全文获取类型
收费全文 | 2921篇 |
免费 | 326篇 |
国内免费 | 27篇 |
出版年
2023年 | 15篇 |
2022年 | 23篇 |
2021年 | 41篇 |
2020年 | 35篇 |
2019年 | 52篇 |
2018年 | 68篇 |
2017年 | 46篇 |
2016年 | 72篇 |
2015年 | 154篇 |
2014年 | 148篇 |
2013年 | 182篇 |
2012年 | 224篇 |
2011年 | 203篇 |
2010年 | 124篇 |
2009年 | 112篇 |
2008年 | 125篇 |
2007年 | 115篇 |
2006年 | 136篇 |
2005年 | 134篇 |
2004年 | 123篇 |
2003年 | 111篇 |
2002年 | 87篇 |
2001年 | 91篇 |
2000年 | 93篇 |
1999年 | 62篇 |
1998年 | 31篇 |
1997年 | 29篇 |
1996年 | 31篇 |
1994年 | 22篇 |
1992年 | 43篇 |
1991年 | 46篇 |
1990年 | 36篇 |
1989年 | 36篇 |
1988年 | 31篇 |
1987年 | 36篇 |
1986年 | 28篇 |
1985年 | 18篇 |
1984年 | 30篇 |
1983年 | 31篇 |
1982年 | 25篇 |
1981年 | 17篇 |
1980年 | 14篇 |
1979年 | 18篇 |
1978年 | 23篇 |
1977年 | 16篇 |
1976年 | 19篇 |
1975年 | 13篇 |
1974年 | 12篇 |
1973年 | 15篇 |
1972年 | 11篇 |
排序方式: 共有3274条查询结果,搜索用时 15 毫秒
71.
We have studied LTA4 and LTB4 synthesis in a cell-free system from RBL-1 cells. All the enzymes leading to the formation of LTB4 from arachidonic acid are localized in the soluble fraction (100, 000 x g supernatant) of these cells. The formation of LTA4 and LTB4 is complete by 10 min. When we varied the arachidonic acid concentration from 1 to 300 μM, the synthesis of LTB4 leveled off at 30 μM and of LTA4 at 100 μM while 5-HETE had not reached a plateau at 300 μM. This enzyme system has the capacity to generate relatively large amounts of 5-HETE and LTA4 and only a relatively small amount of LTB4. Therefore, the rate limiting step is not the 5-lipoxygenase, the first step in the pathway, but the conversion of LTA4 to LTB4. This is in contrast to cyclooxygenase pathway where the first step is rate limiting. A second addition of arachidonic acid at submaximal concentration for LTA4 synthesis did not produce any additional LTA4 or LTB4. Further study of this phenomenon showed that the 5-lipoxygenase and LTA-synthase were inactivated with time by preincubation with arachidonic acid and that peroxy fatty acids seem to be the inactivating species. 相似文献
72.
We investigated the effects of the non-phorbol tumor promoter okadaic acid on human leukemia K562 cells. It was found that okadaic acid potently and reversibly inhibited cell growth, with a nearly complete inhibition of thymidine uptake seen at about 10 nM. The cytotoxicity of okadaic acid was characterized by a marked mitotic arrest of the cells exhibiting scattered chromosomes and abnormal anaphase-like structures, a phenomenon distinct from the typical metaphase arrest caused by colchicine. Okadaic acid (10-1,000 nM) greatly stimulated phosphorylation of a number of nuclear proteins in K562 cells. Phosphorylation of many of the same proteins was also stimulated by 12-O-tetradecanoylphorbol-13-O-acetate, a protein kinase C activator. The present findings, consistent with recent reports that okadaic acid is a potent inhibitor of protein phosphatases 1 and 2A (PP1 and PP2A) shown to be essential for normal mitosis, provided evidence for the first time that okadaic acid inhibition of PP1/PP2A resulted in enhanced nuclear protein phosphorylation and subsequent mitotic arrest. 相似文献
73.
Membrane interactions of tetradecapeptide toxin mastoparan (MP) and analogues (MP-3, MP-X and polistes MP), as indicated by inhibition of various enzymatic and cellular activities, were investigated. MP-3 was found to be the least active in inhibiting protein kinase C (PKC; activated by phosphatidylserine vesicles, synaptosomal membranes or phorbol ester), synaptosomal membrane Na,K-ATPase and proliferation and viability of leukemia HL60 cells. MP-3, however, was as active as others in inhibiting PKC activated by arachidonate monomers and phorbol ester binding. The unique properties of MP-3, the [des-Ile1-Asn2]-analogue of MP, might be related to its low functional amphiphilicity compared to others and useful in further delineating biological activities associated with or regulated by membranes. 相似文献
74.
