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   388篇
  国内免费   1篇
  完全免费   93篇
  2018年   7篇
  2017年   3篇
  2016年   5篇
  2015年   3篇
  2014年   7篇
  2013年   3篇
  2012年   16篇
  2011年   6篇
  2010年   6篇
  2009年   13篇
  2008年   10篇
  2007年   6篇
  2006年   9篇
  2005年   6篇
  2004年   5篇
  2003年   2篇
  2002年   3篇
  2001年   43篇
  2000年   34篇
  1999年   49篇
  1998年   10篇
  1997年   11篇
  1996年   11篇
  1995年   10篇
  1994年   13篇
  1993年   11篇
  1992年   23篇
  1991年   14篇
  1990年   14篇
  1989年   21篇
  1988年   7篇
  1987年   10篇
  1986年   7篇
  1985年   4篇
  1984年   10篇
  1983年   7篇
  1982年   2篇
  1981年   2篇
  1980年   2篇
  1979年   8篇
  1978年   6篇
  1977年   6篇
  1976年   5篇
  1975年   3篇
  1974年   4篇
  1973年   3篇
  1972年   7篇
  1971年   4篇
  1970年   3篇
  1969年   3篇
  1968年   1篇
  1967年   1篇
  1958年   1篇
  1957年   1篇
  1954年   1篇
排序方式: 共有482条查询结果,搜索用时 62 毫秒
1.
We have prepared three monoclonal antibodies against human epidermal keratins. These antibodies were highly specific for keratins and, in combination, recognized all major epidermal keratins of several mammalian species. We have used these antibodies to study the tissue distribution of epidermis-related keratins. In various mammalian epithelia, the antibodies recognized seven classes of keratins defined by their immunological reactivity and size. The 40, 46 and 52 kilodalton (kd) keratin classes were present in almost all epithelia; the 50 kd and 58 kd keratin classes were detected in all stratified squamous epithelia, but not in any simple epithelia; and the 56 kd and 65-67 kd keratin classes were unique to keratinized epidermis. Thus the expression of specific keratin classes appeared to correlate with different types of epithelial differentiation (simple versus stratified; keratinized versus nonkeratinized).  相似文献
2.
Ethanol-type fermentation from carbohydrate in high rate acidogenic reactor   总被引:46,自引:0,他引:46  
It has been found, in this study, that a new ethanol-type fermentation can be obtained in a continuous flow, high-rate acidogenic reactor receiving molasses as the feed. The operating pH must be maintained at about 4.5 to avoid onset of propionic fermentation. The acidogenic reactor had a VSS level of 20 g/L and its organic loading was as high as 80 to 90 kg COD/m(3) d. The operating ORP was around -250 mV. The ethanol-type fermentation was characterized by a simultaneous production of acetic acid and ethanol, while the yield of propionic was minimal even at a high organic loading rate of 80 to 90 kg COD/m(3) d, and also, the hydrogen partial pressure was as high as 50 kPa. Thus, this study has shown that the production of propionic acid is not always related to high hydrogen partial pressure. When the operating pH was increased to 5.5, the yield of propionic acid became significant. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 428-433, 1997.  相似文献
3.
One hallmark of Alzheimer disease is the accumulation of amyloid beta-peptide in the brain and its deposition as plaques. Mice transgenic for an amyloid beta precursor protein (APP) mini-gene driven by a platelet-derived (PD) growth factor promoter (PDAPP mice), which overexpress one of the disease-linked mutant forms of the human amyloid precursor protein, show many of the pathological features of Alzheimer disease, including extensive deposition of extracellular amyloid plaques, astrocytosis and neuritic dystrophy. Active immunization of PDAPP mice with human amyloid beta-peptide reduces plaque burden and its associated pathologies. Several hypotheses have been proposed regarding the mechanism of this response. Here we report that peripheral administration of antibodies against amyloid beta-peptide, was sufficient to reduce amyloid burden. Despite their relatively modest serum levels, the passively administered antibodies were able to enter the central nervous system, decorate plaques and induce clearance of preexisting amyloid. When examined in an ex vivo assay with sections of PDAPP or Alzheimer disease brain tissue, antibodies against amyloid beta-peptide triggered microglial cells to clear plaques through Fc receptor-mediated phagocytosis and subsequent peptide degradation. These results indicate that antibodies can cross the blood-brain barrier to act directly in the central nervous system and should be considered as a therapeutic approach for the treatment of Alzheimer disease and other neurological disorders.  相似文献
4.
