全文获取类型
收费全文 | 835190篇 |
免费 | 99818篇 |
国内免费 | 517篇 |
专业分类
935525篇 |
出版年
2016年 | 8388篇 |
2015年 | 12127篇 |
2014年 | 14412篇 |
2013年 | 20548篇 |
2012年 | 22728篇 |
2011年 | 22931篇 |
2010年 | 15198篇 |
2009年 | 14377篇 |
2008年 | 20443篇 |
2007年 | 21558篇 |
2006年 | 20307篇 |
2005年 | 19644篇 |
2004年 | 19469篇 |
2003年 | 18773篇 |
2002年 | 18340篇 |
2001年 | 36123篇 |
2000年 | 36921篇 |
1999年 | 29604篇 |
1998年 | 10456篇 |
1997年 | 11017篇 |
1996年 | 10465篇 |
1995年 | 10045篇 |
1994年 | 9974篇 |
1993年 | 9913篇 |
1992年 | 25193篇 |
1991年 | 24695篇 |
1990年 | 24186篇 |
1989年 | 23493篇 |
1988年 | 22047篇 |
1987年 | 21162篇 |
1986年 | 19947篇 |
1985年 | 20145篇 |
1984年 | 16879篇 |
1983年 | 14822篇 |
1982年 | 11558篇 |
1981年 | 10778篇 |
1980年 | 10046篇 |
1979年 | 16711篇 |
1978年 | 13223篇 |
1977年 | 12134篇 |
1976年 | 11473篇 |
1975年 | 12849篇 |
1974年 | 13682篇 |
1973年 | 13534篇 |
1972年 | 12580篇 |
1971年 | 11244篇 |
1970年 | 9804篇 |
1969年 | 9433篇 |
1968年 | 8625篇 |
1967年 | 7415篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
R E Byrne D Polacek J I Gordon A M Scanu 《The Journal of biological chemistry》1984,259(23):14537-14543
The proteolytic activity directed against apolipoprotein A-II (apo-A-II) which is released from human blood polymorphonuclear cells (PMN) when they are incubated with human plasma high-density lipoprotein-3 (HDL3) was studied to assess the properties and site specificity of the enzyme. When 125I-apo-A-II-labeled HDL3 was incubated with the PMN protease at 37 degrees C, a complete cleavage of apo-A-II was observed which paralleled the formation of bands of approximately 11,000 and 7,000 daltons by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 7,000-dalton component had the following N-terminal sequence: NH2-Thr-Asp-Tyr-Gly-Lys-Asp-Leu-Met-Glu-Lys. This corresponds to residues 19 through 28 of the intact apo-A-II monomer. Methoxysuccinyl (MeO-Suc)-Ala-Ala-Pro-Val-chloromethylketone-(CH2Cl) caused a 90% inhibition of apo-A-II hydrolysis at the highest concentration tested (6 X 10(-4)M). Besides apo-A-II, the PMN enzyme also hydrolyzed a synthetic substrate, MeO-Suc-Ala-Ala-Pro-Val-4-nitroanilide and its 4-methylcoumaryl-7-amide analogue. The protease appeared to have a mass of 28,000 daltons as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the [3H]diisopropylfluorophosphate-labeled PMN enzyme. That the PMN enzyme which cleaves apo-A-II is an elastase was derived from the following criteria: 1) cleavage at the Val-X bond in apo-A-II and in the two synthetic substrates studied; 2) prevention of the cleavage by MeO-Suc-Ala-Ala-Pro-Val-CH2Cl, a known specific elastase inhibitor; and 3) a mass comparable to that reported for a pure PMN elastase. These studies establish that apolipoproteins can be suitable substrates for enzymes of the elastase family. 相似文献
962.
Partial purification of the maturation-promoting factor MPF from unfertilized eggs of Xenopus laevis 总被引:5,自引:0,他引:5
P Nguyen-Gia M Bomsel J P Labrousse C L Gallien H Weintraub 《European journal of biochemistry》1986,161(3):771-777
A 200-fold purification of the maturation-promoting factor or MPF from unfertilized eggs of Xenopus laevis is reported for the first time. Purification was achieved by three successive column chromatographies on hydroxyapatite, trisacryl blue and L-arginine-agarose. The presence of MPF was assessed by the usual maturation criteria after injections of test material into immature stage VI unstimulated X. laevis oocytes: the precocious appearance of the maturation spot (within 45-120 min), the germinal vesicle breakdown, the presence of the first polar body and the second metaphase spindle. Purification was monitored by the decrease of the minimal amount of protein injected in a constant volume (50 nl) required to induce 50% frequency of germinal vesicle breakdown. This amount decreased from 500 ng in the crude extract to 2.5 ng in the 200-fold purified material. Analysis by SDS-PAGE of the crude extract showed about 40 Coomassie-blue-stained polypeptides with molecular masses ranging from 300 kDa to 20 kDa, whereas in the 200-fold purified MPF only 5 stained polypeptides were revealed, with molecular masses of 62, 53, 49, 39 and 37 kDa. In vitro phosphorylations for the detection of kinase activities for endogenous and exogenous substrates were monitored by analysis of autoradiograms of SDS-PAGE, after treatment of fractions with [gamma-32P]ATP. Only inactive fractions eluted from columns ahead of MPF, and fractions containing MPF activity were tested. Phosphorylation of numerous stained polypeptides was demonstrated in the crude MPF extract and exogenous substrates such as phosvitin, casein and histone type II-AS were also strongly phosphorylated. In the MPF fraction, purified on hydroxyapatite, a polypeptide of 53 kDa was more highly and specifically phosphorylated and the presence of kinase activities was observed for the above three exogenous substrates. In the 100-fold and 200-fold purified MPF, phosphorylation of endogenous substrates could not be shown and kinase activities for the above three substrates were drastically decreased as compared with the crude and purified MPF obtained after hydroxyapatite column chromatography. However, neither endogenous phosphorylations nor kinase activities with the above exogenous substrates could be shown in inactive fractions eluted ahead of MPF at the different purification steps. Some characteristics of the purified material are also described in this paper. 相似文献
963.
