全文获取类型
收费全文 | 2032篇 |
免费 | 97篇 |
国内免费 | 2篇 |
出版年
2022年 | 8篇 |
2021年 | 17篇 |
2020年 | 14篇 |
2019年 | 18篇 |
2018年 | 30篇 |
2017年 | 16篇 |
2016年 | 38篇 |
2015年 | 76篇 |
2014年 | 71篇 |
2013年 | 145篇 |
2012年 | 143篇 |
2011年 | 118篇 |
2010年 | 73篇 |
2009年 | 100篇 |
2008年 | 128篇 |
2007年 | 143篇 |
2006年 | 127篇 |
2005年 | 136篇 |
2004年 | 131篇 |
2003年 | 128篇 |
2002年 | 124篇 |
2001年 | 13篇 |
2000年 | 10篇 |
1999年 | 17篇 |
1998年 | 27篇 |
1997年 | 26篇 |
1996年 | 26篇 |
1995年 | 23篇 |
1994年 | 11篇 |
1993年 | 22篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1987年 | 16篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 6篇 |
1983年 | 10篇 |
1982年 | 5篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1965年 | 3篇 |
1963年 | 2篇 |
排序方式: 共有2131条查询结果,搜索用时 15 毫秒
101.
In starfish, the peptide hormone gonad-stimulating substance (GSS) secreted from nervous tissue stimulates oocyte maturation to induce 1-methyladenine (1-MeAde) production by ovarian follicle cells. The SALMFamide family is also known to an echinoderm neuropeptide. The present study examined effect of SALMFamide 1 (S1) on oocyte maturation of starfish Asterina pectinifera. Unlike GSS, S1 did not induce spawning in starfish ovary. In contrast, S1 was found to inhibit GSS secretion from radial nerves by treatment with high K+ concentration. Fifty percent inhibition was obtained by 0.1 mM S1. S1 did not have any effect on GSS- and 1-MeAde-induced oocyte maturation. Following incubation with a S1 antibody and subsequently with rhodamine-conjugated second antibody, neural networks were observed in ovaries. The networks were restricted mainly to their surface with little evidence of immunoreactivity inside the basement membranes. This indicates that neural networks are distributed in the ovarian wall. The result further suggests that S1 plays a role in oocyte maturation to regulate GSS secretion from the nervous system. 相似文献
102.
A mixture of steamed soybean and boiled rice with seeded Aspergillus oryzae was naturally fermented without addition of yeasts or Lactobacilli, and kept matured for 12 months at room temperature. Chemical analysis of this rice-koji miso sample for lipid changes during maturation showed that triacylglycerol was gradually decomposed into free fatty acid, with distinct formation of fatty acid ethyl ester which, six months after the start of fermentation, came to account for 35.0% of total lipid. The ester was constituted primarily with linoleic acid (ca. 50%) and oleic acid (ca. 20%), no appreciable change in this proportion being observed during maturation. Also, the proportion was unique in that this did not reflect the fatty acid composition in a mixture of the two materials. It is possible to monitor the maturation of the rice-koji miso by following up the increase with time in fatty acid ethyl ester. 相似文献
103.
Tokunaga H Mitsuo K Ichinose S Omori A Ventosa A Nakae T Tokunaga M 《Applied and environmental microbiology》2004,70(8):4424-4431
It has been known that halophilic bacteria often show natural resistance to antibiotics, dyes, and toxic metal ions, but the mechanism and regulation of this resistance have remained unexplained. We have addressed this question by identifying the gene responsible for multidrug resistance. A spontaneous ofloxacin-resistant mutant derived from the moderately halophilic bacterium Chromohalobacter sp. strain 160 showed a two- to fourfold increased resistance to structurally diverse compounds, such as tetracycline, cefsulodin, chloramphenicol, and ethidium bromide (EtBr), and tolerance to organic solvents, e.g., hexane and heptane. The mutant produced an elevated level of the 58-kDa outer membrane protein. This mutant (160R) accumulated about one-third the level of EtBr that the parent cells did. An uncoupler, carbonyl cyanide m-chlorophenylhydrazone, caused a severalfold increase in the intracellular accumulation of EtBr, with the wild-type and mutant cells accumulating nearly equal amounts. The hrdC gene encoding the 58-kDa outer membrane protein has been cloned. Disruption of this gene rendered the cells hypersusceptible to antibiotics and EtBr and led to a high level of accumulation of intracellular EtBr. The primary structure of HrdC has a weak similarity to that of Escherichia coli TolC. Interestingly, both drug resistance and the expression of HrdC were markedly increased in the presence of a high salt concentration in the growth medium, but this was not observed in hrdC-disrupted cells. These results indicate that HrdC is the outer membrane component of the putative efflux pump assembly and that it plays a major role in the observed induction of drug resistance by salt in this bacterium. 相似文献
104.
