全文获取类型
收费全文 | 31711篇 |
免费 | 1720篇 |
国内免费 | 1535篇 |
出版年
2023年 | 451篇 |
2022年 | 737篇 |
2021年 | 1003篇 |
2020年 | 932篇 |
2019年 | 1365篇 |
2018年 | 1119篇 |
2017年 | 719篇 |
2016年 | 763篇 |
2015年 | 1041篇 |
2014年 | 2182篇 |
2013年 | 2616篇 |
2012年 | 1665篇 |
2011年 | 2048篇 |
2010年 | 1550篇 |
2009年 | 1473篇 |
2008年 | 1656篇 |
2007年 | 1623篇 |
2006年 | 1366篇 |
2005年 | 1295篇 |
2004年 | 1104篇 |
2003年 | 960篇 |
2002年 | 839篇 |
2001年 | 507篇 |
2000年 | 474篇 |
1999年 | 443篇 |
1998年 | 422篇 |
1997年 | 325篇 |
1996年 | 347篇 |
1995年 | 350篇 |
1994年 | 380篇 |
1993年 | 258篇 |
1992年 | 290篇 |
1991年 | 228篇 |
1990年 | 195篇 |
1989年 | 165篇 |
1988年 | 136篇 |
1987年 | 134篇 |
1986年 | 129篇 |
1985年 | 208篇 |
1984年 | 221篇 |
1983年 | 181篇 |
1982年 | 196篇 |
1981年 | 153篇 |
1980年 | 153篇 |
1979年 | 122篇 |
1978年 | 101篇 |
1977年 | 78篇 |
1976年 | 71篇 |
1974年 | 47篇 |
1973年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
101.
UV-visible and 13C NMR measurements described in the literature and our 31P NMR measurements support the following mechanism of proton transfer reactions in aqueous solutions of pyridoxamine phosphate: Only the tautomeric equilibrium between neutral form, A
N, and zwitterion, A
Z, which is analogous to the tautomeric equilibrium of 3-hydroxypyridine in aqueous solution, is important, and that equilibrium does not change upon the dissociation of the second phosphate proton. With these simplifying assumption, we have simulated the relaxation spectrum of the proton transfer reactions of pyridoxamine phosphate in water using parameters from analogous reactions and compared it with our ultrasound and temperature jump measurements. We have found that the relaxation process measured by the temperature jump experiment is mainly caused by the overall reaction A
N=A
Z (or A
N
-
=A
Z
-
) and the ultrasound absorption at the isoelectric point between pK2 and pK3 is mainly caused by the overall reaction
. 相似文献
102.
Cytoplasmic Poly(ADP-Ribose) Polymerase Associated with Free Messenger Ribonucleoprotein Particles in Rat Brain 总被引:1,自引:0,他引:1
Poly(ADP-ribose) polymerase associated with free cytoplasmic messenger ribonucleoprotein particles (free mRNP particles) carrying messenger RNA has been characterized in rat brain. There were first-order kinetics for NAD with an apparent Km for NAD of 90.5 +/- 0.70 microM and Vmax of 19.7 +/- 2.8 pmol ADP-ribose incorporated min-1 mg protein-1. Five poly(ADP-ribose) protein acceptors were identified in the Mr 37,000-120,000 range. It is hypothesized that ADP-ribosylation of specific free mRNP proteins might play a role in the derepression and translation of the silent mRNAs of free mRNP particles. 相似文献
103.
