全文获取类型
收费全文 | 2519篇 |
免费 | 224篇 |
国内免费 | 1篇 |
专业分类
2744篇 |
出版年
2022年 | 23篇 |
2021年 | 30篇 |
2020年 | 13篇 |
2019年 | 18篇 |
2018年 | 21篇 |
2017年 | 22篇 |
2016年 | 54篇 |
2015年 | 89篇 |
2014年 | 94篇 |
2013年 | 125篇 |
2012年 | 163篇 |
2011年 | 173篇 |
2010年 | 105篇 |
2009年 | 100篇 |
2008年 | 143篇 |
2007年 | 159篇 |
2006年 | 117篇 |
2005年 | 149篇 |
2004年 | 154篇 |
2003年 | 124篇 |
2002年 | 149篇 |
2001年 | 38篇 |
2000年 | 22篇 |
1999年 | 26篇 |
1998年 | 40篇 |
1997年 | 36篇 |
1996年 | 35篇 |
1995年 | 26篇 |
1994年 | 32篇 |
1993年 | 25篇 |
1992年 | 23篇 |
1991年 | 17篇 |
1990年 | 19篇 |
1989年 | 15篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1985年 | 22篇 |
1984年 | 20篇 |
1983年 | 24篇 |
1982年 | 20篇 |
1981年 | 14篇 |
1980年 | 13篇 |
1979年 | 11篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 20篇 |
1974年 | 14篇 |
1973年 | 10篇 |
1972年 | 10篇 |
1969年 | 11篇 |
排序方式: 共有2744条查询结果,搜索用时 0 毫秒
31.
Switch-like Responses of Two Cholesterol Sensors Do Not Require Protein Oligomerization in Membranes
Many cellular processes are sensitive to levels of cholesterol in specific membranes and show a strongly sigmoidal dependence on membrane composition. The sigmoidal responses of the cholesterol sensors involved in these processes could arise from several mechanisms, including positive cooperativity (protein effects) and limited cholesterol accessibility (membrane effects). Here, we describe a sigmoidal response that arises primarily from membrane effects due to sharp changes in the chemical activity of cholesterol. Our models for eukaryotic membrane-bound cholesterol sensors are soluble bacterial toxins that show an identical switch-like specificity for endoplasmic reticulum membrane cholesterol. We show that truncated versions of these toxins fail to form oligomers but still show sigmoidal binding to cholesterol-containing membranes. The nonlinear response emerges because interactions between bilayer lipids control cholesterol accessibility to toxins in a threshold-like fashion. Around these thresholds, the affinity of toxins for membrane cholesterol varies by >100-fold, generating highly cooperative lipid-dependent responses independently of protein-protein interactions. Such lipid-driven cooperativity may control the sensitivity of many cholesterol-dependent processes. 相似文献
32.
Dearnaley John D.W.; Levina Natalia N.; Lew Roger R.; Heath I. Brent; Goring Daphne R. 《Plant & cell physiology》1997,38(9):985-999
We have investigated Ca2+-involving cell signaling, plasma membranepotentials and conductances and callose formation during earlystages of pollination of papillae of Brassica napus. Using fluorescenceimaging of calcium green-1, we found that application of a rangeof pollen types and controls all rapidly produced small localizedpeaks in papillar cytoplasmic [Ca2+]. This response was morefrequent in compatible than incompatible interactions and wascorrelated with subsequent hydration of the applied pollen grains,indicating that it may be a differential prerequisite of thecompatible signaling pathway leading to successful pollinations.In contrast, a slight trend to increased plasma membrane conductance(but with no indications of action potential-like responses)and also callose deposition in papillae adjacent to pollen grainsfollowed pollination in both SC and SI interactions, indicatingthat alterations in plasma membrane permeability and callosedeposition during early phases of pollination are not primarydeterminants of the fate of attempted pollinations.
2On leave of absence from A.N. Bach Institute of Biochemistry,Russian Academy of Sciences, Leninsky Pr., 33, Moscow, Russia,117071 相似文献
33.
