全文获取类型
收费全文 | 1286285篇 |
免费 | 106347篇 |
国内免费 | 1665篇 |
出版年
2021年 | 14246篇 |
2020年 | 13400篇 |
2019年 | 17077篇 |
2018年 | 18055篇 |
2017年 | 16686篇 |
2016年 | 28581篇 |
2015年 | 43582篇 |
2014年 | 51755篇 |
2013年 | 78029篇 |
2012年 | 36280篇 |
2011年 | 23911篇 |
2010年 | 43501篇 |
2009年 | 45067篇 |
2008年 | 22787篇 |
2007年 | 20109篇 |
2006年 | 25799篇 |
2005年 | 26666篇 |
2004年 | 25836篇 |
2003年 | 23452篇 |
2002年 | 21355篇 |
2001年 | 29214篇 |
2000年 | 26206篇 |
1999年 | 26906篇 |
1998年 | 23655篇 |
1997年 | 23530篇 |
1996年 | 23073篇 |
1995年 | 22439篇 |
1994年 | 22422篇 |
1993年 | 21332篇 |
1992年 | 24302篇 |
1991年 | 22910篇 |
1990年 | 21401篇 |
1989年 | 22529篇 |
1988年 | 20520篇 |
1987年 | 19560篇 |
1986年 | 18561篇 |
1985年 | 20606篇 |
1984年 | 20865篇 |
1983年 | 18458篇 |
1982年 | 19638篇 |
1981年 | 19010篇 |
1980年 | 17680篇 |
1979年 | 17006篇 |
1978年 | 16351篇 |
1977年 | 16004篇 |
1976年 | 15212篇 |
1975年 | 14556篇 |
1974年 | 14990篇 |
1973年 | 15346篇 |
1972年 | 12854篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Dominika Staniec Miroslaw Ksiazek Ida B. Th?gersen Jan J. Enghild Aneta Sroka Danuta Bryzek Matthew Bogyo Magnus Abrahamson Jan Potempa 《The Journal of biological chemistry》2015,290(45):27248-27260
Porphyromonas gingivalis is a peptide-fermenting asaccharolytic periodontal pathogen. Its genome contains several genes encoding cysteine peptidases other than gingipains. One of these genes (PG1055) encodes a protein called Tpr (thiol protease) that has sequence similarity to cysteine peptidases of the papain and calpain families. In this study we biochemically characterize Tpr. We found that the 55-kDa Tpr inactive zymogen proteolytically processes itself into active forms of 48, 37, and 33 kDa via sequential truncations at the N terminus. These processed molecular forms of Tpr are associated with the bacterial outer membrane where they are likely responsible for the generation of metabolic peptides required for survival of the pathogen. Both autoprocessing and activity were dependent on calcium concentrations >1 mm, consistent with the protein''s activity within the intestinal and inflammatory milieus. Calcium also stabilized the Tpr structure and rendered the protein fully resistant to proteolytic degradation by gingipains. Together, our findings suggest that Tpr is an example of a bacterial calpain, a calcium-responsive peptidase that may generate substrates required for the peptide-fermenting metabolism of P. gingivalis. Aside from nutrient generation, Tpr may also be involved in evasion of host immune response through degradation of the antimicrobial peptide LL-37 and complement proteins C3, C4, and C5. Taken together, these results indicate that Tpr likely represents an important pathogenesis factor for P. gingivalis. 相似文献
2.
Gopinath Kulasekaran Nadya Nossova Andrea L. Marat Ingrid Lund Christopher Cremer Maria S. Ioannou Peter S. McPherson 《The Journal of biological chemistry》2015,290(29):17999-18008
Connecdenn 1/2 are DENN (differentially expressed in normal and neoplastic cells) domain-bearing proteins that function as GEFs (guanine nucleotide exchange factors) for the small GTPase Rab35. Disruption of connecdenn/Rab35 function leads to defects in the recycling of multiple cargo proteins from endosomes with altered cell function, yet the regulation of connecdenn GEF activity is unexplored. We now demonstrate that connecdenn 1/2 are autoinhibited such that the purified, full-length proteins have significantly less Rab35 binding and GEF activity than the isolated DENN domain. Both proteins are phosphorylated with prominent phosphorylation sites between residues 500 and 600 of connecdenn 1. A large scale proteomics screen revealed that connecdenn 1 is phosphorylated at residues Ser-536 and Ser-538 in an Akt-dependent manner in response to insulin stimulation of adipocytes. Interestingly, we find that an Akt inhibitor reduces connecdenn 1 interaction with Rab35 after insulin treatment of adipocytes. Remarkably, a peptide flanking Ser-536/Ser-538 binds the DENN domain of connecdenn 1, whereas a phosphomimetic peptide does not. Moreover, connecdenn 1 interacts with 14-3-3 proteins, and this interaction is also disrupted by Akt inhibition and by mutation of Ser-536/Ser-538. We propose that Akt phosphorylation of connecdenn 1 downstream of insulin activation regulates connecdenn 1 function through an intramolecular interaction. 相似文献
3.