Hyperplastic gastric tumors induced by activated macrophages in COX-2/mPGES-1 transgenic mice 总被引:6,自引:0,他引:6
Cyclooxygenase-2 (COX-2), the rate-limiting enzyme for prostanoid biosynthesis, plays a key role in gastrointestinal carcinogenesis. Among various prostanoids, prostaglandin E2 (PGE2) appears to be most responsible for cancer development. To investigate the role of PGE2 in gastric tumorigenesis, we constructed transgenic mice simultaneously expressing COX-2 and microsomal prostaglandin E synthase (mPGES)-1 in the gastric epithelial cells. The transgenic mice developed metaplasia, hyperplasia and tumorous growths in the glandular stomach with heavy macrophage infiltrations. Although gastric bacterial counts in the transgenic mice were within the normal range, treatment with antibiotics significantly suppressed activation of the macrophages and tumorous hyperplasia. Importantly, the antibiotics treatment did not affect the macrophage accumulation. Notably, treatment of the transgenic mice with lipopolysaccharides induced proinflammatory cytokines through Toll-like receptor 4 in the gastric epithelial cells. These results indicate that an increased level of PGE2 enhances macrophage infiltration, and that they are activated through epithelial cells by the gastric flora, resulting in gastric metaplasia and tumorous growth. Furthermore, Helicobacter infection upregulated epithelial PGE2 production, suggesting that the COX-2/mPGES-1 pathway contributes to the Helicobacter-associated gastric tumorigenesis. 相似文献
105.
Nakayama K Kawato H Watanabe J Ohtsuka M Yoshida K Yokomizo Y Sakamoto A Kuru N Ohta T Hoshino K Yoshida K Ishida H Cho A Palme MH Zhang JZ Lee VJ Watkins WJ 《Bioorganic & medicinal chemistry letters》2004,14(2):475-479
The addition of substituents to the pyridopyrimidine scaffold of MexAB-OprM specific efflux pump inhibitors was explored. As predicted by a pharmacophore model, the incorporation substituents at the 2-position improved potency. Piperidines were found to be optimal, and further introduction of polar groups without compromising the activity was shown to be feasible. Careful positioning of the essential acidic moiety of the pharmacophore relative to the scaffold led to the discovery of vinyl tetrazoles with still greater potency. 相似文献
106.
Human alpha-1,3-fucosyltansferase (FucT) encoded by the FUT6 gene was displayed at the cell surface of yeast cells engineered using the yeast cell wall protein Pir1 or Pir2, and the FucT activity was detected at the surface of cells producing the Pir1-HA-FUT6 or Pir2-FLAG-FUT6 fusion proteins. To obtain higher activity, we engineered the host yeast cells in which endogenous PIR genes of the PIR1-4 gene family were disrupted. Among the disruptants, the pir1Delta pir2Delta pir3Delta strain with the PIR1-HA-FUT6 fusion gene showed the highest FucT activity, which was about three-fold higher than that of the wild-type strain. Furthermore, the co-expression of both the Pir1-HA-FUT6 and the Pir2-FLAG-FUT6 fusions showed an approximately 1.5-fold higher activity than that in the cell wall displaying Pir1-HA-FUT6 alone. The present method was thus effective for producing yeast cells that can easily synthesize various oligosaccharides, such as Le(x) and sLe(x), using Pir-glycosyltransferase fusions in combination with the deletion of endogenous PIR genes. 相似文献
107.
Sclerostin is a novel secreted osteoclast-derived bone morphogenetic protein antagonist with unique ligand specificity 总被引:6,自引:0,他引:6
Kusu N Laurikkala J Imanishi M Usui H Konishi M Miyake A Thesleff I Itoh N 《The Journal of biological chemistry》2003,278(26):24113-24117
Sclerosteosis is a progressive sclerosing bone dysplasia. Sclerostin (the SOST gene) was originally identified as the sclerosteosis-causing gene. However, the physiological role of sclerostin remains to be elucidated. Sclerostin was intensely expressed in developing bones of mouse embryos. Punctuated expression of sclerostin was localized on the surfaces of both intramembranously forming skull bones and endochondrally forming long bones. Sclerostin-positive cells were identified as osteoclasts. Recombinant sclerostin protein produced in cultured cells was efficiently secreted as a monomer. We examined effects of sclerostin on the activity of BMP2, BMP4, BMP6, and BMP7 for mouse preosteoblastic MC3T3-E1 cells. Sclerostin inhibited the BMP6 and BMP7 activity but not the BMP2 and BMP4 activity. Sclerostin bound to BMP6 and BMP7 with high affinity but bound to BMP2 and BMP4 with lower affinity. In conclusion, sclerostin is a novel secreted osteoclast-derived BMP antagonist with unique ligand specificity. We suggest that sclerostin negatively regulates the formation of bone by repressing the differentiation and/or function of osteoblasts induced by BMPs. Since sclerostin expression is confined to the bone-resorbing osteoclast, it provides a mechanism whereby bone apposition is inhibited in the vicinity of resorption. Our findings indicate that sclerostin plays an important role in bone remodeling and links bone resorption and bone apposition. 相似文献
108.