Peroxisome proliferator-activated receptor γ (PPAR-γ) is a key regulator of fatty acid metabolism, promoting its storage in adipose tissue and reducing circulating concentrations of free fatty acids. Activation of PPAR-γ has favorable effects on measures of adipocyte function, insulin sensitivity, lipoprotein metabolism, and vascular structure and function. Despite these effects, clinical trials of thiazolidinedione PPAR-γ activators have not provided conclusive evidence that they reduce cardiovascular morbidity and mortality. The apparent disparity between effects on laboratory measurements and clinical outcomes may be related to limitations of clinical trials, adverse effects of PPAR-γ activation, or off-target effects of thiazolidinedione agents. This review addresses these issues from a clinician's perspective and highlights several ongoing clinical trials that may help to clarify the therapeutic role of PPAR-γ activators in cardiovascular disease. 相似文献
34.
Whalen, M. K., Gurai, S. K., Zahed-Kargaran, H. and Pluth, J. M. Specific ATM-Mediated Phosphorylation Dependent on Radiation Quality. Radiat. Res. 170, 353-364 (2008).To determine whether the physical differences between high- and low-LET radiation are reflected in the biological responses of exposed cells, we detailed phospho-protein profiles of three proteins functional in radiation repair and signal transduction. Detailing gamma-H2AX, pATF2 Ser(490/498) and pSMC1 Ser(957) kinetics after X-ray and iron-ion exposure also provides a window into understanding the underlying cellular responses. Phosphorylated forms of these proteins have been documented to co-localize at sites of double-strand breaks (DSBs) after low-LET radiation exposures, and two of these phosphorylations, pATF2 and pSMC1, are specifically dependent on ATM. Flow cytometry-based methods were used to quantify total levels of each phospho-protein at various times after irradiation. As expected, we observed a greater induction and persistence in gamma-H2AX after iron-ion (high-LET) exposure compared to X-ray (low-LET) exposure. In contrast, pATF2 and pSMC1 showed markedly lower induction levels after iron-ion exposure compared to equivalent doses of X rays. Quantification of pATF2 and pSMC1 foci revealed fewer cells containing foci and fewer foci per cell after iron-ion compared to X-ray exposure. These findings suggest that ATM responds to DSBs induced by high-LET radiation differently from DSBs induced by low-LET radiation. 相似文献
35.
Populations often contain discrete classes or morphs (e.g., sexual dimorphisms, wing dimorphisms, trophic dimorphisms) characterized by distinct patterns of trait expression. In quantitative genetic analyses, the different morphs can be considered as different environments within which traits are expressed. Genetic variances and covariances can then be estimated independently for each morph or in a combined analysis. In the latter case, morphs can be considered as separate environments in a bivariate analysis or entered as fixed effects in a univariate analysis. Although a common approach, we demonstrate that the latter produces downwardly biased estimates of additive genetic variance and heritability unless the quantitative genetic architecture of the traits concerned is perfectly correlated between the morphs. This result is derived for four widely used quantitative genetic variance partitioning methods. Given that theory predicts the evolution of genotype‐by‐environment (morph) interactions as a consequence of selection favoring different trait combinations in each morph, we argue that perfect correlations between the genetic architectures of the different morphs are unlikely. A sampling of the recent literature indicates that the majority of researchers studying traits expressed in different morphs recognize this and do estimate morph‐specific quantitative genetic architecture. However, ca. 16% of the studies in our sample utilized only univariate, fixed‐effects models. We caution against this approach and recommend that it be used only if supported by evidence that the genetic architectures of the different morphs do not differ. 相似文献
36.
Type A botulinum neurotoxin is one of the most lethal of the seven serotypes and is increasingly used as a therapeutic agent
in neuromuscular dysfunctions. Its toxic function is related to zinc-endopeptidase activity of the N-terminal light chain
(LC) on synaptosome-associated protein-25 kDa (SNAP-25) of the SNARE complex. To understand the determinants of substrate
specificity and assist the development of strategies for effective inhibitors, we used site-directed mutagenesis to investigate
the effects of 13 polar residues of the LC on substrate binding and catalysis. Selection of the residues for mutation was
based on a computational analysis of the three-dimensional structure of the LC modeled with a 17-residue substrate fragment
of SNAP-25. Steady-state kinetic parameters for proteolysis of the substrate fragment were determined for a set of 16 single
mutants. Of the mutated residues non-conserved among the serotypes, replacement of Arg-230 and Asp-369 by polar or apolar
residues resulted in drastic lowering of the catalytic rate constant (k
cat), but had less effect on substrate affinity (K
m). Substitution of Arg-230 with Lys decreased the catalytic efficiency (k
cat/K
m) by 50-fold, whereas replacement by Leu yielded an inactive protein. Removal of the electrostatic charge at Asp-369 by mutation
to Asn resulted in 140-fold decrease in k
cat/K
m. Replacement of other variable residues surrounding the catalytic cleft (Glu-54, Glu-63, Asn-66, Asp-130, Asn-161, Glu-163,
Glu-170, Glu-256), had only marginal effect on decreasing the catalytic efficiency, but unexpectedly the substitution of Lys-165
with Leu resulted in fourfold increase in k
cat/K
m. For comparison purposes, two conserved residues Arg-362 and Tyr-365 were investigated with substitutions of Leu and Phe,
respectively, and their catalytic efficiency decreased 140- and 10-fold, respectively, whereas substitution of the tyrosine
ring with Asn abolished activity. The altered catalytic efficiencies of the mutants were not due to any significant changes
in secondary or tertiary structures, or in zinc content and thermal stability. We suggest that, despite the large minimal
substrate size for catalysis, only a few non-conserved residues surrounding the active site are important to render the LC
competent for catalysis or provide conformational selection of the substrate. 相似文献
37.
