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   425篇
  免费   48篇
  2021年   5篇
  2020年   4篇
  2017年   4篇
  2016年   8篇
  2015年   13篇
  2014年   13篇
  2013年   10篇
  2012年   23篇
  2011年   22篇
  2010年   12篇
  2009年   18篇
  2008年   21篇
  2007年   22篇
  2006年   20篇
  2005年   15篇
  2004年   15篇
  2003年   22篇
  2002年   19篇
  2001年   13篇
  2000年   18篇
  1999年   10篇
  1998年   16篇
  1997年   7篇
  1996年   5篇
  1994年   11篇
  1992年   12篇
  1991年   7篇
  1990年   7篇
  1989年   5篇
  1988年   4篇
  1987年   4篇
  1986年   5篇
  1985年   7篇
  1984年   2篇
  1983年   4篇
  1982年   5篇
  1981年   2篇
  1980年   3篇
  1979年   9篇
  1978年   5篇
  1977年   3篇
  1976年   2篇
  1974年   3篇
  1973年   2篇
  1972年   2篇
  1971年   6篇
  1969年   5篇
  1967年   2篇
  1966年   3篇
  1965年   2篇
排序方式: 共有473条查询结果,搜索用时 15 毫秒
101.
The B cell-restricted antigen HD39, whose cell surface expression is limited to resting and activated human B lymphocytes, is described in this report. The monoclonal antibody HD39 detects a two-chain glycoprotein with apparent molecular weights of 130,000 and 140,000. During B cell ontogeny, HD39 is first expressed in the cytoplasm of bone marrow derived pre-B cells, then appears on the cell surface of sIgM+ B cells, and finally on the majority of sIgM+ sIgD+ resting B cells. After activation in vitro, the expression of HD39 on the cell surface first increases, and then the antigen is lost as cells begin to differentiate. HD39 is weakly expressed on very few non-T cell ALL and B cell CLL, on approximately 50% of B cell lymphomas, and not on Waldenstr?m's macroglobulinemias and myelomas. In contrast, it is strongly expressed on all hairy cell leukemias. Its limited cell surface expression in B cell ontogeny suggests that HD39 may be important in the events that regulate the activation of the human resting B lymphocytes.  相似文献   
102.
103.
Feller  Urs 《Plant & cell physiology》1981,22(6):1095-1104
Endopeptidase activity against azocasein had a higher temperatureoptimum (50°C) in leaf extracts than in cotyledon extracts(37°C). The temperature optima for aminopeptidase (46°C)and for carboxypeptidase (53°C) were similar in leaf andcotyledon extracts. The endopeptidase activity showed an excellentstability in crude extracts from leaves even at 37°C, whilethe endopeptidase in cotyledon extracts was less stable. Carboxypeptidasewas very stable in both leaf and cotyledon extracts. Aminopeptidasewas the least stable of the enzymes investigated and its inactivationrate depended on the source of the extract. A moderate stabilitywas observed in extracts of leaves or of ungerminated seeds,but this enzyme was rapidly inactivated in cotyledon extractsat pH 5.4. At pH 7.5 aminopeptidase remained active longer thanat pH 5.4. From experiments with mixed extracts it could beconcluded that in cotyledons an aminopeptidase inactivatingfactor was formed during germination. This factor was heat sensitive,excluded by Sephadex G-25, precipitated by 75% ammonium sulfateand inhibited by tosyl-L-lysine chloromethyl ketone. These datasuggest that the factor is a protein and considering the similarproperties it appears possible that it is the endopeptidaseformed during germination. (Received May 15, 1981; Accepted July 18, 1981)  相似文献   
104.
105.
Affinities of the catalytic subunit (C1) of Saccharomyces cerevisiae cAMP-dependent protein kinase and of mammalian cGMP-dependent protein kinase were determined for the protein kinase inhibitor (PKI) peptide PKI(6-22)amide and seven analogues. These analogues contained structural alterations in the N-terminal alpha-helix, the C-terminal pseudosubstrate portion, or the central connecting region of the PKI peptide. In all cases, the PKI peptides were appreciably less active as inhibitors of yeast C1 than of mammalian C alpha subunit. Ki values ranged from 5- to 290-fold higher for the yeast enzyme than for its mammalian counterpart. Consistent with these results, yeast C1 exhibited a higher Km for the peptide substrate Kemptide. All of the PKI peptides were even less active against the mammalian cGMP-dependent protein kinase than toward yeast cAMP-dependent protein kinase, and Kemptide was a poorer substrate for the former enzyme. Alignment of amino acid sequences of these homologous protein kinases around residues in the active site of mammalian C alpha subunit known to interact with determinants in the PKI peptide [Knighton, D. R., Zheng, J., Ten Eyck, L. F., Xuong, N-h, Taylor, S. S., & Sowadski, J. M. (1991) Science 253, 414-420] provides a structural basis for the inherently lower affinities of yeast C1 and cGMP-dependent protein kinase for binding peptide inhibitors and substrates. Both yeast cAMP-dependent and mammalian cGMP-dependent protein kinases are missing two of the three acidic residues that interact with arginine-18 in the pseudosubstrate portion of PKI. Further, the cGMP-dependent protein kinase appears to completely lack the hydrophobic/aromatic pocket that recognizes the important phenylalanine-10 residue in the N-terminus of the PKI peptide, and binding of the inhibitor by the yeast protein kinase at this site appears to be partially compromised.  相似文献   
106.
