全文获取类型
收费全文 | 3284篇 |
免费 | 114篇 |
国内免费 | 64篇 |
出版年
2023年 | 16篇 |
2022年 | 24篇 |
2021年 | 19篇 |
2020年 | 32篇 |
2019年 | 42篇 |
2018年 | 34篇 |
2017年 | 37篇 |
2016年 | 41篇 |
2015年 | 163篇 |
2014年 | 347篇 |
2013年 | 313篇 |
2012年 | 55篇 |
2011年 | 159篇 |
2010年 | 56篇 |
2009年 | 165篇 |
2008年 | 123篇 |
2007年 | 135篇 |
2006年 | 124篇 |
2005年 | 109篇 |
2004年 | 142篇 |
2003年 | 139篇 |
2002年 | 107篇 |
2001年 | 77篇 |
2000年 | 76篇 |
1999年 | 82篇 |
1998年 | 86篇 |
1997年 | 67篇 |
1996年 | 52篇 |
1995年 | 86篇 |
1994年 | 81篇 |
1993年 | 68篇 |
1992年 | 43篇 |
1991年 | 27篇 |
1990年 | 38篇 |
1989年 | 37篇 |
1988年 | 33篇 |
1987年 | 16篇 |
1986年 | 22篇 |
1985年 | 31篇 |
1984年 | 35篇 |
1983年 | 28篇 |
1982年 | 15篇 |
1981年 | 26篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1973年 | 3篇 |
排序方式: 共有3462条查询结果,搜索用时 15 毫秒
1.
E. Smyk-Randall O. R. Brown A. Wilke A. Eisenstark D. H. Flint 《Free radical biology & medicine》1993,14(6):609-613
The effects of near ultraviolet (NUV) light on a NUV chromophore-containing oxidant-sensitive enzyme, dihydroxyacid dehydratase (DHAD), were measured in seven strains of Escherichia coli. The strains differed in production of the oxidant-defense enzymes, superoxide dismutases (Fe-SOD and Mn-SOD), and catalases HPI and HPII. With the stress of aerobic growth but without NUV exposure, the strains lacking either Fe or Mn SOD or both SODs had 57%, 25%, and 12%, respectively, of the DHAD-specific activity of the parent (K12) strain. Under the same conditions, the catalase strains that were wild type, overproducing, and deficient had comparable DHAD-specific activities. When aerobic cultures were exposed for 30 min to NUV with a fluence of 216 J/m2/s at 310–400 nm, the percentage decreases in DHAD-specific activities were similar (ranging from 75% to 89%) in strains with none, either, or both SODs missing, and in the catalase-overproducing strain. However, the decreases were only 58% and 52% in the strain with catalase missing and in its parent, respectively. The NUV-induced loss of DHAD enzyme activity was not accompanied by any detectable loss of the DHAD protein as measured by polyclonal antibody to DHAD. 相似文献
2.
3.
本文提出了一种检测非真空电束辐射的间接作用的一种方法,根据氧化型和还原型的细胞色素C的吸收光谱不同,从而测量非真空电子束作用对应的辐射分解的自由基总量。 相似文献
4.
Enhanced plant regeneration in grain and sweet sorghum by asparagine,proline and cefotaxime 总被引:7,自引:0,他引:7
Summary Cefotaxime ( 50 and 100 mg/1 ), a cephalosporin antibiotic and the amino acids asparagine and proline (200 mg/l) enhanced the production of embryogenic callus, increased the frequency of plant regeneration, and delayed the loss of regeneration potential in immature embryo-derived callus cultures ofSorghum bicolor (L.) Moench. Although these compounds did not promote callus induction or growth of callus, they influenced plant regeneration considerably in 10 low responding genotypes of grain and high anthocyanin containing sweet sorghums. 相似文献
5.
We describe a solid vegetable oil–water gel structure which is stabilized through the use of low concentrations of monoglycerides,
containing no added trans fats or saturated fats. The resulting structure consists of oil droplets encapsulated in self-assembled crystalline monoglyceride
multilayers, surrounded by a continuous water phase. Acute ingestion human feeding trials indicated that the serum triglyceride
loading was significantly lower after ingestion of the structured gel rather than a simple oil–water mixture, resulting in
an attenuated increase in serum insulin. This indicates the effectiveness of encapsulation in modulating blood lipid and insulin
response in humans, and suggests a strategy that can be employed for the controlled release of food macronutrients.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
6.
