全文获取类型
收费全文 | 2274篇 |
免费 | 381篇 |
国内免费 | 1篇 |
专业分类
2656篇 |
出版年
2021年 | 25篇 |
2018年 | 18篇 |
2016年 | 19篇 |
2015年 | 47篇 |
2014年 | 58篇 |
2013年 | 63篇 |
2012年 | 107篇 |
2011年 | 81篇 |
2010年 | 58篇 |
2009年 | 60篇 |
2008年 | 89篇 |
2007年 | 95篇 |
2006年 | 70篇 |
2005年 | 72篇 |
2004年 | 86篇 |
2003年 | 79篇 |
2002年 | 66篇 |
2001年 | 88篇 |
2000年 | 80篇 |
1999年 | 53篇 |
1998年 | 30篇 |
1997年 | 27篇 |
1996年 | 25篇 |
1995年 | 36篇 |
1994年 | 27篇 |
1993年 | 40篇 |
1992年 | 62篇 |
1991年 | 63篇 |
1990年 | 56篇 |
1989年 | 52篇 |
1988年 | 62篇 |
1987年 | 39篇 |
1986年 | 47篇 |
1985年 | 47篇 |
1984年 | 48篇 |
1983年 | 32篇 |
1982年 | 33篇 |
1981年 | 36篇 |
1980年 | 37篇 |
1979年 | 51篇 |
1978年 | 41篇 |
1977年 | 36篇 |
1976年 | 35篇 |
1975年 | 18篇 |
1974年 | 24篇 |
1973年 | 24篇 |
1972年 | 23篇 |
1971年 | 29篇 |
1970年 | 17篇 |
1966年 | 19篇 |
排序方式: 共有2656条查询结果,搜索用时 15 毫秒
1.
L-myo-Inositol-1-phosphate synthase has been found to have at least a 5-fold preference for the beta-anomer of its natural substrate D-Glc-6-P. The alpha-anomer appears to be an inhibitor of the reaction and may be converted to product as well. As well as showing an enzymatic preference for the equatorial C-1 hydroxyl of D-Glc-6-P, our results suggest that it is the pyranose form of D-Glc-6-P that binds to the enzyme and that ring-opening is an enzymatic step. We have also found D-2-dGlc-6-P, D-2-F-2-dGlc-6-P, and D-Man-6-P each to be both competitive inhibitors and substrates that are converted to inositol phosphates by the synthase. D-Allose-6-P is a weak inhibitor of the enzyme, but not a substrate. D-Gal-6-P is neither substrate nor inhibitor. Thus the specificity of the synthase with respect to single position epimers of D-Glc-6-P increases in the order C1 less than C2 much less than C3 less than C4. 相似文献
2.
3.
4.
5.
The existence of an optimal range of cytosolic free calcium for insulin-stimulated glucose transport in rat adipocytes 总被引:9,自引:0,他引:9
B Draznin K Sussman M Kao D Lewis N Sherman 《The Journal of biological chemistry》1987,262(30):14385-14388
We have examined the effects of extracellular and intracellular Ca2+ concentrations upon basal and insulin-stimulated 2-deoxyglucose uptake in isolated rat adipocytes. In the absence of extracellular Ca2+, both basal and insulin-stimulated glucose uptake were significantly reduced. Insulin-stimulated glucose transport was optimal at 1 and 2 mM Ca2+. Further increases in extracellular Ca2+ concentration (3 mM) significantly diminished insulin-stimulated glucose uptake. When intracellular Ca2+ concentrations were augmented by ionomycin (1 microM), insulin-stimulated glucose uptake was significantly reduced at extracellular Ca2+ concentrations of 2 and 3 mM. The levels of intracellular free Ca2+ concentrations were then measured with Ca2+ indicator fura-2. The correlation between the levels of intracellular free Ca2+ and the magnitude of insulin-stimulated glucose uptake revealed that the optimal effect of insulin is observed at Ca2+ levels between 140 and 370 nM. At both extremes outside of this window, both low and high levels of intracellular Ca2+ result in diminished cellular responsiveness to insulin. These data suggest that intracellular calcium concentrations may exert a dual role in the regulation of cellular sensitivity to insulin. First, there must exist a minimal concentration of intracellular calcium to promote insulin action. Second, increased levels of intracellular calcium may provide a critical signal for diminution of insulin action. 相似文献
6.
Abstract: Electroconvulsive shock (ECS) administrations repeated for 10 consecutive days cause an elevation in the opioid content of the rat brain. Two different endogenous opioids, enkephalin and humoral-endorphin, undergo independent changes that differ in both their time course and intracerebral localization. These metabolic changes parallel long-term behavioral modifications such as the development and dissipation of tolerance to the analgesic effect of ECS. The activation of two different, independent, endogenous opioid systems by ECS is in agreement with previous behavioral and pharmacological studies. 相似文献
7.
Nonlymphoid, stromal cells in the mouse thymus are believed to be important in T cell maturation and have been proposed to play a central role in the acquisition of major histocompatibility complex (MHC) restriction and self-tolerance by maturing thymocytes. Both cortical and medullary epithelial cells in the thymus express high levels of class II (A) major histocompatibility antigens (MHC Ags). We show here that a specific subset of these A– epithelial cells express a transformation-associated antigen (6C3Ag) found previously on the surfaces of Abelson murine leukemia virus-transformed pre-B cells and on those bone marrow-derived stromal cell clones which support normal and preneoplastic pre-B cell proliferation. Among solid lymphoid organs, only the thymus contains 6C3Ag1 cells and within the thymus, this antigen is found exclusively on A– epithelial cells in cortical regions. It is striking that the expression of the 6C3Ag on thymic epithelium is developmentally regulated, suggesting a role for this lymphostromal antigen in the maturation of the thymic microenvironment. 相似文献
8.
9.
10.
Amino acid replacements resulting from super-suppression of nonsense mutants of iso-1-cytochrome c from yeast 总被引:25,自引:0,他引:25
The eight class I, set 1 super-suppressor genes, SUP2, SUP3, SUP4, SUP5, SUP6, SUP7, SUP8 and SUP11 are not closely linked and map at distinct loci throughout the genome of yeast. Each of these suppressors causes the production of 5 to 10% of the normal amount of iso-1-cytochrome c when it is individually coupled to the ochre (UAA) mutant cy1-2. All eight iso-1-cytochromes c contain a residue of tyrosine at position 20 which corresponds to the site of the ochre codon. Several of these super-suppressors also were shown to act on cy1-9, but at a much lower efficiency. It was shown that iso-1-cytochrome c from one of the suppressed cy1-9 strains contains a tyrosine at position 2, which corresponds to the site of the ochre codon in this mutant. It is suggested that the gene product of the eight super-suppressors is tyrosine transfer RNA. 相似文献