全文获取类型
收费全文 | 7259篇 |
免费 | 185篇 |
国内免费 | 217篇 |
出版年
2023年 | 46篇 |
2022年 | 50篇 |
2021年 | 68篇 |
2020年 | 102篇 |
2019年 | 131篇 |
2018年 | 133篇 |
2017年 | 87篇 |
2016年 | 130篇 |
2015年 | 153篇 |
2014年 | 322篇 |
2013年 | 519篇 |
2012年 | 188篇 |
2011年 | 421篇 |
2010年 | 294篇 |
2009年 | 433篇 |
2008年 | 432篇 |
2007年 | 463篇 |
2006年 | 344篇 |
2005年 | 365篇 |
2004年 | 364篇 |
2003年 | 259篇 |
2002年 | 221篇 |
2001年 | 151篇 |
2000年 | 131篇 |
1999年 | 138篇 |
1998年 | 141篇 |
1997年 | 133篇 |
1996年 | 103篇 |
1995年 | 118篇 |
1994年 | 101篇 |
1993年 | 121篇 |
1992年 | 76篇 |
1991年 | 77篇 |
1990年 | 63篇 |
1989年 | 59篇 |
1988年 | 69篇 |
1987年 | 68篇 |
1986年 | 50篇 |
1985年 | 69篇 |
1984年 | 58篇 |
1983年 | 58篇 |
1982年 | 87篇 |
1981年 | 57篇 |
1980年 | 53篇 |
1979年 | 41篇 |
1978年 | 21篇 |
1977年 | 23篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1973年 | 13篇 |
排序方式: 共有7661条查询结果,搜索用时 15 毫秒
1.
Takemi Yoshida Akio Uchida Toshinori Yamamoto Yukio Kuroiwa 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,677(2):280-286
The activities of mitochondrial type A and B monoamine oxidase were determined in the liver of rats fed a diet containing 2-acetylaminofluorene (AAF). Three days after the initiation of AAF-feeding, there was a significant decrease of type B monoamine oxidase activity without affect on type A enzyme. The decreased activity of type B monoamine oxidase, which reached a minimum after three weeks, was sustained for as long as AAF-feeding was continued. Sex-related difference in response to AAF was seen in the rat with respect to the onset and the intensity of the decreased type B monoamine oxidase activity, male rats being more sensitive to the carcinogen than female rats. In contrast to the in vivo effect, AAF showed a potent inhibitory effect on type A monoamine oxidase, rather than on type B enzyme, when added in vitro. The pI50 values were estimated to be 7.5 against type A monoamine oxidase and 4.1 against type B enzyme, respectively. The in vitro inhibition of both types of monoamine oxidase by AAF was competitive. The Ki values for AAF were calculated to be 9.51 · 10?9 M for type A monoamine oxidase and 1.30 · 10?5 M for type B enzyme, respectively. In accordance with the potent inhibitory effect of AAF on type A monoamine oxidase in vitro, a single administration of the carcinogen, at a dose of 50 mg/kg, resulted in a marked and temporal decrease of the enzyme activity in the mitochondria of male rat liver. Recovery of the decreased type B monoamine oxidase activity was slow, and the enzyme activity did not return to control levels, even if rats were fed the basal diet for 2 or 4 weeks after the cessation of AAF-feeding. 相似文献
2.
Harsur M. Jajda 《Archives Of Phytopathology And Plant Protection》2013,46(12):1464-1478
Three varieties of Arachis hypogeae, GG 11, GG 20 and GG 24, were compared for resistance against A. niger. GG 20 showed the least disease severity. Infection with A. niger resulted in a rapid increase in NADPH oxidase, Glutathione reductase (GR) and salicylic acid in all the three varieties, indicating hyper increase of reactive oxygen species (ROS) and activation of phenyl propanoid pathway. Ferric reducing antioxidant power value was found to be decreasing due to infection in all the three varieties, confirming the role of ROS in pathogenesis. Since A. niger was found to cause pathogenesis by oxidative stress, the treatment of zinc was given as an antioxidant and its effect was studied. The application of zinc inhibited NADPH oxidase and GR activity in the control as well as in the infected GG 11 and GG 24 varieties but induced in the tolerant variety GG 20. Because zinc treatment could control the ROS in GG 11 and GG 24 varieties, disease severity was reduced but in GG 20 variety, zinc treatment aggravated ROS levels and also the disease severity. The protein profile of GG 20 in comparison to GG 11 and GG 24 varieties revealed one oligomeric protein of 110 kD as one of the responsible factors for its resistance. Total oil and its iodine value were found little higher in GG 20 variety than in other two varieties. It was found that the control of ROS could control the A. niger infection in Arachis hypogeae. 相似文献
3.
