全文获取类型
收费全文 | 103406篇 |
免费 | 3375篇 |
国内免费 | 56篇 |
专业分类
106837篇 |
出版年
2021年 | 418篇 |
2018年 | 1411篇 |
2017年 | 1449篇 |
2016年 | 3447篇 |
2015年 | 7225篇 |
2014年 | 6906篇 |
2013年 | 6802篇 |
2012年 | 6402篇 |
2011年 | 3546篇 |
2010年 | 3080篇 |
2009年 | 2864篇 |
2008年 | 1800篇 |
2007年 | 1593篇 |
2006年 | 1506篇 |
2005年 | 7486篇 |
2004年 | 6172篇 |
2003年 | 4223篇 |
2002年 | 1823篇 |
2001年 | 1696篇 |
2000年 | 936篇 |
1999年 | 1876篇 |
1998年 | 482篇 |
1992年 | 2073篇 |
1991年 | 2132篇 |
1990年 | 2157篇 |
1989年 | 2098篇 |
1988年 | 2020篇 |
1987年 | 1875篇 |
1986年 | 1691篇 |
1985年 | 1704篇 |
1984年 | 1133篇 |
1983年 | 869篇 |
1982年 | 483篇 |
1981年 | 444篇 |
1980年 | 382篇 |
1979年 | 1095篇 |
1978年 | 787篇 |
1977年 | 629篇 |
1976年 | 661篇 |
1975年 | 901篇 |
1974年 | 1014篇 |
1973年 | 1037篇 |
1972年 | 972篇 |
1971年 | 951篇 |
1970年 | 842篇 |
1969年 | 854篇 |
1968年 | 744篇 |
1967年 | 766篇 |
1966年 | 597篇 |
1965年 | 441篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Translocation of proteins across membranes 总被引:1,自引:0,他引:1
992.
Lipopolysaccharide core mutants of Salmonella typhimurium containing D-glycero-D-manno-heptose 总被引:1,自引:0,他引:1
Mutants resistant to several hydrophobic membrane antagonists were isolated from a "deep rough" (rfaC) mutant of Salmonella typhimurium. The resistance was due to an alteration in the core region lipopolysaccharide composition as evidenced by altered bacteriophage and complement sensitivity and by compositional analysis. The principal change in carbohydrate composition was the predominance of the unusual heptose isomer D-glycero-D-manno-heptose. The unusually wide pleiotropic phenotype of this organism is suggested to be due to a fundamental change in the properties of the bacterial outer membrane. 相似文献
993.
Effect of interferon treatment on blockade of protein synthesis induced by poliovirus infection 总被引:1,自引:0,他引:1
Treatment of HeLa cells with lymphoblastoid interferon leads to a drastic inhibition of infective poliovirus. Even relatively high concentrations of human lymphoblastoid interferon HuIFN-alpha (Ly) (400 IU/ml) do not prevent destruction of the cell monolayer after most of the cells have been infected with poliovirus. Analysis of macromolecular synthesis in a single step growth cycle of poliovirus in interferon-treated cells detected no viral protein synthesis. In spite of this inhibition of viral translation, the shut-off of host protein synthesis in interferon-treated cells is apparent when they are infected both at low and high multiplicities. Although viral RNA synthesis is inhibited considerably in cells treated with interferon, a certain amount is detected, suggesting that some viral replication takes place. Analysis of membrane permeability after poliovirus infection shows a leakage to 86Rb+ ions and modification of membrane permeability to the translation inhibitor hygromycin B at the moment when the bulk of virus protein synthesis occurs. These changes are delayed and even prevented if cells are pretreated with interferon. A situation is described in which host protein synthesis is shut-down with no major changes in membrane permeability, as studied by the two tests mentioned above. Prevention of viral gene expression by inactivation with ultraviolet light of the input virus or by treatment with cycloheximide blocks the shut-off of protein synthesis. This does not occur in the presence of 3 mM guanidine. These observations are in agreement with the idea that some poliovirus protein synthesis takes place in interferon-treated cells and this early gene expression is necessary to block cellular protein synthesis. 相似文献
994.
