首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4387篇
  免费   249篇
  国内免费   5篇
  2022年   17篇
  2021年   49篇
  2020年   18篇
  2019年   28篇
  2018年   39篇
  2017年   49篇
  2016年   80篇
  2015年   109篇
  2014年   110篇
  2013年   296篇
  2012年   224篇
  2011年   262篇
  2010年   142篇
  2009年   146篇
  2008年   248篇
  2007年   233篇
  2006年   242篇
  2005年   256篇
  2004年   213篇
  2003年   198篇
  2002年   214篇
  2001年   118篇
  2000年   115篇
  1999年   106篇
  1998年   55篇
  1997年   73篇
  1996年   38篇
  1995年   35篇
  1994年   46篇
  1993年   33篇
  1992年   78篇
  1991年   67篇
  1990年   62篇
  1989年   54篇
  1988年   81篇
  1987年   50篇
  1986年   42篇
  1985年   32篇
  1984年   38篇
  1983年   29篇
  1982年   41篇
  1981年   25篇
  1980年   26篇
  1979年   26篇
  1978年   31篇
  1976年   15篇
  1975年   24篇
  1974年   17篇
  1973年   24篇
  1972年   14篇
排序方式: 共有4641条查询结果,搜索用时 46 毫秒
131.
Immunoreactive alpha B crystallin and a 28-kDa protein in an extract of human pectoral muscle were precipitated by (NH4)2SO4 at 40% saturation, and coeluted during column chromatography on DEAE-Sepharose and on Bio-Gel A-5m. The two proteins were separated on a column of S-Sepharose HP in the presence of 7 M urea. Further chromatography of each of the two resultant fractions on a column of Superdex 75 pg and on a TSK-SP 5PW column in the presence of urea yielded preparations of alpha B crystallin and the 28-kDa protein each of which gave a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The final preparation of 28-kDa protein contained at least two subtypes, which were separable on the TSK-SP column. However, fragmentation patterns of the two major 28-kDa proteins after digestion with endoproteinase Asp-N were identical. Amino acid sequences of peptides formed by cleavage of the purified 28-kDa protein and alpha B crystallin were identical to those of particular regions of the deduced amino acid sequences of human small heat shock protein (HSP28) and lens alpha B crystallin, respectively. Using an immunoassay method, with antibodies raised in rabbits, we found that HSP28 was present in all human tissues tested and at high levels (greater than 1 micrograms/mg protein) in the heart and other tissues composed of striated and smooth muscles. HSP28, found with alpha B crystallin, in extracts of several human and bovine tissues was trapped on and coeluted with alpha B crystallin from an affinity column prepared with antibodies against alpha B crystallin. This result suggests that the two proteins are associated in cells.  相似文献   
132.
Esaka M  Fujisawa K  Goto M  Kisu Y 《Plant physiology》1992,100(1):231-237
Ascorbate oxidase expression in pumpkin (Cucurbita spp.) tissues was studied. Specific ascorbate oxidase activities in pumpkin leaf and stem tissues were about 2 and 1.5 times that in the fruit tissues, respectively. In seeds, little ascorbate oxidase activity was detected. Northern blot analyses showed an abundant ascorbate oxidase mRNA in leaf and stem tissues. Fruit tissues had lower levels of ascorbate oxidase mRNA than leaf and stem tissues. Ascorbate oxidase mRNA was not detected in seeds. Specific ascorbate oxidase activity gradually increased during early seedling growth of pumpkin seeds. The increase was accompanied by an increase in ascorbate oxidase mRNA. When ascorbate oxidase activity in developing pumpkin fruits was investigated, the activities in immature fruits that are rapidly growing at 0, 2, 4, and 7 d after anthesis were much higher than those in mature fruits at 14 and 30 d after anthesis. The specific activity and mRNA of ascorbate oxidase markedly increased after inoculation of pumpkin fruit tissues into Murashige and Skoog's culture medium in the presence of an auxin such as 2,4-dichlorophenoxyacetic acid (2,4-D) but not in the absence of 2,4-D. In the presence of 10 mg/L of 2,4-D, ascorbate oxidase mRNA was the most abundant. Thus, ascorbate oxidase is induced by 2,4-D. These results indicate that ascorbate oxidase is involved in cell growth. In pumpkin callus, ascorbate oxidase activity could be markedly increased by adding copper. Furthermore, immunological blotting showed that the amount of ascorbate oxidase protein was also increased by adding copper. However, northern blot analyses showed that ascorbate oxidase mRNA was not increased by adding copper. We suggest that copper may control ascorbate oxidase expression at translation or at a site after translation.  相似文献   
133.
T Niwa  T Koshiyama  J Goto  T Nambara 《Steroids》1992,57(11):522-529
3-N-Acetylglucosaminides of unconjugated, glycine- and taurine-conjugated bile acids have been synthesized. Bile acids appropriately protected were condensed with acetochloroglucosamine through the 3 alpha-hydroxyl group by means of the Koenigs-Knorr reaction using cadmium carbonate as a catalyst. Subsequent borohydride reduction and/or alkaline hydrolysis provided desired 3-N-acetylglucosaminides of unconjugated bile acids. Glycine-conjugates were obtained from N-acetylglucosaminides of unconjugated bile acids and ethyl glycinate by the carbodiimide method. The preparation of N-acetylglucosaminides of taurine-conjugates was attained by the Koenigs-Knorr reaction of bile acid p-nitrophenyl esters followed by condensation with taurine. 7-N-Acetylglucosaminides of ursodeoxycholates were prepared in a similar fashion. The convenient synthesis of 3-N-acetylglucosaminides of unconjugated bile acids is also described.  相似文献   
134.
