Autoinhibition mechanism of proto-Dbl

The dbl oncogene encodes a prototype member of the Rho GTPase guanine nucleotide exchange factor (GEF) family. Oncogenic activation of proto-Dbl occurs through truncation of the N-terminal 497 residues. The C-terminal half of proto-Dbl includes residues 498 to 680 and 710 to 815, which fold into the Dbl homology (DH) domain and the pleckstrin homology (PH) domain, respectively, both of which are essential for cell transformation via the Rho GEF activity or cytoskeletal targeting function. Here we have investigated the mechanism of the apparent negative regulation of proto-Dbl imposed by the N-terminal sequences. Deletion of the N-terminal 285 or C-terminal 100 residues of proto-Dbl did not significantly affect either its transforming activity or GEF activity, while removal of the N-terminal 348 amino acids resulted in a significant increase in both transformation and GEF potential. Proto-Dbl displayed a mostly perinuclear distribution pattern, similar to a polypeptide derived from its N-terminal sequences, whereas onco-Dbl colocalized with actin stress fibers, like the PH domain. Coexpression of the N-terminal 482 residues with onco-Dbl resulted in disruption of its cytoskeletal localization and led to inhibition of onco-Dbl transforming activity. The apparent interference with the DH and PH functions by the N-terminal sequences can be rationalized by the observation that the N-terminal 482 residues or a fragment containing residues 286 to 482 binds specifically to the PH domain, limiting the access of Rho GTPases to the catalytic DH domain and masking the intracellular targeting function of the PH domain. Taken together, our findings unveiled an autoinhibitory mode of regulation of proto-Dbl that is mediated by the intramolecular interaction between its N-terminal sequences and PH domain, directly impacting both the GEF function and intracellular distribution.     

The mechanism of hidromeiosis

Summary The problem of hidromeiosis is its mechanism.Hidromeiosis is a rapidly reversible process and an active sweat gland and a wetted skin are necessary conditions for its development. The threshold for hidromeiosis is lower for unacclimatized than acclimatized males. The facts suggest that accumulation of water in the skin and depression of the eccrine sweat gland may be involved in the explanation of the condition.Histochemical studies of the sweat glands during the development of hidromeiosis should elucidate curious individual differences in the manner in which the depression of sweating develops and the role of the sweat gland in the process.

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Zusammenfassung Der Wirkungsmechanismus der Hidromiosis (progressiver Abfall der Schweissmenge bei hoher Umgebungstemperatur) ist noch nicht aufgeklärt. Aktive Schweissdrüsen und nasse Haut sind für die Auslösung der Hidromiosis notwendig; der Vorgang is leicht reversibel. Der Hidromiosis-Schwellenwert ist bei nicht-akklimatisierten Männern niedriger als bei akklimatisierten. Diese Tatsachen führen zu der Annahme,dass die Ansammlung von Wasser in der Haut und die Unterdrückung der Sekretion der Schweissdrüsen für die Erklärung des Vorgangs eine Rolle spielen können. Durch histochemische Untersuchungen der Schweissdrüsen während Hidromiosis sollte es möglich sein die sonderbaren individuellen Unterschiede in der Art wie sich der Abfall des Schwitzens entwickelt und die Holle der Schweissdrüsen dabei aufzuklären.

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Résumé Le mécanisme de l'hidromiose (diminution progressive de la quantité de sueur lors de températures externes éleyées) n'est pas encore éclairci.Des glandes sudoripares actives et une peau humide sont nécessaires pour que l'hidromiose se produise; le processus est aisément réversible.Le senil de température de l'hidromiose est plus bas pour des hommes non acclimatés. Les faits font supposer que l'accumulation d'eau dans la peau et la gène de la sécrétion des glandes sudoripares peuvent jouer un rÔle dans l'explication du processus. L'examen histochimique des glandes sudoripares pendant l'hidromiose devrait expliquer les remarquables différences individuelles dans l'apparition de la diminution de sudation et le rÔle des glandes sudoripares.

