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   472篇
  免费   29篇
  国内免费   16篇
  517篇
  2023年   3篇
  2022年   8篇
  2021年   11篇
  2020年   28篇
  2019年   30篇
  2018年   27篇
  2017年   10篇
  2016年   6篇
  2015年   6篇
  2014年   34篇
  2013年   39篇
  2012年   23篇
  2011年   36篇
  2010年   19篇
  2009年   23篇
  2008年   23篇
  2007年   21篇
  2006年   15篇
  2005年   12篇
  2004年   1篇
  2003年   6篇
  2002年   5篇
  2001年   6篇
  1999年   3篇
  1998年   2篇
  1997年   2篇
  1996年   5篇
  1995年   1篇
  1994年   2篇
  1993年   5篇
  1992年   4篇
  1991年   5篇
  1990年   1篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1986年   2篇
  1985年   6篇
  1984年   11篇
  1983年   4篇
  1982年   8篇
  1981年   10篇
  1980年   9篇
  1979年   12篇
  1978年   5篇
  1977年   5篇
  1976年   7篇
  1975年   4篇
  1974年   5篇
  1973年   3篇
排序方式: 共有517条查询结果,搜索用时 15 毫秒
71.
alpha-Synuclein (AS) is a main component of Lewy bodies in midbrain dopamine neurons pathologically characteristic of Parkinson's disease. We show that heat shock protein (Hsp) 70 inhibits AS fibril formation via preventing the formation of prefibrillar AS (PreAS), binding with PreAS to impede nuclei formation, and binding with nuclei to retard fibril elongation. Also, Hsp70 suppresses the PreAS-induced permeabilization of vesicular membrane through interactions with PreAS. The substrate-binding domain alone is sufficient for Hsp70 to inhibit AS fibril formation. The binding of Hsp70 with PreAS only requires the substrate-binding subdomain, and the binding with AS nuclei requires the C-terminal lid subdomain as well. The results may form the molecular basis for elucidating the mechanism of AS fibril formation and the crucial roles of chaperones in protecting proteins from toxic conversion in many conformational diseases.  相似文献   
72.
73.
Su Z  Gu X 《Gene》2012,504(1):102-106
Gene duplications and alternative splicing (AS) isoforms are two widespread types of genetic variations that can facilitate diversification of protein function. A number of studies claimed that after gene duplication, two AS isoforms with differential functions can be 'fixed', respectively, in each of the duplicate copies. This simple 'functional-sharing' hypothesis was recently challenged by Roux and Robinson-Rechavi (2011). Instead, they proposed a more sophisticated hypothesis, invoking that less alternative splicing genes tend to be duplicated more frequently, and single-copy genes are younger than duplicate genes, or the 'duplicability-age' hypothesis for short. In this letter, we show that all these genome-wide analyses of AS isoforms actually did not provide clear-cut evidence to nullify the basic idea of functional-sharing hypothesis. After updating our understanding of genome-wide alternative splicing, duplicability and CNV (copy number variation), we argue that the foundation of the duplicability-age hypothesis remains to be justified carefully. Finally, we suggest that a better approach to resolving this controversy is the correspondence analysis of indels (insertions and deletions) between duplicate genes to the genomic exon-intron structure, which can be used to experimentally test the effect of functional-sharing hypothesis.  相似文献   
74.
The present report is a part of our continuing efforts to explore the utility of the rat epidermal keratinocyte organotypic culture (ROC) as an alternative model to human skin in transdermal drug delivery and skin irritation studies of new chemical entities and formulations. The aim of the present study was to compare the stratum corneum lipid content of ROC with the corresponding material from human skin. The lipid composition was determined by thin-layer chromatography (TLC) and mass-spectrometry, and the thermal phase transitions of stratum corneum were studied by differential scanning calorimetry (DSC). All major lipid classes of the stratum corneum were present in ROC in a similar ratio as found in human stratum corneum. Compared to human skin, the level of non-hydroxyacid-sphingosine ceramide (NS) was increased in ROC, while α-hydroxyacid-phytosphingosine ceramide (AP) and non-hydroxyacid-phytosphingosine ceramides (NP) were absent. Also some alterations in fatty acid profiles of ROC ceramides were noted, e.g., esterified ω-hydroxyacid-sphingosine contained increased levels of oleic acid instead of linoleic acid. The fraction of lipids covalently bound to corneocyte proteins was distinctly lower in ROC compared to human skin, in agreement with the results from DSC. ROC underwent a lipid lamellar order to disorder transition (T2) at a slightly lower temperature (68 °C) than human skin (74 °C). These differences in stratum corneum lipid composition and the thermal phase transitions may explain the minor differences previously observed in drug permeation between ROC and human skin.  相似文献   
75.
In common bean, asparagine synthetase (AS; EC 6.3.5.4) is encoded by three members of a multigene family called PVAS1, PVAS2 and PVAS3. Two of these genes, PVAS1 and PVAS2, have been extensively studied, but little is known about PVAS3, remaining unclear whether PVAS3 function is redundant to the other AS or if it plays a specific role in Phaseolus vulgaris metabolism. In this work, we used a molecular approach to characterize PVAS3 expression and to gain some knowledge about its physiological function. We showed that, in contrast to PVAS1 and PVAS2, PVAS3 was expressed in all organs analyzed. Interestingly, PVAS3 was the AS gene most highly expressed in nodules, leaves and pods at the earliest stages of development, and its expression decreased as these organs developed. Expression of PVAS3 parallels the accumulation of AS protein and the asparagine content during the earliest stages of nodule, leaf and pod development, suggesting an important role for PVAS3 in the synthesis of asparagine in that period. Furthermore, PVAS3 was not repressed by light, as most class-II AS genes. Surprisingly, fertilization of nodulated plants with nitrate or ammonium, conditions that induce PVAS1 and PVAS2 and the shift from ureides to amide synthesis, repressed the expression of PVAS3 in nodules and roots. The possible implications of this regulation are discussed.  相似文献   
76.
