全文获取类型
收费全文 | 132788篇 |
免费 | 8494篇 |
国内免费 | 5513篇 |
出版年
2023年 | 2416篇 |
2022年 | 2922篇 |
2021年 | 4015篇 |
2020年 | 3985篇 |
2019年 | 5005篇 |
2018年 | 4669篇 |
2017年 | 3550篇 |
2016年 | 3117篇 |
2015年 | 3795篇 |
2014年 | 6857篇 |
2013年 | 8719篇 |
2012年 | 5132篇 |
2011年 | 7582篇 |
2010年 | 5359篇 |
2009年 | 6045篇 |
2008年 | 6345篇 |
2007年 | 6424篇 |
2006年 | 5676篇 |
2005年 | 5151篇 |
2004年 | 4786篇 |
2003年 | 4019篇 |
2002年 | 3791篇 |
2001年 | 3183篇 |
2000年 | 2776篇 |
1999年 | 2855篇 |
1998年 | 2719篇 |
1997年 | 2436篇 |
1996年 | 1855篇 |
1995年 | 1582篇 |
1994年 | 1386篇 |
1993年 | 1233篇 |
1992年 | 1109篇 |
1991年 | 1041篇 |
1990年 | 843篇 |
1989年 | 772篇 |
1988年 | 714篇 |
1987年 | 691篇 |
1986年 | 567篇 |
1985年 | 1002篇 |
1984年 | 1569篇 |
1983年 | 1288篇 |
1982年 | 1313篇 |
1981年 | 1041篇 |
1980年 | 1061篇 |
1979年 | 936篇 |
1978年 | 665篇 |
1977年 | 566篇 |
1976年 | 537篇 |
1975年 | 464篇 |
1974年 | 433篇 |
排序方式: 共有10000条查询结果,搜索用时 138 毫秒
991.
Stuart J. Hamill David Y. Cooper Heinz Schleyer Otto Rosenthal 《Archives of biochemistry and biophysics》1983,224(2):614-624
The time-course kinetics of the cytochrome P-450-catalyzed dealkylations of the exogenous compounds benzphetamine, ethylmorphine, codeine, and 7-ethoxycoumarin were compared to the hydroxylation of the endogenous compound testosterone. Using liver microsomes from phenobarbital-induced rats, the time course of the demethylations of ethylmorphine, codeine, and especially benzphetamine was characterized by a fast initial phase of enzymatic activity and then a steady decline in the rate throughout the remainder of the reaction. In contrast, under the same experimental conditions, both the dealkylation of 7-ethoxycoumarin and the hydroxylation of testosterone showed no initial fast phase of activity and a constant rate of product formation for most of the remainder of the time course. The difference also held for the carbon monoxide inhibition studies in which the degree of inhibition of the demethylation reactions by a variety of CO:O2 mixtures was time dependent, in contrast to the constant, time-independent degree of CO inhibition of the other two reactions. The kinetics of the demethylation reactions could not be explained by enzyme destruction, back reaction, or product adduct formation and were further confirmed by measurements of the rate of O2 utilization and NADPH oxidation. The complexity of the demethylation reaction should be taken into consideration in any detailed studies of the monooxygenation reaction system. 相似文献
992.
Renny T. Franceschi Hector F.De Luca Donna L. Mercado 《Archives of biochemistry and biophysics》1983,222(2):504-517
The interaction of the 1α,25-dihydroxyvitamin D3 receptor with immobilized calf thymus DNA has been compared with its sedimentation properties on hypotonic sucrose gradients. Forty to sixty percent of total hormone:receptor complexes formed at 4 °C were retained by DNA-cellulose and could be eluted by 0.18 to 0.2 m KCl. In contrast, heating preparations to 25 °C rapidly and irreversibly converted receptor to a form which bound hormone and DEAE-cellulose normally, but was unable to associate with DNA. Similarly, the ability of receptor to aggregate to a 6 S species was labile at 25 °C. Stabilization of receptor in the DNA binding aggregating form was accomplished using Ca2+, Mg2+, Mn2+, or Na2MoO4 while several protease and phosphatase inhibitors were ineffective. An examination of DNA binding properties of aggregating and nonaggregating receptor forms revealed that only receptor competent to enter into aggregates could bind DNA suggesting that a functional nucleic acid binding site, and, hence, a nucleic acid interaction is necessary for aggregate formation. Consistent with this view, an RNA:receptor interaction appears to be involved in formation of the 6 S complex since removal of RNA by ribonuclease treatment or purification of receptor reduced aggregation, an effect that could be reversed by addition of purified RNA. 相似文献
993.
