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   476篇
  免费   12篇
  2023年   1篇
  2022年   5篇
  2021年   8篇
  2020年   4篇
  2019年   9篇
  2018年   10篇
  2017年   6篇
  2016年   15篇
  2015年   13篇
  2014年   30篇
  2013年   27篇
  2012年   40篇
  2011年   60篇
  2010年   32篇
  2009年   14篇
  2008年   32篇
  2007年   26篇
  2006年   23篇
  2005年   24篇
  2004年   28篇
  2003年   26篇
  2002年   11篇
  2001年   5篇
  2000年   1篇
  1999年   2篇
  1998年   2篇
  1997年   4篇
  1996年   2篇
  1995年   2篇
  1994年   2篇
  1993年   1篇
  1992年   1篇
  1991年   2篇
  1990年   1篇
  1988年   3篇
  1987年   2篇
  1984年   2篇
  1983年   1篇
  1982年   1篇
  1980年   4篇
  1979年   2篇
  1977年   1篇
  1976年   1篇
  1975年   1篇
  1972年   1篇
排序方式: 共有488条查询结果,搜索用时 46 毫秒
161.
Aspergillus niger LOCK 62 produces an antifungal chitinase. Different sources of chitin in the medium were used to test the production of the chitinase. Chitinase production was most effective when colloidal chitin and shrimp shell were used as substrates. The optimum incubation period for chitinase production by Aspergillus niger LOCK 62 was 6?days. The chitinase was purified from the culture medium by fractionation with ammonium sulfate and affinity chromatography. The molecular mass of the purified enzyme was 43?kDa. The highest activity was obtained at 40?°C for both crude and purified enzymes. The crude chitinase activity was stable during 180?min incubation at 40?°C, but purified chitinase lost about 25?% of its activity under these conditions. Optimal pH for chitinase activity was pH 6–6.5. The activity of crude and purified enzyme was stabilized by Mg2+ and Ca2+ ions, but inhibited by Hg2+ and Pb2+ ions. Chitinase isolated from Aspergillus niger LOCK 62 inhibited the growth of the fungal phytopathogens: Fusarium culmorum, Fusarium solani and Rhizoctonia solani. The growth of Botrytis cinerea, Alternaria alternata, and Fusarium oxysporum was not affected.  相似文献   
162.
Allele and haplotype segregation at the polymorphic sites within the promoter (23indel) and intron 1 (12indel) regions of the PRNP gene was analyzed in Polish Holstein-Friesian cattle. More 23del/del homozygotes and fewer 23ins/ins homozygotes than expected were observed in the offspring of ♂ 23ins/del × ♀ 23ins/del parents. In the offspring of ♂ 23ins/del × ♀ 23del/del parents and ♂ 23del/del × ♀ 23ins/del parents, a trend toward more 23del/del animals and fewer 23ins/del animals than expected was noted. At the 12indel polymorphic site, the only trend found was one toward fewer 12ins/ins genotypes and more 12ins/del and 12del/del genotypes than expected in the offspring of ♂ 12ins/del × ♀ 12ins/del parents. An analysis of haplotype segregation revealed more 23del-12del/23del-12del diplotypes and fewer 23ins-12ins/23ins-12ins diplotypes at the significance threshold than expected in the offspring of ♂ 23ins-12ins/23del-12del × ♀ 23ins-12ins/23del-12del parents.  相似文献   
163.
Increased ROS generation by the overload by metabolic substrates mitochondria paralleled by decrease of antioxidant activity are typical events found in metabolic syndrome and diabetes type 2. Metabolites of beta-carotene (BC) such as retinoic acid (RA), as well as low concentration of reactive oxygen species (ROS) modify the mitochondrial bioenergetic function. The aim of the study was to investigate the effect of beta-carotene on mitochondrial activity in human preadipocytes. BC used in concentrations, 10 or 30 μM, decreased mitochondrial membrane potential, inhibited mitochondrial respiration and decreased cellular ATP content. We conclude, that BC, the known antioxidant may decrease oxidative phosphorylation capacity of mitochondria.  相似文献   
164.
Cardiac transverse (t)‐tubules are altered during disease and may be regulated by stretch‐sensitive molecules. The relationship between variations in the degree and duration of load and t‐tubule structure remains unknown, as well as its implications for local Ca2+‐induced Ca2+ release (CICR). Rat hearts were studied after 4 or 8 weeks of moderate mechanical unloading [using heterotopic abdominal heart–lung transplantation (HAHLT)] and 6 or 10 weeks of pressure overloading using thoracic aortic constriction. CICR, cell and t‐tubule structure were assessed using confocal‐microscopy, patch‐clamping and scanning ion conductance microscopy. Moderate unloading was compared with severe unloading [using heart‐only transplantation (HAHT)]. Mechanical unloading reduced cardiomyocyte volume in a time‐dependent manner. Ca2+ release synchronicity was reduced at 8 weeks moderate unloading only. Ca2+ sparks increased in frequency and duration at 8 weeks of moderate unloading, which also induced t‐tubule disorganization. Overloading increased cardiomyocyte volume and disrupted t‐tubule morphology at 10 weeks but not 6 weeks. Moderate mechanical unloading for 4 weeks had milder effects compared with severe mechanical unloading (37% reduction in cell volume at 4 weeks compared to 56% reduction after severe mechanical unloading) and did not cause depression and delay of the Ca2+ transient, increased Ca2+ spark frequency or impaired t‐tubule and cell surface structure. These data suggest that variations in chronic mechanical load influence local CICR and t‐tubule structure in a time‐ and degree‐dependent manner, and that physiological states of increased and reduced cell size, without pathological changes are possible.  相似文献   
165.