4.
Ulrich Weser Karl-Heinz Sellinger Edmund Lengfelder Walther Werner Joachim Strähle 《Biochimica et Biophysica Acta (BBA)/General Subjects》1980,631(2):232-245
The copper complex of indomethacin (1-(p-chlorobenzoyl)-5-methoxy-2-methyl-indole acetate), a common anti-inflammatory drug, was prepared and characterized. Crystal structure determination revealed the dimeric form of the 1:2 complex, namely Cu2(indomethacin)4 · L2, in the unit cell. Suprisingly, the copper-copper distance (263 pm) was very close to metallic copper (256 pm). The two coordination sites in the copper-copper axis can be readily replaced by superoxide. An intriguing similarity to Cu2(acetate)4 was seen.Due to the lipophilic nature of the indomethacin ligand, this copper complex reacted with superoxide in aprotic solvents. The superoxide dismutating activity was successfully demonstrated in Me2SO/water and acetonitrile/water mixtures using the nitro-blue tetrazolium assay and pulse radiolysis. The second-order rate constant of 6 · 109 M?1 · s?1 in strictly aqueous systems dropped only slightly to 1.1 · 109 M?1 · s?1 when aprotic solvents were used. This is the fastest rate constant ever observed for a copper-dependent dismutation of superoxide. The KO2-induced lipid peroxidation in both erythrocytes and liver microsomes was suppressed by 70% in the presence of 1 · 10?10 mol · ml?1 of Cu2(indomethacin)4. The inhibitory action dropped to 25% when Cu2Zn2superoxide dismutase was employed. The formation of copper · indomethacin in rat serum after administration of indomethacin was shown in vitro and in vivo. 相似文献
5.
In the large arteries, it is believed that elastin provides the resistance to stretch at low pressure, while collagen provides the resistance to stretch at high pressure. It is also thought that elastin is responsible for the low energy loss observed with cyclic loading. These tenets are supported through experiments that alter component amounts through protease digestion, vessel remodeling, normal growth, or in different artery types. Genetic engineering provides the opportunity to revisit these tenets through the loss of expression of specific wall components. We used newborn mice lacking elastin (Eln−/−) or two key proteins (lysyl oxidase, Lox−/−, or fibulin-4, Fbln4−/−) that are necessary for the assembly of mechanically-functional elastic fibers to investigate the contributions of elastic fibers to large artery mechanics. We determined component content and organization and quantified the nonlinear and viscoelastic mechanical behavior of Eln−/−, Lox−/−, and Fbln4−/− ascending aorta and their respective controls. We confirmed that the lack of elastin, fibulin-4, or lysyl oxidase leads to absent or highly fragmented elastic fibers in the aortic wall and a 56–97% decrease in crosslinked elastin amounts. We found that the resistance to stretch at low pressure is decreased only in Eln−/− aorta, confirming the role of elastin in the nonlinear mechanical behavior of the aortic wall. Dissipated energy with cyclic loading and unloading is increased 53–387% in Eln−/−, Lox−/−, and Fbln4−/− aorta, indicating that not only elastin, but properly assembled and crosslinked elastic fibers, are necessary for low energy loss in the aorta. 相似文献
6.
Copper is an essential yet toxic metal ion. To satisfy cellular requirements, while, at the same time, minimizing toxicity,
complex systems of copper trafficking have evolved in all cell types. The best conserved and most widely distributed of these
involve Atx1-like chaperones and P1B-type ATPase transporters. Here, we discuss current understanding of how these chaperones bind Cu(I) and transfer it to the
Atx1-like N-terminal domains of their cognate transporter. 相似文献
7.
