全文获取类型
收费全文 | 2728篇 |
免费 | 146篇 |
国内免费 | 17篇 |
专业分类
耳鼻咽喉 | 17篇 |
儿科学 | 54篇 |
妇产科学 | 74篇 |
基础医学 | 470篇 |
口腔科学 | 92篇 |
临床医学 | 245篇 |
内科学 | 649篇 |
皮肤病学 | 78篇 |
神经病学 | 233篇 |
特种医学 | 32篇 |
外科学 | 271篇 |
综合类 | 11篇 |
一般理论 | 2篇 |
预防医学 | 232篇 |
眼科学 | 18篇 |
药学 | 196篇 |
中国医学 | 7篇 |
肿瘤学 | 210篇 |
出版年
2023年 | 38篇 |
2022年 | 78篇 |
2021年 | 159篇 |
2020年 | 76篇 |
2019年 | 117篇 |
2018年 | 119篇 |
2017年 | 79篇 |
2016年 | 72篇 |
2015年 | 87篇 |
2014年 | 129篇 |
2013年 | 167篇 |
2012年 | 267篇 |
2011年 | 246篇 |
2010年 | 123篇 |
2009年 | 102篇 |
2008年 | 146篇 |
2007年 | 157篇 |
2006年 | 140篇 |
2005年 | 114篇 |
2004年 | 118篇 |
2003年 | 91篇 |
2002年 | 67篇 |
2001年 | 19篇 |
2000年 | 20篇 |
1999年 | 17篇 |
1998年 | 13篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1995年 | 8篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 12篇 |
1990年 | 9篇 |
1989年 | 3篇 |
1988年 | 9篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 12篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1972年 | 3篇 |
1970年 | 1篇 |
1967年 | 1篇 |
1964年 | 1篇 |
1959年 | 2篇 |
排序方式: 共有2891条查询结果,搜索用时 31 毫秒
91.
Tessa Moses Jacob Pollier Lorena Almagro Dieter Buyst Marc Van Montagu María A. Pedre?o José C. Martins Johan M. Thevelein Alain Goossens 《Proceedings of the National Academy of Sciences of the United States of America》2014,111(4):1634-1639
The saikosaponins comprise oleanane- and ursane-type triterpene saponins that are abundantly present in the roots of the genus Bupleurum widely used in Asian traditional medicine. Here we identified a gene, designated CYP716Y1, encoding a cytochrome P450 monooxygenase from Bupleurum falcatum that catalyzes the C-16α hydroxylation of oleanane- and ursane-type triterpenes. Exploiting this hitherto unavailable enzymatic activity, we launched a combinatorial synthetic biology program in which we combined CYP716Y1 with oxidosqualene cyclase, P450, and glycosyltransferase genes available from other plant species and reconstituted the synthesis of monoglycosylated saponins in yeast. Additionally, we established a culturing strategy in which applying methylated β-cyclodextrin to the culture medium allows the sequestration of heterologous nonvolatile hydrophobic terpenes, such as triterpene sapogenins, from engineered yeast cells into the growth medium, thereby greatly enhancing productivity. Together, our findings provide a sound base for the development of a synthetic biology platform for the production of bioactive triterpene sapo(ge)nins.Triterpene saponins are secondary metabolites that exhibit a large structural diversity and wide range of biological activities in many plant species (1, 2). Saponins are glycosides of sapogenins, which are composed of 30 carbon atoms arranged in 4- or 5-ring structures that are “decorated” by functional groups. Saponins are synthesized by multiple glycosylations of the sapogenin building blocks that are produced by multiple cytochrome P450-dependent monooxygenase (P450) or oxidoreductase-mediated modifications of basic backbones, such as β-amyrin (oleanane type), α-amyrin (ursane type), lupeol, and dammarenediol. These backbones are generated by oxidosqualene cyclase (OSC)-mediated cyclization of 2,3-oxidosqualene, which is also an intermediate in the synthesis of sterols in eukaryotes (3, 4). Both saponins and sapogenins include biologically active compounds or serve as starter molecules for the generation of novel, potentially bioactive structures by synthetic modification (5–7).The genus Bupleurum consists of perennial herbs that are used in Asian traditional medicine, either alone or