viable. As compared with wildtype cells, fibroblasts from Cdc25b-/- mice grew vigorously in culture and arrested normally in response to DNA damage. Female Cdc25b-/- mice were sterile, and Cdc25b-/- oocytes remained arrested at prophase with low MPF activity. Microinjection of wildtype Cdc25b mRNA into Cdc25b-/- oocytes caused activation of MPF and resumption of meiosis. Thus, Cdc25b-/- female mice are sterile because of permanent meiotic arrest resulting from the inability to activate MPF. Cdc25b is therefore essential for meiotic resumption in female mice. Mice lacking Cdc25b provide the first genetic model for studying the mechanisms regulating prophase arrest in vertebrates. 相似文献
977.
Germline mutations in FH predispose to dominantly inherited uterine fibroids, skin leiomyomata and papillary renal cell cancer 总被引:26,自引:0,他引:26
Tomlinson IP Alam NA Rowan AJ Barclay E Jaeger EE Kelsell D Leigh I Gorman P Lamlum H Rahman S Roylance RR Olpin S Bevan S Barker K Hearle N Houlston RS Kiuru M Lehtonen R Karhu A Vilkki S Laiho P Eklund C Vierimaa O Aittomäki K Hietala M Sistonen P Paetau A Salovaara R Herva R Launonen V Aaltonen LA;Multiple Leiomyoma Consortium 《Nature genetics》2002,30(4):406-410
978.
Protecting against promiscuity: the regulatory role of insulators 总被引:11,自引:0,他引:11
979.
The superoxide-generating NADPH oxidase: structural aspects and activation mechanism 总被引:31,自引:0,他引:31
Vignais PV 《Cellular and molecular life sciences : CMLS》2002,59(9):1428-1459
Flavocytochrome b
558
is the catalytic core of the respiratory-burst oxidase, an enzyme complex that catalyzes the NADPH-dependent reduction of
O2 into the superoxide anion O2
- in phagocytic cells. Flavocytochrome b
558
is anchored in the plasma membrane. It is a heterodimer that consists of a large glycoprotein gp91phox (phox for phagocyte oxidase) (β subunit) and a small protein p22phox (α subunit). The other components of the respiratory-burst oxidase are water-soluble
proteins of cytosolic origin, namely p67phox, p47phox, p40phox and Rac. Upon cell stimulation, they assemble with the membrane-bound
flavocytochrome b
558
which becomes activated and generates O2
-. A defect in any of the genes encoding gp91phox, p22phox, p67phox or p47phox results in chronic granulomatous disease, a
genetic disorder characterized by severe and recurrent infections, illustrating the role of O2
- and the derived metabolites H2O2 and HOCl in host defense against invading microorganisms. The electron carriers, FAD and hemes b, and the binding site for NADPH are confined to the gp91phox subunit of flavocytochrome b
558
. The p22phox subunit serves as a docking site for the cytosolic phox proteins. This review provides an overview of current
knowledge on the structural organization of the O2
--generating flavocytochrome b
558
, its kinetics, its mechanism of activation and the regulation of its biosynthesis. Homologues of gp91phox, called Nox and
Duox, are present in a large variety of non-phagocytic cells. They exhibit modest O2
--generating oxidase activity, and some act as proton channels. Their role in various aspects of signal transduction is currently
under investigation and is briefly discussed.
Received 28 May 2002; received after revision 20 June 2002; accepted 24 June 2002 相似文献
980.
Role of hydrogen peroxide and oxidative stress in healing responses 总被引:11,自引:0,他引:11
Rojkind M Domínguez-Rosales JA Nieto N Greenwel P 《Cellular and molecular life sciences : CMLS》2002,59(11):1872-1891