We report a novel micro-potentiometric hemoglobin (Hb) immunosensor based on electrochemically synthesized polypyrrole (PPy)–gold nanoparticles (AuNPs) composite. PPy–AuNPs film with AuNPs uniformly distributed in it was deposited on gold electrode surface by a simple and direct procedure, without the addition of any nanoparticles or reducing agent. And this generic method makes it possible to deposite different polymers on miniaturized electrodes. With the existence of AuNPs, the antibody immobilization onto the electrode surface was facilitated. Morphology study by field emission scanning electron microscope (FE-SEM) confirms the presence of AuNPs in PPy. Based on an ion-sensitive field-effect transistors (ISFETs) integrated chip, a micro-potentiometric immunosensor for Hb and hemoglobin-A1c (HbA1c) has been constructed. The sensor response was linear over the concentration range 60–180 μg/ml Hb and 4–18 μg/ml HbA1c. The Hb concentration in whole blood samples has also been analysed, with a linear dose–response behavior between 125 and 197 μg/ml and a sensitivity of 0.20 mV μg−1 ml. The measuring ranges of the developed Hb and HbA1c immunosensors meet the clinical demand for measuring the HbA1c/Hb ratio of 5–20%. This sensor results in simple and rapid differential measurement of Hb and HbA1c, and has great potential to become an inexpensive and portable device for monitoring of diabetes.  相似文献   
77.
Lui JC  Kong SK 《FEBS letters》2006,580(8):1965-1970
The involvement of caspase-3 and its failure in the induction of DNA fragmentation during erythropoiesis were investigated with TF-1 cells. During erythroid differentiation, caspase-3 activation and cleavage of caspase-3 substrates such as ICAD (inhibitor of caspase-activated DNase) were detected without concomitant phosphatidyl-serine (PS) externalization and DNA fragmentation. These observations are in contrast to our understanding that DNA is degraded by CAD (caspase-activated DNase) when ICAD is cleaved by caspase-3. Our study demonstrates that CAD is downregulated at the mRNA and protein level during the erythroid differentiation in TF-1 cells. This provides a mechanism for the first time how cells avoid DNA fragmentation with activated caspase-3.  相似文献   
78.
Recombinant human erythropoietin (rhEPO), the prototype erythropoiesis-stimulating agent developed in the 1980s, was among the first recombinant human proteins to be marketed for clinical use in the oncology setting. Anemia is a frequent concern in patients with cancer receiving myelosuppressive chemotherapy and the availability of rhEPO as an alternative to red blood cell transfusions to treat symptomatic anemia created excitement among clinicians, particularly during an era of mounting concern for transfusion-transmissible infections. Early studies of rhEPO for chemotherapy-induced anemia in patients with non-myeloid malignancies showed these agents improved hemoglobin levels and reduced transfusion rates. rhEPO therapy was reported to decrease fatigue and improve quality of life, although the magnitude and clinical meaningfulness of these effects have been debated. More recent clinical trials since 2003 linking rhEPO therapy to increased risk of tumor progression, thrombo-vascular events and mortality prompted implementation of use restrictions to minimize potential for harm. Scientific research to understand the basic mechanisms of the biologic effects of erythropoietin at the cellular receptor and signaling level has revealed pleiotropic cytokine effects extending beyond erythropoiesis regulation. The importance of erythropoietin receptor signaling in normal, non-erythroid tissues and in pre-clinical tumor models has been under intense investigation and scrutiny, as potential mechanisms of the adverse outcomes associated with rhEPO therapy have been debated. Further research will be required to clarify the complex interplay between the diverse hematopoietic and non-hematopoietic effects of erythropoietin in normal and malignant tissues and to optimize the clinical use of rhEPO in the supportive care of cancer patients.  相似文献   
79.
80.
Bacillus circulans xylanase (BcX) is a single-domain family 11 glycoside hydrolase. Using NMR-monitored titrations, we discovered that an inactive variant of this enzyme, E78Q-BcX, bound xylooligosaccharides not only within its pronounced active site (AS) cleft, but also at a distal surface region. Chemical shift perturbation mapping and affinity electrophoresis, combined with mutational studies, identified the xylan-specific secondary binding site (SBS) as a shallow groove lined by Asn, Ser, and Thr residues and with a Trp at one end. The AS and SBS bound short xylooligosaccharides with similar dissociation constants in the millimolar range. However, the on and off-rates to the SBS were at least tenfold faster than those of kon approximately 3x10(5) M(-1) s(-1) and koff approximately 1000 s(-1) measured for xylotetraose to the AS of E78Q-BcX. Consistent with their structural differences, this suggests that a conformational change in the enzyme and/or the substrate is required for association to and dissociation from the deep AS, but not the shallow SBS. In contrast to the independent binding of small xylooligosaccharides, high-affinity binding of soluble and insoluble xylan, as well as xylododecaose, occurred cooperatively to the two sites. This was evidenced by an approximately 100-fold increase in relative Kd values for these ligands upon mutation of the SBS. The SBS also enhances the activity of BcX towards soluble and insoluble xylan through a significant reduction in the Michaelis KM values for these polymeric substrates. This study provides an unexpected example of how a single domain family 11 xylanase overcomes the lack of a carbohydrate-binding module through the use of a secondary binding site to enhance substrate specificity and affinity.  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