全文获取类型
收费全文 | 11772篇 |
免费 | 949篇 |
国内免费 | 385篇 |
专业分类
13106篇 |
出版年
2023年 | 100篇 |
2022年 | 193篇 |
2021年 | 292篇 |
2020年 | 238篇 |
2019年 | 272篇 |
2018年 | 320篇 |
2017年 | 272篇 |
2016年 | 361篇 |
2015年 | 558篇 |
2014年 | 612篇 |
2013年 | 754篇 |
2012年 | 849篇 |
2011年 | 784篇 |
2010年 | 497篇 |
2009年 | 383篇 |
2008年 | 571篇 |
2007年 | 558篇 |
2006年 | 510篇 |
2005年 | 469篇 |
2004年 | 424篇 |
2003年 | 432篇 |
2002年 | 362篇 |
2001年 | 322篇 |
2000年 | 322篇 |
1999年 | 274篇 |
1998年 | 116篇 |
1997年 | 89篇 |
1996年 | 72篇 |
1995年 | 57篇 |
1994年 | 86篇 |
1993年 | 59篇 |
1992年 | 161篇 |
1991年 | 146篇 |
1990年 | 131篇 |
1989年 | 152篇 |
1988年 | 121篇 |
1987年 | 110篇 |
1986年 | 105篇 |
1985年 | 98篇 |
1984年 | 66篇 |
1983年 | 73篇 |
1982年 | 45篇 |
1979年 | 52篇 |
1977年 | 75篇 |
1976年 | 47篇 |
1975年 | 43篇 |
1974年 | 56篇 |
1973年 | 41篇 |
1971年 | 40篇 |
1969年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Human erythrocyte glucose-6-phosphate dehydrogenase. Identification of a reactive lysyl residue labelled with pyridoxal 5'-phosphate 总被引:5,自引:0,他引:5
L Camardella C Caruso B Rutigliano M Romano G Di Prisco F Descalzi-Cancedda 《European journal of biochemistry》1988,171(3):485-489
Human erythrocyte glucose-6-phosphate dehydrogenase contains a reactive lysyl residue, which can be labelled with pyridoxal 5'-phosphate. The binding of one mole of pyridoxal 5'-phosphate per mole of enzyme subunit produces substantial inactivation. The substrate glucose-6-phosphate prevents the loss of activity, suggesting that the reaction site is close to the substrate-binding site. A tryptic peptide containing the pyridoxal-5'-phosphate-binding lysyl residue has been isolated and characterised. The reactive lysyl residue has been identified in the glucose-6-phosphate dehydrogenase amino acid sequence. Comparison with glucose-6-phosphate dehydrogenase from other sources shows a high homology with a peptide containing a reactive lysyl residue, isolated from the enzyme from Saccharomyces cerevisiae; glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides also contains a region highly homologous with the sequence around the reactive lysyl residue in the human enzyme. The results of this communication provide the first direct evidence for the association of an essential catalytic function with a specific region of the molecule of human erythrocyte glucose-6-phosphate dehydrogenase. 相似文献
42.
The technique of whole mount spreading is used to investigate the SC of three species of Asellidae (isopod crustaceans), Asellus aquaticus, Proasellus coxalis and Proasellus meridianus, which display considerable differences in genomic DNA content.The three species, originally considered to belong to the same genus Asellus, were subsequently assigned to two separate genera: Asellus and Proasellus. The SCs of the three species differ in morphological details related to the shape of the centromere region, the attachments to the nuclear envelope, the width of the central region and the presence of twists of the lateral elements. Furthermore, they display some differences in the degree of compaction of genomic DNA in the mitotic chromosomes. The greatest differences are found between A. aquaticus and P. coxalis, while P. meridianus has several features in common with either species. 相似文献
43.
