全文获取类型
收费全文 | 6496篇 |
免费 | 534篇 |
国内免费 | 3篇 |
出版年
2023年 | 32篇 |
2021年 | 112篇 |
2020年 | 80篇 |
2019年 | 94篇 |
2018年 | 115篇 |
2017年 | 111篇 |
2016年 | 165篇 |
2015年 | 284篇 |
2014年 | 285篇 |
2013年 | 364篇 |
2012年 | 462篇 |
2011年 | 433篇 |
2010年 | 338篇 |
2009年 | 267篇 |
2008年 | 384篇 |
2007年 | 352篇 |
2006年 | 318篇 |
2005年 | 327篇 |
2004年 | 276篇 |
2003年 | 254篇 |
2002年 | 265篇 |
2001年 | 106篇 |
2000年 | 74篇 |
1999年 | 94篇 |
1998年 | 96篇 |
1997年 | 63篇 |
1996年 | 74篇 |
1995年 | 56篇 |
1994年 | 55篇 |
1993年 | 63篇 |
1992年 | 61篇 |
1991年 | 67篇 |
1990年 | 51篇 |
1989年 | 50篇 |
1988年 | 48篇 |
1987年 | 58篇 |
1986年 | 47篇 |
1985年 | 57篇 |
1984年 | 37篇 |
1983年 | 29篇 |
1982年 | 41篇 |
1981年 | 27篇 |
1980年 | 34篇 |
1979年 | 28篇 |
1978年 | 29篇 |
1977年 | 31篇 |
1976年 | 38篇 |
1974年 | 30篇 |
1973年 | 26篇 |
1971年 | 28篇 |
排序方式: 共有7033条查询结果,搜索用时 500 毫秒
41.
Human alpha 2-macroglobulin (alpha 2M) of Mr approximately 720,000 is a proteinase inhibitor whose four identical subunits are arranged to form two adjacent inhibitory units. At present, the spatial arrangement of the two subunits which form one inhibitory unit (the functional "half-molecule") is not known. Treatment of alpha 2M with either 0.5 mM dithiothreitol (DTT) or 4 M urea results in dissociation of the native tetramer into two half-molecules of Mr approximately 360,000. These half-molecules retain trypsin inhibitory activity, but in each case, the reaction results in reassociation of the half-molecules to produce tetramers of Mr approximately 720,000. However, when reacted with plasmin, the preparations of half-molecules have different properties. DTT-induced half-molecules protect the activity of plasmin from inhibition by soybean trypsin inhibitor (STI) without reassociation, while urea-induced half-molecules show no ability to protect plasmin from reaction with STI. High-performance size-exclusion chromatography and sedimentation velocity ultracentrifugation studies were then used to estimate the Stokes radius (Re) of alpha 2M and both DTT- and urea-induced half-molecules of alpha 2M. The Re of tetrameric alpha 2M was 88-94 A, while that of DTT-induced half-molecules was 57-60 A and urea-induced half-molecules 75-77 A. These results demonstrate that DTT- and urea-induced half-molecules have fundamentally different molecular dimensions as well as inhibitory properties. The hydrodynamic data suggest that the urea-induced half-molecule is a "rod"-like structure, although it is not possible to predict the three-dimensional structure of this molecule with the available data.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
42.
Robin L. Clark 《Hydrobiologia》1986,143(1):63-69
Pollen analysis is widely used to reconstruct vegetation and land use histories, but can also provide sedimentological information. At Burrinjuck Reservoir, in south-eastern Australia, annual grass pollen peaks are used to distinguish each year's sediment, even when there are no visible laminations. In conjunction with other dating methods, this allows the determination of year by year influxes of all sediment components. Pollen grains in the Burrinjuck sediments are shown to be predominantly waterborne so that they can be used to trace sediment to its source in particular vegetation stands. Pollen concentration and the proportion of damaged pollen might also distinguish sediment eroded from topsoils and that from subsoils. Pollen analysis can thus be used to locate specific erosion events in both time and space. 相似文献
43.
Summary The leaf and root nitrate reductase activities were measured in 7 day-old barley seedlings by anoxic nitrite accumulation in darkness, during 48h after the transfer from a N-starved medium to a 1.5 mM K15NO3 medium. Thisin situ nitrate reduction was compared with the15N incorporation in the reduced N fraction of the whole seedlings.The nitrate reduction integrated fromin situ measurements was lower than the reduced15N accumulation. The rootin situ nitrate reductase activity seemed to account for only the third of the real root nitrate reduction, which may have been responsible for the overall underestimation. This discrepancy was partly explained by the ability of the root to reduce nitrite in an anoxic environment.These results suggest that, after correction of thein situ estimation of the nitrate reduction. the roots contribute to about 50% of the total assimilation. 相似文献
44.