One of the major enzymatic cell defenses providing protection from oxidative injury is the TrxR-Trx system. It consists of NADPH and thioredoxin reductase (TrxR), which maintain thioredoxin (Trx) in a reduced state. Perturbing the TrxR-Trx system with the selective TrxR inhibitor auranofin (AuF; 2,3,4,6-tetra-O-acetyl-1-thio-β-D-glucopyranosato-S-(triethylphosphine) gold) induces oxidative stress by keeping Trx in its oxidized state. We have prepared a family of tri- and tetra-oligopeptides derived from the canonical CxxC motif of the Trx active site and a modified CxC motif. These Trx-mimetic compounds are N- and C-terminal-blocked peptides that consist of two cysteine residues that flank the two-amino-acid CxxC motif (CB4 and CB6) or the single-amino-acid CxC motif (CB3). Catecholamine (CA) secretion in bovine chromaffin cells, which is a highly redox sensitive process, is abolished by AuF. The Trx-mimetic peptides effectively restore CA secretion, as monitored by amperometry in single cells. They also prevent the AuF-induced phosphorylation of p38 mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal kinase. In PC12 cells, the alleviation of AuF-induced ERK1/2-MAPK phosphorylation by Trx-like peptides parallels their effect of restoring CA secretion. CB3, CB4, and CB6 act intracellularly and are significantly more potent than the traditional antioxidants NAC, GSH, DTT, AD4 (NAC-amide), and ascorbic acid. Taken together, the CxxC and CxC peptides represent a new family of potent and selective redox compounds that could serve as potential candidates for prevention and treatment of oxidative-stress-related disorders. 相似文献
38.
39.
Szasz T Thompson JM Watts SW 《American journal of physiology. Heart and circulatory physiology》2008,295(3):H1341-H1350
Reactive oxygen species (ROS) are important mediators in vascular biology. Venous function, although relevant to cardiovascular disease, is still understudied. We compared aspects of ROS metabolism between a major artery (the aorta) and a major vein (the vena cava, VC) of the rat, with the hypothesis that venous ROS metabolism would be overall increased compared with its arterial counterpart. Superoxide and hydrogen peroxide (H2O2) release in basal conditions was higher in VC compared with aorta. The antioxidant capacity for H2O2 was also higher in VC than in aorta. Exogenous superoxide induced a higher contraction in VC compared with aorta. Protein expression of three major ROS metabolizing enzymes, xanthine oxidase (XO), CuZn-SOD, and catalase, was higher in VC compared with aorta. Because XO seemed a likely source of the higher VC ROS levels, we examined it further and found higher mRNA expression and activity of XO in VC compared with aorta. We also investigated the impact of XO inhibition by allopurinol on aorta and VC functional responses to norepinephrine, ANG II, ET-1, and ACh. Maximal ET-1-mediated contraction was decreased by allopurinol in VC but not in the aorta. Our results suggest that there are overall differences in ROS metabolism between aorta and VC, with the latter operating normally at a higher set point, releasing but also being able to handle, higher ROS levels. We propose XO to be an important source for these differences. The result of this particular comparison may be reflective of a general arteriovenous contrast. 相似文献
40.
Elena Taycher Andreas Rolfs Yanhui Hu Dongmei Zuo Stephanie E Mohr Janice Williamson Joshua LaBaer 《BMC bioinformatics》2007,8(1):198