全文获取类型
收费全文 | 2062篇 |
免费 | 78篇 |
国内免费 | 5篇 |
出版年
2023年 | 4篇 |
2022年 | 9篇 |
2021年 | 4篇 |
2020年 | 2篇 |
2019年 | 10篇 |
2018年 | 5篇 |
2017年 | 5篇 |
2016年 | 6篇 |
2015年 | 8篇 |
2014年 | 30篇 |
2013年 | 31篇 |
2012年 | 24篇 |
2011年 | 12篇 |
2010年 | 8篇 |
2009年 | 13篇 |
2008年 | 14篇 |
2007年 | 14篇 |
2006年 | 6篇 |
2005年 | 9篇 |
2004年 | 189篇 |
2003年 | 47篇 |
2002年 | 5篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 7篇 |
1998年 | 73篇 |
1997年 | 60篇 |
1996年 | 40篇 |
1995年 | 50篇 |
1994年 | 5篇 |
1993年 | 10篇 |
1992年 | 4篇 |
1991年 | 10篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 14篇 |
1985年 | 279篇 |
1984年 | 295篇 |
1983年 | 130篇 |
1982年 | 244篇 |
1981年 | 91篇 |
1980年 | 64篇 |
1979年 | 57篇 |
1978年 | 85篇 |
1977年 | 34篇 |
1976年 | 36篇 |
1975年 | 36篇 |
1974年 | 28篇 |
1973年 | 12篇 |
排序方式: 共有2145条查询结果,搜索用时 187 毫秒
81.
Guanine aminohydrolase (GAH) (E.C. 3.5.4.3) was purified by affinity chromatography on 9-(p-β-aminoethoxyphenyl)guanine-Sepharose to a specific activity of 35.5 units/mg. The molecular weight of the enzyme was estimated to be 110,000 by gel filtration. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS) showed that the enzyme was composed of subunits with molecular weights of approximately 52,000. Data from SDS-gel electrophoresis in a discontinuous buffer system and from isoelectric focusing in the presence of 8-m urea indicated that more than one type of subunit were present. This was consistent with multiple forms of the native enzyme seen by electrophoresis and isoelectric focusing in polyacrylamide gels. The isoelectric points for the different forms of GAH were in the range of 4.65–4.85. Amino acid analyses showed cysteine to be the minimum amino acid and gave a calculated molecular weight for GAH of 53,016 when the assumption that there were four cysteines per subunit was made. Guanine, 8-azaguanine, and 6-thioguanine served as substrates for the enzyme but 3-deazaguanine, a potent competitive inhibitor of GAH, did not. Fluoride ion inhibited the enzyme in a noncompetitive manner, and this inhibition decreased as pH increased. Variation of the kinetic parameters with pH suggested that hydroxide ion might be the second substrate and that a functional group on the enzyme with a pKa near 5.6 was involved in the reaction. The enzyme was inactivated by treatment with p-hydroxymercurobenzoate and by photooxidation in the presence of rose bengal. Two plausible mechanisms are proposed for the reaction catalyzed by GAH. 相似文献
82.
Nitrogen and ammonia assimilation in the cyanobacteria: regulation of glutamine synthetase. 总被引:4,自引:0,他引:4
Glutamine synthetase, the first enzyme of the ammonia assimilatory pathway, has been purified from Anabaena sp. CA by use of established procedures and by affinity chromatography as a final step. No adenylylation system controlling glutamine synthetase activity was found. The enzyme shows a marked specificity for Mg2+ in the biosynthetic assay and Mn2+ in the transferase assay. Under physiological conditions, Co2+ produces a large stimulatory effect on the Mg2+-dependent biosynthetic activity. The enzyme is inhibited by the feedback modifiers l-alanine, glycine, l-serine, l-aspartate, and 5′-AMP. Inhibition by l-serine and l-aspartate is linear, noncompetitive with respect to l-glutamate with apparent Ki values of 3 and 13 mm, respectively. Cumulative inhibition is seen with mixtures of l-serine, l-aspartate, and 5′-AMP. The results indicate that, in vivo, divalent cation availability and the presence of feedback inhibitors may play the dominant role in regulating glutamine synthetase activity and hence ammonia assimilation in nitrogen-fixing cyanobacteria. 相似文献
83.
An affinity column for the purification of thymidine kinase is described. The ligand in this column is a glycoprotein isolated from rat kidney. This glycoprotein inhibits phosphorylation of thymidine in cultured cells and in a cell-free assay system. With an affinity column containing the glycoprotein as a ligand, a 24-fold purification of thymidine kinase from an ammonium sulfate fraction of a crude tissue extract can be obtained. Thymidine kinase eluted from the affinity column migrates as one major band on polyacrylamide and as one diffuse major band on sodium dodecyl sulfate-polyacrylamide. The affinity column, with thymidine kinase bound to the inhibitor, can also be used as an assay system. When the glycoprotein is covalently attached to Sepharose, it retains its binding capacity for thymidine kinase but has apparently lost its ability to inhibit the enzyme. Thymidine kinase eluted from the affinity column is again sensitive to the glycoprotein. It seems to be a carbohydrate moiety of the glycoprotein that is responsible for the inhibition. 相似文献
84.