     

Bisphosphonate mechanism of action

Nitrogen-containing bisphosphonates (N-BPs) are potent inhibitors of bone resorption widely used in the treatment of osteoporosis and other bone degrading disorders. At the tissue level, N-BPs reduce bone turnover, increase bone mass and mineralization, measured clinically as a rise in bone mineral density, increase bone strength and reduce fracture risk. At the cellular level, N-BPs, localize preferentially at sites of bone resorption, where mineral is exposed, are taken up by ostoclasts and inhibit osteoclast activity. The bone formation that follows incroporates the N-BP in the matrix, where it becomes pharmacologically inactive until released at a future time during bone remodeling. At the molecular level, N-BPs inhibit an enzyme in the cholesterol synthesis pathway, farnesyl diphosphate synthase. As a result, there is a reduction in the lipid geranylgeranyl diphosphate, which prenylates GTPases required for cytoskeletal organization and vesicular traffic in the osteoclast, leading to osteoclast inactivation.     

肿瘤发生发展的分子机理(4)

细胞由静息状态进入到细胞增殖期并不断进展是受到十分精确调控的，一旦细胞进入到G1后期这个“限制点”，将不可逆转地进入到S期，进行DNA复制。细胞周期蛋白、细胞周期蛋白依赖激酶(cyclindependent kinases，CDKs)和Rb蛋白都是调节细胞通过“限制点”的重要成分。这些蛋白可以通过控制细胞     

N-glycosidase mechanism of Trichosanthin

Ｔｒｉｃｈｏｓａｎｔｈｉｎ(ＴＣＳ)ｉｓａｔｙｐｅＩｒｉｂｏｓｏｍｅｉｎａｃｔｉｖａｔｉｎｇｐｒｏｔｅｉｎ(ＲＩＰ)[1].ＴｈｅｍｅｃｈａｎｉｓｍｏｆｉｎａｃｔｉｖｅｒｉｂｏｓｏｍｅｂｅｌｏｎｇｓｔｏｔｈａｔｏｆＲＮＡＮｇｌｙｃｏｓｉｄａｓｅ[2].ＴｈｅａｃｔｉｖｅｐｏｃｋｅｔｏｆＮｇｌｙｃｏｓｉｄａｓｅｈａｓｂｅｅｎｅｓｔａｂｌｉｓｈｅｄｏｎｔｈｅｓｕｒｆａｃｅｖａｃａｎｔｂｅｔｗｅｅｎｔｗｏｄｏｍａｉｎｓｔｈｒｏｕｇｈｃｒｙｓｔａｌｓｔｒｕｃｔｕｒｅｄｅｔｅｒｍｉｎａｔｉｏｎ[3]ａｎｄｔｈｅａｃｔｉ…     

Molecular mechanism of preconditioning

During the last 20 years, since the appearance of the first publication on ischemic preconditioning (PC), our knowledge of this phenomenon has increased exponentially. PC is defined as an increased tolerance to ischemia and reperfusion induced by previous sublethal period ischemia. This is the most powerful mechanism known to date for limiting the infract size. This adaptation occurs in a biphasic pattern (i) early preconditioning (lasts for 2-3 h) and (ii) late preconditioning (starting at 24 h lasting until 72-96 h after initial ischemia). Early preconditioning is more potent than delayed preconditioning in reducing infract size. Late preconditioning attenuates myocardial stunning and requires genomic activation with de novo protein synthesis. Early preconditioning depends on adenosine, opioids and to a lesser degree, on bradykinin and prostaglandins, released during ischemia. These molecules activate G-protein-coupled receptor, initiate activation of K(ATP) channel and generate oxygen-free radicals, and stimulate a series of protein kinases, which include protein kinase C, tyrosine kinase, and members of MAP kinase family. Late preconditioning is triggered by a similar sequence of events, but in addition essentially depends on newly synthesized proteins, which comprise iNOS, COX-2, manganese superoxide dismutase, and possibly heat shock proteins. The final mechanism of PC is still not very clear. The present review focuses on the possible role signaling molecules that regulate cardiomyocyte life and death during ischemia and reperfusion.     

