全文获取类型
收费全文 | 6948篇 |
免费 | 786篇 |
国内免费 | 1篇 |
出版年
2021年 | 75篇 |
2020年 | 64篇 |
2019年 | 67篇 |
2018年 | 100篇 |
2017年 | 78篇 |
2016年 | 122篇 |
2015年 | 200篇 |
2014年 | 214篇 |
2013年 | 323篇 |
2012年 | 380篇 |
2011年 | 344篇 |
2010年 | 239篇 |
2009年 | 224篇 |
2008年 | 316篇 |
2007年 | 329篇 |
2006年 | 274篇 |
2005年 | 260篇 |
2004年 | 282篇 |
2003年 | 244篇 |
2002年 | 259篇 |
2001年 | 158篇 |
2000年 | 191篇 |
1999年 | 155篇 |
1998年 | 107篇 |
1997年 | 75篇 |
1996年 | 72篇 |
1995年 | 63篇 |
1994年 | 50篇 |
1993年 | 61篇 |
1992年 | 136篇 |
1991年 | 119篇 |
1990年 | 163篇 |
1989年 | 101篇 |
1988年 | 109篇 |
1987年 | 106篇 |
1986年 | 93篇 |
1985年 | 94篇 |
1984年 | 84篇 |
1983年 | 61篇 |
1982年 | 93篇 |
1981年 | 77篇 |
1980年 | 72篇 |
1979年 | 89篇 |
1978年 | 77篇 |
1977年 | 72篇 |
1976年 | 54篇 |
1975年 | 68篇 |
1974年 | 55篇 |
1973年 | 55篇 |
1972年 | 53篇 |
排序方式: 共有7735条查询结果,搜索用时 15 毫秒
81.
Claudio Fortis Elisabetta Ferrero Mauro Biffi Silvia Heltai Carlo Besana Eraldo Bucci Moreno Tresoldi Claudio Rugarli 《Cancer immunology, immunotherapy : CII》1991,34(2):128-132
Summary Burkitt lymphoma (BL) lines can be grouped according to phenotypic characteristics. Group I cells exhibit the phenotype of resting B cells and grow as single cells. Such lines can be Epstein-Barr-virus(EBV)-negative or -positive. Group II and group III cells are always EBV-positive, they express B cell activation markers, grow in aggregates and resemble in varying degrees lymphoblastoid cell lines (LCL). We studied three groups of BL lines for their capacity to interact with allogeneic lymphocytes. The results showed that as long as the lines have the group I phenotype, they do not stimulate allogeneic T lymphocytes irrespective whether they carry the EBV genome. The group II and III cells are stimulatory. Generally there was no correlation between sensitivity to lymphocyte-mediated lysis and the phenotype of the lines. In one set of lines, the group I cells had higher sensitivity to both natural killer and lymphokine-activated killer effectors compared to the group II or III lines. However, such correlation could not be seen with the other two sets of lines. Among the phenotypic features investigated, expression of the adhesion molecules LFA-1 and LFA-3 correlated with the tendency for cell aggregation. 相似文献
82.
Diagnostic significance of coexpression of intermediate filaments in fine needle aspirates of human tumors 总被引:1,自引:0,他引:1
A study was undertaken of the diagnostic significance of the coexpression of intermediate filaments in fine needle aspirates of human tumors. Three types of coexpression were found: (1) true coexpression, in which tumor cells simultaneously express more than one intermediate filament protein; (2) pseudocoexpression, in which various tumor cell types from histogenetically different parts of a complex tumor show different results; and (3) false coexpression, in which tumor cells with one or two types of intermediate filaments are present together with benign cells expressing a different filament type. True coexpression of vimentin and keratin was documented in renal cell carcinomas, endometrial carcinomas, certain thyroid carcinomas and Hürthle cell adenomas. Coexpression of keratin and neurofilaments was seen in Merkel cell carcinomas, and coexpression of desmin and vimentin was found in leiomyosarcomas. Keratin, vimentin and neurofilament expression was seen in medullary thyroid carcinomas, and keratin, vimentin and glial fibrillary acidic protein expression was observed in pleomorphic adenomas of the salivary gland. Pseudocoexpression was noted in synovial sarcoma, epithelioid sarcoma, benign cystosarcoma phyllodes of the breast, teratocarcinoma, malignant granular cell tumor, progonoma, Wilms' tumor and triton tumor. Sources of false coexpression are also discussed. 相似文献
83.
