首页 | 官方网站   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   334篇
  免费   16篇
生物科学   350篇
  2022年   2篇
  2021年   6篇
  2020年   5篇
  2017年   5篇
  2016年   8篇
  2015年   18篇
  2014年   24篇
  2013年   27篇
  2012年   24篇
  2011年   33篇
  2010年   14篇
  2009年   9篇
  2008年   16篇
  2007年   14篇
  2006年   13篇
  2005年   17篇
  2004年   17篇
  2003年   12篇
  2002年   9篇
  2001年   4篇
  2000年   3篇
  1999年   4篇
  1998年   10篇
  1996年   4篇
  1995年   3篇
  1993年   2篇
  1992年   2篇
  1990年   1篇
  1988年   3篇
  1986年   1篇
  1985年   2篇
  1984年   3篇
  1983年   2篇
  1982年   2篇
  1980年   2篇
  1979年   2篇
  1978年   6篇
  1977年   4篇
  1975年   1篇
  1974年   1篇
  1971年   1篇
  1969年   2篇
  1968年   1篇
  1965年   1篇
  1963年   1篇
  1961年   1篇
  1956年   1篇
  1955年   2篇
  1951年   1篇
  1950年   1篇
排序方式: 共有350条查询结果,搜索用时 15 毫秒
1.
2.
Racemic 5-methoxy-2-methyl-2-dipropylaminotetralin ( 3 ) has been prepared by a short synthetic route, in which the N,N-dipropyliminium perchlorate of 5-methoxy-2-tetralone ( 4 ) is a key intermediate. Racemic 3 was resolved by crystallization of the corresponding diastereomeric di-p-toluoyltartrates. The enantiomeric excess (%ee) of the phenolic derivatives of (+)-(R)- and (?)-(S)-3 [(+)-(R)- and (?)-(S)-2] was determined by 1HNMR spectroscopic analysis of the corresponding diastereomeric (?)-(R)-1,1′-binaphthyl-2,2′-diylphosphoric acid salts utilizing 13C satellites. X-ray crystallography established the absolute configuration of (?)-(S)-2 · HCl. The enantiomers of 2 were tested for hippocampal output of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and dihydroxyphenylacetic acid in rats by use of in vivo microdialysis. The (?)-(S)-enantiomer appeared to affect 5-HT-turnover, whereas (+)-(R)- 2 was inactive. Results obtained provide support for the previously reported hypothesis that the inactivity of (?)-(S)- 2 at central DA receptors is caused by the steric bulk of the C(2)-methyl group. This makes it possible to define a “DA D2 receptor essential volume.” © 1993 Wiley-Liss, Inc.  相似文献   
3.
In normal rats treated with 1,25(OH)2D3 or 24,25(OH)2D3, serum Ca2+, ALP, PRL and GH are significantly altered. In order to study the primary effect of vitamin D3 analogues on target organ function, rat UMR 106 osteosarcoma and GH3 pituitary adenoma cells in monolayer culture were exposed accordingly.Surprisingly, prolonged exposure of these cell lines to physiological levels of either 1,25(OH)2D3 or 24,25(OH)2D3 did not significantly affect the secretory parameters (ALP, PRL or GH) tested. However, 1,25(OH)2D3 exposure significantly reduced PTH- and Gpp(NH)p-elicited AC as well as Gpp(NH)p-stimulated PLC activities in the UMR 106 cells. These changes were accompanied by an increase and decrease in the membrane contents of the G-protein subunits G36 and Gq/11, respectively. In contrast, 24,25(OH)2D3 remained without significant biological effect on these signalling systems despite concomitantly augmented levels of G36. TRH- and Gpp(NH)p-elicited PLC activities in the GH3 cells were significantly reduced by 1,25(OH)2D3 with a concurrent reduction in cellular amounts of Gq/11, however, 24,25(OH)2D3 did not significantly alter any signalling systems nor G-proteins analyzed.It is concluded that the osteoblastic and pituitary cell secretion of ALP, PRL and GH remain unaffected by the presence of 1,25(OH)2D3 and 24,25(OH)2D3, despite distinct alterations in components of G-protein mediated signalling pathways. Hence, other factors like ambient Ca2+ may be responsible for the perturbed secretory patterns of ALP and PRL seen in vitamin D3 treated rats.Abbreviations AC adenylate cyclase - ALP alkaline phosphatase - BGP osteocalcin - BSA bovine serum albumin - DA dopamine - DAG diacylglycerol - GH growth hormone - GHRH growth hormone releasing hormone - Gpp(NH)p guanosine 5-[-imido]triphosphate - G-protein guanine nucleotide-binding regulatory protein - Gs etc. Gs protein -subunit - IP3 inositol 1,4,5 trisphosphate - OAF osteoclast activating factor - PGE2 prostaglandin E2 - PKA & PKC protein kinase A & C - PLC phospholipase C - PRL prolactin - PTH parathyroid hormone - SRIF somatostatin - TRH thyrotropin releasing hormone - VIP vasoactive intestinal peptide - 25(OH)D3 25 hydroxy vitamin D3 - 1,25(OH)2D3 1·25 dihydroxy vitamin D3 - 24,25(OH)2D3 24,25 dihydroxy vitamin D3  相似文献   
4.
5.
The glycosyltransferases (GTs) are an important and functionally diverse family of enzymes involved in glycan and glycoside biosynthesis. Plants have evolved large families of GTs which undertake the array of glycosylation reactions that occur during plant development and growth. Based on the Carbohydrate‐Active enZymes (CAZy) database, the genome of the reference plant Arabidopsis thaliana codes for over 450 GTs, while the rice genome (Oryza sativa) contains over 600 members. Collectively, GTs from these reference plants can be classified into over 40 distinct GT families. Although these enzymes are involved in many important plant specific processes such as cell‐wall and secondary metabolite biosynthesis, few have been functionally characterized. We have sought to develop a plant GTs clone resource that will enable functional genomic approaches to be undertaken by the plant research community. In total, 403 (88%) of CAZy defined Arabidopsis GTs have been cloned, while 96 (15%) of the GTs coded by rice have been cloned. The collection resulted in the update of a number of Arabidopsis GT gene models. The clones represent full‐length coding sequences without termination codons and are Gateway® compatible. To demonstrate the utility of this JBEI GT Collection, a set of efficient particle bombardment plasmids (pBullet) was also constructed with markers for the endomembrane. The utility of the pBullet collection was demonstrated by localizing all members of the Arabidopsis GT14 family to the Golgi apparatus or the endoplasmic reticulum (ER). Updates to these resources are available at the JBEI GT Collection website http://www.addgene.org/ .  相似文献   
