全文获取类型
收费全文 | 4772篇 |
免费 | 500篇 |
国内免费 | 1篇 |
出版年
2021年 | 64篇 |
2019年 | 42篇 |
2017年 | 40篇 |
2016年 | 70篇 |
2015年 | 152篇 |
2014年 | 156篇 |
2013年 | 249篇 |
2012年 | 262篇 |
2011年 | 275篇 |
2010年 | 166篇 |
2009年 | 144篇 |
2008年 | 220篇 |
2007年 | 217篇 |
2006年 | 236篇 |
2005年 | 243篇 |
2004年 | 221篇 |
2003年 | 194篇 |
2002年 | 201篇 |
2001年 | 107篇 |
2000年 | 105篇 |
1999年 | 111篇 |
1998年 | 67篇 |
1997年 | 53篇 |
1996年 | 46篇 |
1995年 | 48篇 |
1994年 | 44篇 |
1993年 | 60篇 |
1992年 | 77篇 |
1991年 | 73篇 |
1990年 | 64篇 |
1989年 | 79篇 |
1988年 | 59篇 |
1987年 | 68篇 |
1986年 | 66篇 |
1985年 | 55篇 |
1984年 | 82篇 |
1983年 | 52篇 |
1982年 | 50篇 |
1981年 | 42篇 |
1980年 | 42篇 |
1979年 | 53篇 |
1978年 | 46篇 |
1977年 | 39篇 |
1976年 | 39篇 |
1975年 | 54篇 |
1974年 | 46篇 |
1973年 | 37篇 |
1970年 | 36篇 |
1969年 | 30篇 |
1966年 | 34篇 |
排序方式: 共有5273条查询结果,搜索用时 15 毫秒
151.
Purification and characterization of fatty acid beta-oxidation enzymes from Caulobacter crescentus.
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Acetoacetyl coenzyme A (acetoacetyl-CoA) thiolase, an enzyme required for short-chain fatty acid degradation, has been purified to near homogeneity from Caulobacter crescentus. The relative heat stability of this enzyme allowed it to be separated from beta-ketoacyl-CoA thiolase. The purification scheme minus the heating step also permitted the copurification of crotonase and 3-hydroxyacyl-CoA dehydrogenase. These activities are in a multienzyme complex in Escherichia coli, but a similar complex was not observed in C. crescentus. Instead, separate proteins differing in enzymatic activity were detected, analogous to the beta-oxidation enzymes that have been isolated from Clostridium acetobutylicum and from mitochondria of higher eucaryotes. In these cells, as appears to be the case with C. crescentus, the individual enzymes form multimers of identical subunits. 相似文献
152.
G B Quistad S Suwanrumpha M A Jarema M J Shapiro W S Skinner G C Jamieson A Lui E W Fu 《Biochemical and biophysical research communications》1990,169(1):51-56
The structures are given for five paralytic acylpolyamines from the venom of the funnel web spider, Agelenopsis aperta. The acyl moieties are derived from (3-indolyl)acetic acid, (4-hydroxy-3-indolyl)acetic acid, and 4-hydroxybenzoic acid. The polyamine portions of the toxins are novel. Three toxins (AG489, AG505, and AG452) contain 1, 5, 9, 13, 18, 22-hexaazadocosane which is unique as a natural polyamine because of its length and hydroxylation at the 5-aza position. The polyamine portions of two other alpha-agatoxins (AG488 and AG504) are unusual also, containing guanidinooxy moieties. 相似文献
153.
A membrane-associated form of phosphate-dependent glutaminase was derived from sonicated mitochondria and purified essentially free of gamma-glutamyl transpeptidase activity. Increasing concentrations of phosphate cause a sigmoidal activation of the membrane-bound glutaminase. Phosphate also causes a similar effect on the rate of glutaminase inactivation by the two affinity labels, L-2-amino-4-oxo-5-chloropentanoic acid and 6-diazo-5-oxo-L-norleucine, as observed previously for the solubilized and purified enzyme. Therefore the two forms of glutaminase undergo similar phosphate-induced changes in conformation. A sensitive radioactive assay was developed and used to determine the kinetics of glutamate inhibition of the membrane-associated glutaminase. The Km for glutamine decreases from 36 to 4 mM when the phosphate concentration is increased from 5 to 100 mM. Glutamate is a competitive inhibitor with respect to glutamine at both high and low concentrations of phosphate. However, the Ki for glutamate is increased from 5 to 52 mM with increasing phosphate concentration. Therefore glutamine and glutamate interact with the same site on the glutaminase, but the specificity of the site is determined by the available phosphate concentration. 相似文献
154.
The ability to visualize nucleic acid secondary structure has become quite important since the advent of computer prediction and biochemical techniques that depict such structures. Manually drawing the conformations can be quite time consuming and tedious. Thus, the ability to draw with the aid of a computer the secondary structure of nucleic acid molecules is quite advantageous. This paper describes an interactive algorithm that permits one to generate such drawings which may then be used for further analysis and/or publications. 相似文献
155.
Data on Gilles de la Tourette syndrome are analyzed by multiple threshold models in inheritance that incorporate sex effect. The polygenic-multifactorial model is rejected. Single major locus inheritance can account for the data, although many of the occurrences of Tourette are due to nongenetic phenocopies. In both models, males and females share a common genetic environmental liability, but the less prevalent sex, that is, females, has a higher genetic loading for the disorder. The predicted population prevalences in the single major locus model are 2.3% for males and 0.8% for females. The implications for genetic and biological research in Tourette syndrome are discussed. 相似文献
156.