Y Natsumeda Y Yamada Y Yamaji G Weber 《Biochemical and biophysical research communications》1988,153(1):321-327
Tiazofurin, an anti-cancer drug, which induces remissions in human leukemia, and ribavirin, an anti-viral agent, bind at separate sites (NADH and IMP-XMP sites, respectively) on the target enzyme, IMP dehydrogenase. Now we show that the binding to IMP dehydrogenase of these drugs at two separate sites is translated into synergistic inhibition of de novo guanylate biosynthesis and synergistic toxicity in rat hepatoma 3924A cells. These results may be utilized in the chemotherapy of neoplastic diseases and in the treatment of hepatitis virus infection and hepatocellular carcinoma. 相似文献
84.
Isolation of a domain of villin retaining calcium-dependent interaction with G-actin, but devoid of F-actin fragmenting activity 总被引:2,自引:0,他引:2
Villin is an F-actin binding protein located in the microfilament bundle of intestinal epithelial cell microvilli. Extensive in vitro proteolysis with Staphylococcus aureus V8 protease results in the production of a stable domain (apparent Mr 44000) which can be isolated due to its Ca2+-dependent interaction with G-actin bound to immobilized DNase-I, the standard procedure for the purification of villin. This 44-kDa fragment retains a single Ca2+ binding site with an apparent Kd = 2 X 10(-6) M, binds to G-actin, and inhibits the rate of actin polymerization. However, the 44-kDa domain does not shown any Ca2+-activated severing activity nor does it compete with villin for F-actin binding. These results suggest that villin contains three domains: headpiece containing an F-actin binding site, 44-kDa fragment containing a G-actin binding site, and an amino-terminal fragment responsible for the Ca2+-dependent severing activity. 相似文献
85.
Moderate concentrations of ethanol reduce the velocity of uptake of three representative Na+-symport systems (D-glucose, L-alanine, L-ascorbate), whether electrogenic (the first two) or electroneutral (L-ascorbate). This 'inhibition' is observed only if these transport systems are tested in the presence of an initial Na+ gradient (out greater than in); no inhibition is found in tracer-equilibrium exchange measurements. A representative Na+-independent system (D-fructose) is not inhibited by ethanol. 'Passive diffusion' (measured as uptake of L-glucose) is increased somewhat by alcohol. All these observations can be rationalized [as suggested by Tillotson et al. (1981) Arch. Biochem. Biophys. 207, 360-370] by an effect of ethanol on passive diffusion, which leads to a faster collapse of the Na+ gradient, with the resulting reduction of the uptake velocities of Na+-dependent transport systems when tested with the added driving force of an Na+ out----in gradient. 相似文献
86.
Biochemical and partial sequence data reveal the two-domain structure of p36. A loose structure of some 30 residues at the amino-terminus contains the phosphorylatable tyrosine and the binding site for the p11 regulatory chain. The following p33 domain retains the lipid-binding site as well as the Ca2+ site which influences the spectral properties of the single tryptophan and one tyrosine. The combined sequence data covering about 25% of the molecule identify p36 as a unique polypeptide. 相似文献
87.
A model of a stage-V monkey Sertoli cell was reconstructed from electron micrographs taken of semiserial sections. The configuration (type-B) was one in which spermatids were positioned near the lumen, their heads occupying shallow cylindrical recesses at the apical portion of the Sertoli cell. The cell volume was calculated to be 4,100 micron3, the surface area 2,400.68 micron2, and the surface-to-volume ratio 0.58:1. The reconstructed cell extended from the basal lamina to the tubular lumen and was generally of the tall columnar type although its surface contour was highly irregular. The dimensions of the cell [centripetal (68.46 micron), circumferential (18.40 micron), and longitudinal (21.63 micron)] were determined and cell surfaces designated. Relative and absolute surface areas of the reconstructed cell which faced other Sertoli cells, germ cells, basal lamina, and tubular lumen were calculated. Junctions and surface specializations were enumerated, catalogued, and depicted on diagrams of the cell surface. Where appropriate, type-A rat and type-B monkey Sertoli cells were compared and discussed. Morphometry was utilized to analyze the relative surface areas of germ cells adjoining the reconstructed cell to determine the percentage of their surface facing cellular and acellular elements, and these data were compared to data obtained for the rat. 相似文献
88.