We studied the stability of the genomic distribution of six retrotransposon families in long-term and short-term cultures of Drosophila cells. In a subclone derived from Kc cells, no significant rearrangements were detected over an 8 year period. On the contrary, extensive reshuffling and amplification of transposon families were observed in recently established cell lines. These results show that in cultured Drosophila cells transposition appears to be restricted to the transition from the embryo to continuous cell lines. 相似文献
44.
45.
Immunological detection of proteins phosphorylated at tyrosine in cells stimulated by growth factors or transformed by retroviral-oncogene-coded tyrosine kinases 总被引:10,自引:0,他引:10
M F Di Renzo R Ferracini L Naldini S Giordano P M Comoglio 《European journal of biochemistry》1986,158(2):383-391
The receptors for polypeptide growth factors and proteins coded by oncogenes of the src family are endowed with protein kinase activity and share the uncommon property of autophosphorylating at tyrosine residues. It is unclear whether the tyrosine kinase activity is also directed towards other targets of physiological significance. In this work, phosphotyrosine antibodies were used to detect, by Western blots and immunoprecipitation, proteins phosphorylated at tyrosine in fibroblasts either stimulated by growth factors (PDGF and EGF) or transformed by oncogene-coded tyrosine kinases. In stimulated cells the antibodies detected the autophosphorylated receptors, but only trace amounts of other proteins phosphorylated at tyrosine. In fibroblasts transformed by retroviral oncogenes (v-src, v-abl, v-fps or v-fes) proteins other than the corresponding oncogene-coded kinase, were found. A p70 was found to be heavily phosphorylated in fibroblasts transformed by v-src, v-fes and v-fps. A p130 and a p36 were found in cells transformed by v-src and v-abl. A unique p70 was phosphorylated in v-abl-transformed fibroblasts. These proteins were also phosphorylated in vitro in an immunocomplex kinase reaction. This reaction was blocked by the specific kinase inhibitors. These data strongly suggest that tyrosine kinases phosphorylate protein targets other than themselves. These targets are barely detectable in normal cells stimulated by growth factors, where the kinase activity is triggered rapidly and transiently. By contrast, a number of intracellular proteins phosphorylated at tyrosine accumulate in cells transformed by v-onc-coded kinases, endowed with constitutive and non-regulated enzymatic activity. 相似文献
46.
Nucleid acid metabolism in the LAM of the rat, both before and after castration, and during testosterone treatment, was investigated. RNA synthesis was increased by testosterone treatment, to varying degree, in adult and in prepubertal castrated rats, and was not merely dependent on the degree of hypertrophy of the LAM. The DNA content and the incorporation rate of formate-14C into DNA showed a characteristic profile under the same conditions: the atrophy of LAM following castration and its subsequent restoration appeared to be accompanied by variations in nuclei number. The possible role of testosterone in DNA duplication and in cell mitosis is hypothesized here; further investigation must be integrated by careful morphometric observation. 相似文献
47.
A. Filla G. De Michele V. Brescia Morra V. Palma A. Di Lauro G. Di Geronimo G. Campanella 《Journal of neurochemistry》1986,46(2):422-424
Glutamate dehydrogenase (GDH) activity was studied in 17 regions of six human brains. Duration and conditions of the postmortem period did not affect enzyme activity. Specific activity ranged between 103 and 377 nmoles/min/mg protein at 25 degrees C and it was 10-fold higher than that found in leukocytes. Apart from exclusively white matter regions (corpus callosum and centrum ovale), there was a moderate regional distribution (2.5-fold variation), with highest values in the inferior olive and hypothalamus, and lowest in the cerebellum and lenticular nucleus. With alpha-ketoglutarate (alpha-KG), NADH, or NH4+ as variable substrate, the apparent Km values in human brain were Km alpha-KG = 1.9 X 10(-3) M, KmNADH = 0.21 X 10(-3) M, and KmNH4+ = 28 X 10(-3) M, and in leukocytes they were Km alpha-KG = 1.7 X 10(-3) M, KmNADH = 0.24 X 10(-3) M, and KmNH4+ = 28 X 10(-3) M. The effects of cofactors, inhibitor, and pH were similar in brain and leukocyte GDH. 相似文献
48.
