全文获取类型
收费全文 | 17194篇 |
免费 | 1421篇 |
国内免费 | 248篇 |
出版年
2024年 | 23篇 |
2023年 | 181篇 |
2022年 | 195篇 |
2021年 | 404篇 |
2020年 | 326篇 |
2019年 | 365篇 |
2018年 | 721篇 |
2017年 | 571篇 |
2016年 | 634篇 |
2015年 | 585篇 |
2014年 | 675篇 |
2013年 | 1036篇 |
2012年 | 1449篇 |
2011年 | 1554篇 |
2010年 | 823篇 |
2009年 | 569篇 |
2008年 | 1125篇 |
2007年 | 984篇 |
2006年 | 959篇 |
2005年 | 737篇 |
2004年 | 742篇 |
2003年 | 715篇 |
2002年 | 630篇 |
2001年 | 448篇 |
2000年 | 577篇 |
1999年 | 316篇 |
1998年 | 166篇 |
1997年 | 128篇 |
1996年 | 115篇 |
1995年 | 107篇 |
1994年 | 103篇 |
1993年 | 96篇 |
1992年 | 101篇 |
1991年 | 87篇 |
1990年 | 61篇 |
1989年 | 47篇 |
1988年 | 45篇 |
1987年 | 43篇 |
1986年 | 25篇 |
1985年 | 48篇 |
1984年 | 34篇 |
1983年 | 27篇 |
1982年 | 37篇 |
1981年 | 27篇 |
1980年 | 28篇 |
1979年 | 25篇 |
1978年 | 23篇 |
1977年 | 19篇 |
1974年 | 17篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
Morphological comparisons are made from the study of 64 specimens belonging to the following species:Squalus acanihias, S. blainvillei, S. megalops andS. acutirostris. The results suggest conspecificity betweenS. acutipinnis andS. megalops. The differences betweenS. blainvillei andS. megalops in the E-Atlantic are stressed. The Indo-Pacific species calledS. blainvillei by Chen et al. (1979) is regarded in this paper as probably an undescribed species. Some consideration on the status of several nominal species and the species grouping withinSqualus are also made. 相似文献
72.
73.
74.
75.
Nieves M. Aparicio-Pagés Hein W. Verspaget Salvador A. Peña Cornelis B. H. W. Lamers 《Cancer immunology, immunotherapy : CII》1989,28(4):301-304
Summary The present study was undertaken to study natural killer (NK) cell activity in patients with colorectal cancer at peripheral and local levels. Mononuclear cells were isolated from uninvolved colorectal mucosa, tumor tissue and peripheral blood, and tested against the colon carcinoma cell line CaCo-2 and the erythroleukemia cell line K-562. Peripheral blood NK cell activity from the patients showed similar levels compared with healthy controls, whereas, mononuclear cells of tumor tissue were found to have a significantly decreased NK cell activity compared to the normal intestinal mucosa (P<0.01). No relation was found between the NK cell activity and the advancement of the disease according to the Duke's stage. Interferon- (IFN-) stimulated the NK cell activity of the mononuclear cells from blood, mucosa and tumor. However, the increase of NK cell activity after IFN- stimulation was lower in the tumor compared to the mucosa (P<0.02). The lectin, phytohaemagglutinin, increased the cytotoxicity of mononuclear cells from blood, mucosa and tumor to a similar level. These results suggest that patients with colorectal tumors exhibit a normal NK cell activity in peripheral blood and intestinal mucosa; however, a diminished NK cell activity exists at the tumor level. Although mononuclear cells isolated from the tumor have a normal response to lectin stimulation they show hyporesponsiveness to IFN- stimulation with regard to their NK cell activity. 相似文献
76.
Rats were fed a diet containing ammonium for up to 6 months. High ammonia levels were attained in brain. The amount of polymerized tubulin in microtubules increased, while the amount of free tubulin remained unchanged. Polymerization of tubulin from brain of ammonium fed rats (30 min, 37°C) was approximately 60% of control. Depolymerization of the microtubules was also affected and took approximately 3 times longer than in controls. These results indicate that both assembly and disassembly of tubulin in brain are impaired by high ammonia levels. Interestingly, the amount of microtubule-associated proteins was not affected. 相似文献
77.