Suzuki N Nakatsuka H Mochizuki M Nishi N Kadoya Y Utani A Oishi S Fujii N Kleinman HK Nomizu M 《The Journal of biological chemistry》2003,278(46):45697-45705
Laminin alpha chains (alpha1-alpha5 chains) have diverse chain-specific biological functions. The LG4 modules of laminin alpha chains consist of a 14-stranded beta-sheet (A-N) sandwich structure. Several biologically active sequences have been identified in the connecting loop regions. Here, we evaluated the biological activities of the loop regions of the E and F strands in the LG4 modules using five homologous peptides from each of the mouse alpha chains (EF-1: DYATLQLQEGRLHFMFDLG, alpha1 chain 2747-2765; EF-2: DFGTVQLRNGFPFFSYDLG, alpha2 chain 2808-2826; EF-3: RDSFVALYLSEGHVIFALG, alpha3 chain 2266-2284; EF-4: DFMTLFLAHGRLVFMFNVG, alpha4 chain 1511-1529; EF-5: SPSLVLFLNHGHFVAQTEGP, alpha5 chain 3304-3323). These homologous peptides showed chain-specific cell attachment and neurite outgrowth activities. Well organized actin stress fibers and focal contacts with vinculin accumulation were observed in fibroblasts attached on EF-1, whereas fibroblasts on EF-2 and EF-4 showed filopodia with ruffling. Fibroblast attachment to EF-2 and EF-4 was mediated by syndecan-2. In contrast, EF-1 promoted alpha2beta1 integrin-mediated fibroblast attachment and inhibited fibroblast attachment to a recombinant laminin alpha1 chain LG4-5. The receptors for EF-3 and EF-5 are unknown. Further, when the active core sequence of EF-1 was cyclized, utilizing two additional cysteine residues at both the N and C termini through a disulfide bridge, the cyclic peptide significantly enhanced integrin-mediated cell attachment. These results indicate that integrin-mediated cell attachment to the EF-1 sequence is conformation-dependent and that the loop structure is important for the activity. The homologous peptides, which promote either integrin- or syndecan-mediated cell attachment, may be useful for understanding the cell type- and chain-specific biological activities of the laminins. 相似文献
109.
Bink RJ Habuchi H Lele Z Dolk E Joore J Rauch GJ Geisler R Wilson SW den Hertog J Kimata K Zivkovic D 《The Journal of biological chemistry》2003,278(33):31118-31127
Heparan sulfate proteoglycans function in development and disease. They consist of a core protein with attached heparan sulfate chains that are altered by a series of carbohydrate-modifying enzymes and sulfotransferases. Here, we report on the identification and characterization of a gene encoding zebrafish heparan sulfate 6-O-sulfotransferase (hs6st) that shows high homology to other heparan sulfate 6-O-sulfotransferases. When expressed as a fusion protein in cultured cells, the protein shows specific 6-O-sulfotransferase activity and preferentially acts on the iduronosyl N-sulfoglycosamine. In the developing embryo, hs6st is expressed in the brain, the somites, and the fins; the same structures that were affected upon morpholino-mediated functional knockdown. Morpholino injections significantly inhibited 6-O- but not 2-O-sulfation as assessed by HPLC. Morphants display disturbed somite specification independent of the somite oscillator mechanism and have impaired muscle differentiation. In conclusion, our results show that transfer of sulfate to specific positions on glycosaminoglycans is essential for muscle development. 相似文献
110.
Irie A Habuchi H Kimata K Sanai Y 《Biochemical and biophysical research communications》2003,308(4):858-865
Although genetic studies have suggested that heparan sulfate (HS) is involved in bone morphogenetic protein (BMP)-mediated embryonic morphogenesis, it is unclear whether HS is directly involved in BMP-mediated signaling. Here, we investigate the involvement of HS in BMP-7 signaling. We show that HS and heparin chains specifically bind to BMP-7. Digestion of cell-surface HS with heparitinase interferes with BMP-7-mediated Smad phosphorylation in ROS 17/2.8 osteoblastic cells. Inhibiting sulfation of cell-surface HS with chlorate also causes interruption of Smad phosphorylation. Addition of exogenous heparin to ROS 17/2.8 cells prevents BMP-7-mediated Smad phosphorylation rather than enhances the BMP-7 signal, suggesting that HS should be anchored on the plasma membrane for BMP signaling. Moreover, BMP-7 binding to ROS 17/2.8 cells is inhibited by chlorate treatment and exogenous application of heparin. These results demonstrate that BMP-7 specifically binds to cell-surface HS and the BMP-7-HS interaction is required for BMP-7 signaling. 相似文献