It was found that an optically active copolyester, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), denoted as P(3HB-co-3HV), is synthesized by Alcaligenes eutrophus H16 from several amino acids under various fermentation conditions. The optimum condition for the biosynthesis from one amino acid, threonine, was investigated and its biosynthetic pathway was discussed on the basis of the relation between the fermentation condition and the co-monomer composition of the produced polyesters.  相似文献   
135.
We have characterized seven human renal cell carcinoma cell lines established from primary sites of five patients between 1987 and 1989. Two lines, OUR-20P and OUR-20S, were derived from the OUR-20 cells by cloning with a dilution method 3 months after the primary culture. These three cell lines were tumorigenic in athymic nude mice when inoculated subcutaneously. Examined by a dye uptake method, OUR-20 was highly sensitive to interferon-alpha (IFN-alpha); OUR-20P, OUR-20S and OUR-30 showed slight sensitivities, while the other three cell lines were insensitive. All seven cell lines have been maintained for more than 2 years and over 50 passages in vitro. Cytogenetic analyses performed 1.5 to 3 years after the starts of primary cultures indicated that all seven cell lines, which exhibited different morphologies in phase-contrast micrographs, were aneuploid with modal chromosome numbers 41 to 89.  相似文献   
136.
Chick cardiac membranes were affinity labelled by cross-linking to membrane-bound 125I-endothelin-1 with disuccinimidyl tartarate. SDS/PAGE and autoradiographic analysis of the 125I-endothelin-1-labelled material in the presence or absence of 2-mercaptoethanol revealed one major labelled band, corresponding to a molecular mass of 53 kDa, whose appearance was dose-dependently inhibited by the addition of unlabelled endothelin-1 (1-100 nM). Subtracting the molecular mass of 125I-endothelin-1 and disuccinimidyl tartarate, the binding protein appeared to have a molecular mass of 50 kDa. To investigate further the molecular properties of endothelin receptor, the 125I-endothelin-1-endothelin-receptor complex was solubilized from chick cardiac membranes using the detergent digitonin. Sucrose gradient sedimentation of the solubilized complex indicated a sedimentation coefficient of 13 S, whereas the complex of (+)-[3H]PN200-110, a dihydropyridine derivative, and dihydropyridine-sensitive Ca2+ channels sedimented at 22 S. A monoclonal antibody raised against dihydropyridine-sensitive Ca2+ channels from the chick brain did not immunoprecipitate the 125I-endothelin-1-endothelin-receptor complex. These data suggest that endothelin receptor is clearly distinct from dihydropyridine-sensitive Ca2+ channels and endothelin has its own specific 50-kDa receptor.  相似文献   
137.
Human embryonic lung diploid fibroblast, IMR-90 cells secreted a tumor cytotoxic factor. The fibroblast-derived tumor cytotoxic factor (F-TCF) has a cytotoxic activity to Sarcoma 180 and a cytostatic and degenerative activities to KB cells. F-TCF has been purified about 540,000-fold with 23.3% recovery from 75 liters of the conditioned medium containing 5% newborn calf serum. The purified F-TCF is a basic glycoprotein with isoelectric point values of 7.4 to 8.6. It was stable in the pH range from 6.0 to 9.0 and was stable at the heating temperature of 60 degrees C for 10 min, but completely inactivated by reducing it with 2-mercaptoethanol. F-TCF has molecular weight of 76 to 80 kD on SDS-PAGE under non-reducing conditions and is a heterodimer consisting of a large alpha subunit with 52 to 56 kD and a small beta subunit with 30 to 34 kD. F-TCF was identified as one of human hepatocyte growth factors by the physicochemical properties including N terminal and a few internal amino acid sequences. We have confirmed that F-TCF has an ability to dramatically stimulate DNA synthesis in adult rat hepatocytes in the low dose range of 1 to 10 ng/ml.  相似文献   
138.
Novel restriction fragment length polymorphisms (RFLPs) in inbred rats were revealed with the human N-ras gene as probe. Three fragments hybridizing to the probe were detected by Southern blot hybridization under highly stringent conditions, and one of the fragments showed variation in inbred rat strains. Furthermore, on hybridization under low-stringency conditions, an additional fragment hybridizing to the probe was observed, and this fragment also showed interstrain variation. These two variant fragments showed different distributions in 27 inbred rat strains and segregated in backcross progeny as codominant alleles of independent single autosomal loci. Therefore, the loci for these RFLPs were named Nras-1 and Nras-2, respectively. Analyses of linkages between the RFLPs and 11 other loci revealed that the Nras-2 locus was closely linked to the c locus (3.7 +/- 2.6%), which belongs to rat linkage group I.  相似文献   
139.
A new acid carboxypeptidase was purified fromAspergillus oryzae grown on solid bran culture medium. The purified enzyme was found to be homogeneous by disc gel electrophoresis at pH 9.4 and isoelectric focusing. The enzyme was termedA. oryzae acid carboxypeptidase O-1 with isoelectric point 4.08. The substrate specificity of the new enzyme was investigated with proangiotensin, angiotensin, and bradykinin. Even when the proline was present at the penultimate position of the peptide, the enzyme rapidly hydrolyzed the carboxyterminal Pro-X (X=amino acid) peptide bond. TheK m andk cat values for angiotension (–Pro7–Phe8) at pH 3.7 and 30°C were 0.2 mM and 1.7 sec–1, respectively.  相似文献   
140.
Summary The direct innervation of the pial blood vessels by serotonin neurons has been demonstrated with a modified peroxidase-antiperoxidase technique in the mammalian central nervous system. The pia mater covering the ventrolateral surface of the medulla oblongata is innervated by numerous varicose serotonin fibers originating from the serotonin neurons of the lower brainstem. Scattered serotonin fibers were observed in the pia mater in every part of the brain and spinal cord.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号