Francisco J. Campoy Maria Dolores Cánovas Encarnación Muñoz-Delgado Cecilio J. Vidal 《Neurochemical research》1989,14(2):197-204
Incubation of membranes derived from sarcotubular system of rabbit skeletal muscle with increasing concentrations of Triton X-100 produced both stimulation of the AChE activity and solubilization of this enzyme. Mild proteolytic treatment of microsomal membranes produced a several fold activation of the still membrane-bound acetylcholinesterase (AChE) activity. Attempts were made to solubilize AChE from microsomal membranes by proteolytic treatment. About 30–40% of the total enzyme activity could be solubilized by means of trypsin or papain. Short trypsin treatment of the microsomal membranes produced first an activation of the membrane-bound enzyme followed by solubilization. Incubation of muscle microsomes for a short time with papain yielded a significant portion of soluble enzyme. Membrane-bound enzyme activation was measured after a prolonged incubation period. These results are compared with those of solubilization obtained by treatment of membranes with progressive concentrations of Triton X-100. The occurrence of molecular forms in protease-solubilized AChE was investigated by means of centrifugation analysis and slab gel electrophoresis. Centrifugation on sucrose gradients revealed two main components of 4.4S and 10–11S in either trypsin or papain-solubilized AChE. These components behaved as hydrophilic species whereas the Triton solubilized AChE showed an amphipatic character. Application of slab gel electrophoresis showed the occurrence of forms with molecular weights of 350,000; 175,000; 165,000; 85,000 and 76,000. The stimulation of membrane-bound AChE by detergents or proteases would indicate that most of the enzyme molecules or their active sites are sequestered into the lipid bilayer through lipid-protein or protein-protein interactions and these are broken by proteolytic digestion of the muscle microsomes. 相似文献
78.
Synaptosomes were isolated from cerebrums of rats fed standard (20% protein) or protein-free diets for 30 days. Arrhenius plots of their (Na+/K+)ATPase activities revealed a transition temperature of 25.5°C for control rats and 23.4°C for rats on protein-free diet, indicating that the latter increases synaptosomal membrane fluidity. The only change observed in the composition of the synaptosomal membranes was a 26% decrease of sialic acid. In synaptosomes from rats on protein-free diet the uptake of tyrosine was slightly reduced while that of glutamate was not affected. However, the exit of glutamate was reduced. 相似文献
79.
Fibrinogen Lille, a congenital dysfibrinogenemia, has been reported to arise from a mutation from Asp to Asn at position 7 of the A alpha chain of human fibrinogen, thereby reducing the thrombin-catalyzed rate of hydrolysis of the Arg(16)-Gly(17) peptide bond of this chain. Synthetic peptides of relevant portions of the wild-type and mutant A alpha chains were prepared, and the thrombin-catalyzed rates of hydrolysis of their Arg(16)-Gly(17) peptide bonds were determined. In addition, transferred NOE measurements were made to deduce their conformations, when complexed to bovine thrombin. The kinetics data showed little difference in the hydrolysis rates between the wild-type and mutant peptides, and the NMR data indicate no difference in the bound conformation of these two peptides. Therefore, electrostatic (or salt-bridge) interactions between Asp(7) and thrombin do not influence the bound conformations of these peptides. Asp(7) may interact with a remote residue of fibrinogen, not present in these synthetic peptides, or there may be additional mutations beyond A alpha (1-20) which have not been detected in fibrinogen Lille. Alternatively, when thrombin binds to fibrinogen at its secondary binding site, its primary (active) site may display different reactivities toward wild-type fibrinogen and fibrinogen Lille. 相似文献
80.
The COOH-terminal region of hirudin represents an independent functional domain that binds to an anion-binding exosite of thrombin and inhibits the interaction of thrombin with fibrinogen and regulatory proteins in blood coagulation. The thrombin-bound structure of the peptide fragment, hirudin 55-65, has been determined by use of transferred NOE spectroscopy [Ni, F., Konishi, Y., & Scheraga, H. A. (1990) Biochemistry 29, 4479-4489]. The stability of the thrombin-bound conformation has been characterized further by a combined NMR and theoretical analysis of the conformational ensemble accessible by the hirudin peptide. Medium- and long-range NOE's were found for the free hirudin peptide in aqueous solution and in a mixture of dimethyl sulfoxide and water at both ambient (25 degrees C) and low (0 degrees C) temperatures, suggesting that ordered conformations are highly populated in solution. The global folding of these conformations is similar to that in the thrombin-bound state, as indicated by NOE's involving the side-chain protons of residues Phe(56), Ile(59), Pro(60), Tyr(63), and Leu(64). Residues Glu(61), Glu(62), Tyr(63), and Leu(64) all contain approximately 50% of helical conformations calculated from the ratio of the sequential dNN and d alpha N NOE's. Among the helical ensemble, active 3(10)-helical conformations were found by an analysis of the medium-range [(i,i+2) and (i,i+3)] NOE's involving the last six residues of the peptide. An analysis of the side-chain rotamers revealed that, upon binding to thrombin, there may be a rotation around the alpha CH-beta CH bond of Ile(59) such that Ile(59) adopts a gauche- (chi 1 = +60) conformation in contrast to the highly populated trans (chi 1 = -60) found for Ile(59) in the free peptide. However, the thrombin-bound conformation of the hirudin peptide is still an intrinsically stable conformer, and the preferred conformational ensemble of the peptide contains a large population of the active conformation. The apparent preference for a gauche- (chi 1 = +60) side-chain conformation of Ile(59) in the bound state may be explained by the existence of a positively charged arginine residue among the hydrophobic residues in the thrombin exosite. 相似文献