G A Omburo J M Kuo L S Mullins F M Raushel 《The Journal of biological chemistry》1992,267(19):13278-13283
The bacterial phosphotriesterase has been found to require a divalent cation for enzymatic activity. This enzyme catalyzes the detoxification of organophosphorus insecticides and nerve agents. In an Escherichia coli expression system significantly higher concentrations of active enzyme could be produced when 1.0 mM concentrations of Mn2+, Co2+, Ni2+, and Cd2+ were included in the growth medium. The isolated enzymes contained up to 2 equivalents of these metal ions as determined by atomic absorption spectroscopy. The catalytic activity of the various metal enzyme derivatives was lost upon incubation with EDTA, 1,10-phenanthroline, and 8-hydroxyquinoline-5-sulfonic acid. Protection against inactivation by metal chelation was afforded by the binding of competitive inhibitors, suggesting that at least one metal is at or near the active site. Apoenzyme was prepared by incubation of the phosphotriesterase with beta-mercaptoethanol and EDTA for 2 days. Full recovery of enzymatic activity could be obtained by incubation of the apoenzyme with 2 equivalents of Zn2+, Co2+, Ni2+, Cd2+, or Mn2+. The 113Cd NMR spectrum of enzyme containing 2 equivalents of 113Cd2+ showed two resonances at 120 and 215 ppm downfield from Cd(ClO4)2. The NMR data are consistent with nitrogen (histidine) and oxygen ligands to the metal centers. 相似文献
75.
A universal T cell epitope-containing peptide from hepatitis B surface antigen can enhance antibody specific for HIV gp120. 总被引:4,自引:0,他引:4
J L Greenstein V C Schad W H Goodwin A B Brauer B K Bollinger R D Chin M C Kuo 《Journal of immunology (Baltimore, Md. : 1950)》1992,148(12):3970-3977
Peptide-based vaccines that directly target T cell or B cell epitopes may have significant advantages over conventional vaccines. Further, synthetic chimeric peptides that combine strong T cell epitopes with poorly immunogenic, but immunodominant, B cell epitopes or strain-conserved B cell epitopes may be useful in eliciting antibody to such important regions. Here we characterize a human T cell epitope analyzed in 54 individuals immunized with a hepatitis B virus surface Ag vaccine. Primary cultures from a total of 59 immunized donors were assessed for their ability to respond to hepatitis B virus surface Ag and peptides, and five were non-responders (8.5%). T cell lines were established from the remaining 54 responders. Of the responders, it was found that the peptide representing amino acids 19 through 33 (19-33) elicited significant proliferation in lines derived from 50 donors. This "universal" T cell epitope, which was recognized in donors of many different HLA-DR and -DQ haplotypes, was then used to construct a chimeric peptide containing 19-33 and the third V region loop structure (V3 loop) of HIV-1 envelope gp 120, in an attempt to augment the immune response to the V3 loop peptide. The V3 loop is the region to which significant neutralizing antibody is directed. Thus, a strong immune response to a synthetic peptide that contains the strain-conserved V3 loop region could have significant therapeutic implications. The V3 loop/19-33 peptide was then used to prime mice, to determine whether V3 loop-specific antibody could be induced. The peptide elicited potent 19-33-specific proliferation in T cells isolated from draining lymph nodes, and in six of six mice anti-V3 loop antibody was elicited. Further, V3 loop/19-33-primed animals made significant levels of antibody that bound rgp120. These data suggest that, when a major T cell epitope is synthesized in tandem with the V3 loop, a significant immune response against the loop can be elicited. Thus, given the finding that neutralizing antibody may play a role in the control and/or prevention of HIV infection, an HIV vaccine composed of a T cell epitope-containing peptide may prove effective. In addition, this type of approach can be generalized to the design of peptide-based vaccines. 相似文献
76.
