全文获取类型
收费全文 | 14617篇 |
免费 | 1235篇 |
国内免费 | 2篇 |
专业分类
15854篇 |
出版年
2024年 | 21篇 |
2023年 | 153篇 |
2022年 | 98篇 |
2021年 | 239篇 |
2020年 | 222篇 |
2019年 | 227篇 |
2018年 | 586篇 |
2017年 | 482篇 |
2016年 | 517篇 |
2015年 | 431篇 |
2014年 | 469篇 |
2013年 | 841篇 |
2012年 | 1183篇 |
2011年 | 1338篇 |
2010年 | 695篇 |
2009年 | 459篇 |
2008年 | 992篇 |
2007年 | 901篇 |
2006年 | 876篇 |
2005年 | 689篇 |
2004年 | 693篇 |
2003年 | 655篇 |
2002年 | 587篇 |
2001年 | 417篇 |
2000年 | 536篇 |
1999年 | 270篇 |
1998年 | 143篇 |
1997年 | 106篇 |
1996年 | 101篇 |
1995年 | 82篇 |
1994年 | 81篇 |
1993年 | 82篇 |
1992年 | 70篇 |
1991年 | 68篇 |
1990年 | 45篇 |
1989年 | 37篇 |
1988年 | 37篇 |
1987年 | 32篇 |
1986年 | 18篇 |
1985年 | 44篇 |
1984年 | 33篇 |
1983年 | 31篇 |
1982年 | 34篇 |
1981年 | 24篇 |
1980年 | 24篇 |
1979年 | 25篇 |
1978年 | 22篇 |
1977年 | 18篇 |
1974年 | 17篇 |
1973年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Insulin-like growth factor (IGF)-binding protein-2 (IGFBP-2) is altered in different diseases and might be used as an indication of its severity. The aims of our study were to investigate: (1) the developmental pattern of the serum IGFBP-2 concentration at birth and during childhood and adolescence; (2) whether the serum IGFBP-2 level could be a marker for the diagnosis and evolution of diseases where the growth hormone (GH)-IGF axis is altered, and (3) whether this binding protein shows a relationship with IGF-I, its free fraction, IGFBP-1 and -3. We report reference values for 55 normal full-term newborns and 221 normal children who were divided into 5 groups according to their Tanner stage. Serum levels were higher in newborns when compared with Tanner stages I-V (p < 0.001, ANOVA), with no further changes throughout development. Furthermore, we studied IGFBP-2 levels in 24 children with congenital GH deficiency (GHD), 26 with acute lymphoblastic leukemia (ALL), 75 obese children, and 60 girls with anorexia nervosa (AN) at diagnosis and during a follow-up period. IGFBP-2 at diagnosis was increased in GHD, ALL and AN, and decreased in obesity (p < 0.05, ANOVA). During the follow-up, IGFBP-2 concentrations tended to normalize. IGFBP-2 correlated positively with IGFBP-1 and negatively with IGF-I and IGFBP-3 in normal subjects and at diagnosis of the pathologies studied. Although IGFBP-2 functions are not well understood, these results suggest a possible role for this protein in diseases where the GH-IGF axis is altered. 相似文献
82.
Artichoke leaf extract was studied in human leukocytes for activity against oxidative stress using flow cytometry and dichlorofluorescin diacetate as a fluorescence probe. It produces a concentration-dependent inhibition of oxidative stress when cells are stimulated with agents that generate reactive oxygen species (ROS): hydrogen peroxide, phorbol-12-myristate-13-acetate (PMA), and N-formyl-methionyl-leucyl-phenylalanine (FMLP). Cynarin, caffeic acid, chlorogenic acid, and luteolin, constituents of artichoke leaf extract, also show a concentration-dependent inhibitory activity in the above models, contributing to the antioxidant activity of the extract in human neutrophils. 相似文献
83.
Carlos Peña Silvia López Andrés García Guadalupe Espín Angel Romo-Uribe Daniel Segura 《Annals of microbiology》2014,64(1):39-47
The aim of this study was to characterize the influence of the aeration conditions on the production of PHB and its molecular mass in a mutant strain of Azotobacter vinelandii (OPN), which carries a mutation on ptsN, the gene encoding enzyme IIANtr, previously shown to increase the accumulation of PHB. Cultures of A. vinelandii wild-type strain OP and its mutant derivative strain OPN were grown in 500-mL flasks, containing 100 and 200 mL of PY sucrose medium. PHB production and its molecular mass were analyzed at the end of the culture. The molecular mass (MM) was significantly influenced by the aeration conditions and strain used. A polymer with a higher molecular weight was produced under low aeration conditions for both strains. A maximal molecular mass of 2,026 kDa (equivalent to 3,670 kDa measured by GPC) was obtained with strain OPN cultured under low-aeration conditions, reaching a value two-fold higher than that obtained from the parental strain OP (MM?=?1,013 kDa) grown under the same conditions. Aeration conditions and the ptsN mutation influence the molecular mass of the PHB produced by A. vinelandii affecting in turn its physico-chemical properties. 相似文献
84.
