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   392篇
  免费   59篇
  2019年   4篇
  2017年   5篇
  2016年   8篇
  2015年   14篇
  2014年   14篇
  2013年   13篇
  2012年   13篇
  2011年   12篇
  2010年   9篇
  2009年   4篇
  2008年   11篇
  2007年   5篇
  2006年   11篇
  2005年   18篇
  2004年   10篇
  2003年   10篇
  2002年   10篇
  2001年   5篇
  2000年   9篇
  1999年   10篇
  1998年   7篇
  1997年   7篇
  1996年   13篇
  1995年   6篇
  1993年   6篇
  1992年   4篇
  1991年   12篇
  1990年   11篇
  1989年   7篇
  1988年   7篇
  1987年   12篇
  1986年   8篇
  1985年   9篇
  1984年   8篇
  1983年   4篇
  1982年   4篇
  1981年   8篇
  1980年   4篇
  1979年   4篇
  1978年   8篇
  1977年   10篇
  1975年   8篇
  1974年   17篇
  1973年   8篇
  1972年   9篇
  1971年   8篇
  1970年   6篇
  1969年   8篇
  1968年   8篇
  1966年   6篇
排序方式: 共有451条查询结果,搜索用时 31 毫秒
101.
1. A rapid procedure, involving ion-exchange chromatography on DEAE-cellulose and affinity chromatography on GTP-Sepharose, was used to purify glutamate dehydrogenase from ox brain and liver. 2. Preparations purified in this way differed from those of the ox liver enzyme that were obtained from commercial suppliers in their mobilities on polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate. This difference appears to result from the occurrence of limited proteolysis during the preparation of the latter enzyme samples. 3. N-Terminal sequence analysis showed the presence of four amino acid residues in the enzyme prepared in this study that were not present in those obtained from the commercial sources and which have not been reported in previous studies on the sequence of the ox liver enzyme. 4. A preliminary examination of the enzyme prepared in this way indicated that the Michaelis constants for the substrates are similar to those obtained from the commercial preparation, but that the response to allosteric effectors was modified.  相似文献   
102.
1. Treatment of a partly purified preparation of rat liver monoamine oxidase with the chaotropic agent sodium perchlorate caused the enzyme to migrate as a single band of activity of polyacrylamide-gel electrophoresis, whereas the untreated enzyme separated into a number of bands. 2. Treatment with the chaotropic agent caused no loss of enzyme activity towards benzylamine, dopamine or tyramine. 3. The activities of the untreated preparation towards different substrates were inhibited to different extents by heat treatment and by some inhibitors. No such differences could be detected after the enzyme preparation had been treated with sodium perchlorate. 4. Lipid material, which could be separated by gel filtration, was liberated from the enzyme preparation by sodium perchlorate treatment. 5. The molecular weight of the treated enzyme was found to be 380000+/-38000. 6. Perchlorate treatment altered the solubility of the enzyme. 7. A continuous assay method for monoamine oxidase is described.  相似文献   
103.
Negatively co-operative ligand binding   总被引:9,自引:8,他引:1       下载免费PDF全文
Simple systems are considered in which the binding of a ligand at a single site exhibits a doubly sigmoid curve when saturation is plotted against the logarithm of ligand concentration, i.e. where a fraction of the site exhibits one dissociation constant and the rest exhibits another. The condition for this to occur is that the ligand should also combine at another site on the binding molecule with comparable affinity and that the binding at one site should markedly lower affinity at the other. The protonation of simple compounds such as cysteine and 3-hydroxypyridine is taken as an example of this process and the equations derived are also applied to the binding of substrates to enzymes.  相似文献   
104.
On the phospholipids of Bacillus megaterium   总被引:12,自引:0,他引:12  
  相似文献   
105.
106.
A series of cholesterol hydroperoxides has been prepared and tested as inactivators of calmodulin. Two previously undescribed compounds, tentatively identified as 20-(R)-25-dihydroperoxy-5-cholesten-3 beta-ol and its 20-(S) isomer inactivate calmodulin with 50% loss of activity at 5-10 microM. Cholesterol derivatives with a single hydroperoxy group at C-20 or C-25 are less effective, while 7 alpha-hydroperoxy-cholesterol and 25-hydroxy-cholesterol are inactive. The side-chain hydroperoxide compounds were isolated from a mixture shown earlier to suppress formation of fatty streaks in aortas of rabbits fed a diet supplemented with cholesterol.  相似文献   
107.