Cerebroside sulfatase activator deficiency induced metachromatic leukodystrophy 总被引:16,自引:2,他引:14
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
R L Stevens A L Fluharty H Kihara M M Kaback L J Shapiro B Marsh K Sandhoff G Fischer 《American journal of human genetics》1981,33(6):900-906
Two siblings of consanguineous parents had presented with a variety of findings indicative of juvenile metachromatic leukodystrophy (MLD). However, instead of the expected profound deficiency of arylsulfatase A (ARS A), their enzyme levels were about half-normal, and enzyme from fibroblasts had properties identical with the properties of enzyme from normal fibroblasts. Nevertheless, the hydrolysis of cerebroside sulfate by growing fibroblasts was markedly attenuated. Supplementation of the fibroblasts with cerebroside sulfatase activator normalized the response in the loading test. These results imply that the fibroblasts, and by extension the patients, are deficient in activator. Although the defective catabolism of cerebroside sulfate and the clinical manifestations in these patients mimic MLD, the molecular basis is distinct from the classical forms of the disorder. 相似文献
157.
Glucosephosphate isomerase (EC 5.3.1.9) of Schistosoma mansoni is inhibited competitively by a number of tetrose, pentose, and hexose phosphates with inhibitor constant (Ki) values in the range of 0.5 to 400 μM. The most potent inhibitor is 5-phospho-d-arabinonate which resembles the cis-enediolate transition state intermediate of the reaction. These analogs were also found to be effective inhibitors of the production of lactate from glucose by suitably supplemented worm homogenates. The rank order of potency of inhibition of glycolysis was inversely related to the magnitudes of the Ki values for glucosephosphate isomerase. These Ki values were similar to those previously reported for mammalian glucosephosphate isomerase, suggesting similarities in the steric and electronic characteristics of the active sites of these isofunctional enzymes. This conclusion was further supported by the observed pH dependence of the inhibition by 5-phospho-d-arabinonate. Although glucosephosphate isomerase is not a rate-limiting enzyme of glycolysis, in the conventional sense, its selective inhibition could be of chemotherapeutic importance, in part because of the accumulation in glycolyzing systems of glucose 6-phosphate which is a potent feedback inhibitor of hexokinase. 相似文献
158.
Mildred Gordon Eugene G. Morris Freddy Stark William L. Williams Ron J. Young 《Molecular reproduction and development》1982,6(4):353-363
Epididymal sperm of the mouse, rat, and guinea pig and ejaculated sperm of rabbits are cleaved at the head-tail junction by an extract of Nereis virens. Annelids are extracted with water and the extract is purified by ion exchange chromatography. Electron microscopy shows that the extract acts on the filaments connecting the capitulum of the tail with the basal plate lining the nuclear envelope. Following detachment, the basal plate remains with the head. The extract contains proteases as indicated by hydrolysis of tosyl arginine methyl ester (TAME), benzoyl arginine ethyl ester (BAEE), and Azocoll, a general protease substrate. The hydrolysis of TAME is inhibited by tosyl lysine chloromethyl ketone (TLCK), a trypsin inhibitor, but TLCK does not prevent head-tail separation by the Nereis extract. Similarly tosyl phenylalanine chloromethyl ketone (TPCK), a chymotrypsin inhibitor, and phosphoramidon and leucyltryptophan, both thermolysin and acrolysin inhibitors — singly or in combination — do not prevent hydrolysis of Azocoll. Head-tail separation activity of the extract was inhibited by dithiothreitol, which reduces disulfide bonds, and phenylmethyl sulfonyl fluoride, an inhibitor of serine proteases. These results indicate that the extract is a mixture of proteases, one being a serine protease similar to trypsin. Digestion of the connecting filaments with the pure proteases, trypsin and Staphylococcus aureus V8 protease, has yielded the following information on the proteins of the filaments. The accessibility of arginine and/or lysine peptide bonds to enzyme action is highest in rat sperm filaments, whereas those in the filaments of mouse, rabbit, and guinea pig sperm are less accessible than in the rat. Another possibility is that the total content of arginine and/or lysine varies between the species. The most dramatic difference is the enzymatic action on glutamyl peptide bonds of the filaments, the order being: mouse 〉 rat 〉 rabbit, with guinea pig sperm filaments completely resistant over the time course of the experiment. 相似文献
159.
Changes in cell dimensions during amino acid starvation of Escherichia coli. 总被引:3,自引:3,他引:0
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Electron microscopic analysis was used to study cells of Escherichia coli B and K-12 during and after amino acid starvation. The results confirmed our previous conclusion that cell division and initiation of DNA replication occur at a smaller cell volume after amino acid starvation. Although during short starvation periods, the number of constricting cells decreased due to residual division, it appears that during prolonged starvation, cells of E. coli B and K-12 were capable of initiating new constrictions. During amino acid starvation, cell diameter decreased significantly. The decrease was reversed only after two generation times after the resumption of protein synthesis and was larger in magnitude than that previously observed before division (F. J. Trueba and C. L. Woldringh, J. Bacteriol. 142:869-878, 1980). This decrease in cell diameter correlates with synchronization of cell division which has been shown to occur after amino acid starvation. 相似文献
160.
Tethered, flying crickets perform stereotyped steering movements when stimulated with sound. Electrophysiological recordings show that identifiable motoneurones respond to acoustic stimulation in a manner which reflects the phonotactic behaviour of the intact cricket. Both steering behaviour and its neural correlates reflect the fine temporal structure of the acoustic stimulus. Decapitation of the cricket eliminates steering movements in response to sound. 相似文献