Larval hemolymph tyrosinase activity in Drosophila melanogaster was detected with high performance liquid chromatography with electrochemical detection. The enzyme hydroxylated L-tyrosine, and oxidized the diphenol substrates L-dopa and dopamine. In larvae of a selected immune-reactive strain the rates of tyrosine hydroxylation, dopa oxidation, and dopamine oxidation were markedly increased during the early stages of melanotic encapsulation of the eggs of the parasitic wasp Leptopilina boulardi. Tyrosinase activity was not modified in parasitized larvae of a selected susceptible strain of D. melanogaster, in which hosts the parasitoids developed unmolested. During the same period of parasitization, the amount of free tyrosine in immune reactive larvae was approximately three times higher than in susceptible hosts. These data indicate that the tyrosinase system of the immune reactive strain is activated during parasitization, and this results in the synthesis of some precursors which ultimately produce a melanotic and sclerotic capsule around the eggs of the parasite. Based on known genetic information of the enzyme system in Drosophila, it appears that at least two genes may be involved in the activation process, one associated with the proenzyme for monophenol oxidase activity, and the second with the proenzyme for diphenol oxidase activity. 相似文献
8.
Using benzo(a)pyrene (BaP) as a probe for aryl hydrocarbon hydroxylase (AHH) activity, differences in mixed-function oxidase (MFO) activity were observed using microspectrofluorimetry in single living cells during long term treatment with 3-methylcholanthrene (3-MC) or carbaryl. Although these two compounds differ in chemical structure, similar effects were observed in 3T3 cell populations. The results suggest that the two compounds activate the same enzymatic system and that individual cells of a supposed homogeneous cell population are not equally sensitive to xenobiotics, i.e. subpopulations were observed which have differences in AHH activity. 相似文献
9.
Laran T. Jensen J. M. Peltier Dennis R. Winge 《Journal of biological inorganic chemistry》1998,3(6):627-631
Mammalian metallothioneins (MT) are known to maximally bind 12 copper ions in two six-Cu(I) ion clusters. Using electrospray
ionization mass spectrometry of MT at pH 4.5, a four-Cu(I) ion cluster was observed intermediate to a fully formed six Cu(I)
in a single domain or a fully formed Cu12MT species. The four-Cu(I) cluster was observed in both MT1 and MT3 isoforms. Addition of increasing amounts of Cu(I) to MT
at pH 4.5 resulted in prominent ions whoses masses were consistent with apo-MT, Cu4MT, Cu6MT, and Cu12MT. The cooperativity of cluster formation was reduced at pH 2.5. Addition of Cu(I) to apo-MT at a reduced pH resulted in
a series of ions consistent with Cu4 to Cu12MT species. However, formation of the tetracopper MT species remained cooperative at low pH, suggesting that this species
is very stable. To determine whether the tetracopper cluster was formed in either the α or β domain, domain peptides of MT3
were used. Addition of Cu(I) to the apo β domain resulted in a peak consistent with the formation of a four-Cu(I) cluster.
This is consistent with reports that Cu(I) ions bind preferentially to the β domain of MTs.
Received: 2 June 1998 / Accepted: 21 August 1998 相似文献
10.
Ezzatollah Keyhani Jacqueline Keyhani 《Biochimica et Biophysica Acta (BBA)/General Subjects》1980,633(2):211-227
Heme a was not detected either in mitochondria isolated from copper-deficient yeast or in the intact cells. Nevertheless, the intracellular concentration of free porphyrins indicated that the pathway of porphyrin and heme synthesis was not impaired in copper-deficient cells. The immunoprecipitated apo-oxidase from copper-deficient cells revealed an absorption spectrum with maxima at 645, 592, 559, 519 and 423 nm, similar to that of purified porphyrin a. When solubilized mitochondria from [3H]leucine and δ-amino[14C]levulinic acid-labeled copper-deficient yeast cells were incubated with rabbit antiserum against cytochrome c oxidase, a precipitate was obtained. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis of this immunoprecipitate showed [3H]leucine associated with six bands and δ-amino[14C]levulinic acid resolved in a single band. HCl fractionation of copper-deficient mitochondria labeled with δ-amino[14C]levulinic acid showed a high specific radioactivity in the fraction extracted by 20% HCl, a solvent which extracts porphyrin a. Thinlayer chromatography of the radioactivity found in 20% HCl showed an RF value identical to that of purified porphyrin a. When δ-amino[3H]levulinic acid-labeled, copper-deficient yeast cells are grown in copper-supplemented medium, the porphyrin a accumulated in copper-deficient cells wa converted into heme a, and this conversion was prevented by cycloheximidine.These observations suggest that porphyrin a is present in the apo-oxidase of copper-deficient cells, but that the conversion to heme a does not occur. This conversion reaction appears to be a point in the biosynthetic pathway of cytochrome c oxidase which is blocked by copper deficieny. 相似文献