Lin Ding An Song Meng Dai Min Xu Wanwan Sun Baihui Xu Jichao Sun Tiange Wang Yu Xu Jieli Lu Weiqing Wang Yufang Bi Guang Ning 《Journal of lipid research》2015,56(4):920-926
Lipoprotein (a) [Lp(a)], an LDL-like particle, has been proposed as a causal risk factor for CVD among general populations. Meanwhile, both serum Lp(a) and diabetes increase the risk of CVD. However, the relationship between serum Lp(a) and T2D is poorly characterized, especially in the Asian population. Therefore, we conducted a cross-sectional study in 10,122 participants aged 40 years or older in Jiading District, Shanghai, China. Our study found that the prevalence of T2D was decreased from 20.9% to 15.0% from the lowest quartile to the highest quartile of serum Lp(a) concentrations (P for trend <0.0001). Logistic regression analyses showed that the odds ratios and 95% confidence intervals of prevalent T2D for quartiles 2–4 versus quartile 1 were 0.86 (0.73–1.01), 0.88 (0.75–1.04), and 0.76 (0.64–0.90) (P for trend = 0.0002), after adjustment for traditional confounding factors. Moreover, the risks for prevalent prediabetes, insulin resistance, and hyperinsulinemia were also decreased from the lowest to the top quartile. This inverse association between serum Lp(a) and T2D was not appreciably changed after we adjusted hypoglycemic medications or excluded the subjects with hypoglycemic and/or lipid-lowering agents and/or a history of self-reported CVD. 相似文献
4.
Polyphenols from green tea prevent antineuritogenic action of Nogo‐A via 67‐kDa laminin receptor and hydrogen peroxide 下载免费PDF全文
Usha Gundimeda Thomas H. McNeill Barsegh A. Barseghian William S. Tzeng David V. Rayudu Enrique Cadenas Rayudu Gopalakrishna 《Journal of neurochemistry》2015,132(1):70-84
Axonal regeneration after injury to the CNS is hampered by myelin‐derived inhibitors, such as Nogo‐A. Natural products, such as green tea, which are neuroprotective and safe for long‐term therapy, would complement ongoing various pharmacological approaches. In this study, using nerve growth factor‐differentiated neuronal‐like Neuroscreen‐1 cells, we show that extremely low concentrations of unfractionated green tea polyphenol mixture (GTPP) and its active ingredient, epigallocatechin‐3‐gallate (EGCG), prevent both the neurite outgrowth‐inhibiting activity and growth cone‐collapsing activity of Nogo‐66 (C‐terminal domain of Nogo‐A). Furthermore, a synergistic interaction was observed among GTPP constituents. This preventive effect was dependent on 67‐kDa laminin receptor (67LR) to which EGCG binds with high affinity. The antioxidants N‐acetylcysteine and cell‐permeable catalase abolished this preventive effect of GTPP and EGCG, suggesting the involvement of sublethal levels of H2O2 in this process. Accordingly, exogenous sublethal concentrations of H2O2, added as a bolus dose (5 μM) or more effectively through a steady‐state generation (1–2 μM), mimicked GTPP in counteracting the action of Nogo‐66. Exogenous H2O2 mediated this action by bypassing the requirement of 67LR. Taken together, these results show for the first time that GTPP and EGCG, acting through 67LR and elevating intracellular sublethal levels of H2O2, inhibit the antineuritogenic action of Nogo‐A.
5.
6.
7.
8.
9.
10.