Reactions in vitro of the DNA polymerase-primase from Xenopus laevis eggs. A role for ATP in chain elongation 总被引:5,自引:0,他引:5
A form of DNA polymerase alpha was purified several thousandfold from a protein extract of Xenopus laevis eggs. The enzyme effectively converts, in the presence of ribonucleoside triphosphates, a circular single-stranded phage fd DNA template into a double-stranded DNA form and, therefore, must be associated with a DNA primase. We first show by gel electrophoresis in the presence of sodium dodecyl sulfate that both enzymatic activities, DNA polymerase and primase, most probably reside on a greater than 100 000-Da subunit of the DNA polymerase holoenzyme. We then assayed the polymerase-primase at various template/enzyme ratios and found that the DNA complementary strand sections synthesized in vitro belong to defined size classes in the range of 600-2000 nucleotides, suggesting preferred start and/or stop sites on the fd DNA template strand. We show that the stop sites coincide with stable hairpin structures in fd DNA. We have used a fd DNA template, primed by a restriction fragment of known size, to show that the polymerase-primase stops at the first stable hairpin structure upstream from the 3'-OH primer site when the reaction was carried out at 0.1 mM ATP. However, at 2 mM ATP the enzyme was able to travers this and other stop sites on the fd DNA template strand leading to the synthesis of 2-4 times longer DNA strands. Our results suggest a role for ATP in the polymerase-primase-catalyzed chain-elongation reaction. 相似文献
995.
UDPglucosyltransferase and its kinetic fluorimetric assay 总被引:1,自引:0,他引:1
A rapid, kinetic assay for UDPglucosyltransferase has been developed using 1-naphthol as substrate. It is based on the continuous fluorimetric monitoring of 1-naphthyl glucoside formation during the reaction at physiological pH. The conjugate is easily distinguished from aglycone, since their fluorimetric properties differ. Glucoside biosynthesis in vitro by microsomal preparations isolated from the gut and fat body of cockroaches Periplaneta americana and Leucophaea maderae, and from the green gland and hepatopancreas of the crayfish Astacus astacus, has been demonstrated. The effects of buffer, pH, MgCl2, UDP-glucuronic acid, UDP-N-acetylglucosamine, sodium cholate and sonication on the enzyme activity have been assessed. The kinetic parameters of 1-naphthol and UDP-glucose have also been determined. 相似文献
996.
W Stoffel H Hillen W Schr?der R Deutzmann 《Hoppe-Seyler's Zeitschrift für physiologische Chemie》1983,364(10):1455-1466
The amino-acid sequence of bovine myelin lipophilin (proteolipid apoprotein, Folch-protein) has been completed. Lipophilin is a 276 amino acid residues containing, extremely hydrophobic membrane protein with molecular mass 30,000 Da. The sequence determination was based on automated Edman degradation of four tryptophan and four cyanogen bromide fragments and of proteolytic peptides of complete lipophilin as well as the fragments obtained by chemical cleavage. Four additional sequences were determined which led to the completion of the primary structure. Lipophilin is esterified at threonine-198 by long chain fatty acids (palmitic, stearic and oleic acid). The attachment site has been established at the same threonine residue in three different peptides isolated from thermolysinolytic, papainolytic and chymotrypsinolytic hydrolysates. This threonine residue is part of a hydrophilic segment of lipophilin. The covalent fatty acyl bond is being discussed together with important structural and functional properties of this membrane protein which can be derived from sequence information. New separation and purification methods of hydrophobic and hydrophilic polypeptides for this sequence determination (fractional solubilization, silica gel exclusion, high-performance liquid chromatography) had to be elaborated as indispensable tools. They are generally applicable to the structural analysis of hydrophobic membrane proteins. Four long (26, 29, 40 and 36 residues) and one medium long (12 residues) hydrophobic segments are separated by four predominantly positively and one negatively charged hydrophilic segments. On the basis of structural data a model for the membrane integration of lipophilin is proposed. 相似文献
997.