David E. Moody Jacqueline L. James Edward A. Smuckler 《Biochemical and biophysical research communications》1980,97(2):673-679
Treatment of male rats with carbon tetrachloride, bromotrichloromethane, chloroform, 1,2-dibromoethane, 1-bromo-2-chloroethane, and 1,2-dibromo-3-chloropropane results in a decrease in cytochrome P-450 content and alterations in the relative content of fatty acids in hepatic microsomes. A high correlation was found between the loss of cytochrome P-450, the decrease in arachidonic acid (r=0.93), and the increases in linoleic (r=?0.91) and oleic acids (r=?0.89). 相似文献
85.
Levels of guanosine 3′,5′-cyclic monophosphate (cGMP) were determined by radioimmunoassay in adherence-purified, oil-induced guinea pig peritoneal exudate macrophages, after extraction of the cells with perchloric acid, purification on Dowex AG1-X8, and acetylation. We found that: (i) Basal cGMP levels were strictly dependent on the concentration of extracellular Ca2+ (0.33 ± 0.03 pmol/mg macrophage protein in Ca2+-free medium and 2.49 ± 0.42 pmol/mg in 1.8 mM Ca2+). (ii) The stimulatory effect of Ca2+ on cGMP levels was prevented by tetracaine. (iii) The cGMP content of macrophages was not elevated by incubation with the ionophore A23187 at extracellular Ca2+ concentrations varying between 0 and 1.8 mM. (iv) Macrophage cGMP levels were increased markedly (up to 40-fold) by incubation of the cells with the nitric oxide (NO)-generating agents, sodium azide, hydroxylamine, sodium nitrite, and sodium nitroprusside. (v) Stimulation of cGMP accumulation by NO-generating agents occurred within 30 sec, was Ca2+-independent, and developed in the presence and absence of the phosphodiesterase inhibitor, isobutyl-methylxanthine. (vi) A minimal elevation in the macrophage cGMP level (less than 2-fold) was induced by ascorbic acid but no significant increases were induced by the following agents, found effective in other cells: serotonin, acetylcholine, carbamylcholine, phorbol myristate acetate, arachidonic acid, Superoxide dismutase, and nitrate reductase. 相似文献
86.
A reliable assay of uridylate sequences longer than 10 is described. The procedure is based on the hybridization of [3H]poly(A) with poly(U) or oligo(U) sequences in high ionic conditions and a subsequent degradation of single stranded polynucleotides with purified Taka-diastase. A 1:2 complex between poly(A) and poly(U) is formed on which on poly(U) strand is digested by Taka-diastase. The procedure is especially suitable for the detection and quantitation of Un (n greater than 10) in RNA preparation. 相似文献
87.
The synthesis of four chiral NAD+ models 1 and their 1,4-dihydro analogs 2 is described. From the temperature dependence of the 1H-nmr spectra it is concluded that for these compounds two preferred conformations I and II, differing slightly in energy, exist. Both conformations are “folded” with the more or less parallel p-anisyl and pyridine groups mutually gauche, but in I the pyridine group is rotated by about 180° as compared with II, thus leading to a conspicuous difference in orientation of the substituent Z (NH2CO, C6H5NHSO2, (CH2)4NSO2, or (C4H8ON)SO2) in the pyridine ring toward the anisyl group. The most stable conformation (I) has Z closest to the center of the p-anisyl group. In 360-MHz spectra of the dihydropyridines at low temperature (?10°C), slow interconversion of I and II leads to the observation of an XY pattern for the C-4 methylene protons of the 1,4-dihydropyridine system. The anisochronity in this methylene group is caused mainly by the anisotropy of the neighboring p-anisyl group. 相似文献
88.
M Schiffer F A Westholm N Panagiotopoulos A Solomon 《Journal of molecular biology》1978,124(1):287-290
A complete human κ-type Bence—Jones protein (Fin) has been isolated and crystallized. Immunochemical and physicochemical characterization of protein Fin indicates that it is of the κ-chain subgroup, κII, and that it consists of two non-covalently bound intact monomers having a molecular weight of ~23,000 Crystals of Bence—Jones protein Fin obtained from ammonium sulfate solutions have the orthorhombic space group P212121 with cell dimensions . The asymmetric unit consists of a dimer of molecular weight ~46,000. 相似文献
89.
James P. Kutney John Balsevich Robert Carruthers Arie Markus Michael J. McGrath Robert N. Young Brian R. Worth 《Bioorganic chemistry》1978,7(3):289-302
Syntheses of the C8 and C10 olefinic units cis- and trans-5-ethyl-1-iodo-hex-4-enes and cis- and trans-7-ethyl-3-iodo-oct-6-enes are described. The Wittig coupling of such units with derivatives of α- and β-thujaketonic acids to give analogs of insect juvenile hormones is discussed. 相似文献
90.
D.J. Back A.M. Breckenridge Francesca E. Crawford M.LE. Orme P.H. Rowe T.P. Sloan Eileen Smith 《Steroids》1978,32(4):423-433
The disappearance of ethinylestradiol from the blood of rabbits has been studied, following the intravenous administration of this steroid. The disappearance followed two exponentials, the first having a half life () of 5.5 min and the second, apparently terminal exponential was also rapid (). The plasma clearance was 150 ml/min which suggests almost total clearance of this steroid during a single passage through the liver. Bile contained a significant concentration of EE conjugates and thus this steroid could undergo enterohepatic recirculations. A large oral dose of unlabelled EE, given prior to intravenous administra tion of tritiated EE, considerably altered the pharmacokinetics of the latter by saturating both phase one metabolism (changes of the steroid nucleus) and the secretion of conjugates into bile. It was not clear whether phase two metabolism (conjugation) was also saturated. 相似文献