全文获取类型
收费全文 | 7746篇 |
免费 | 401篇 |
国内免费 | 328篇 |
出版年
2023年 | 90篇 |
2022年 | 149篇 |
2021年 | 194篇 |
2020年 | 195篇 |
2019年 | 271篇 |
2018年 | 248篇 |
2017年 | 165篇 |
2016年 | 148篇 |
2015年 | 161篇 |
2014年 | 416篇 |
2013年 | 558篇 |
2012年 | 341篇 |
2011年 | 421篇 |
2010年 | 301篇 |
2009年 | 352篇 |
2008年 | 360篇 |
2007年 | 412篇 |
2006年 | 380篇 |
2005年 | 333篇 |
2004年 | 254篇 |
2003年 | 256篇 |
2002年 | 220篇 |
2001年 | 140篇 |
2000年 | 173篇 |
1999年 | 115篇 |
1998年 | 102篇 |
1997年 | 103篇 |
1996年 | 110篇 |
1995年 | 77篇 |
1994年 | 66篇 |
1993年 | 73篇 |
1992年 | 91篇 |
1991年 | 69篇 |
1990年 | 64篇 |
1989年 | 54篇 |
1988年 | 49篇 |
1987年 | 51篇 |
1986年 | 33篇 |
1985年 | 77篇 |
1984年 | 149篇 |
1983年 | 88篇 |
1982年 | 99篇 |
1981年 | 88篇 |
1980年 | 88篇 |
1979年 | 67篇 |
1978年 | 42篇 |
1977年 | 39篇 |
1976年 | 41篇 |
1975年 | 33篇 |
1974年 | 26篇 |
排序方式: 共有8475条查询结果,搜索用时 25 毫秒
71.
Selenocysteine lyase activity was detected in crude extracts from a cysteine-requiring mutant ofEscherichia coli K-12. The level of activity was the same whether cells had been grown aerobically or anaerobically, with or without selenocysteine.
Selenocysteine lyase catalyzes the conversion of selenocysteine to alanine and elemental Se, a reaction that is followed by
a nonenzymatic reduction of the Se to hydrogen selenide. Both of these end products were identified in this study. With cysteine
as the substrate, alanine and H2S were formed, but only at levels 50% less than the products formed from selenocysteine. Selenocysteine lyase has been identified
in a number of mammals and bacteria; its presence in a cysK mutant ofE. coli K-12 suggests a common route whereby hydrogen selenide, derived from selenocysteine, can then be assimilated into selenoproteins. 相似文献
72.
Photolabeling of staphylococcal alpha-toxin from within rabbit erythrocyte membranes 总被引:3,自引:0,他引:3
M Thelestam C Jolivet-Reynaud J E Alouf 《Biochemical and biophysical research communications》1983,111(2):444-449
Intrinsic membrane proteins of rabbit red blood cells were labeled with the photoreactive amphipatic reagent 12-(4-azido-2-nitrophenoxy) stearoyl (1-14C) glucosamine, which inserts into the hydrophobic membrane region and generates a reactive nitrene upon ultraviolet irradiation. Photolabeling of membrane-bound staphylococcal alpha-toxin after lysis of probe-treated rabbit red blood cells by this toxin implies its penetration into the hydrophobic region of the outer leaflet of the membrane. In contrast clostridial theta-toxin and staphylococcal delta-toxin were not labeled, but extraction of intrinsic membrane proteins by delta-toxin was evidenced. 相似文献
73.
A S Abramovitz J Y Hong V Randolph 《Biochemical and biophysical research communications》1983,117(1):22-29
We have previously reported the purification of polypeptides from soybean which are potent inhibitors of superoxide production by human neutrophils. We now report that neither oxygen uptake nor hydrogen peroxide production by stimulated neutrophils is affected by these inhibitors. Furthermore, the E-1 and E-3 polypeptides inhibit ferricytochrome c reduction by a xanthine oxidase superoxide generation system. The inhibitory activity of E-3 in the model system is blocked by 1 mM KCN while E-1 is only slightly cyanide sensitive. Atomic absorption analysis of E-1 and E-3 polypeptides reveal copper in the latter and manganese in the former. Thus, E-3 is a copper-containing superoxide dismutase while E-1 appears to be a manganese-containing superoxide dismutase. 相似文献
74.
Marion T. Hierowski 《FEBS letters》1983,154(1):92-96
Quantitative analyses of LH-RH-like membrane receptors were performed in five tumors from the transplantable Dunning R3372H rat prostatic adenocarcinoma. The binding of D-Trp6-LH-RH, an agonist of LH-RH, was observed in all 5 tumors. The antagonist [Ac-Dp-Cl-Phe1,2,D-Trp3,D-Lys6,D-Ala10]-LH-RH was bound to 4 tumors. The apparent equilibrium dissociation constant (Kd) for D-Trp6-LH-RH receptor was from 2.6–3.9 × 10?10 M. The apparent equilibrium Bmax values (maximum number of binding sites) were from 17.2–86.0 fmol/mg membrane protein for D-Trp6-LH-RH receptor. The Kd for the antagonist was from 2.4–2.7 × 10?10 M and the Bmax values were from 35.5–66.0 fmol/mg membrane protein. Similar binding studies performed in 6 normal rat prostates showed no binding capacities. 相似文献
75.