R Arem R Chayoth T D Shenkenberg S I Miller M C Chou J B Field 《Archives of biochemistry and biophysics》1983,225(1):66-74
An initial incubation of dog thyroid slices with 0.1 or 1 microM acetylcholine (ACH) for at least 2 h decreases its subsequent stimulation of [1-14C]glucose oxidation. Refractoriness persists for as long as 6 h in the absence of ACH. While new protein synthesis is essential for recovery, it is not necessary for its induction. Refractoriness is prevented when 25 microM tropicamide, an atropine-like drug, is present from the beginning of the initial incubation, but not when it is added after 2 h of incubation of slices with ACH, indicating that at this time ACH is no longer necessary for refractoriness. During refractoriness induced by ACH, stimulation of glucose oxidation by thyroid-stimulating hormone, prostaglandin E1, dibutyryl cyclic AMP, and cholera toxin, but not menadiol, is also significantly diminished. Incubation of thyroid slices with ACH does not modify its stimulation of iodide organification or 32Pi incorporation into phospholipids. These results suggest that the desensitization is not due to changes in the ACH receptor but rather to intracellular metabolic effects. This phenomenon may be important in the regulation of cholinergic effects on the thyroid. 相似文献
994.
Multiple enzyme forms of isocitrate lyase from various sources have been frequently reported. Protease action after cell rupture was sporadically claimed to explain the observed multiple enzyme forms. In this communication studies which are consistent with a protease action in vitro on isocitrate lyase of Pinus pinea germinating seeds are reported. Moreover, changes in DEAE-Sephacel patterns, mainly related to the age of germination, were observed. Differences regarding the heat stability of the detected enzyme forms were also found. The results indicate that isocitrate lyase from P. pinea may be detected in at least three different forms, one of which is heat stable and may be obtained only at the early stages of germination. 相似文献
995.
In intact, uncoupled type B chloroplasts from spinach, added ATP causes a slow light-induced decline () of chlorophyll a fluorescence at room temperature. Fluorescence spectra were recorded after fast cooling to 77 K and normalized with fluorescein as an internal standard. Related to the fluorescence quenching at room temperature, an increase in Photosystem (PS) I fluorescence (F735) and a decrease in PS II fluorescence (F695) were observed in the low-temperature spectra. The change in the ratio was abolished by the presence of methyl viologen. Fluorescence induction at 77 K of chloroplasts frozen in the quenched state showed lowered variable (Fv) and initial (F0) fluorescence at 690 nm and an increase in F0 at 735 nm. The results are interpreted as indicating an ATP-dependent change of the initial distribution of excitation energy in favor of PS I, which is controlled by the redox state of the electron-transport chain and, according to current theories, is caused by phosphorylation of the light-harvesting complex. 相似文献
996.
Chlorotetracycline inhibits the uncoupled oxidation of exogenous NADH by Jerusalem artichoke (Helianthus tuberosus L.) mitochondria extensively (over 80%) and rapidly (inhibition complete in 10 s) in the presence of added Ca2+. Half-maximal inhibition is observed at 15 μM chlorotetracycline in the presence of 2 mM Ca2+. The oxidation of succinate is only affected marginally by chlorotetracycline plus Ca2+. The inhibition of NADH oxidation and the fluorescence of CTC are well correlated. Mn2+ is the only other cation which shows an (increased) inhibition in the presence of chlorotetracycline. The inhibition by Ca2+ and chlorotetracycline disappears at acid pH, and the pH optimum in their presence is 6.4. The inhibition caused by other lipid-soluble Ca2+-chelators is not reversible or is enhanced by the addition of excess Ca2+. In contrast, inhibition caused by relatively water-soluble chelators is completely reversed by added Ca2+. It is suggested that a neutral 1:2 complex is formed between Ca2+ and chlorotetracycline which can substitute for Ca2+ bound at sites in the lipophilic phase of the inner mitochondrial membrane, which are essential for the activity of the external NADH dehydrogenase. 相似文献
997.
