首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4193篇
  免费   254篇
  国内免费   2篇
  2023年   34篇
  2022年   57篇
  2021年   119篇
  2020年   99篇
  2019年   108篇
  2018年   104篇
  2017年   115篇
  2016年   204篇
  2015年   239篇
  2014年   234篇
  2013年   316篇
  2012年   365篇
  2011年   338篇
  2010年   243篇
  2009年   182篇
  2008年   256篇
  2007年   258篇
  2006年   216篇
  2005年   192篇
  2004年   148篇
  2003年   154篇
  2002年   133篇
  2001年   26篇
  2000年   28篇
  1999年   31篇
  1998年   42篇
  1997年   23篇
  1996年   25篇
  1995年   19篇
  1994年   15篇
  1993年   12篇
  1992年   5篇
  1991年   11篇
  1990年   16篇
  1989年   6篇
  1988年   6篇
  1987年   8篇
  1986年   5篇
  1985年   6篇
  1984年   5篇
  1983年   8篇
  1982年   3篇
  1981年   5篇
  1980年   7篇
  1979年   4篇
  1978年   3篇
  1977年   7篇
  1975年   3篇
  1974年   2篇
  1971年   1篇
排序方式: 共有4449条查询结果,搜索用时 15 毫秒
991.
We examined the substrate specificity of the carboxydipeptidase activity of neprilysin (NEP) using fluorescence resonance energy transfer (FRET) peptides containing ortho-aminobenzoyl (Abz) and 2,4-dinitrophenyl (Dnp) as a donor/acceptor pair. Two peptide series with general sequences Abz-RXFK(Dnp)-OH and Abz-XRFK(Dnp)-OH (X denotes the position of the altered amino acid) were synthesized to study P1 (cleavage at the X-F bond) and P2 (cleavage at R-F bond) specificity, respectively. In these peptides a Phe residue was fixed in P1' to fulfill the well-known NEP S1' site requirement for a hydrophobic amino acid. In addition, we explored NEP capability to hydrolyze bradykinin (RPPGFSPFR) and its fluorescent derivative Abz-RPPGFSPFRQ-EDDnp (EDDnp=2,4-dinitrophenyl ethylenediamine). The enzyme acts upon bradykinin mainly as a carboxydipeptidase, preferentially cleaving Pro-Phe over the Gly-Phe bond in a 9:1 ratio, whereas Abz-RPPGFSPFRQ-EDDnp was hydrolyzed at the same bonds but at an inverted proportion of 1:9. The results show very efficient interaction of the substrates' C-terminal free carboxyl group with site S2' of NEP, confirming the enzyme's preference to act as carboxydipeptidase at substrates with a free carboxyl-terminus. Using data gathered from our study, we developed sensitive and selective NEP substrates that permit continuous measurement of the enzyme activity, even in crude tissue extracts.  相似文献   
992.
In a recent study we showed that two proteinases (CMS2MS2 and CMS2MS3) from Carica candamarcensis enhance mammalian cell proliferation. The aim of the present study is the determination of the primary structure of CMS2MS2 and prediction of its three-dimensional structure. The protein contains 214 residues, including the catalytic triad composed of Cys(25), His(159), and Asn(175). A phylogenetic tree analysis demonstrated that CMS2MS2 ranks closer to chymopapain than to papain. The overall predicted three-dimensional structure is similar to proteinases from the papain family. These results suggest that minor structural differences within CMS2MS2 must account for its proliferative action.  相似文献   
993.