Formation of the Neurotransmitter Glycine from the Anticonvulsant Milacemide Is Mediated by Brain Monoamine Oxidase B 总被引:5,自引:1,他引:4
Philippe Janssens de Varebeke Robert Cavalier Michèle David-Remacle Moussa B. H. Youdim 《Journal of neurochemistry》1988,50(4):1011-1016
Milacemide (2-n-pentylaminoacetamide) is a secondary monoamine that in the brain is converted to glycinamide and glycine. This oxidative reaction was suspected to involve the reaction of monoamine oxidase (MAO). Using mitochondrial preparations from tissues that contain MAO-A and -B (rat brain and liver), MAO-A (human placenta), and MAO-B (human platelet and bovine adrenal chromaffin cell), it has been established that mitochondria containing MAO-B rather than MAO-A oxidize (H2O2 production and glycinamide formation) milacemide. The apparent Km (30-90 microM) for milacemide oxidation by mitochondrial MAO-B preparations is significantly lower than that for milacemide oxidation by mitochondrial MAO-A (approximately 1,300 microM). In vitro MAO-B (l-deprenyl and AGN 1135) rather than MAO-A (clorgyline) selectively inhibited the oxidation of milacemide. These in vitro data are matched by ex vivo experiments where milacemide oxidation was compared to oxidation of serotonin (MAO-A) and beta-phenylethylamine (MAO-B) by brain mitochondria prepared from rats pretreated with clorgyline (0.5-10 mg/kg) and l-deprenyl (0.5-10 mg/kg). Furthermore, in vivo experiment demonstrated that l-deprenyl selectively increased the urinary excretion of [14C]milacemide and the total radioactivity with a concomitant decrease of [14C]glycinamide. Such changes were not observed after clorgyline treatment, but were evident only at doses beyond clorgyline selectivity. The present data therefore demonstrate that milacemide is a substrate for brain MAO-B, and its conversion to glycinamide, further transformed to the inhibitory neurotransmitter, glycine, mediated by this enzyme may contribute to its pharmacological activities. 相似文献
104.
Jean-Paul Kan Régis Steinberg Jérome Leclercq Paul Worms Kathleen Biziere 《Journal of neurochemistry》1988,50(4):1137-1144
In rodents, SR 95191 [3-(2-morpholinoethylamino)-4-cyano-6-phenylpyridazine] has been shown to be active in animal models of depression. The profile of activity of SR 95191 suggests that the compound is a selective and short-acting type A monoamine oxidase (MAO) inhibitor (MAOI) in vivo. In the present study, the interaction of SR 95191 with MAO-A and MAO-B activity was further examined in vivo and in vitro. In brain, liver, and duodenum of pretreated rats, SR 95191 selectively inhibited MAO-A (ED50 = 3-5 mg/kg, p.o.), whereas MAO-B was only weakly inhibited for doses as high as 300 mg/kg, p.o. In vivo, SR 95191 (1-100 mg/kg, p.o.) antagonized, in a dose-dependent fashion, the irreversible inhibition of brain and liver MAO-A induced by phenelzine. Finally, dopamine and 5-hydroxytryptamine depleted from their striatal stores by tetrabenazine were able to displace SR 95191 from the active site of MAO-A. However, ex vivo, kinetic studies showed that the inhibitory effect of SR 95191 (1-10 mg/kg) towards MAO-A was noncompetitive and was unchanged after dilution or dialysis. In vitro, the inhibition of brain MAO-A, but not MAO-B, by SR 95191 was time dependent, with a 19-fold decrease in the IC50 values being observed over a 30-min incubation period (140 to 7.5 microM). At this time, the SR 95191-induced inhibition of MAO-A was not removed by repeated washings. When the reaction was started by adding the homogenate without prior preincubation with SR 95191, the inhibition of brain MAO-A was fully competitive (Ki = 68 microM).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
105.
Inhibition by Cyclic AMP of Phorbol Ester-Potentiated Norepinephrine Release from Guinea Pig Brain Cortical Synaptosomes 总被引:8,自引:4,他引:4
Hisato Shuntoh Kohtaro Taniyama Hisashi Fukuzaki Chikako Tanaka 《Journal of neurochemistry》1988,51(5):1565-1572
The involvement of Ca2+/phospholipid-dependent protein kinase (protein kinase C, PKC) and cyclic AMP-dependent protein kinase in the K+-evoked release of norepinephrine (NE) was studied using guinea pig brain cortical synaptosomes preloaded with [3H]NE. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent activator of PKC, enhanced the K+-evoked release of [3H]NE, in a concentration-dependent manner, but with no effect on the spontaneous outflow and uptake of [3H]NE in the synaptosomes. The apparent affinity of the evoked release for added calcium but not the maximally evoked release was increased by TPA (10(-7) M). Inhibitors of PKC, polymyxin B, and a more potent inhibitor, staurosporine, counteracted the TPA-induced potentiation of the evoked release. Both forskolin and dibutyryl cyclic AMP (DBcAMP) enhanced the evoked release, but reduced the TPA-potentiated NE release. A novel inhibitor of cyclic AMP-dependent protein kinase, KT5720, blocked both the forskolin-induced increase in the evoked release and its inhibition of TPA-induced potentiation in the evoked release, thereby suggesting that forskolin or DBcAMP counteracts the Ca2+-dependent release of NE by activating cyclic AMP-dependent protein kinase. These results suggest that the activation of PKC potentiates the evoked release of NE and that the activation of cyclic AMP-dependent protein kinase acts negatively on the PKC-activated exocytotic neurotransmitter release process in brain synaptosomes of the guinea pig. 相似文献
106.