The structural organization of Tetrahymena pyriformis is such that its cilia are remote from the main centers of lipid metabolism. As a result, the ciliary membrane lipid composition of cells exposed to low-temperature stress is initially unaffected by the significant metabolic changes induced in microsomal membranes. Nevertheless, changes in the ciliary membrane lipid composition can be detected during the first 4 h of cold exposure. A combination of in vivo and in vitro experiments has provided strong evidence for a substantial retailoring of ciliary phospholipid molecular species in situ in the absence of any importation of lipids from the cell interior or change in overall ciliary fatty acid composition. The mechanism responsible for the ciliary lipid changes is independent of the one(s) triggering internal acclimation responses. Our observations establish for the first time that chilling stress can simultaneously induce separate and distinctive lipid modification responses in different parts of a cell. This finding could be important in identifying the molecular ‘sensor’ capable of actuating stress-induced lipid changes. 相似文献
998.
Crosslinking of chromosomal proteins to DNA in HeLa cells by UV gamma radiation and some antitumor drugs 总被引:1,自引:0,他引:1
Z M Banjar L S Hnilica R C Briggs J Stein G Stein 《Biochemical and biophysical research communications》1983,114(2):767-773
Immunochemical techniques were used to investigate the protein-DNA crosslinking by ultraviolet (UV) and gamma radiation as well as 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) or cis- and trans-diamminedichloroplatinum II (cis-DDP and trans-DDP). Antisera to 0.35 M NaCl extract and 0.35 M NaCl residue of HeLa nuclei were employed. Both gamma and UV irradiation, exposure to cis- or trans-DDP and, to a lesser extent, BCNU, resulted in crosslinking of various antigens to the DNA. Although several antigens were crosslinked by all the employed agents, other exhibited agent-specific crosslinking patterns. 相似文献
999.
Chemotactic factor causes rapid decreases in phosphatidylinositol,4,5-bisphosphate and phosphatidylinositol 4-monophosphate in rabbit neutrophils 总被引:20,自引:0,他引:20
M Volpi R Yassin P H Naccache R I Sha'afi 《Biochemical and biophysical research communications》1983,112(3):957-964
Stimulation of rabbit neutrophils prelabeled with 32P by the synthetic chemotactic peptide f-Met-Leu-Phe induces a rapid decrease in the radioactivity in both phosphatidylinositol, 4,5 bis phosphate and phosphatidylinositol 4-monophosphate. The mean +/- standard error of the mean values of the maximum decrease in phosphatidylinositol, 4,5 bis phosphate occurred at 10 seconds following stimulation and is equal to 19 +/- 3% of the control value. The corresponding value for phosphatidylinositol 4-monophosphate occurred at 60 seconds following stimulation and is equal to 37 +/- 7% of the control value. On the other hand, the radioactivity in phosphatidic acid and lysophospholipids increased continuously with time following stimulation. The relationship of these changes to calcium release and neutrophil activation is discussed. 相似文献
1000.
8-Bromoguanosine 2'- and 3'-phosphates have been shown to bind to RNase T1 with the same affinity as the corresponding guanosine phosphates, inducing difference absorption and circular dichroism spectra similar to those induced by the guanosine phosphates. Since the brominated ligands have reduced electron density on N-7 of the guanine ring and syn-fixed conformation due to a bulky, electron-withdrawing Br substituent on C-8, the difference spectra are not attributable to the protonation on N-7 and to the restriction of the ligand to syn-conformation as proposed previously. 相似文献