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   2926篇
  免费   180篇
  国内免费   1篇
  3107篇
  2022年   16篇
  2021年   38篇
  2020年   16篇
  2019年   19篇
  2018年   32篇
  2017年   32篇
  2016年   51篇
  2015年   117篇
  2014年   135篇
  2013年   196篇
  2012年   172篇
  2011年   184篇
  2010年   119篇
  2009年   137篇
  2008年   178篇
  2007年   176篇
  2006年   203篇
  2005年   202篇
  2004年   228篇
  2003年   189篇
  2002年   163篇
  2001年   36篇
  2000年   35篇
  1999年   45篇
  1998年   45篇
  1997年   33篇
  1996年   23篇
  1995年   27篇
  1994年   22篇
  1993年   23篇
  1992年   19篇
  1991年   15篇
  1990年   18篇
  1989年   15篇
  1988年   19篇
  1987年   8篇
  1986年   10篇
  1985年   14篇
  1984年   10篇
  1983年   8篇
  1982年   13篇
  1981年   15篇
  1979年   8篇
  1978年   8篇
  1977年   4篇
  1976年   8篇
  1973年   3篇
  1971年   3篇
  1968年   2篇
  1966年   3篇
排序方式: 共有3107条查询结果,搜索用时 0 毫秒
1.
2.
Amino acid sequence of the nonsecretory ribonuclease of human urine   总被引:7,自引:0,他引:7  
The amino acid sequence of a nonsecretory ribonuclease isolated from human urine was determined except for the identity of the residue at position 7. Sequence information indicates that the ribonucleases of human liver and spleen and an eosinophil-derived neurotoxin are identical or very closely related gene products. The sequence is identical at about 30% of the amino acid positions with those of all of the secreted mammalian ribonucleases for which information is available. Identical residues include active-site residues histidine-12, histidine-119, and lysine-41, other residues known to be important for substrate binding and catalytic activity, and all eight half-cystine residues common to these enzymes. Major differences include a deletion of six residues in the (so-called) S-peptide loop, insertions of two, and nine residues, respectively, in three other external loops of the molecule, and an addition of three residues at the amino terminus. The sequence shows the human nonsecretory ribonuclease to belong to the same ribonuclease superfamily as the mammalian secretory ribonucleases, turtle pancreatic ribonuclease, and human angiogenin. Sequence data suggest that a gene duplication occurred in an ancient vertebrate ancestor; one branch led to the nonsecretory ribonuclease, while the other branch led to a second duplication, with one line leading to the secretory ribonucleases (in mammals) and the second line leading to pancreatic ribonuclease in turtle and an angiogenic factor in mammals (human angiogenin). The nonsecretory ribonuclease has five short carbohydrate chains attached via asparagine residues at the surface of the molecule; these chains may have been shortened by exoglycosidase action.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
3.
In the course of study on the mechanism of the tartaric acid formation from 5-ketogluconic acid, a new intermediary substance with mauve color to Abdel-Akhel and Smith’s reagent was isolated from intact cell culture liquid. The chemical structure of this substance was determined as 1,2-dihydroxyethyl hydrogen L(+) tartrate from the results of hydrolysis experiments and from the identifications of the constituents of the molecule, and named “pretaric acid.” Tartaric acid was evidently produced from pretaric acid by intact cell culture. Clearly, then, pretaric acid appears to be an intermediate in the formation of tartaric acid from 5-ketogluconic acid. The authors assumed that in the formation of pretaric acid from 5-ketogluconic acid, a Baeyer-Villiger type oxidation occurred.  相似文献   
4.
Summary Two-dimensional gel electrophoresis, at high and low temperatures, and gel mobilities of circularly permuted DNA segments showed a large bending locus about 50 bp downstream from the right border of the 245 by oriC box, a minimal essential region of autonomous replication on the Escherichia coli chromosome. Bending was strongly enhanced by Dam methylation. In DNA from a Dam strain, the mobility anomaly arising from altered conformation was much reduced, but was raised to the original level by methylation in vivo or in vitro. Enhancement of the mobility anomaly was also observed by hybrid formation of the Dam strand with the Dam+ strand. Near the bending center, GATC, the target of Dam methylase, occurs seven times arranged essentially on the same face of the helix with 10.5 by per turn. We concluded that small bends at each Dam site added up to the large bending detectable by gel electrophoresis.  相似文献   
5.
To isolate and identify the plasma factor which stimulates prostaglandin I 2 production by rat aortic ring, a human plasma fraction which showed a major stimulating activity on prostaglandin I 2 production was purified by ultrafiltrate, Sephadex G-10 gel filtration and QAE-Sephadex column chromatography. The purified plasma factor was identified as acid by its ultraviolet and infrared absorption spectroscopy, and 1H nmr and 13C nmr spectroscopy. The stimulating activity of the purified plasma factor and that of authentic uric acid coincided with each other. The stimulating potency of uric acid at its physiological concentration in human plasma (about 50 μg/ml) was half of the deproteinized human plasma, and was about 30 fold stronger than that of L-tryptophan, a cofactor of prostaglandin hyperoxidase.  相似文献   
6.
Antipsychotics represent high affinity for sigma receptors and sigma-like drugs often have the psychotomimetic properties. Besides, the receptors are unevenly distributed in human brain. These findings suggest that sigma receptors might be involved in the pathophysiology of schizophrenia. Sigma receptors in rat and human brain were measured with [3H]-1, 3, di-o-tolylguanidine (DTG) and non-specific binding of [3H]DTG was determined in the presence of 10–5M haloperidol. Monovalent and divalent cations strongly inhibited [3H]DTG binding. Glutamate, aspartate and glycine also decreased the binding to human cerebral membranes. With post-mortem brain samples from 12 schizophrenics and 10 controls, sigma receptors were measured in 17 areas of cerebral cortex. Sigma receptors binding showed the regional differences in the cortex, but no significant differences between schizophrenics and controls were observed except the superior parietal cortex where the binding significantly increased in the schizophrenic group. These results suggest that sigma receptors in cerebral cortices might not be directly concerned with the pathophysiological role in schizophrenia.Dedicated to Dr. Morris Aprison. Received too late for publication in special issue.  相似文献   
7.
