全文获取类型
收费全文 | 3522篇 |
免费 | 203篇 |
国内免费 | 1篇 |
专业分类
3726篇 |
出版年
2021年 | 24篇 |
2020年 | 18篇 |
2019年 | 20篇 |
2018年 | 35篇 |
2017年 | 31篇 |
2016年 | 45篇 |
2015年 | 97篇 |
2014年 | 105篇 |
2013年 | 189篇 |
2012年 | 160篇 |
2011年 | 184篇 |
2010年 | 98篇 |
2009年 | 111篇 |
2008年 | 170篇 |
2007年 | 145篇 |
2006年 | 185篇 |
2005年 | 162篇 |
2004年 | 163篇 |
2003年 | 178篇 |
2002年 | 174篇 |
2001年 | 110篇 |
2000年 | 101篇 |
1999年 | 87篇 |
1998年 | 58篇 |
1997年 | 52篇 |
1996年 | 44篇 |
1995年 | 39篇 |
1994年 | 37篇 |
1993年 | 34篇 |
1992年 | 71篇 |
1991年 | 52篇 |
1990年 | 43篇 |
1989年 | 51篇 |
1988年 | 44篇 |
1987年 | 45篇 |
1986年 | 47篇 |
1985年 | 54篇 |
1984年 | 45篇 |
1983年 | 35篇 |
1982年 | 36篇 |
1981年 | 38篇 |
1980年 | 25篇 |
1979年 | 42篇 |
1978年 | 20篇 |
1976年 | 27篇 |
1975年 | 19篇 |
1974年 | 26篇 |
1973年 | 20篇 |
1972年 | 20篇 |
1971年 | 16篇 |
排序方式: 共有3726条查询结果,搜索用时 0 毫秒
1.
We have previously shown that replacing the P1-site residue (Ala) of chicken ovomucoid domain 3 (OMCHI3) with a Met or Lys results in the acquisition of inhibitory activity toward chymotrypsin or trypsin, respectively. However, the inhibitory activities thus induced are not strong. In the present study, we introduced additional amino acid replacements around the reactive site to try to make the P1-site mutants more effective inhibitors of chymotrypsin or trypsin. The amino acid replacement Asp-->Tyr at the P2' site of OMCHI3(P1Met) resulted in conversion to a 35000-fold more effective inhibitor of chymotrypsin with an inhibitor constant (K(i)) of 1. 17x10(-11) M. The K(i) value of OMCHI3(P1Met, P2'Ala) indicated that the effect on the interaction with chymotrypsin of removing a negative charge from the P2' site was greater than that of introducing an aromatic ring. Similarly, enhanced inhibition of trypsin was observed when the Asp-->Tyr replacement was introduced into the P2' site of OMCHI3(P1Lys). Two additional replacements, Asp-->Ala at the P4 site and Arg-->Ala at the P3' site, made the mutant a more effective inhibitor of trypsin with a K(i) value of 1. 44x10(-9) M. By contrast, Arg-->Ala replacement at the P3' site of OMCHI3(P1Met, P2'Tyr) resulted in a greatly reduced inhibition of chymotrypsin, and Asp-->Ala replacement at the P4 site produced only a small change when compared with a natural variant of OMCHI3. These results clearly indicate that not only the P1-site residue but also the characteristics, particularly the electrostatic properties, of the amino acid residues around the reactive site of the protease inhibitor determine the strength of its interactions with proteases. Furthermore, amino acids with different characteristics are required around the reactive site for strong inhibition of chymotrypsin and trypsin. 相似文献
2.
3.
Yusuke Nakamura Michio Ogawa Takahiro Nishide Mitsuru Emi Goro Kosaki Seiichi Himeno Kenichi Matsubara 《Gene》1984,28(2):263-270
The nucleotide sequences of the cloned human salivary and pancreatic α-amylase cDNAs correspond to the continuous mRNA sequences of 1768 and 1566 nucleotides, respectively. These include all of the amino acid coding regions. Salivary cDNA contains 200 bp in the 5′-noncoding region and 32 in the 3′-noncoding region. Pancreatic cDNA contains 3 and 27 bp of 5′- and 3′-noncoding regions, respectively. The nucleotide sequence humology of the two cDNAs is 96% in the coding region, and the predicted amino acid sequences are 94% homologous.Comparison of the sequences of human α-amylase cDNAs with those previously obtained for mouse α-amylase genes (Hagenbuchle et al., 1980; Schibler et al., 1982) showed the possibility of gene conversion between the two genes of human α-amylase. 相似文献
4.
The activities of glutamic oxaloacetic transaminase and Ca++ ion-activated ATPase of muscle in the adult rats fed a protein-free diet for 8, 16 and 24 days were measured in order to clarify their metabolic responses with respect to reserve proteins. It was found that these enzyme activities, or presumably their enzyme proteins, decreased at the stage as early as the 8th day of protein depletion following the same pattern as seen in reserve proteins. Their responses, particularly those in unit activity, were somewhat different from each other. The metabolic significance of those responses was discussed in relation to protein nutrition. 相似文献
5.