6.
The Population Study of Women in Gothenburg, Sweden is an ongoing prospective study of female residents who were recruited from the local registry in 1968–1969 when they were 38–60 years old. The data presented here were collected from 361 healthy women who underwent a baseline physical examination including a supplementary dietary history interview and returned for a second general health examination 6 years later. This report identifies a subgroup of 57 women who were sedentary during their leisure time and appear to have been particularly susceptible to gaining weight as a function of the fat content of their diets. Specifically, longitudinal analysis of body weights in the whole sample revealed a statistical interaction between leisure-time physical activity and habitual dietary fat intake (energy-%), as reported at the baseline examination, in the prediction of subsequent weight change. Further stratified analysis suggested that weight changes were significantly dependent on dietary fat intake among the sedentary women only. High energy intake also predicted weight gain in the sedentary group, although the predictive value for a high-fat diet was of marginal significance after adjusting for total energy consumption. These results suggest that sedentary recreational activity plus a low-fat diet may have a combined contribution to weight change that is not equivalent to the sum of the separate effects. Such a synergy between two modifiable lifestyle factors seems highly relevant for prevention of obesity.  相似文献   
7.
Type IB topoisomerases cleave and rejoin DNA through a DNA-(3'-phosphotyrosyl)-enzyme intermediate. A constellation of conserved amino acids (Arg-130, Lys-167, Arg-223, and His-265 in vaccinia topoisomerase) catalyzes the attack of the tyrosine nucleophile (Tyr-274) at the scissile phosphodiester. Previous studies implicated Arg-223 and His-265 in transition state stabilization and Lys-167 in proton donation to the 5'-O of the leaving DNA strand. Here we find that Arg-130 also plays a major role in leaving group expulsion. The rate of DNA cleavage by vaccinia topoisomerase mutant R130K, which was slower than wild-type topoisomerase by a factor of 10(-4.3), was stimulated 2600-fold by a 5'-bridging phosphorothiolate at the cleavage site. The catalytic defect of the R130A mutant was also rescued by the 5'-S modification (190-fold stimulation), albeit to a lesser degree than R130K. We surmise that Arg-130 plays dual roles in transition state stabilization and general acid catalysis. Whereas the R130A mutation abolishes both functions, R130K permits the transition state stabilization function (via contact of lysine with the scissile phosphate) but not the proton transfer function. Our results show that the process of general acid catalysis is complex and suggest that Lys-167 and Arg-130 comprise a proton relay from the topoisomerase to the 5'-O of the leaving DNA strand.  相似文献   
8.
Although lipid signaling has been shown to serve crucial roles in mammals and plants, little is known about this process in filamentous fungi. Here we analyze the contribution of phospholipase D (PLD) and its product phosphatidic acid (PA) in hyphal morphogenesis and growth of Epichloë festucae and Neurospora crassa, and in the establishment of a symbiotic interaction between E. festucae and Lolium perenne. Growth of E. festucae and N. crassa PLD deletion strains in axenic culture, and for E. festucae in association with L. perenne, were analyzed by light-, confocal- and electron microscopy. Changes in PA distribution were analyzed in E. festucae using a PA biosensor and the impact of these changes on the endocytic recycling and superoxide production investigated. We found that E. festucae PldB, and the N. crassa ortholog, PLA-7, are required for polarized growth and cell fusion and contribute to ascospore development, whereas PldA/PLA-8 are dispensable for these functions. Exogenous addition of PA rescues the cell-fusion phenotype in E. festucae. PldB is also crucial for E. festucae to establish a symbiotic association with L. perenne. This study identifies a new component of the cell-cell communication and cell fusion signaling network for hyphal morphogenesis and growth of filamentous fungi.  相似文献   
9.
The movement patterns of three commercially important wrasse (Labridae) species inside a small marine protected area (~ 0.15 km2) on the west coast of Norway were analysed over a period of 21 months. The mean distance between capture and recapture locations varied between 10 and 187 m, and was species and season specific. The extent of movement was not related to body size or sex. These results imply that a network of small strategically located marine protected areas can be used as management tools to protect wrasses from size- and sex-selective fishing mortality.  相似文献   
10.