8.
The effects of 4-bromo-5-(3-ethoxy-4-methoxybenzylamino)-3(2H)-pyridazinone (NZ-107) on immediate type hypersensitivity reactions in rats and guinea-pigs were studied. 1. When NZ-107, at a dose of 50 mg/kg (i.p.) or 100 mg/kg (orally), was administered to rats, 48-h homologous passive cutaneous anaphylaxis (PCA) reaction and histamine-, leukotriene C4 (LTC4)- and leukotriene D4 (LTD4)-induced skin reactions were suppressed by the agent. 2. NZ-107 (10(-6) g/ml) inhibited both LTC4- and LTD4-induced contractions of isolated rat stomach smooth muscle. 3. NZ-107 inhibited antigen-induced histamine release from rat peritoneal mast cells by 26% at a concentration of 10(-4) g/ml. 4. NZ-107, at doses of 25 and 50 mg/kg (orally), significantly inhibited guinea-pig 3-h heterologous PCA reaction. 5. NZ-107 inhibited antigen-induced histamine release from guinea-pig lung tissue by 17% and 48% at concentrations of 5 x 10(-5) and 10(-4) g/ml, respectively. 6. NZ-107, at doses of 25 and 50 mg/kg (i.p.), inhibited antigen-induced bronchoconstriction and eosinophil accumulation in the bronchoalveolar lavage fluid (BALF) of guinea-pigs. These results suggest that NZ-107 has anti-allergic action including inhibition of eosinophil accumulation in an antigen-challenged airway lesion in rats and guinea-pigs. The anti-allergic action of this agent is thought to be due to its action as a histamine and LT antagonist and its consequent inhibition of antigen-induced histamine release.  相似文献   
9.
Considerable inherent variations in the relation between macropterous and brachypterous wing forms, and nymphal density were found in field populations of the brown planthopper, Nilaparvata lugens Stål (Homoptera: Delphacidae), collected from various locations in Japan. When compared under uniform laboratory rearing conditions, most of the female populations exhibited higher ratios of macropters with increasing nymphal density, but some showed extremely high proportion of brachypters and the others were highly macropterous, over broad ranges of density. These results indicate the possibility that the planthoppers in Japan, which are known not to persist in winter, are derived from different migration sources.About ten generations of successive selection for brachyptery from a population showing usual density-dependent wing morphism generated populations similar to highly brachypterous ones mentioned above. Genetic analysis of the inheritance of wing morphism revealed that brachyptery in the females was controlled by a single pair of dominant alleles. However, in the males wing forms did not segregate so clearly in the crossing experiments. This suggests that wing morphism in N. lugens in under sex-limited inheritance.
Einwanderung von Nilaparvata lugens mit unterscheidlicher Reaktion auf Populationsdischte bei der Flügelausbildung
Zusammenfassung In Feldpopulationen von Nilaparvata lugens Stål., welche in verschiedenen Regionen Japans gesammelt wurden, bestand in der Beziehung zwischen makropteren bzw. brachypteren Flügelformen und der Larvendichte eine beträchtliche Variation. Unter einheitlichen Zuchtbedingungen im Laboratorium stieg der Makropterenanteil bei den meisten Weibchenpopulationen mit steigender Temperatur; bei einigen Populationen hingegen war entweder der Brachypterenanteil oder der Makropterenanteil extrem hoch und zwar über weite Dichtebereiche. Dies deutet auf die Möglichkeit hin, dass die Zikade in Japan, wo sie bekanntlich nicht überwintert, jeweils aus verschiedenen Quellen einwandert.Wenn eine Population mit der üblichen dichteabhängigen Flügelausbildung 10 Generationen lang auf Brachypterie selektioniert wurde, entstanden Populationen, die den erwähnten hochbrachypteren Populationen aus dem Feld glichen. Die genetische Analyse der Vererbung der Brachypterie ergab, dass bei Weibchen ein einzelnes dominantes Allel verantwortlich ist. Bei Männchen dagegen trennten sich bei Kreuzungsexperimenten die Flügelformen nicht so klar. Dies deuted auf Unterschiede zwischen den Geschlechtern bei der Vererbung der Flügelformen.
  相似文献   
10.
The carboxyl group in a ribonuclease from Rhizopus sp. (RNase Rh) was modified by a water-soluble carbodiimide, 1-cyclohexyl-3-(2-morpholinyl-(4)-ethyl)carbodiimide p-toluenesulfonate (CMC). From the relation between the extent of modification and the enzymatic activity, it was concluded that at least the modification of two carboxyl groups seemed to induce the loss in enzymatic activity. In the presence of 1 M cytidine, RNase Rh activity was protected from the CMC-modification. Under conditions in which the enzyme was inactivated to 20% activity, about 70% of the enzymatic activity was retained in the presence of cytidine. The inactivation of the RNase Rh pre-treated with CMC in the presence of cytidine with [14C]CMC indicated that the RNase Rh lost its enzymatic activity with the incorporation of about one [14C]CMC. Therefore, it could be concluded that one carboxyl group is involved in the active site of RNase Rh. The binding of the CMC-modified RNase Rh with 2'-AMP was studied spectrophotometrically. The affinity of the modified RNase Rh towards 2'-AMP decreased markedly upon CMC modification.  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