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   3498篇
  免费   100篇
  国内免费   3篇
  3601篇
  2024年   19篇
  2023年   12篇
  2022年   49篇
  2021年   67篇
  2020年   46篇
  2019年   72篇
  2018年   111篇
  2017年   70篇
  2016年   140篇
  2015年   195篇
  2014年   221篇
  2013年   245篇
  2012年   307篇
  2011年   275篇
  2010年   192篇
  2009年   183篇
  2008年   222篇
  2007年   194篇
  2006年   137篇
  2005年   142篇
  2004年   115篇
  2003年   113篇
  2002年   85篇
  2001年   73篇
  2000年   82篇
  1999年   53篇
  1998年   17篇
  1997年   17篇
  1996年   11篇
  1995年   12篇
  1994年   5篇
  1993年   9篇
  1992年   13篇
  1991年   15篇
  1990年   16篇
  1989年   14篇
  1988年   7篇
  1987年   2篇
  1986年   4篇
  1985年   8篇
  1984年   2篇
  1983年   7篇
  1982年   2篇
  1981年   2篇
  1978年   3篇
  1975年   2篇
  1974年   2篇
  1972年   2篇
  1970年   1篇
  1969年   3篇
排序方式: 共有3601条查询结果,搜索用时 15 毫秒
1.
We previously reported the presence of nitric oxide synthase (NOS) in Staphylococcus aureus ATCC6538P whose activity was induced by methanol. In the present study, the methanol-induced NOS was purified 900-fold from S. aureus by means of Mono Q ion exchange column, 2',5'-ADP-agarose affinity column, and Superdex 200HR gel permeation column chromatography. The purified bacterial NOS showed two protein bands with 67 and 64 kDa molecular mass on SDS-PAGE. However, the molecular mass of the NOS was 135 kDa on Superdex 200HR gel permeation column chromatography, indicating that the native enzyme exists as a heterodimer. This bacterial NOS had K(m) value of 13.4x10(-6) M for L-arginine and V(max) of 35.3 nmol min(-1) mg(-1) protein. In addition, reduced nicotinamide adenine dinucleotide phosphate, flavin adenine dinucleotide, flavin mononucleotide, tetrahydrobiopterin, calmodulin and Ca(2+) were required as cofactors in the conversion of L-arginine to L-citrulline, and NOS inhibitors selectively inhibited the activity of the purified NOS.  相似文献   
2.
The biochemical oxygen demand (BOD) determination was studied using a novel flow injection analysis (FIA) system with encapsulated Saccharomyces cerevisiae cells and an oxygen electrode and was compared with conventional 5-day BOD tests. S. cerevisiae cells were packed in a calcium alginate capsule at a dry cell weight of 250 g/l of capsule core. The level of dissolved oxygen (DO) was reduced due to the enhanced respiratory activity of the microbial cells when the injected nutrient passed through the bioreactor. The decrease in DO (ΔDO) was intensified with the amount of microbial cells packed in the bioreactor. However, the specific ΔDO decreased as the amount of cells loaded in the bioreactor increased. The ΔDO value was dependent on the pH and temperature of the mobile phase and reached its maximum value at 35°C and pH 7–8. Also, ΔDO became larger at longer response times as the flow rate of the mobile phase decreased. The measurement of ΔDO was repeated more than six times consecutively using a 20-ppm standard glucose and glutamic acid solution, which confirmed the reproducibility with a standard deviation of 0.95%. A strong linear correlation between ΔDO and BOD was also observed. The 5-day BOD values of actual water and wastewater samples were in accordance with the BOD values obtained by this FIA method using encapsulated S. cerevisiae cells. Unlike the cell-immobilized bead system, there was no contamination of the bioreactor resulting from any leak of yeast cells from the sensor capsules during BOD measurements.  相似文献   
3.