Glycinebetaine (betaine) affords osmoprotection in bacteria, plants and animals, and protects cell components against harsh conditions in vitro. This and a compelling body of other evidence have encouraged the engineering of betaine production in plants lacking it. We have installed the metabolic step for oxidation of choline, a ubiquitous substance, to betaine in three diverse species, Arabidopsis, Brassica napus, and tobacco (Nicotiana tabacum), by constitutive expression of a bacterial choline oxidase gene. The highest levels of betaine in independent transgenics were 18.6, 12.8, and 13 micromol g(-1) dry weight, respectively, values 10- to 20-fold lower than the levels found in natural betaine producers. However, choline-fed transgenic plants synthesized substantially more betaine. Increasing the choline supplementation further enhanced betaine synthesis, up to 613 micromol g(-1) dry weight in Arabidopsis, 250 micromol g(-1) dry weight in B. napus, and 80 micromol g(-1) dry weight in tobacco. These studies demonstrate the need to enhance the endogenous choline supply to support accumulation of physiologically relevant amounts of betaine. A moderate stress tolerance was noted in some but not all betaine-producing transgenic lines based on relative shoot growth. Furthermore, the responses to stresses such as salinity, drought, and freezing were variable among the three species.  相似文献
5.
The expression of certain mRNAs from human immunodeficiency virus type 1 (HIV-1) is controlled by the viral transactivator Rev, a nucleolar protein that binds a cis-acting element in these mRNAs. Rev is encoded by two viral exons that specify amino acids 1 to 26 and 27 to 116, respectively. Earlier studies have mapped essential regions of the protein that are encoded in the second exon. By further mutational analysis of Rev, we have now identified a novel locus encoded by the first exon that also is essential for transactivation in vivo. Defined by mutations at residues 14 to 20, this locus coincides with a cluster of positively charged and nonpolar amino acids that is conserved in Rev proteins of all known primate immunodeficiency viruses. Rev proteins that contained mutations at this site were defective in both nuclear localization and transactivation and did not function as trans-dominant inhibitors of wild-type Rev. Fusion of these mutants to a heterologous nuclear protein complemented the defect in localization but did not restore biological activity. Our findings suggest that this N-terminal locus may play a direct role in transactivation, perhaps contributing to essential protein-protein interactions or forming part of the RNA-binding domain of Rev.  相似文献
6.
Disintegrins: a family of integrin inhibitory proteins from viper venoms   总被引:31,自引:0,他引:31  
Disintegrins represent a new class of low molecular weight, RGD-containing, cysteine-rich peptides isolated from the venom of various snakes. They interact with the beta 1 and beta 3 families of integrins and their potency is at least 500-2000 times higher than short RGDX peptides. Analysis of the amino acid sequences of 14 different disintegrins suggests that the RGD sequence, in the spatial configuration determined by the appropriate pairing of the cysteine residues, functions as a cell recognition site. However, certain nonconserved amino acids appear to modify the activity of disintegrins, their specificity for various receptors, and their ability to compete specifically with various ligands.  相似文献
7.
8.
We report the purification of a presynaptic "particle web" consisting of approximately 50 nm pyramidally shaped particles interconnected by approximately 100 nm spaced fibrils. This is the "presynaptic grid" described in early EM studies. It is completely soluble above pH 8, but reconstitutes after dialysis against pH 6. Interestingly, reconstituted particles orient and bind PSDs asymmetrically. Mass spectrometry of purified web components reveals major proteins involved in the exocytosis of synaptic vesicles and in membrane retrieval. Our data support the idea that the CNS synaptic junction is organized by transmembrane adhesion molecules interlinked in the synaptic cleft, connected via their intracytoplasmic domains to the presynaptic web on one side and to the postsynaptic density on the other. The CNS synaptic junction may therefore be conceptualized as a complicated macromolecular scaffold that isostatically bridges two closely aligned plasma membranes.  相似文献
9.
The co-carcinogenic compound phorbol 12-myristate 13-acetate but not its inactive analogue 4 alpha-phorbol 12,13-didecanoate causes the phosphorylation of several rabbit neutrophil polypeptides whose molecular weights and isoelectric points (pI) are as follows: Mr = 40,000, pI = 6.4; Mr = 50,000, pI = 4.9; Mr = 55,000, pI = 6.3; Mr = 64,000, pI = 6.0; Mr = 70,000, pI = 5.6; Mr = 90,000, pI = 6.0. Most of these phosphorylated proteins are located exclusively in the cytosol; the 64,000 molecular weight protein is found both in the cytosol and the cytoskeleton, and the 40,000 molecular weight protein is found in the nuclear pellet. The 50,000 molecular weight protein is also phosphorylated in whole cells by the chemotactic peptide fMet-Leu-Phe and in cell-free systems by protein kinase C. Using limited proteolysis, one phosphopeptide fragment was phosphorylated by the three stimuli. In addition, phorbol 12-myristate 13-acetate but not 4 alpha-phorbol 12,13-didecanoate causes cell aggregation and the exocytotic release of the specific granules of rabbit neutrophils. In contrast, both compounds increase the amount of actin associated with the cytoskeleton. The divalent cation ionophore A23187 at low concentration and the compound phorbol 12-myristate 13-acetate act synergistically in causing neutrophil degranulation. Lysosomal enzyme release and the phosphorylation of the 50,000 molecular weight polypeptide produced by phorbl 12-myristate 13-acetate are inhibited by trifluoperazine, and these two responses seem to be causally related. These results are discussed in terms of the role of 1,2-diacylglycerol and activation of protein kinase C in specific granule release from rabbit neutrophils.  相似文献
10.
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