M Rahmatullah J M Jilka G A Radke T E Roche 《The Journal of biological chemistry》1986,261(14):6515-6523
Studies were conducted on four pyruvate dehydrogenase kinase-containing fractions: purified pyruvate dehydrogenase complex, the dihydrolipoyl transacetylase-protein X-kinase subcomplex (E2.X.K), a kinase fraction (K fraction) prepared from the E2.X.K subcomplex, and a kinase fraction generated by limited trypsin-digestion of E2.X.K. We characterized the gel electrophoresis properties of dissociated subunits (one-dimensional and two-dimensional), the catalytic and ATP binding properties of kinase-containing fractions, and the subunit requirements for kinase binding to and being activated by the transacetylase-protein X subcomplex (E2.X). A significant portion of protein X was retained with the transacetylase core following release of virtually all the kinase. The K fraction had four major bands separated by sodium dodecyl sulfate-slab gel electrophoresis which corresponded to the dihydrolipoyl dehydrogenase, protein X, the trypsin-resistant catalytic subunit of the kinase and a chymotrypsin-resistant subunit which had a high pI and comigrated in one-dimensional systems with the chymotrypsin-sensitive alpha-subunit of the pyruvate dehydrogenase component. While purified kidney complex contained only about three molecules of kinase (determined by [14C]ATP binding), one molecule of E2.X subcomplex activated a large number (greater than 15) molecules of kinase associated with the protein X-containing K fraction. Sephadex G-200 chromatography of the K fraction in the presence of dithiothreitol led to coelution of protein X and kinase subunits. Limited trypsin digestion converted the transacetylase into subdomains and cleaved protein X and the high pI subunit of the kinase. Under those conditions, the intact catalytic subunit of the kinase did not bind to the large inner domain of the transacetylase but could be activated by untreated E2.X subcomplex. Thus, binding of the catalytic subunit of the kinase and its activation by E2.X required either protein X or the lipoyl-bearing outer domain of the transacetylase. In combination, our results suggest that protein X serves to anchor the kinase to the core of the complex. 相似文献
45.
John J. Langone Chandra Das Robin Mainwaring William T. Shearer 《Molecular and cellular biochemistry》1985,65(2):159-170
Summary Protein A of Staphylococcus aureus is an Fc receptor for IgG that has been used as a therapeutic reagent to treat cancer in humans and experimental animals. We used ultracentrifugation combined with analysis of isolated fractions by radioimmunoprecipitation and competitive radioimmunoassay with chicken antibodies that bind free protein A or protein A in complexes but do bind free immunoglobulin reagents to localize and characterize the types of complexes formed with different molar ratios of 125I-protein A and human 131I-IgG alone or in serum, and 1311-Fc fragments. This approach offers a distinct advantage over direct counting of radioactivity in the fractions because resolution of complexes and free reagents is much improved. With excess 131I-IgG or 1311-Fc, all the 125I-protein A is present only in complexes that contained 4 molecules of immunoglobulin reagent and 2 molecules of protein A (4:2 complexes), whereas with excess 125I-protein A the stoichiometry of the complexes was 1:1. We have also shown the preformed 4:2 and 1:1 complexes will interconvert in the presence of added excess protein A or IgG, respectively, and that fresh IgG will exchange with IgG or Fc in preformed complexes. Because protein A has been found to elute from an immobilized reagent used in serotherapy of human cancer and is present in a large excess of IgG, the 4:2 complexes may play an active role in the tumoricidal or toxic reactions observed.Abbreviations SpA
protein A of Staphyloccus aureus
- VBS
EDTA gel, 0.0055 M veronal buffered saline containing 0.01 M EDTA and 0.1% gelatin, pH 7.4
- PBS
0.01 M phosphate buffered saline, pH 7.4 相似文献
46.
Eugene D Robin 《BMJ (Clinical research ed.)》1985,290(6477):1259-1260
47.
Micrococcal nuclease digestion and light scattering are used to compare native chromatins with various histone H1[0] contents. The experimental data show that the higher the H1[0] content, the greater the ability to form compact structures with increasing ionic strength, and the lower the DNA accessibility to micrococcal nuclease. On the contrary, reconstituted samples from H1-depleted chromatin and pure individual H1 fractions behave in such a way that samples reconstituted with pure H1 degree give rise to a looser structure, more accessible to nuclease than samples reconstituted with H1-1. This contradiction suggests that the effect of H1o on chromatin structure must originate from the interaction of this histone with other components in native chromatin among which other histone H1 subfractions are good candidates. 相似文献
48.