The lip2 gene from the antarctic psychotroph Moraxella TA144 was sequenced. The primary structure of the Lip2 preprotein deduced from the nucleotide sequence is composed of 433 amino acids with a predicted Mr of 47,222. This enzyme contains a Ser-centered consensus sequence and a conserved His-Gly dipeptide found in most lipase amino-terminal domains. These sequences are involved in the lipase active site conformation since substitution of the conserved Ser or His residues by Ala and Gln, respectively, results in the loss of both lipase and esterase activities. Structural factors that would allow proper enzyme flexibility at low temperatures are discussed. It is suggested that only subtle changes in the primary structure of these psychrotrophic enzymes can account for their ability to catalyze lipolysis at temperatures close to 0 degrees C.  相似文献   
107.
The lipid modified human N-Ras protein, implicated in human cancer development, is of particular interest due to its membrane anchor that determines the activity and subcellular location of the protein. Previous solid-state NMR investigations indicated that this membrane anchor is highly dynamic, which may be indicative of backbone conformational flexibility. This article aims to address if a dynamic exchange between three structural models exist that had been determined previously. We applied a combination of solid-state nuclear magnetic resonance (NMR) methods and replica exchange molecular dynamics (MD) simulations using a Ras peptide that represents the terminal seven amino acids of the human N-Ras protein. Analysis of correlations between the conformations of individual amino acids revealed that Cys 181 and Met 182 undergo collective conformational exchange. Two major structures constituting about 60% of all conformations could be identified. The two conformations found in the simulation are in rapid exchange, which gives rise to low backbone order parameters and nuclear spin relaxation as measured by experimental NMR methods. These parameters were also determined from two 300 ns conventional MD simulations, providing very good agreement with the experimental data.  相似文献   
108.
Steam-girdling experiments with detached wheat shoots showed that cesium was eliminated from the xylem sap and loaded into the phloem during acropetal transport. This transfer is important for the accumulation of cesium (especially also of the radiopollutants 134Cs and 137Cs) in maturing wheat grains.  相似文献   
109.
The light-regulated chloroplast enzyme phosphoribulokinase (EC 2.7.1. 19) exists in two forms. In darkness this enzyme is present in an oxidized form, which is inactive. It is activated in the light by a thioredoxin-mediated reduction. In extracts from young wheat leaves (Triticum aeestivum L.) phosphoribulokinase as well as some other thioredoxin-modulated enzymes can be activated by the artificial reductant dithiothreitol (DTT). The influence of the activation status and of the substrate ATP on phosphoribulokinase stability was investigated in the presence of endogenous endopeptidases from senescing wheat leaves. Similar experiments were performed with purified phosphoribulokinase from spinach in the presence of exogenous, purified endopeptidases (chymotrypsin and trypsin). Phosphoribulokinase stability was analysed by immunoblotting and activity measurements. Both systems led to similar conclusions. DTT (reductant and ATP (substrate) stabilized phosphoribulokinase in wheat leaf extracts as well as partially purified phosphoribulokinase from spinach. The combination of both effectors was far more protective than either effector alone. DTT had hardly any effect on the degradation of thioredoxin-independent chloroplast enzymes such as glutamate synthase and glutamine synthetase. These results suggest that the activation status and substrate concentrations are not only important for the activity of phosphoribulokinase, but are also relevant for the susceptibility of this enzyme to proteolysis.  相似文献   
110.
Feller  Urs 《Plant & cell physiology》1979,20(8):1577-1583
Nitrogen mobilization and the pattern of proteolytic enzymeswere investigated in leaves and glumes of field-grown winterwheat (Triticum aestivum L.) during maturation. Source/sinkrelations were changed by removal of the ear, the flag leafor the lower leaves shortly after anthesis. Removal of the earwas most effective, resulting in delayed senescence of the flagleaf with the chlorophyll, aminopeptidase and carboxypeptidaseactivities remaining high in contrast to the control, whereasneutral endopeptidase activity increased more slowly. No majorchanges were observed in the second leaf from the top in plantswith either ears or flag leaves removed. Nitrogen mobilizationand proteolytic activities in glumes and the remaining leaveswere influenced only slightly by leaf removal. In earless plants,nitrogen was transported from the second leaf into the leafsheath and stem, but in the flag leaf the total reduced nitrogenremained high and free amino groups increased. The increase in endopeptidase activity was influenced by thesource/sink relations. However, the accumulation of amino groupsand the increasing endopeptidase activity in the flag leaf ofearless plants suggest that the nitrogen sink capacity did notgreatly control protein degradation; it remains to be seen whetherphytohormones, accumulated amino acids or other factors delayedthe increase in endopeptidase activity. (Received September 3, 1979; )  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