in combination with other ingredients, for the treatment of common colds, fever, and inflammatory disorders (8). Saikosaponins constitute the largest class of secondary metabolites in Bupleurum and can account for up to ∼7% of root dry weight. Their accumulation can be further stimulated by jasmonate treatment (9). More than 120 closely related oleanane- and ursane-type saikosaponins have been identified from this genus and the oxidations at various positions suggest the presence of multiple enzymes, mainly P450s, capable of catalyzing specific modifications on the amyrin backbones (8, 10). To date, no P450 or oxidoreductase involved in triterpene saponin biosynthesis has been identified from Bupleurum species.P450s that modify the β-amyrin backbone on C-11; C-12,13; C-16; C-22; C-23; C-28 or C-30 have been characterized from Glycyrrhiza uralensis, Avena strigosa, Medicago truncatula, Glycine max, Vitis vinifera, and Catharanthus roseus (11–18). Hydroxylases from Panax ginseng that oxidize the dammarenediol-II backbone on C-6, C-12, or C-28 (19–21), and a C-20 hydroxylase from Lotus japonicus (22) that modifies lupeol, have also been identified. To characterize these P450s, they have been ectopically expressed in yeast strains either producing β-amyrin or externally supplied with candidate substrates. Similarly, several OSCs have been produced and functionally analyzed in yeast. From these studies, it is clear that yeast cells cannot only be used for the characterization of novel enzymes, but possibly also as a heterologous host for the production of triterpene sapogenins (23). To date only two pilot studies have aimed at engineering of β-amyrin production in yeast (24, 25), but no efforts toward engineering of sustainable production of sapogenins or saponins in yeast have been reported.Here, we identified and characterized CYP716Y1, a P450 from Bupleurum falcatum that corresponds to a C-16α oxidase, designated according to Nelson’s nomenclature (http://drnelson.uthsc.edu/cytochromeP450.html). By designing triterpene-hyperproducing starter strains, optimizing culturing conditions for triterpene synthesis, and using the CYP716Y1 gene in a combinatorial synthetic biology program, we established a platform that allows us to produce and sequester triterpene sapogenins in culture medium and to reconstitute a full saponin synthetic pathway in yeast cells. 相似文献
92.
Experimental data indicate that apoptosis is activated in the aged skeletal muscle, contributing to sarcopenia. We have previously demonstrated that testosterone protects against hydrogen peroxide (H(2)O(2))-induced apoptosis in C2C12 muscle cells. Here we identified molecular events involved in the antiapoptotic effect of testosterone. At short times of exposure to H(2)O(2) cells exhibit a defense response but at longer treatment times cells undergo apoptosis. Incubation with testosterone prior to H(2)O(2) induces BAD inactivation, inhibition of poly(ADP-ribose) polymerase cleavage, and a decrease in BAX levels, and impedes the loss of mitochondrial membrane potential, suggesting that the hormone participates in the regulation of the apoptotic intrinsic pathway. Simultaneous treatment with testosterone, H(2)O(2), and the androgen receptor (AR) antagonist, flutamide, reduces the effects of the hormone, pointing to a possible participation of the AR in the antiapoptotic effect. The data presented allow us to begin to elucidate the mechanism by which the hormone prevents apoptosis in skeletal muscle. 相似文献
93.
94.