Centrifugal filtration through a double layer of silicone oil was applied to determine the intermembrane space of organelles enclosed by double envelope membranes, i.e. proplastids, chloroplasts, mitochondria and amyloplasts. The organelles, capable of transporting adenylates by an adenylate translocator located in the inner envelope membrane, were incubated with increasing concentrations of adenylates while maintaining their specific radioactivities constant. Intermembrane spaces were estimated by extrapolation of radioactivities recovered after filtration of the organelles. The values estimated were compared to those obtained employing the classical method measuring the intake of [14C]-sucrose and [14C]-sorbitol which are impermeable to the inner membranes of organelles. The intermembrane space determined by the present method was shown to be uniformly smaller than the sucrose-permeable space which was always smaller than the sorbitol-permeable space. 相似文献
6.
7.
Feedbacks between nutrient cycling and vegetation predict plant species coexistence and invasion 总被引:5,自引:0,他引:5
To investigate the role of species‐specific litter decomposability in determining plant community structure, we constructed a theoretical model of the codevelopmental dynamics of soil and vegetation. This model incorporates feedback between vegetation and soil. Vegetation changes the nutrient conditions of soil by affecting mineralization processes; soil, in turn, has an impact on plant community structure. The model shows that species‐level traits (decomposability, reproductive and competitive abilities) determine whether litter feedback effects are positive or negative. The feedback determines community‐level properties, such as species composition and community stability against invasion. The model predicts that positive feedback may generate multiple alternative steady states of the plant community, which differ in species richness or community composition. In such cases, the realized state is determined by initial abundance of co‐occurring species. Further, the model shows that the importance of species‐level traits depends on environmental conditions such as system fertility. 相似文献
8.
The rate and period of DNA synthesis during meiotic prophasewere examined using lily microsporocytes. Meiocytes at the earlyleptotene stage were cultured for discrete periods in the presenceof inhibitors of DNA synthesis, deoxyadenosine and nalidixicacid. Deoxyadenosine, which arrests meiotic development at theearly zygotene stage, markedly suppressed DNA synthesis to 35%of control at 2 mM. Nalidixic acid simply reduced the rate ofDNA synthesis, resulting in prolongation of the synthetic period.The relevance of DNA synthesis to meiotic development is discussed. (Received January 12, 1987; Accepted May 7, 1987) 相似文献
9.
Primary structures of two types of alpha-subunit of rat brain Na+,K+,-ATPase deduced from cDNA sequences 总被引:2,自引:0,他引:2
Y Hara O Urayama K Kawakami H Nojima H Nagamune T Kojima T Ohta K Nagano M Nakao 《Journal of biochemistry》1987,102(1):43-58
A rat brain cDNA library was screened by using as a probe a fragment of cDNA encoding the alpha-subunit of human Na+,K+-ATPase. Two different cDNA clones were obtained and analyzed. One of them was concluded to be a cDNA encoding the alpha-subunit of the weakly ouabain-sensitive rat kidney-type Na+,K+-ATPase. The deduced amino acid sequence consists of 1,018 amino acids. The alpha-subunit of the rat kidney-type Na+,K+-ATPase shows 97% homology in amino acid sequence with the alpha-subunit of human, sheep, or pig enzyme and 87% with that of Torpedo. Based on a comparison of the amino acid sequence at the extracellular domain of the alpha-subunit between weakly ouabain-sensitive rat kidney-type enzyme and the ouabain-sensitive human, sheep, pig, or Torpedo enzyme, it was proposed that only two significant amino acid replacements are unique to the rat kidney-type alpha-subunit. Another cDNA clone obtained showed 72% homology in nucleotide sequence with the former cDNA coding the alpha-subunit of the rat kidney-type Na+,K+-ATPase and the deduced amino acid sequence exhibited 85% homology with that of the alpha-subunit of rat kidney-type Na+,K+-ATPase. 相似文献
10.
Porcine polymorphonuclear leukocyte NADPH-cytochrome c reductase generates superoxide in the presence of cytochrome b559 and phospholipid 总被引:1,自引:0,他引:1
F Sakane H Kojima K Takahashi J Koyama 《Biochemical and biophysical research communications》1987,147(1):71-77
NADPH-cytochrome c reductase and cytochrome b559 were purified from the membrane fraction of phorbol myristate acetate-stimulated porcine polymorphonuclear leukocytes. The highly purified reductase oxidized NADPH and generated superoxide when combined with partially purified cytochrome b559 in the presence of phosphatidylcholine. In the same system, but under the anaerobic condition, the reductase was found to reduce cytochrome b559. 相似文献