Thierry Maillet Annie-Claude Roche Fabienne Thérain Michel Monsigny 《Cancer immunology, immunotherapy : CII》1985,19(3):177-182
Summary In vivo localization of a mouse monoclonal antibody (F2-10.23 IgM) binding leukemic L 1210 cells was studied in DBA/2 mice bearing an L 1210 tumor. F(ab)2 fragments were prepared and their specific binding to L 1210 cells was analyzed by flow cytofluorometry. Radiolocalization studies were performed by using 125I- or 131I-labeled IgM monoclonal antibody or its F(ab')2 fragments to ascertain their capacity to visualize the L 1210 tumor. F(ab)2 fragments were cleared more rapidly than the whole IgM; the clearance was as fast in healthy as in tumor-bearing mice. The tumor-to-muscle ratio observed 24 h after injection of 125I-radiolabeled F(ab)2 fragments and 125I-radiolabeled IgM was 10; the radioactivity level in the blood with F(ab)2 fragments was lower than with IgM, and so -camera imaging was workable with F(ab)2 fragments without background substraction. The tumor localization was studied over a period of 5 days by recording the distribution of the iodinated fragments in the tumor-bearing leg compared with that in the normal leg, and by computer analysis of the region of interest. F(ab)2 fragments gave better results than intact IgM in tumor visualization. Nevertheless, the rapid clearance of this antibody or its F(ab)2 fragments make them hardly suitable as carriers of toxic drugs.
Abbreviations used are: MEM Minimum essential medium; SDS sodium dodecylsulfate; PAGE polyacrylamide gel electrophoresis 相似文献
49.
Aileen M. Hyde Robert B. Stagg Robin McEachron Robert L. Nutter 《Cancer immunology, immunotherapy : CII》1985,20(2):97-102
Summary Cell-mediated immunity was investigated in two BALB/c mouse tumor systems using the lymphoblastogenesis test with phytohemagglutinin as the mitogen. This lymphoproliferative response was quantitated using the Stimulation Index (SI). There was little evidence for suppressor cell activity in cell mixing experiments in which spleen cells from #51 cell-injected mice were mixed with spleen cells from normal mice. Following macrophage removal by Sephadex G-10 columns and carbonyl iron ingestion, there were no significant changes in the SI values for spleen cells from the #51 cell-injected mice. In contrast, spleen cells from mice injected with H238 cells, a herpes virus-transformed cell line, had a significantly lower SI value than that of normal mice. Suppressor cell activity was demonstrated in cell mixing experiments in which spleen cells from H238 cell-injected mice were mixed with normal spleen cells. Removal of adherent cells from spleen cells from H238 cell-injected mice by Sephadex G-10 columns restored the SI value to that of normal mice. An increased SI value was also seen after removal of phagocytic cells by carbonyl iron. These results suggested that cells with the functional properties of macrophages played an important part in the immunosuppression observed in the H238 tumor system. Comparison of the two macrophage depletion methods suggested that another cell population was also involved in the suppressive effect. Results of immunofluorescent techniques with anti-Lyt-1 and anti-Lyt-2 monoclonal antibodies show these cells to be Ly 1–, Ly 2,3+ phenotypes of T-lymphocytes. 相似文献
50.
P Braquet N Senn M Fagoo R Garay J P Robin A Esanu E Chabrier T Godfrain 《Comptes rendus de l'Académie des sciences. Série III, Sciences de la vie》1986,302(12):443-448
Lignans are natural products, some of which were recently discovered in animal urines, semen and blood plasma. We investigated the actions of animal lignans obtained by total synthesis or extracted from urines of pregnant women on Na+, K+-ATPase in human red cells and human and guinea-pig heart cell membranes. Some of the tested lignans (enterolactone, prestegane B and 3-O-methyl enterolactone) inhibited Na+, K+-pump activity in human red cells with IC50 ranging from 5 to 9 X 10(-4) M. The IC50 for ouabain (7 X 10(-7) M) was not modified by addition of lignans. Enterolactone inhibited Na+, K+-ATPase activity in human and guinea pig heart membranes. It also displaced [3H]-ouabain binding from human heart with IC50 = 1.5 X 10(-4) M. The apparent dissociation rate constants (kd) of [3H]-ouabain were not different in presence of digoxin or enterolactone. Enterolactone exhibited a poor cross reactivity against antidigoxin antibodies. The aglycones of the lignans studied here were slight inhibitors of the Na+, K+-ATPase. However, we cannot exclude that a glycosyl- (and/or butenolide-) derivative of enterolactone could be one "endogenous ouabain-like" factor. 相似文献