Cynthia Sarniguet Jeannette Toloza Micaella Cipriani Michel Lapier Marisol Vieites Yanis Toledano-Magaña Juan Carlos García-Ramos Lena Ruiz-Azuara Virtudes Moreno Juan Diego Maya Claudio Olea Azar Dinorah Gambino Lucía Otero 《Biological trace element research》2014,159(1-3):379-392
Parasitic illnesses are major causes of human disease and misery worldwide. Among them, both amebiasis and Chagas disease, caused by the protozoan parasites, Entamoeba histolytica and Trypanosoma cruzi, are responsible for thousands of annual deaths. The lack of safe and effective chemotherapy and/or the appearance of current drug resistance make the development of novel pharmacological tools for their treatment relevant. In this sense, within the framework of the medicinal inorganic chemistry, metal-based drugs appear to be a good alternative to find a pharmacological answer to parasitic diseases. In this work, novel ruthenium complexes [RuCl2(HL)(HPTA)2]Cl2 with HL = bioactive 5-nitrofuryl containing thiosemicarbazones and PTA?=?1,3,5-triaza-7-phosphaadamantane have been synthesized and fully characterized. PTA was included as co-ligand in order to modulate complexes aqueous solubility. In fact, obtained complexes were water soluble. Their activity against T. cruzi and E. histolytica was evaluated in vitro. [RuCl2(HL4)(HPTA)2]Cl2 complex, with HL4?=?N-phenyl-5-nitrofuryl-thiosemicarbazone, was the most active compound against both parasites. In particular, it showed an excellent activity against E. histolytica (half maximal inhibitory concentration (IC50)?=?5.2 μM), even higher than that of the reference drug metronidazole. In addition, this complex turns out to be selective for E. histolytica (selectivity index (SI) >38). The potential mechanism of antiparasitic action of the obtained ruthenium complexes could involve oxidative stress for both parasites. Additionally, complexes could interact with DNA as second potential target by an intercalative-like mode. Obtained results could be considered a contribution in the search for metal compounds that could be active against multiple parasites. 相似文献
85.
J. Juárez-Muñoz G. Carrillo-Castañeda R. Arreguin A. Rubluo 《Biodiversity and Conservation》2002,11(5):921-930
The species of Prosopis (Leguminosae) are welladapted to grow in the arid regions of the world. In Mexico, nearly 70%of its territory accounts for arid lands and nine species of this genus arefound in the country which are currently utilized for multiple purposes. Giventhat a sustainable exploitation of this important natural resource is necessary,knowledge of the genetic variability of natural populations is required. Thegenetic variation among and within four wild populations of Prosopisglandulosa, P. juliflora, and two ofP. laevigata, located in the Mexican States of Chihuahua(1), Sinaloa (2), Guanajuato (3) and Hidalgo (4), respectively, was determinedusing random amplified polymorphic DNA (RAPDs). There was a genetic variation of72.48% among these four populations, and within them the estimated geneticvariation was 27.52%. Between the populations of P.laevigata the genetic variation obtained was 7.15%, and within them92.85% (AMOVA, P < 0.001). Based on the assumption ofWright's island model, the genetic variation found among populations correspondsto a gene flow (Nem) of 0.09, while the gene flow detected betweenthe populations of P. laevigata was 3.2. A total of 27 RAPDphenotypes were identified in the four populations; among them, 11 correspondedto P. laevigata. The phylogenetic analyses showed that thethree species are differentiated in three clades and that probably P.juliflora was the origin of the other two species. The protogynoustrait of their flowers and the outcrossing are the main factors that may explainthe genetic variation observed in these populations. 相似文献
86.