Maximum oxygen consumption (VO2max) has been shown to be reduced after periods of simulated weightlessness. To assess the role of the sympathetic nervous system in these reductions, Sprague-Dawley rats were either chemically sympathectomized (SYMX) or injected with saline (SHAM) and assigned to head-down suspension (HDS), horizontal restraint with the hindlimbs weight bearing (HWB), or cage-control (CC) conditions. VO2max, run time (RT), and mechanical efficiency (ME) were measured before suspension and on days 7 and 14. Male and female SHAM HDS groups exhibited reduced measures of VO2max (12-13%) after 7 and 14 days, and this decrease was attenuated in the SYMX and HWB rats. HDS resulted in a significant reduction in RT (9-15%) in both the male and female rats, and ME was significantly reduced after HDS in male and female SYMX and male SHAM rats (23-33%) but not in the female SHAM rats. Lesser reductions in ME were observed in the HWB rats. HDS and HWB were associated with lower body, fat-free, and fat masses, which were similar in male and female rats as well as for the SHAM and SYMX conditions. In a related HDS experiment with normal rats, plasma norepinephrine and epinephrine were increased by 53 and 42% after 7 days, but only epinephrine returned to baseline after 14 days. It was concluded that chemical sympathectomy and/or a weight-bearing stimulus will attenuate the loss in VO2max associated with simulated weightlessness in rats despite similar changes in body mass and composition. The mechanism(s) remains unclear at this time.  相似文献   
108.
To test the hypothesis that endurance training would attenuate the carotid sinus baroreflex in rats, studies were undertaken with 25 nontrained (NT) and 22 trained (T) male Sprague-Dawley rats that were exercised for 11-14 wk. Maximal O2 consumption was significantly increased 10% after training. The left carotid sinus region was functionally isolated in anesthetized animals. Subsequently, static carotid sinus pressure was raised in 20-Torr increments from 95 Torr until a maximal response in systemic arterial pressure and regional blood flows was recorded. Compared with the NT group, baroreflex control of blood pressure and calculated regional resistance of the T animals was less responsive to changes in carotid sinus pressure. Resting blood pressure, heart rate, and changes in peripheral blood flow velocity were similar for the two groups. Peripheral sensitivity to phenylephrine-HCl and hexamethonium bromide were also similar in the T and NT groups. It was concluded that the arterial baroreflex control of blood pressure was attenuated by exercise training. These findings support the concept that the trained individual is at disadvantage during hypotensive episodes and that endurance training will attenuate the sympathetic component of the arterial baroreflex.  相似文献   
109.
The phi-screen, a method of phylogenetic screening, can be employed to detect repetitive sequence families that differentially hybridize between closely related species. Such differences may involve sequence divergence or variations in copy number, including total presence versus absence of a family of repeated DNA. We present the results of a phi-screen comparing the human genome to that of the prosimian, Galago crassicaudatus. Three human repetitive families that are divergent or not present in galago have been detected. One of these families is described in detail; it is similar among the anthropoids but is present in a lower copy number and/or divergent form in prosimians. The family is clearly related to the transposon-like human element (THE) described by Paulson et al. (1985). THEs have long terminal repeats reminiscent of retroviruses but are unique in that they have no sequence similarity to known mammalian retroviruses. The sequence of a solo long terminal repeat, found unassociated with THE internal sequence, is presented. This family member, THE p2, is bordered by a 5-bp target-site repeat and is interrupted by the insertion of an Alu element. A solo THE element sequenced by Wiginton et al. (1986) contains an insertion of Alu at precisely the same position as does THE p2.   相似文献   
110.
The alcohol dehydrogenase (Adh) region from five planitibia subgroup species of Hawaiian picture-wing Drosophila has been cloned. A total of 15 kb of DNA in and around the Adh gene has been compared among the five species. Genetic distances were calculated to determine evolutionary relationships. These distances agree with previous distances determined by protein polymorphism and DNA hybridization techniques and can be interpreted in terms of specific island colonization and speciation (founder) events over the past 5 Myr. Examination of the restriction maps of the cloned Adh region from the five species shows many instances of small deletions, insertion of a transposable element in D. heteroneura, and the existence of a highly variable region on the 3' side of the Adh gene. Clustering relationships and rates of DNA change are calculated and compared with the relationship found for other species of Drosophila.   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