Christine L. Truitt Elizabeth A. Weaver William G. Haldenwang 《Molecular & general genetics : MGG》1988,212(1):166-171
Summary The E-37 gene ctc was inactivated by a site-specific insertion into the Bacillus subtilis chromosome. The resulting mutation inhibited sporulation by 95% at elevated temperatures (48° C). If the ctc
- mutation is placed in a strain that carries a mutation in the closely linked but distinct spoVC gene, ctc now affects both growth and sporulation at elevated temperatures. Growth of the ctc
- spoVC285 strain was transiently inhibited when exponentially growing cultures were shifted from 37° C to 48° C. A similar, but less pronounced growth lag, was also seen in a B. subtilis strain carrying only the spoVC-285 mutation. This finding suggests that both the ctc and spoVC products function in vegetatively growing B. subtilis. 相似文献
107.
Alteration of the synthesis of lipoxygenase in the early stages of soybean cotyledon culture 总被引:1,自引:0,他引:1
Xuemin Wang Gerhard Bookjans Mitch Altschuler Glenn B. Collins David F. Hildebrand 《Physiologia plantarum》1988,72(1):127-132
A detailed study of lipoxygenase (EC 1.13.11.12) synthesis in cotyledons of soybean [ Glycine max (L.) Merr. cv. Century] cultured in vitro for up to 40 h showed that synthesis of this protein, measured by in vivo [35S]-methionine labelling in connection with immunological methods and cell-free translation of mRNA, underwent a large transient reduction in the first 4 h of culturing and gradually increased in the following 36 h. Northern blot hybridizations with lipoxygenase cDNA clones showed that the decrease in translational activity was the consequence of a considerable reduction in lipoxygenase mRNA in the cotyledons. From these results we conclude that the transient decline in lipoxygenase synthesis in excised soybean cotyledons is regulated at the RNA level. Similarly judged from the analysis of patterns of uni-dimensional gel electrophoresis, the synthesis of a few other polypeptides decreased during the first 4 h of culture as well, while several others increased; in cotyledons cultured for 20 to 40 h the protein-synthesis pattern had returned to that in freshly excised cotyledons. An acclimation period of ca 1 day seems to be needed for isolated soybean cotyledons to stabilize and to resume regular RNA and protein synthesis. 相似文献
108.
Victor Chude 《Plant and Soil》1988,112(2):293-295
The profile distribution of total and extractable B was determined in 16 Nigerian cacao-growing soil profiles formed from
different parent materials. Total B for all soils ranged from 8 to 54μgg−1 with a mean of 24μgg−1. The soils formed from sandstones in the rainforest zone contained higher amounts of total B than soils derived from basement
complex. Boron extractable in hot water, in 0.1% CaCl2, and in 1N NH4OAc varied from 0.13 to 1.38, 0.44 to 1.20 0.03 to 0.56μgg−1 respectively. The corresponding means were 0.66, 0.75 and 0.27μgg−1 B. Soils on metamorphic rocks gave the highest values. All extractable B values were related to organic matter while only
CaCl2-extractable B correlated with total B. Generally total and extractable B values were higher in the top soils than in the
subsoils. 相似文献
109.
110.
本文用凝胶直读法、末端鉴定法等相配合,测定了樗蚕(Philosamia cynthia)絲腺5SrRNA的核苷酸顺序:AGACAACGUCCAUACCACGUUGAAAACACCGGUUCUCGUCCGAUCACCGAAGUCAAGCAACGUCGGGCGCGGUCAGUACUUGGAUGGGUGACCGCCUGGGAACACCGCGUGCUGUUGGCUU比较了樗蚕、蓖麻蚕、柞蚕、家蚕、果蝇等5SrRNA结构差异,在分子水平上探讨了昆虫的分化。 相似文献