A gas-exchange structure interacts with the environment andis constantly challenged by contaminants that may elicit defenseresponses, thus compromising its primary function. It is alsoexposed to high concentrations of O2 that can generate reactiveoxygen species (ROS). Revisiting the lung of mammals, an integrativepicture emerges, indicating that this bronchi-alveolar structuredeals with inflammation in a special way, which minimizes compromisingthe gas-exchange role. Depending on the challenge, pro-inflammatoryor antiinflammatory responses are elicited by conserved molecules,such as surfactant proteins A and D. An even broader picturepoints to the participation of airway sensors, responsive toinflammatory mediators, in a loop linking the immunologicaland nervous systems and expanding the role played by respiratoryorgans in functions other than gas-exchange. A byproduct ofexposure to high concentration of O2 is the formation of superoxide(), hydrogen peroxide (H2O2),hydroxyl radical (HO), and other ROS, which are knownto be toxic to different types of cells, including the lungepithelium. A balance between antioxidants and oxidants exists;in pulmonary epithelial cells high intracellular and extracellularlevels of antioxidants are found. Antioxidant adaptations relatedto plant and animal life-styles involve a broad range of overlappingstrategies based on well-conserved molecules. Glutathione (GSH)is an abundant and ubiquitous thiol-tripeptide antioxidant,also present in lungs, whose role in providing information onthe intracellular redox state of animals and plants is wellestablished. In these organisms, GSH influences gene expressionassociated with stress, maximizing defense responses. Severalenzymatic antioxidants, such as glutathione peroxidase (GPx),glutathione reductase, glutathione S-transferase, and glucose6-phosphate dehydrogenase participate in the redox system; inanimals that are stress-tolerant GPx is a key element againstoxidative assaults. Most importantly, alternative roles of ROSas signaling molecules have been found in all plants and animals.For example, alveolar macrophages produce that act as second messengers, in addition tohaving a bactericidal role. The nonradical ROS H2O2 signalsinflammation in mammalian lungs, apoptosis in different animaltissues, and is also involved in stomatal closure, root development,gene expression, and defense responses of plants. Antioxidantadaptations in some water-breathing animals involve the excretionof H2O2 by diffusion through gas-exchange structures. The finebalance among a multitude of factors and cells makes the differencebetween damage and protection in animals and plants. Knowledgeabout the mechanisms and consequences of these molecular interactionsis now starting to be integrated.  相似文献   
994.
The malaria parasite harbors a relict plastid called the apicoplast and its discovery opened a new avenue for drug discovery and development due to its unusual, nonmammalian metabolism. The apicoplast is essential during the asexual intraerythrocytic and hepatic stages of the parasite, and there is strong evidence supporting its essential metabolic role during the mosquito stages of the parasite. Supply of the isoprenoid building blocks isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) is the essential metabolic function of the apicoplast during the asexual intraerythrocytic stages. However, the metabolic role of the apicoplast during gametocyte development, the malaria stages transmitted to the mosquito, remains unknown. In this study, we showed that production of IPP for isoprenoid biosynthesis is the essential metabolic function of the apicoplast during gametocytogenesis, by obtaining normal gametocytes lacking the apicoplast when supplemented with IPP. When IPP supplementation was removed early in gametocytogenesis, developmental defects were observed, supporting the essential role of isoprenoids for normal gametocytogenesis. Furthermore, mosquitoes infected with gametocytes lacking the apicoplast developed fewer and smaller oocysts that failed to produce sporozoites. This finding further supports the essential role of the apicoplast in establishing a successful infection in the mosquito vector. Our study supports isoprenoid biosynthesis as a valid drug target for development of malaria transmission-blocking inhibitors.  相似文献   
995.
Leptin is an adipokine that acts in the central nervous system and regulates energy balance. Animal models and human observational studies have suggested that leptin surge in the perinatal period has a critical role in programming long-term risk of obesity. In utero exposure to maternal hyperglycemia has been associated with increased risk of obesity later in life. Epigenetic mechanisms are suspected to be involved in fetal programming of long term metabolic diseases. We investigated whether DNA methylation levels near LEP locus mediate the relation between maternal glycemia and neonatal leptin levels using the 2-step epigenetic Mendelian randomization approach. We used data and samples from up to 485 mother-child dyads from Gen3G, a large prospective population-based cohort. First, we built a genetic risk score to capture maternal glycemia based on 10 known glycemic genetic variants (GRS10) and showed it was an adequate instrumental variable (β = 0.046 mmol/L of maternal fasting glucose per additional risk allele; SE = 0.007; P = 7.8 × 10−11; N = 467). A higher GRS10 was associated with lower methylation levels at cg12083122 located near LEP (β = −0.072 unit per additional risk allele; SE = 0.04; P = 0.05; N = 166). Direction and effect size of association between the instrumental variable GRS10 and methylation at cg12083122 were consistent with the negative association we observed using measured maternal glycemia. Lower DNA methylation levels at cg12083122 were associated with higher cord blood leptin levels (β = −0.17 log of cord blood leptin per unit; SE = 0.07; P = 0.01; N = 170). Our study supports that maternal glycemia is part of causal pathways influencing offspring leptin epigenetic regulation.  相似文献   
996.
With the aim of determining the occurrence of Cryptosporidium spp., 222 fecal samples were collected from Murrah buffalo calves aged up to 6 mo. Fecal DNA was genotyped with a nested polymerase chain reaction targeting the 18S rRNA gene and sequencing of the amplified fragment. Nested 18S PCR was positive for 48.2% of the samples. Sequence analysis showed that the most frequent species in these animals was Cryptosporidium ryanae, which was present in buffalo calves as young as 5 d. The zoonotic species Cryptosporidium parvum was detected in one animal. An uncommon Cryptosporidium 18S genotype was found in buffaloes.  相似文献   
997.