Regulation of insulin receptor kinase activity by insulin mimickers and an insulin antagonist 总被引:9,自引:0,他引:9
R A Roth D J Cassell B A Maddux I D Goldfine 《Biochemical and biophysical research communications》1983,115(1):245-252
Three agents which mimic insulin action in intact cells (concanavalin A, wheat germ agglutinin, and polyclonal insulin receptor antibody), mimicked insulin's ability to stimulate the kinase activity of purified insulin receptors. In contrast, monoclonal insulin receptor antibody, an antagonist of insulin action, did not stimulate the phosphorylation of the insulin receptor either in intact IM-9 cells or in purified receptor preparations. This antibody, however, antagonized the ability of insulin to stimulate the phosphorylation of the receptor both in intact cells and in the purified receptor. These studies with insulin mimickers and an insulin antagonist are consistent with a role for the kinase activity of the receptor mediating the actions of insulin. 相似文献
76.
In this paper a number of experiments with the purple bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulata is described in which the total fluorescence yield and/or the total fraction of reaction centers closed after a picosecond laser pulse were measured as a function of the pulse intensity. The conditions were such that the reaction centers were either all in the open or all in the closed state before the pulse arrived. These experiments are analysed using the theoretical formalism discussed in the preceding paper (Den Hollander, W.T.F., Bakker J.G.C., and Van Grondelle, R., Biochim. Biophys. Acta 725, 492–507). From the experimental results the number of connected photosynthetic units, λ, the rate of energy transfer between neighboring antenna molecules, kh, and the rate of trapping by an open reaction center, kot, can be estimated. For R. rubrum it is found that λ = 14−17, kh = (1−2)·1012 s−1 and kot = (4−6)·1011 s−1, for Rps. capsulata λ ≈ 30, kh ≈ 4·1011 s−1 and kot ≈ 3·1011 s−1. The findings are discussed in terms of current models for the structure of the antenna and the kinetic properties of the decay processes occurring in these purple bacteria. 相似文献
77.
Chemical modification of Rhodospirillum rubrum chromatophores by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) results in inactivation of photophosphorylation, Mg2+-ATPase, oxidative phosphorylation and ATP-driven transhydrogenase, with apparent first-order kinetics. Other energy-linked reactions such as light-driven transhydrogenase and light-dependent proton uptake were insensitive to NBD-Cl. The Ca2+-ATPase activity of the soluble coupling factor from chromatophores (R. rubrum F1) was inactivated by NBD-Cl with kinetics resembling those described for Mg2+-ATPase and photophosphorylation activities of chromatophores. Both NBD-chromatophores and NBD-R. rubrum F1 fully recovered their activities when subjected to thiolysis by dithioerythritol. Phosphoryl transfer reactions of chromatophores and Ca2+-ATPase activity of R. rubrum F1 were fully protected by 5 mM Pi against modification by NBD-Cl. ADP or ATP afforded partial protection. Analysis of the protection of Ca2+-ATPase activity by Pi indicated that NBD-Cl and Pi are mutually exclusive ligands. Spectroscopic studies revealed that tyrosine and sulfhydryl residues in R. rubrum F1 underwent modification by NBD-Cl. However, the inactivation was only related to the modification of tyrosine groups. 相似文献
78.
Buspirone produces a dose-dependent but short-lived elevation in striatal dopamine (DA) metabolites in the rat. , buspirone possesses an affinity similar to sulpiride for DA receptors (3H-spiperone). A moderate affinity for α1 receptors was also observed while buspirone was inactive at α2, β, muscarinic and serotonin2 receptors. This pharmacological profile as well as previous behavioral data indicate that buspirone may be a potential “atypical” neuroleptic. 相似文献
79.
To gain insight into the mechanism of formation of chromosomal aberrations by the tumor promoter phorbolmyristate acetate (PMA) in human lymphocytes, we investigated the effect of antioxidants and inhibitors of arachidonic acid metabolism. Among the antioxidants bovine erythrocyte CuZn superoxide dismutase, glutathione peroxidase, mannitol (a scavenger of hydroxyl radicals), butylated hydroxytoluene and butylated hydroxyanisole were anticlastogenic while catalase and dimethylfuran (a scavenger of singlet oxygen) were inactive. These results show that the induction of aberrations by PMA occurs via indirect action, i.e. the intermediacy of superoxide and hydroxyl radicals. The following inhibitors of arachidonic acid metabolism were strongly anticlastogenic: the cyclo-oxygenase inhibitors indomethacin and flufenamic acid and the lipoxygenase inhibitor BN1015. Imidazole, nordihydroguaiaretic acid BN 1048 and 5,8,11,14-eicosatetraynoic acid were moderately active. The inhibitor of phospholipase A2, fluocinolone acetonide, was also anticlastogenic.
We conclude that the oxidative metabolism of arachidonic acid is involved in the induction of chromosomal aberrations by PMA in human lymphocytes. However, because of the limited selectivity of these drugs, it is not yet possible to identify unambiguously the step(s) in the arachidonic acid cascade responsible for PMA clastogenicity. 相似文献
80.
The accumulation of transported materials in cut axons is demonstrated by the light and electron microscopic immunocytochemical localization of substance P and enkephalin in the caudal medulla and cervical spinal cord of adult rat. Two days following unilateral knife-cuts in the caudal medulla or spinal (C2-C3) levels, substance P and enkephalin-like immunoreactivity (SPLI and ELI) are detected in lesioned axons located rostral and caudal to the transection. Rostrally, SPLI and ELI are detected in the lateral reticular region and ventrolateral fasciculus corresponding to the location of previously identified bulbospinal pathways. Caudally, previously unidentified, propriospinal pathways showing SPLI are detected in the dorsal columns and in the dorsolateral fasciculus. In contrast, ELI is found caudal to the transection only in the reticular region of the medulla. For both peptides, immunoreactivity is present throughout axons containing numerous large, dense core, and small clear vesicles. These results support the concept of both particulate and soluble modes of transport for substance P and enkephalin within axons of the central nervous system. 相似文献