Four naturally occurring compounds, indole-3-carbinol (I3C), apigenin (Api), ellagic acid (EA) and tannic acid (TA), were tested for their inhibitory effects against 1-nitropyrene- (1-NP) or 1,6-dinitropyrene (1,6-DNP)-induced genotoxicity in Salmonella tester strains and Chinese hamster ovary (CHO) cells. Api and TA strongly inhibited the bacterial mutagenesis induced by nitropyrenes, while 13C and EA had little or no effect. For example, in TA98, 0.2 μmole Api resulted in 48% and 56% inhibition of the mutagenicity induced by 4 nmole 1-NP and 0.035 nmole 1,6-DNP, respectively. With an equal dose, expected, a good correlation was observed between the antimutagenicity of nitropyrenes and their inhibitory effect on nitroreductase activity. This indicated that one of the possible antimutagenic mechanisms of Api or TA was to inactivate the metabolism of nitropyrenes. Two biological end-points, cytotoxicity and sister-chromatid exchange (SCEs), were used to screen the antigenotoxic effects of these compounds in CHO cells. At the sub-cytotoxic dose, 13C, Api and TA all protected against the cytotoxicity induced by 1-NP and 1,6-DNP, but only TA and Api gave a significant reduction of the frequency of SCEs. Moreover, this reduction was found to be highly dose-dependent. 相似文献
77.
L A Mitscher H Telikepalli P B Wang S Kuo D M Shankel G Stewart 《Mutation research》1992,267(2):229-241
Higher plants contain both mutagens and antimutagens and are susceptible to mutagenesis but screening programs for detection of antimutagenesis rarely employ higher plant systems. Short-term bacterial and mammalian tissue culture systems are the norm. Using modified screening tests for detecting antimutagenic agents, higher plants have been shown to contain a variety of structurally novel antimutagenic agents. Systematic bioassay-directed methodology resulted in the isolation in pure form and biological and chemical characterization of the responsible individual active components from various plants. The methodology in use is illustrated by the isolation of cinnamic acid, cinnamyl cinnamate and cinnamyl ricinoleate as the active constituents of the classic medicinal plant product, Styrax asiatica. The methods which may be used to reveal structure-activity relationships and to explore putative molecular modes of action are illustrated with excerpts from the same study. 相似文献
78.
Rapamycin-FKBP specifically blocks growth-dependent activation of and signaling by the 70 kd S6 protein kinases. 总被引:79,自引:0,他引:79
The macrolide rapamycin blocks cell cycle progression in yeast and various animal cells by an unknown mechanism. We demonstrate that rapamycin blocks the phosphorylation and activation of the 70 kd S6 protein kinases (pp70S6K) in a variety of animal cells. The structurally related drug FK506 had no effect on pp70S6K activation but at high concentrations reversed the rapamycin-induced block, confirming the requirement for the rapamycin and FK506 receptor, FKBP. Rapamycin also interfered with signaling by these S6 kinases, blocking serum-stimulated S6 phosphorylation and delaying entry of Swiss 3T3 cells into S phase. Neither rapamycin nor FK506 blocked activation of a distinct family of S6 kinases (RSKs) or the MAP kinases. These studies identify a rapamycin-sensitive signaling pathway, argue for a ubiquitous role for FKBPs in signal transduction, indicate that FK506-FKBP-calcineurin complexes do not interfere with pp70S6K signaling, and show that in fibroblasts pp70S6K, not RSK, is the physiological S6 kinase. 相似文献
79.
80.
The mechanism by which dietary cis-unsaturated fatty acids lower plasma levels of low-density lipoprotein (LDL) cholesterol is unknown. Since plasma membrane incorporation of dietary cis-unsaturated fatty acids is known to alter the function of plasma membrane associated proteins, perhaps by increasing membrane fluidity, we examined LDL receptor function in Hep G2 hepatocytes that were unmodified, enriched with the cis-unsaturated fatty acids oleate or linoleate, or enriched with the saturated fatty acids stearate or palmitate. Hepatocytes enriched in cis-unsaturated fatty acids exhibited augmented LDL binding, uptake, and degradation in comparison to unmodified cells. In contrast, Hep G2 hepatocytes enriched in saturated fatty acids had decreased LDL binding, uptake, and degradation. Enrichment with oleate or linoleate resulted in a decrease in the calculated fatty acyl mole-weighted melting point of the plasma membrane and an increase in plasma membrane fluidity, as measured by the steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene incorporated into the plasma membrane. Conversely, stearate or palmitate enrichment resulted in an increased plasma membrane fatty acyl mole-weighted melting point and decreased plasma membrane fluidity. LDL binding, uptake, and degradation varied with plasma membrane fluidity in a highly correlated manner. Thus, one mechanism by which dietary cis-unsaturated fatty acids lower LDL cholesterol may possibly involve an alteration in membrane lipid composition or membrane fluidity that promotes enhanced LDL receptor function, thereby leading to increased hepatic clearance of LDL. 相似文献