The anti‐plasmodial activity of conformationally restricted analogs of angiotensin II against Plasmodium gallinaceum has been described. To observe activity against another Plasmodium species, invasion of red blood cells by Plasmodium falciparum was analyzed. Analogs restricted with lactam or disulfide bridges were synthesized to determine their effects and constraints in the peptide–parasite interaction. The analogs were synthesized using tert‐butoxycarbonyl and fluoromethoxycarbonyl solid phase methods, purified by liquid chromatography, and characterized by mass spectrometry. Results indicated that the lactam bridge restricted analogs 1 (Glu‐Asp‐Arg‐Orn ‐Val‐Tyr‐Ile‐His‐Pro‐Phe) and 3 (Asp‐Glu‐Arg‐Val‐Orn ‐Tyr‐Ile‐His‐Pro‐Phe) showed activity toward inhibition of ring formation stage of P. falciparum erythrocytic cycle, preventing invasion in about 40% of the erythrocytes. The disulfide‐bridged analog 10 (Cys‐Asp‐Arg‐Cys ‐Val‐Tyr‐Ile‐His‐Pro‐Phe) was less effective yet significant, showing a 25% decrease in infection of new erythrocytes. In all cases, the peptides presented no pressor activity, and hydrophobic interactions between the aromatic and alkyl amino acid side chains were preserved, a factor proven important in efficacy against P. gallinaceum. In contrast, hydrophilic interactions between the Asp1 carboxyl and Arg2 guanidyl groups proved not to be as important as they were in the case of P. gallinaceum, while interactions between the Arg2 guanidyl and Tyr4 hydroxyl groups were not important in either case. The β‐turn conformation was predominant in all of the active peptides, proving importance in anti‐plasmodial activity. This approach provides insight for understanding the importance of each amino acid residue on the native angiotensin II structure and a new direction for the design of potential chemotherapeutic agents. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.  相似文献   
998.
999.
Tropical forests store vast amounts of carbon and are the most biodiverse terrestrial habitats, yet they are being converted and degraded at alarming rates. Given global shortfalls in the budgets required to prevent carbon and biodiversity loss, we need to seek solutions that simultaneously address both issues. Of particular interest are carbon‐based payments under the Reducing Emissions from Deforestation and Forest Degradation (REDD+) mechanism to also conserve biodiversity at no additional cost. One potential is for REDD+ to protect forest fragments, especially within biomes where contiguous forest cover has diminished dramatically, but we require empirical tests of the strength of any carbon and biodiversity cobenefits in such fragmented systems. Using the globally threatened Atlantic Forest landscape, we measured above‐ground carbon stocks within forest fragments spanning 13 to 23 442 ha in area and with different degrees of isolation. We related these stocks to tree community structure and to the richness and abundance of endemic and IUCN Red‐listed species. We found that increasing fragment size has a positive relationship with above‐ground carbon stock and with abundance of IUCN Red‐listed species and tree community structure. We also found negative relationships between distance from large forest block and tree community structure, endemic species richness and abundance, and IUCN Red‐listed species abundance. These resulted in positive congruence between carbon stocks and Red‐listed species, and the abundance and richness of endemic species, demonstrating vital cobenefits. As such, protecting forest fragments in hotspots of biodiversity, particularly larger fragments and those closest to sources, offers important carbon and biodiversity cobenefits. More generally, our results suggest that macroscale models of cobenefits under REDD+ have likely overlooked key benefits at small scales, indicating the necessity to apply models that include finer‐grained assessments in fragmented landscapes rather than using averaged coarse‐grained cells.  相似文献   
1000.
Germ cell apoptosis is important to regulate sperm production in the mammalian testis, but the molecular mechanisms underlying apoptosis are still poorly understood. We have recently shown that in vitro, etoposide induces upregulation of TACE/ADAM17 and ADAM10, two membrane-bound extracellular metalloproteases. Here we show that in vivo these enzymes are involved in etoposide-, but not in heat shock-, induced apoptosis in rat spermatogenesis. Germ cell apoptosis induced by DNA damage was associated with an increase in protein levels and cell surface localization of TACE/ADAM17 and ADAM10. On the contrary, apoptosis of germ cells induced by heat stress, another cell death stimulus, did not change levels or localization of these proteins. Pharmacological in vivo inhibition of TACE/ADAM17 and ADAM10 prevents etoposide-induced germ cell apoptosis. Finally, Gleevec (STI571) a pharmacological inhibitor of p73, a master gene controlling apoptosis induced by etoposide, prevented the increase of TACE/ADAM17 levels. Our results strongly suggest that TACE/ADAM17 participates in in vivo apoptosis of male germ cells induced by DNA damage.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号