Sarati LI Martinez CR Artés N Arreche N López-Costa JJ Balaszczuk AM Fellet AL 《Metabolism: clinical and experimental》2012,61(9):1301-1311
This study investigates whether changes in nitric oxide (NO) production participate in the cardiovascular manifestations of hypothyroidism and whether these changes are age-related. Sprague-Dawley rats aged 2 and 18 months old were treated with 0.02% methimazole (wt/vol) during 28 days. Left ventricular function was evaluated by echocardiography. Measurements of arterial blood pressure, heart rate, nitric oxide synthase (NOS) activity and NOS/caveolin-1 and -3 protein levels were performed. Hypothyroidism enhanced the age-related changes in heart function. Hypothyroid state decreased atrial NOS activity in both young and adult rats, associated with a reduction in protein levels of the three NOS isoforms in young animals and increased caveolin (cav) 1 expression in adult rats. Ventricle and aorta NOS activity increased in young and adult hypothyroid animals. In ventricle, changes in NOS activity were accompanied by an increase in inducible NOS isoform in young rats and by an increase in caveolins expression in adult rats. Greater aorta NOS activity level in young and in adult Hypo rats would derive from the inducible and the endothelial NOS isoform, respectively. Thyroid hormones would be one of the factors involved in the modulation of cardiovascular NO production and caveolin-1 and -3 tissue-specific abundance, regardless of age. Hypothyroidism appears to contribute in a differential way to aging-induced changes in the myocardium and aorta tissues. Low thyroid hormones levels would enhance the aging effect on the heart. Age-related changes in NO production participate in the cardiovascular manifestations of hypothyroidism. 相似文献
95.
96.
Minnucci G Amicarelli G Salmoiraghi S Spinelli O Guinea Montalvo ML Giussani U Adlerstein D Rambaldi A 《Haematologica》2012,97(9):1394-1400
Background The identification of the JAK2V617F mutation is mandatory in the diagnostic work-up of Philadelphia chromosome-negative myeloproliferative neoplasms. Several molecular techniques to detect this mutation are currently available, but each of them has some limits. DESIGN AND METHODS: We set up a novel molecular method for the identification of the JAK2V617F mutation based on an allele-specific loop-mediated amplification, not polymerase chain reaction analysis. This innovative technique amplifies DNA targets under isothermal conditions with high specificity, efficiency and rapidity. The method does not require either a thermal cycler or gel separation and the DNA amplification reaction is visible to the naked eye and can be monitored by turbidimetry. This method was validated on DNA from cell lines as well as from patients with myeloproliferative neoplasms. The results were compared with those obtained by conventional polymerase chain reaction methods. RESULTS: This assay detects, within 1 hour, the JAK2V617F mutation down to an allele burden of 0.1-0.01%. All samples positive by polymerase chain reaction (n=146) proved positive when tested by allele-specific loop-mediated amplification and none of the 80 negative controls gave false positive results. In addition, six patients with essential thrombocythemia previously diagnosed as being JAK2V617F negative by polymerase chain reaction analysis were found to be positive (at a low level) by allele-specific loop-mediated amplification. Furthermore, this assay discriminated the amount of JAK2V617F tumor allele within intervals of positivity, above 50%, between 50% and 10% and below 10%. Conclusions Allele-specific loop-mediated amplification is a simple, robust and easily applicable method for the molecular diagnosis and monitoring of JAK2V617F mutation in patients with chronic myeloproliferative neoplasms. 相似文献
97.
98.
A decline in the mitochondrial functions and aging are two closely related processes. The presence of estrogen and androgen receptors and hormone-responsive elements in the mitochondria represents the starting point for the investigation of the effects of 17β-estradiol and testosterone on the mitochondrial functions and their relationships with aging. Both steroids trigger a complex molecular mechanism that involves crosstalk between the mitochondria, nucleus, and plasma membrane, and the cytoskeleton plays a key role in these interactions. The result of this signaling is mitochondrial protection. Therefore, the molecular components of the pathways activated by the sexual steroids could represent targets for anti-aging therapies. In this review, we discuss previous studies that describe the estrogen- and testosterone-dependent actions on the mitochondrial processes implicated in aging. 相似文献
99.
Lorena C.W. Brito Silvia DalBó Tatiane M. Striechen Jéssica M. Farias Luiz R. Olchanheski Jr. Reila T. Mendes José C.R. Vellosa Giovani M. Fávero Regina Sordi Jamil Assreuy Fábio A. Santos Daniel Fernandes 《Archives of oral biology》2013