S Burman J C Davis M J Weber B A Averill 《Biochemical and biophysical research communications》1986,136(2):490-497
Reaction of the reduced (pink) form of the purple acid phosphatase from beef spleen with excess phosphate at pH 5.0, monitored by optical and low temperature EPR spectroscopy and by measurement of enzymatic activity, results in parallel loss of activity and oxidation of the iron chromophore. Colorimetric and radiochemical (32P) experiments indicate the presence of one mole of tightly bound phosphate in the oxidized (purple) form of the enzyme; this phosphate is released upon reduction. Acid hydrolysis of 32P-phosphate-containing enzyme, followed by high voltage paper electrophoresis, gave no evidence for significant amounts of acid-stable phosphoamino acids. 相似文献
89.
Chromophoric [4-(dimethylamino)cinnamoyl]imidazole reacts with the serine protease alpha-chymotrypsin to form an acyl enzyme. At pHs below 4.0, the acyl enzyme turns over very slowly to yield the free acid. During this slow deacylation it is possible to obtain a very good resonance Raman spectrum of the acyl intermediate by using the 350.7-nm line of the krypton laser. The resonance Raman carbonyl frequency of the covalently bonded substrate and its wavelength at maximum intensity in the absorption spectrum of the acyl enzyme have been taken and used to monitor the active site environment. A comparison has been made of the absorption and Raman spectra of the acyl enzyme and those of the corresponding chromophoric methyl ester, aldehyde, and imidazole model compounds. A linear correlation is found between the wavelength of maximum absorption and the Raman frequency of the carbonyl group over a wide range of solvent conditions for each of the model compounds. By combining the Raman carbonyl frequency with the absorption maximum, we can determine that the bond order changes in the carbonyl bond of the bound substrate are not due to changes in the solvent, since the carbonyl frequency and the absorption maximum of the acyl enzyme do not fall on any of the linear correlations for the model compounds. The unusual spectroscopic properties of the bound substrate appear to be due to some specific enzyme-induced change in the substrate when it is bound at the active site. Thermal unfolding of the acyl enzymes changes both the carbonyl frequency of the acyl enzyme and its absorption maximum to completely different values.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
90.
Catherine Ferrand Dominique Clarous Christine Delteil Michel J. Weber 《Journal of neurochemistry》1986,46(2):349-358
The secretion and cellular localization of the molecular forms of acetylcholinesterase (AChE) were studied in primary cultures of rat sympathetic neurons. When cultured under conditions favoring a noradrenergic phenotype, these neurons synthesized and secreted large quantities of the tetrameric G4, and the dodecameric A12 forms, and minor amounts of the G1 and G2 forms. When these neurons adopted the cholinergic phenotype, i.e., in the presence of muscle-conditioned medium, the development of the cellular A12 form was completely inhibited. These neurons secreted only globular, mainly G4, AChE. Both cellular and secreted A12 AChE in adrenergic cultures aggregated at an ionic strength similar to that of the culture medium, raising the hypothesis that this form was associated with a polyanionic component of basal lamina. In noradrenergic neurons, 60-80% of the catalytic sites were exposed at the cell surface. In particular, 80% of G4 form, but only 60% of the A12 form, was external, demonstrating for the A12 form a sizeable intracellular pool. The hydrophobic character of the molecular forms was studied in relation to their cellular localization. As in muscle cells, most of the G4 form was membrane-bound. Whereas 76% of the cell surface A12 form was solubilized in the aqueous phase by high salt concentrations, only 50% of the intracellular A12 form was solubilized under these conditions. The rest of intracellular A12 could be solubilized by detergents and was thus either membrane-bound or entrapped in vesicles originating from, e.g., the Golgi apparatus. 相似文献