In order to create novel β-glucosidase constructs, 8 kinds of chimeric β-glucosidases were constructed using overlapping polymerase chain reaction (PCR) based on Cellvibrio gilvus (CG) and Thermotoga maritima (TM) genes. Two kinds of novel chimeric β-glucosidases (No. 6 and No. 8 type) were selected and their properties characterized. SDS-PAGE analysis showed that both constructs had a molecular mass of 80 kDa. The optimum pH of No. 6 chimeric β-glucosidase was found to be 3.0 and 5.0, showing varying maximum activity according to the buffer used. No. 8 chimeric enzyme was found to be optimally active at a pH of 4.5 and the optimum temperature of No.6 and No.8 chimeric β-glucosidases was reported to be 60°C, respectively. The Km values of both novel chimeric enzymes were calculated to be 0.012 mM and 0.0082 mM, respectively and the characteristics of the novel chimeric enzymes were to lie between those of the parental enzymes.  相似文献   
4.
Phospholipid hydroperoxide glutathione peroxidase (PHGPx) is an ubiquitous antioxidant enzyme, but the exact expression pattern in mammalian tissues is still unknown. The expression and cellular localization of PHGPx mRNA were examined in male mice using real time-polymerase chain reaction and in situ hybridization techniques. The rank order of PHGPx mRNA expression across tissues exhibiting substantial levels of expression was:testes ≫ heart > cerebrum ≥ ileum > stomach = liver = jejunum ≥ epididymis. In testes, PHGPx mRNA was highly expressed in spermiogenic cells and Leydig cells. The signal was also expressed in the molecular layer, Purkinje cell layer, and white matter of cerebellum, the pituicytes of neurohypophysis, the parafollicular cells and follicular basement membrane of thyroid, the exocrine portion of pancreas, the tubular epithelium of kidney, the smooth muscle cells of arteries, and the red pulp of spleen. In the gastrointestinal tract, PHGPx mRNA expression was mainly observed in the keratinized surface epithelium of forestomach, the submucosal glands and serosa layers, and further the Paneth cells of intestines. PHGPx mRNA appeared to be ubiquitously expressed in the parenchyma of heart, liver, and lung. These results indicate that PHGPx exhibits a cell- and tissue-specific expression pattern in mice.  相似文献   
5.
6.
7.
  总被引:2,自引:0,他引:2  
  相似文献   
8.
9.
DNA repair helicases function in the cell to separate DNA duplexes or remodel nucleoprotein complexes. These functions are influenced by sensing and signaling; the cellular pool of a DNA helicase may contain subpopulations of enzymes carrying different post-translational modifications and performing distinct biochemical functions. Here, we report a novel experimental strategy, single-molecule sorting, which overcomes difficulties associated with comprehensive analysis of heterologously modified pool of proteins. This methodology was applied to visualize human DNA helicase F-box–containing DNA helicase (FBH1) acting on the DNA structures resembling a stalled or collapsed replication fork and its interactions with RAD51 nucleoprotein filament. Individual helicase molecules isolated from human cells with their native post-translational modifications were analyzed using total internal reflection fluorescence microscopy. Separation of the activity trajectories originated from ubiquitylated and non-ubiquitylated FBH1 molecules revealed that ubiquitylation affects FBH1 interaction with the RAD51 nucleoprotein filament, but not its translocase and helicase activities.  相似文献   
10.
Considerable attention has been focused on the role of corticotropin-releasing factor (CRF) in neuropsychiatric disorders and neurodegenerative diseases including epilepsy. Therefore, in the present study, we investigated the temporal and spatial alteration of CRF receptor in the gerbil hippocampal complex in order to characterize the possible changes and associations with different sequelae of spontaneous seizure in these animals. Thirty minutes postictal, a decline in CRF receptor immunoreactivity was observed in the granule cells and hilar neurons. In the subiculum, CRF receptor immunoreactivity was also significantly decreased at this time point. Twenty-four hours after seizure onset, the immunoreactivity in these regions recovered to the pre-seizure level. Moreover, 30 min after seizure in the entorhinal cortex, the density of CRF receptor immunoreactivity began to decrease, particularly in the layers II and III, compared to pre-seizure group. Nevertheless, 24h after seizure onset, CRF receptor immunodensity had recovered to its seizure-sensitive (SS) level. These results suggest that altered CRF receptor expression in the hippocampal complex may affect tissue excitability and seizure activity in SS gerbils.  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