H Vierhapper P Bratusch-Marrain W Waldh?usl P Nowotny S Panzer 《Hormones et métabolisme》1983,15(1):24-29
The influence of different blood glucose concentrations on the arginine (30 g/30 min i.v.) and TRH (400 micrograms i.v.) induced release of growth hormone and prolactin was studied in six male type II-diabetic patients. Blood glucose concentrations were clamped at euglycaemic (4-5 mmol/l) or hyperglycaemic (12-18 mmol/l) levels by means of an automated glucose-controlled insulin infusion system. The response of growth hormone to arginine, and irregular spikes in growth hormone concentrations following TRH seen in the euglycaemic state were suppressed during hyperglycaemia. The suppression of the arginine-induced release of growth hormone by hyperglycaemia was observed both with and without concomitant administration of exogenous insulin. The rise in serum prolactin concentrations in response to arginine was unaffected by hyperglycaemia, whereas the TRH-induced release of prolactin was suppressed. Since arginine induces the release of growth hormone and prolactin via the hypothalamus, while TRH acts at the pituitary level, the glycaemic state appears to exert a modulatory effect on the secretion of growth hormone and prolactin in type II-diabetics at both locations. 相似文献
998.
Clinical factors influencing the absorption of 125I-NPH insulin in diabetic patients 总被引:1,自引:0,他引:1
Clinical factors which might influence the absorption of subcutaneously injected 125I-NPH insulin were studied in 101 diabetics. The disappearance curve was monoexponential after a delay period of 1.5 +/- 0.8 h (mean +/- SD). Lipohypertrophy significantly prolonged insulin absorption (half life (T1/2) = 11.2 +/- 3.1 h, p = 0.0001). Low bicarbonate levels increased the absorption (T1/2 3.9 +/- 2.3 h, p less than 0.05). Lean diabetics had a faster absorption (6.2 +/- 1.9 h) than normal weight diabetics (7.5 +/- 2.0 h, p less than 0.02). Sex, age, diabetes duration and injection depth did not influence T1/2. The half life was significantly inversely correlated to the resting subcutaneous blood flow (r = 0.882, p less than 0.01). The overall interindividual coefficient of variation for insulin absorption in nonketotic diabetics was 27.4%. Also considerable intra-patient day-to-day variation was found (24.5%), and between different injection sites (30.2%). These variations emphasize the drawbacks of conventional insulin therapy in the management of insulin-requiring diabetics. 相似文献
999.
1. Difference spectroscopy studies indicated that tetrahydrodeoxyaflatoxin B1 and aflatoxicol bind slightly to DNA, whereas aflatoxins B2a, G2a, G2 and aflatoxicol bind to bovine and porcine spleen DNAse II but aflatoxins B1, B2, G1 and tetrahydrodeoxyaflatoxin did not. 2. Kinetic studies showed that aflatoxins B1, G1 and B2 activated bovine and porcine spleen DNAse II while aflatoxins B2a, G2a and G2 had an inhibiting effect. 3. Dissociation constants for the enzyme: substrate-aflatoxin complexes (KAS) as well as the inhibition constants (Ki) were obtained from kinetic studies. 相似文献
1000.
I Hovestadt M Ernst H M?nig H Fischer 《International journal of radiation biology and related studies in physics, chemistry, and medicine》1983,44(6):563-573
The metabolic burst accompanying phagocytosis of granulocytes (PMN) leads to the generation of activated oxygen species such as O-2, H2O2, 1O2 and OH; which give rise to chemiluminescence (CL) in the presence of luminol. Reliable CL-measurements of stimulated PMN can be carried out in freshly drawn mouse blood, when photon counts are related to the number of PMN. Effects of low dose total body X-irradiation were studied using C57B1/6 mice. It was found that 24 and 48 hours after irradiation (0.24-0.95 Gy) CL of whole blood was slightly decreased. If however CL-counts were related to the number of PMN, an enhanced CL per single granulocyte was recorded. The administration of cystamine leads to an immune stimulating effect of unirradiated animals. In animals, who received 0.95 Gy a distinct radioprotective effect of cystamine can be observed. 相似文献