G-proteins transduce signals along diverse pathways, but the factors involved in pathway selection are largely unknown. Here, we have studied the ability of Gαq to select between two effectors—mammalian inositide-specific phospholipase Cβ (PLCβ) and phosphoinositide-3-kinase (PI3K)—in human embryonic kidney 293 cells. These studies were carried out by measuring interactions between eCFP- and eYFP-tagged proteins using Forster resonance energy transfer in the basal state and during stimulation. Instead of association of Gαq with effectors through diffusion and exchange, we found separate and stable pools of Gαq-PLCβ and Gαq-PI3K complexes existing throughout the stimulation cycle. These separate complexes existed despite the ability of Gαq to simultaneously bind both effectors as determined by in vitro measurements using purified proteins. Preformed G-protein/effector complexes will limit the number of pathways that a given signal will take, which may simplify predictive models.  相似文献   
166.
PDZ domains are ubiquitous protein-protein interaction modules which bind short, usually carboxyterminal fragments of receptors, other integral or membrane-associated proteins, and occasionally cytosolic proteins. Their role in organizing multiprotein complexes at the cellular membrane is crucial for many signaling pathways, but the rules defining their binding specificity are still poorly understood and do not readily explain the observed diversity of their known binding partners. Two homologous RhoA-specific, multidomain nucleotide exchange factors PDZRhoGEF and LARG contain PDZ domains which show a particularly broad recognition profile, as suggested by the identification of five diverse biological targets. To investigate the molecular roots of this phenomenon, we constructed a phage display library of random carboxyterminal hexapeptides. Peptide variants corresponding to the sequences identified in library selection were synthesized and their affinities for both PDZ domains were measured and compared with those of peptides derived from sequences of natural partners. Based on the analysis of the binding sequences identified for PDZRhoGEF, we propose a sequence for an 'optimal' binding partner. Our results support the hypothesis that PDZ-peptide interactions may be best understood when one considers the sum of entropic and dynamic effects for each peptide as a whole entity, rather than preferences for specific residues at a given position.  相似文献   
167.
Semiarid sagebrush ecosystems are being transformed by wildfire, rangeland improvement techniques, and exotic plant invasions, but the effects on ecosystem C and N dynamics are poorly understood. We compared ecosystem C and N pools to 1 m depth among historically grazed Wyoming big sagebrush, introduced perennial crested wheatgrass, and invasive annual cheatgrass communities, to examine whether the quantity and quality of plant inputs to soil differs among vegetation types. Natural abundance δ15N isotope ratios were used to examine differences in ecosystem N balance. Sagebrush-dominated sites had greater C and N storage in plant biomass compared to perennial or annual grass systems, but this was predominantly due to woody biomass accumulation. Plant C and N inputs to soil were greatest for cheatgrass compared to sagebrush and crested wheatgrass systems, largely because of slower root turnover in perennial plants. The organic matter quality of roots and leaf litter (as C:N ratios) was similar among vegetation types, but lignin:N ratios were greater for sagebrush than grasses. While cheatgrass invasion has been predicted to result in net C loss and ecosystem degradation, we observed that surface soil organic C and N pools were greater in cheatgrass and crested wheatgrass than sagebrush-dominated sites. Greater biomass turnover in cheatgrass and crested wheatgrass versus sagebrush stands may result in faster rates of soil C and N cycling, with redistribution of actively cycled N towards the soil surface. Plant biomass and surface soil δ15N ratios were enriched in cheatgrass and crested wheatgrass relative to sagebrush-dominated sites. Source pools of plant available N could become 15N enriched if faster soil N cycling rates lead to greater N trace gas losses. In the absence of wildfire, if cheatgrass invasion does lead to degradation of ecosystem function, this may be due to faster nutrient cycling and greater nutrient losses, rather than reduced organic matter inputs.  相似文献   
168.
169.
INTRODUCTION: The aim of this study was to examine the association of Pro12Ala PPARgamma2 polymorphism with anthropometric and biochemical parameters defining the risk for the development of metabolic syndrome in a healthy population of men. MATERIAL AND METHODS: The study group consisted of 176 healthy men, aged 25-65 years (average 54.16 years). Polymorphisms of the PPAR-g gene (Pro12Ala, Ala12Ala, Pro12Pro) were explored using the PCR-RFLP method. Plasma glucose, insulin, total cholesterol, LDL, HDL and TG were measured using commercially available kits. RESULTS: The genotypic distribution of the Pro12Ala polymorphism was as follows: Pro/Ala 69.8% (n = 123), Ala/Ala 28.4% (n = 50) and Pro/Pro 1.8% (n = 3). The Pro12Ala and Ala12Ala subjects did not differ in any of the measured variables. The non-obese (BMI < 30 kg/m(2), n = 117) and obese subpopulations (BMI > 30 kg/m(2), n = 56) did not significantly differ in the distribution of the genotypes. In the nonobese subpopulation, the homozygous Ala12 carriers (n = 38, 32.4%) had higher systolic blood pressure, plasma triglycerides, insulin levels and HOMA-IR. CONCLUSIONS: We conclude that despite the high frequency of the Ala allele at the PPAR-gamma2 gene in our population of Polish men, the Ala12 allele does not appear to improve insulin sensitivity or have an influence on the occurrence of obesity. It remains to be explained by larger studies if this polymorphism carries any risk of the development of metabolic abnormalities in non-obese men.  相似文献   
170.
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