Catalytic mechanism of thioltransferase

To evaluate potential catalytic mechanism for thioltransferase thiol-disulfide exchange reactions, seven pig liver mutants were constructed by site-directed mutagenesis. All the expressed enzymes, including wild-type and mutants with the exception of the inactive mutant, ETT-C22S, were variably inhibited by iodoacetamide, and similar results were obtained when these enzymes were preincubated with GSH. However, when preincubated with S-sulfocysteine or hydroxyethyl disulfide, the activity of the enzymes was totally or partially protected against inhibition by iodoacetamide, with the exception of the mutants, ETT-C25S and ETT-C25A. When simultaneously pretreated with GSH and S-sulfocysteine, all enzymes were highly protected. Isoelectric focusing analysis of the above preincubation mixtures showed that different enzyme-substrate intermediates occurred. Using radioactively labeled substrates, [U-14C]cystine and [glycine-2-3H] GSH, enzyme-substrate intermediates were detected. These data indicate that reduced thioltransferase reacts first with disulfide substrates, then with a thiol substrate, e.g. GSH. The formation of either enzyme-substrate mixed disulfide or protein intramolecular disulfide protected the enzyme from inactivation by iodoacetamide. Based on the experimental results, alternative methods of the catalytic mechanism for thioltransferases are proposed.     

Translocation mechanism of ribosomes

The paper summarizes studies of the molecular mechanism of the dynamic function of the ribosome, i. e. translocation, performed in the author's laboratory during the past decade. The hypothesis of the locking-unlocking of the ribosomal subparticles and the kinematical model of the working ribosome, the processes of spontaneous (factor-free) and factor-dependent translocation, the sequence of events in the factor-dependent translocation, the energetics of translocation and the contribution of the elongation factors with GTP are considered. The following conclusions are made: (1) the translocation mechanism is intrinsic to the structural organization of the ribosome itself but not introduced by the protein elongation factors; (2) the transpeptidation reaction is one of the sources of energy for the work of the translocation mechanism; (3) the protein elongation factors with GTP impart additional energy to the ribosome, including that for translocation, and thus ensure excess power which is realized, in particular, in the increase of the translocation rate and its resistance against inhibitors and hindrances; (4) the promoting role of the elongation factors with GTP does not proceed by a direct conjugation of GTP hydrolysis with translocation, but through the affinity of the elongation factors to the ribosome, with a subsequent compensation of the affinity at the expense of GTP cleavage.     

壳聚糖抑菌机制的初步研究

壳聚糖在医学、食品、环保、日化用品等领域有着广泛而重要的应用.近年来,壳聚糖由于对不同的菌类都具有良好的抑菌效果而被研究者们密切关注.然而,有关壳聚糖抑菌机制的研究却并不多,其抑菌机制也没有被完全阐明.在本研究中,我们发现很多金属离子可以对壳聚糖的抑菌效果产生影响,高浓度金属离子(0.5%)可以使壳聚糖完全丧失抑菌活性.还发现金黄色葡萄球菌和白色念珠菌在壳聚糖的作用下会发生钾离子和ATP的渗漏,而且五万分子量的壳聚糖引起钾离子和ATP的渗漏大约比五千分子量壳聚糖多2到4倍.不同分子量的壳聚糖对金黄色葡萄球菌和白色念珠菌都具有较好的抑菌效果,但是引起钾离子和ATP的渗漏量却存在很大差异,这说明小分子量壳聚糖很可能存在与大分子量壳聚糖不同的抑菌机制.     

Gelation mechanism of agarose

The non-Newtonian behavior and dynamic viscoelasticity of a series of aqueous solutions of agarose were measured with a rheogoniometer. The flow curve, at 25°, of agarose solution approximated to plastic behavior at 0.1, 0.13, and 0.15% concentrations. Gelation occurred at concentration of 0.13% at low temperature (0°). The dynamic modulus of agarose showed a very high value at low temperature, and increased with an increase in temperature, showing a maximum value at 30°, then it decreased. In the presence of NaCl, KCl, CaCl₂, and MgCl₂ for a solution of agarose at 0.08% concentration, the transition temperature, at which dynamic modulus decreased rapidly, was observed at 60°. Gelation was also observed at low temperature (0°) in acid and alkaline range after reaching pH values of 2.3 and 9.5, respectively, by addition of 100m HCl, H₂SO₄, NaOH, and Ca(OH)₂ to a 0.08% agarose solution. A possible mode of intra- and inter-molecular hydrogen bonding within and between the agarose molecules in aqueous solution is proposed.