Introduction

Spondyloarthritis (SpA), an interrelated group of rheumatic diseases, has been suggested to be triggered by bacterial infections prior to the development of an autoimmune response that causes inflammation of the spinal and peripheral joints. Because human heat shock protein 60 (HSP60), recently renamed HSPD1, and bacterial HSP60 are highly homologous, immunological cross-reactivity has been proposed as a mechanism of disease initiation. However, previous investigations of the humoral immune response to HSP60 in SpA patients have lacked determination of immunoglobulin G (IgG) subclasses and patient follow-up. In this study, we have focused on these parameters in a cohort of axial SpA patients with a well-established set of clinical characteristics, including MRI changes and human leukocyte antigen B27.

Methods

IgG subclass antibodies (IgG1, IgG2, IgG3 and IgG4) against recombinant HSP60 of three reactive arthritis-related bacteria; human HSP60; and the microorganisms Chlamydia trachomatis and C. pneumoniae were determined by ELISA. Serum samples collected from 2004 to 2006 and in 2010 and 2011 from 39 axial SpA patients were analyzed and compared with samples from 39 healthy controls. The Mann-Whitney U test and Wilcoxon matched pairs test were used to compare the antibody levels in different and paired groups, respectively. P < 0.01 was considered significant. The Spearman nonparametric correlation was used to determine correlation between antibody levels and between antibody levels and the disease parameters.

Results

Elevated levels of IgG1 and IgG3 to human HSP60 and IgG1 to HSP60 of Salmonella enterica Enteritidis were observed in SpA patients compared with healthy controls at both time points. The antibody levels were almost constant over time for IgG1, whereas high levels of IgG3 to human HSP60 tended to decrease over time. The antibody response to human HSP60 was predominantly of the IgG3 subclass, and patients with high levels of IgG3 to this antigen had low levels of IgG1, indicating an inverse association. Different IgG subclasses were produced against bacterial and human HSP60 in the same serum sample, IgG1 and IgG3, respectively, indicating that there was no cross-reaction.

Conclusions

A significant association was observed between axial SpA and the presence of IgG1/IgG3 antibodies to human HSP60 and of IgG1 to S. enterica Enteritidis and C. trachomatis. Generation of antibodies to human HSP60 was independent of the presence of antibodies to bacterial HSP60. No association was observed between clinical and MRI changes with antibodies over time. Altogether, such antibodies do not reflect the disease activity in these patients.This study has been approved by the Regional Research Ethics Committee of Central Jutland, Denmark. Trial registration numbers: 20050046 and 20100083  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-23

京公网安备 11010802026262号