Our previous studies have shown that endothelin-1 (ET-1) stimulates catalase activity in endothelial cells and in lambs with acute increases in pulmonary blood flow (PBF), without altering gene expression. The purpose of this study was to investigate the molecular mechanism by which this occurs. Exposing pulmonary arterial endothelial cells to ET-1 increased catalase activity and decreased cellular hydrogen peroxide (H2O2) levels. These changes correlated with an increase in serine-phosphorylated catalase. Using the inhibitory peptide δV1.1, this phosphorylation was shown to be protein kinase Cδ (PKCδ) dependent. Mass spectrometry identified serine 167 as the phosphorylation site. Site-directed mutagenesis was used to generate a phospho-mimic (S167D) catalase. Activity assays using recombinant protein purified from Escherichia coli or transiently transfected COS-7 cells demonstrated that S167D catalase had an increased ability to degrade H2O2 compared to the wild-type enzyme. Using a phospho-specific antibody, we were able to verify that pS167 catalase levels are modulated in lambs with acute increases in PBF in the presence and absence of the ET receptor antagonist tezosentan. S167 is located on the dimeric interface, suggesting it could be involved in regulating the formation of catalase tetramers. To evaluate this possibility we utilized analytical gel filtration to examine the multimeric structure of recombinant wild-type and S167D catalase. We found that recombinant wild-type catalase was present as a mixture of monomers and dimers, whereas S167D catalase was primarily tetrameric. Further, the incubation of wild-type catalase with PKCδ was sufficient to convert wild-type catalase into a tetrameric structure. In conclusion, this is the first report indicating that the phosphorylation of catalase regulates its multimeric structure and activity. 相似文献
7.
Glutathione redox balance—defined as the ratio GSH/GSSG—is a critical regulator of cellular redox state, and declines in this ratio are closely associated with oxidative stress and disease. However, little is known about the impact of genetic variation on this trait. Previous mouse studies suggest that tissue GSH/GSSG is regulated by genetic background and is therefore heritable. In this study, we measured glutathione concentrations and GSH/GSSG in liver and kidney of 30 genetically diverse inbred mouse strains. Genetic background caused an approximately threefold difference in hepatic and renal GSH/GSSG between the most disparate strains. Haplotype association mapping determined the loci associated with hepatic and renal glutathione phenotypes. We narrowed the number of significant loci by focusing on those located within protein-coding genes, which we now consider to be candidate genes for glutathione homeostasis. No candidate genes were associated with both hepatic and renal GSH/GSSG, suggesting that genetic regulation of GSH/GSSG occurs predominantly in a tissue-specific manner. This is the first quantitative trait locus study to examine the genetic regulation of glutathione concentrations and redox balance in mammals. We identified novel candidate genes that have the potential to redefine our knowledge of redox biochemistry and its regulation and inform future therapeutic applications. 相似文献
8.
9.
Interactions between α-tocopherol,polyunsaturated fatty acids,and lipoxygenases during embryogenesis
α-Tocopherol is a lipid-soluble antioxidant that is specifically required for reproduction and embryogenesis. However, since its discovery, α-tocopherol's specific biologic functions, other than as an antioxidant, and the mechanism(s) mediating its requirement for embryogenesis remain unknown. As an antioxidant, α-tocopherol protects polyunsaturated fatty acids (PUFAs) from lipid peroxidation. α-Tocopherol is probably required during embryonic development to protect PUFAs that are crucial to development, specifically arachidonic (ARA) and docosahexaenoic (DHA) acids. Additionally, ARA and DHA are metabolized to bioactive lipid mediators via lipoxygenase enzymes, and α-tocopherol may directly protect, or it may mediate the production and/or actions of, these lipid mediators. In this review, we discuss how α-tocopherol (1) prevents the nonspecific, radical-mediated peroxidation of PUFAs, (2) functions within a greater antioxidant network to modulate the production and/or function of lipid mediators derived from 12- and 12/15-lipoxygenases, and (3) modulates 5-lipoxygenase activity. The application and implication of such interactions are discussed in the context of α-tocopherol requirements during embryogenesis. 相似文献
10.
Cellulase, an enzymatic complex that synergically promotes the degradation of cellulose to glucose and cellobiose, free or adsorbed onto Si/SiO2 wafers at 60 °C has been employed as catalyst in the hydrolysis of microcrystalline cellulose (Avicel), microcrystalline cellulose pre-treated with hot phosphoric acid (CP), cotton cellulose (CC) and eucalyptus cellulose (EC). The physical characteristics such as index of crystallinity (IC), degree of polymerization (DP) and water sorption values were determined for all samples. The largest conversion rates of cellulose into the above-mentioned products using free cellulase were observed for samples with the largest water sorption values; conversion rates showed no correlation with either IC or DP of the biopolymer. Cellulose with large water sorption value possesses large pore volumes, hence higher accessibility. The catalytic efficiency of immobilized cellulase could not be correlated with the physical characteristics of cellulose samples. The hydrolysis rates of the same cellulose samples with immobilized cellulase were lower than those by the free enzyme, due to the diffusion barrier (biopolymer chains approaching to the immobilized enzyme) and less effective contact between the enzyme active site and its substrate. Immobilized cellulase, unlike its free counterpart, can be recycled at least six times without loss of catalytic activity, leading to higher overall cellulose conversion. 相似文献