Protein kinase C-associated kinase (PKK) mediates Bcl10-independent NF-kappa B activation induced by phorbol ester 总被引:1,自引:0,他引:1
Muto A Ruland J McAllister-Lucas LM Lucas PC Yamaoka S Chen FF Lin A Mak TW Núñez G Inohara N 《The Journal of biological chemistry》2002,277(35):31871-31876
Protein kinase C-associated kinase (PKK) is a recently described kinase of unknown function that was identified on the basis of its specific interaction with PKC beta. PKK contains N-terminal kinase and C-terminal ankyrin repeats domains linked to an intermediate region. Here we report that the kinase domain of PKK is highly homologous to that of two mediators of nuclear factor-kappa B (NF-kappa B) activation, RICK and RIP, but these related kinases have different C-terminal domains for binding to upstream factors. We find that expression of PKK, like RICK and RIP, induces NF-kappa B activation. Mutational analysis revealed that the kinase domain of PKK is essential for NF-kappa B activation, whereas replacement of serine residues in the putative activation loop did not affect the ability of PKK to activate NF-kappa B. A catalytic inactive PKK mutant inhibited NF-kappa B activation induced by phorbol ester and Ca(2+)-ionophore, but it did not block that mediated by tumor necrosis factor alpha, interleukin-1 beta, or Nod1. Inhibition of NF-kappa B activation by dominant negative PKK was reverted by co-expression of PKC beta I, suggesting a functional association between PKK and PKC beta I. PKK-mediated NF-kappa B activation required IKK alpha and IKK beta but not IKK gamma, the regulatory subunit of the IKK complex. Moreover, NF-kappa B activation induced by PKK was not inhibited by dominant negative Bimp1 and proceeded in the absence of Bcl10, two components of a recently described PKC signaling pathway. These results suggest that PKK is a member of the RICK/RIP family of kinases, which is involved in a PKC-activated NF-kappa B signaling pathway that is independent of Bcl10 and IKK gamma. 相似文献
87.
88.
M. T. Fernández-Espinar S. Vallés F. Piñaga J. A. Pérez-González D. Ramón 《Applied microbiology and biotechnology》1996,45(3):338-341
Using recombinant DNA techniques, an Aspergillus nidulans multicopy transformant for the gene xlnB coding for the minor X24 xylanase has been constructed. When grown on glucose as sole carbon source this transformant secretes 114 U of xylanase (mg
protein)-1. In this culture condition, X24 is the only xylanase secreted and the predominant protein in the culture filtrate. This strategy has been used to purify
the X24 enzyme to homogeneity. The purified xylanase showed a single band on sodium dodecyl sulphate/ polyacrylamide gel electrophoresis
with a molecular mass of 24 kDa and had an isoelectric point of approximately 3.5. The enzyme was a non-debranching endo-1,4-β-xylan
xylanohydrolase highly specific for xylans and showed optimal activity at pH 5.5 and 52°C. The X24 xylanase had a Michaelis constant, K
m, of 12.43 mg oat spelt xylan ml-1 and a V
max of 1639 μmol min-1 (mg protein)-1.
Received: 17 May 1995/Received last revision: 25 September 1995/Accepted: 29 September 1995 相似文献
89.
Francisco G. Acién Fernández Celeste Brindley Alias José A. Sánchez Pérez José M. Fernández Sevilla María J. Ibáñez González Emilio Molina Grima 《Journal of applied phycology》2003,15(2-3):229-237
An integrated process for the indoor production of 13C labelled PUFA from Phaeodactylum tricornutum is presented. The core of the process is a bubble column photobioreactor from which the exhaust gas from the reactor is returned to the culture by a low pressure compressor. To avoid accumulation of dissolved oxygen in the culture medium, the exhaust gas is bubbled through a sodium sulphite solution before returning it to the reactor. Carbon is removed from the medium before inoculating the alga, then labelled 13CO2 is injected for pH control and carbon supply. The reactor has been operated in semicontinuous mode at a dilution rate of 0.01 h–1, a biomass productivity of 0.1 g L–1 d–1 being obtained. Under this conditions both pH and dissolved oxygen were correctly controlled and the adequacy of the system for autotrophic production of labelled biomass was demonstrated. Analysis by GC-MS revealed that the fatty acids content of the biomass obtained was 10% d.wt., the content of eicosapentaenoic acid was 2.5% d.wt. All the fatty acids were labelled, more that 90% of the carbon present in these fatty acids was 13C. Element analysis of biomass and supernatant showed that 59.5% of injected carbon was assimilated into the biomass whereas 33% remained in the supernatant, and 7.5% remained undetected. Due to the high cost of 13CO2 different strategies for the optimisation of labelled carbon use are proposed. 相似文献
90.
Jennifer Ho John Ondos Holly Ning Sharon Smith Teri Kreisl Fabio Iwamoto Joohee Sul Lyndon Kim Kate McNeil Andra Krauze Uma Shankavaram Howard A. Fine Kevin Camphausen 《PloS one》2013,8(8)