Physiological profile of world-class high-altitude climbers 总被引:1,自引:0,他引:1
O Oelz H Howald P E Di Prampero H Hoppeler H Claassen R Jenni A Bühlmann G Ferretti J C Brückner A Veicsteinas 《Journal of applied physiology》1986,60(5):1734-1742
The functional characteristics of six world-class high-altitude mountaineers were assessed 2-12 mo after the last high-altitude climb. Each climber on one or several occasions had reached altitudes of 8,500 m or above without supplementary O2. Static and dynamic lung volumes and right and left echocardiographic measurements were found to be within normal limits of sedentary controls (SC). Muscle fiber distribution was 70% type I, 22% type IIa, and 7% type IIb. Mean muscle fiber cross-sectional area was significantly smaller than that of SC (-15%) and of long-distance runners (LDR, -51%). The number of capillaries per unit cross-sectional area was significantly greater than that of SC (+ 40%). Total mitochondrial volume was not significantly different from that of SC, but its subsarcolemmal component was equal to that of LDR. Average maximal O2 consumption was 60 +/- 6 ml X kg-1 X min-1, which is between the values of SC and LDR. Average maximal anaerobic power was 28 +/- 2.5 W X kg-1, which is equal to that of SC and 40% lower that that of competitive high jumpers. All subjects were characterized by resting hyperventilation both in normoxia and in moderate (inspired O2 partial pressure = 77 Torr) hypoxia resulting in higher oxyhemoglobin saturation levels in hypoxia. The ventilatory response to four tidal volumes of pure O2 was similar to that of SC. It is concluded that elite high-altitude climbers do not have physiological adaptations to high altitude that justify their unique performance. 相似文献
49.
Hemoglobin is encapsulated in liposomes of different lipid composition. The resulting dispersion consists primarily of multilamellar liposomes (hemosomes) of a wide particle size distribution (diameter ranging mainly between 0.1 and 1 micron). The encapsulation efficiency is significantly larger with liposomes containing negatively charged lipids as compared to liposomes made of phosphatidylcholine. The integrity of the phospholipid bilayer is maintained in the presence of hemoglobin. The reaction rate of CO binding to encapsulated hemoglobin is reduced compared to that of free hemoglobin, but it is still greater than that observed in red blood cells. Hemoglobin encapsulated in liposomes made from negatively charged phospholipids is less stable than hemoglobin entrapped in isoelectric phosphatidylcholine. The instability of hemoglobin is due to the protein interacting with the negatively charged lipid bilayer. This interaction leads in turn to hemoglobin denaturation, possibly involving the dissociation of the heme group from the heme-globin complex. The nature of the negatively charged phospholipid is important in promoting the interaction with hemoglobin, the effect being in the order phosphatidic acid greater than phosphatidylinositol congruent to phosphatidylglycerol greater than phosphatidylserine. The presence of equimolar amounts of cholesterol in the phospholipid bilayer has a stabilizing effect on hemoglobin. This effect is pronounced with saturated phospholipids, but it is also observed, though to a lesser extent, with unsaturated ones, indicating that the bilayer fluidity has a modulating effect. The presence of cholesterol possibly interferes with secondary interactions following the binding of hemoglobin to the negatively charged lipid bilayer. 相似文献
50.
In the present research we have extended our work on the presence of binding sites for prolactin in the rabbit brain focusing our attention on the brain areas with high dopamine cell bodies density. Among these areas the hypothalamus showed the highest specific binding of labeled ovine prolactin (oPRL). Clearly detectible specific binding was observed also in substantia nigra, whereas in other brain regions the specific binding was very small, except for the striatum where a low but not negligible binding was found in female rabbits. The binding of 125I-oPRL showed a hormonal specificity and Scatchard analysis of the binding showed no clear difference in dissociation constant (Kd) between hypothalamus, nigra and striatum. 相似文献