The accumulation of polyphosphate in Acinetobacter spp. 总被引:5,自引:0,他引:5
964.
Diurnal variations in response of rat liver tyrosine aminotransferase activity to food intake 总被引:1,自引:0,他引:1
Effects of fasting and refeeding on the hepatic tyrosine aminotransferase activity were examined in rats that had been fed during the night. The tyrosine aminotransferase activity showed clear diurnal variations, with a maximal activity after the feeding time. The tyrosine aminotransferase rhythm persisted even under starvation, though the amplitude decreased remarkably. When the starved rats were refed at night, the tyrosine aminotransferase activity increased rapidly to a high level, but it increased slowly to a rather lower level when they were refed in daytime. 相似文献
965.
A method is described for the subcellular fractionation of goldfish xanthophores. The procedure produces relatively pure fractions of caroteniod droplets, pterinosomes, cytosol and what appears to be plasma membrane. The presence of a distinct pattern of proteins is shown to be associated with the carotenoid droplets. Treatment of the xanthophores with ACTH affects the buoyant density of some carotenoid droplets and stimulates the phosphorylation of a polypeptide associated with the carotenoid droplets. 相似文献
966.
Ten asthmatic children with a history of cough and wheeze after drinking a cola drink performed histamine inhalation tests before and 30 minutes after a drink of Pepsi-Cola, soda water, and water on three separate study days. There was no significant change in baseline peak expiratory flow after any of the three drinks. Sensitivity to histamine was increased after the cola drink (p less than 0.005) but was not significantly different after soda water or water. The detection of change in sensitivity to histamine appears to be a simple and effective method of testing for food sensitivity in asthma. 相似文献
967.
An in vivo/in vitro evaluation of teratogenic action 总被引:1,自引:0,他引:1
Several compounds were administered to pregnant Wistar-derived rats either 24 or four hours prior to the recovery of day 10 embryos for in vitro culture in Waymouth's medium and fetal calf serum. The compounds tested were 2-amino-1,3,4-thiadiazole (thiadiazole), cadmium sulfate, 1,2-dibromo-2,2-dichloroethyl dimethyl phosphate (dibrom), 2-(sec-Butyl)-4,6-dinitrophenol (dinoseb), led nitrate, polybrominated biphenyls (PBB), sodium arsenate, and trypan blue. After 24 hours in culture, two thirds of the embryos were recovered for examination. The remaining one third were continued in culture until 42 hours. Recovered embryos were examined for rotation of the embryonic axis, heart rate, establishment of the visceral yolk sac circulation, somite number, growth of the limb buds, closure of the neural tube, and development of the allantois and amnion. All tested compounds inhibited the rate of development in vitro. 相似文献
968.
W J Theuvenet T Nolthenius-Puylaert Z L Giedrojc Juraha J M Borghouts R van Twisk 《Plastic and reconstructive surgery》1984,74(4):539-543
Idiopathic calcinosis of the scrotum is a rare disease that may cause massive deformation of the scrotal wall. The first patient we present was also known to have neurofibromatosis. In the second patient we describe, nodules of idiopathic calcinosis of the scrotum were seen with walls that evidenced no epithelial lining, as well as calcification in epithelial cysts. At present, the only possible treatment is excision, and we excised the afflicted skin without problems in primary wound closure. 相似文献
969.
Three acetotrophicMethanosarcina species, which included marine, nonmarine, and thermophilic strains, were grown on acetate in a 10-liter pH auxostat. Specific growth rates and molar growth yields were constant throughout growth. Cell yields were up to 18-fold greater than previously reported. These properties of the pH auxostat indicate that it is a preferred culture method for the biochemical study of methanogenesis from acetate. 相似文献
970.
R Iu Iukhananov A I Ma?ski? Iu V Burov 《Biulleten' eksperimental'no? biologii i meditsiny》1985,100(11):588-591
Addition of ethanol to rat brain homogenate containing opiate receptors inhibits at a concentration of 50 mM the stereospecific binding of 3H-naloxone at 37 degrees C but not at 0 degree C, with the ID50 being 462 mM under these conditions. The temperature-dependent inhibition of the ligand binding suggests that ethanol does not compete with naloxone for specific binding sites of opiate receptors and changes the structure of lipids in biological membranes. Scatchard's analysis has demonstrated that apart from a decrease in the number of highly affinity binding sites of 3H-naloxone, the total amount of the binding sites remains unchanged both in the presence and absence of ethanol and constitutes 453 and 549 fmol/mg protein. It is assumed that ethanol might interconvert highly and low-affinity binding sites. Analysis of the effect of ethanol on 3H-naloxone binding with opiate receptors contained by synaptic membranes obtained from animals with varying predisposition to voluntary alcoholization has shown that ethanol inhibits to a greater degree ligand binding with membranes obtained from rats predisposed to alcoholization. The possibility of the involvement of receptors in the biochemical mechanisms by which the initial alcoholic motivation is effected is under discussion. 相似文献