全文获取类型
收费全文 | 1985篇 |
免费 | 101篇 |
出版年
2024年 | 1篇 |
2023年 | 9篇 |
2022年 | 15篇 |
2021年 | 53篇 |
2020年 | 28篇 |
2019年 | 39篇 |
2018年 | 47篇 |
2017年 | 45篇 |
2016年 | 69篇 |
2015年 | 90篇 |
2014年 | 110篇 |
2013年 | 121篇 |
2012年 | 182篇 |
2011年 | 160篇 |
2010年 | 110篇 |
2009年 | 100篇 |
2008年 | 152篇 |
2007年 | 108篇 |
2006年 | 108篇 |
2005年 | 117篇 |
2004年 | 113篇 |
2003年 | 95篇 |
2002年 | 78篇 |
2001年 | 9篇 |
2000年 | 7篇 |
1999年 | 23篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 6篇 |
1995年 | 12篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 9篇 |
1989年 | 13篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有2086条查询结果,搜索用时 31 毫秒
161.
Yano S Haruta H Ikeda T Kikuchi T Murakami M Moriguchi M Wakayama M 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2011,879(29):3247-3252
D-Aminoacylase from Alcaligenes xylosoxydans subsp. xylosoxydans A-6 (AxD-NAase) offers a novel biotechnological application, the production of D-amino acid from the racemic mixture of N-acyl-DL-amino acids. However, its substrate specificity is biased toward certain N-acyl-D-amino acids. To construct mutant AxD-NAases with substrate specificities different from those of wild-type enzyme, the substrate recognition site of the AxD-NAase was rationally manipulated based on computational structural analysis and comparison of its primary structure with other D-aminoacylases with distinct substrate specificities. Mutations of amino acid residues, Phe191, Leu298, Tyr344, and Met346, which interact with the side chain of the substrate, induced marked changes in activities toward each substrate. For example, the catalytic efficiency (k(cat)/K(m)) of mutant F191W toward N-acetyl-D-Trp and N-acetyl-D-Ala was enhanced by 15.6- and 1.5-folds, respectively, compared with that of the wild-type enzyme, and the catalytic efficiency (k(cat)/K(m)) of mutant L298A toward N-acetyl-D-Trp was enhanced by 4.4-folds compared with that of the wild-type enzyme. Other enzymatic properties of both mutants, such as pH and temperature dependence, were the same as those of the wild-type enzyme. The F191W mutant in particular is considered to be useful for the enzymatic production of D-Trp which is an important building block of some therapeutic drugs. 相似文献
162.
Maeda S Arai Y Higuchi H Tomoyasu Y Mizuno R Takahashi T Miyawaki T 《Redox report : communications in free radical research》2011,16(3):114-120
Iron, a source of oxidative stress, plays a major role in the pathology of neurodegenerative disease. In Alzheimer's disease, the hippocampus is vulnerable to oxidative stress, leading to impairment in memory formation. In our previous study, a brain oxidative reaction was induced after intraperitoneal injection of ferric nitrilotriacetate (Fe-NTA). However, since only a small amount of iron reached the brain in the previous study, Fe-NTA was administered into the hippocampus using an osmotic pump in this study. After continuous injection of Fe-NTA for 2 weeks, a high level of apoptotic change was induced in the hippocampus, in accordance with the iron localization. After injection for 4 weeks, the hippocampus was totally destroyed. A small amount of iron infiltrated into the cerebral cortex and the striatum, and deposition was observed at the choroid plexus and ependymal cells. However, no apoptotic reaction or clear tissue injury was observed in these areas. In addition, muscarinic acetylcholine receptors (M1, M2, and M4) were decreased in both the cortex and hippocampus while it increased in the striatum. Thus, the hippocampus is likely vulnerable to oxidative stress from Fe-NTA, and the oxidative stress is considered to bring the disturbance in the muscarinic acetylcholine receptors. 相似文献
163.
A part of eukaryotic tRNA genes harbor an intron at one nucleotide 3' to the anticodon, so that removal of the intron is an essential processing step for tRNA maturation. While some tRNA introns have important roles in modification of certain nucleotides, essentiality of the tRNA intron in eukaryotes has not been tested extensively. This is partly because most of the eukaryotic genomes have multiple genes encoding an isoacceptor tRNA. Here, we examined whether the intron of tRNA-Trp(CCA) genes, six copies of which are scattered on the genome of yeast, Saccharomyces cerevisiae, is essential for growth or translation of the yeast in vivo. We devised a procedure to remove all of the tRNA introns from the yeast genome iteratively with marker cassettes containing both positive and negative markers. Using this procedure, we removed all the introns from the six tRNA-Trp(CCA) genes, and found that the intronless strain grew normally and expressed tRNA-Trp(CCA) in an amount similar to that of the wild-type genes. Neither incorporation of (35)S-labeled amino acids into a TCA-insoluble fraction nor the major protein pattern on SDS-PAGE/2D gel were affected by complete removal of the intron, while expression levels of some proteins were marginally affected. Therefore, the tRNA-Trp(CCA) intron is dispensable for growth and bulk translation of the yeast. This raises the possibility that some mechanism other than selective pressure from translational efficiency maintains the tRNA intron on the yeast genome. 相似文献
164.
Komatsu T Hiasa M Miyaji T Kanamoto T Matsumoto T Otsuka M Moriyama Y Omote H 《The international journal of biochemistry & cell biology》2011,43(6):913-918
Human multidrug and toxic compound extrusion 2 (hMATE2) is a kidney-specific isoform of hMATE1, an exporter of toxic organic cations (OCs) of exogenous and endogenous origins at the final excretion step in the kidneys and liver (Otsuka et al., 2005), and contains a splicing variant, MATE2K, that has an exon of hMATE2 deleted (Masuda et al., 2006). In the present study, we characterized the degree of expression and the transport properties of hMATE2. Quantitative PCR analysis with probes specific for hMATE2 indicated the presence of hMATE2 mRNA in the kidneys, which corresponded to 39% of total mRNA encoding both hMATE2 and hMATE2K. hMATE2-specific antibodies immunostained the renal urinary tubules. Upon expression in HEK293 cells, hMATE2 was localized in intracellular vesicular structures, and thus transport activity of tetraethylammonium (TEA), a typical substrate for MATE transporters, by the cells was not detected. The hMATE2 protein was purified and reconstituted into liposomes. An artificially imposed pH gradient (ΔpH) across the proteoliposomal membrane drove the uptake of TEA. Dissipation of ΔpH by ammonium sulfate effectively inhibited the TEA uptake, while that of the membrane potential by valinomycin had little effect. The profiles of cis-inhibition of TEA transport by hMATE2 and hMATE2K are similar to each other. Thus, both hMATE2 and hMATE2K equally operate in the human kidneys to extrude OCs into the urine. 相似文献
165.
Intraspecific variation in seed production schedule and its effects on seed characteristics were examined in the deciduous
oak species Quercus serrata in Iwate Prefecture, northern Japan. Seed production schedule differed markedly among 31 individual trees. The median dates
of seed production differed by over 1 month between the earliest and latest individuals. Relationships between seed production
schedule and seed characteristics were investigated at two different scales: the among-tree scale and the within-tree scale.
At the within-tree scale, analyses revealed that the seeds dispersed by a tree later in the season tended to be larger in
size and lower in tannin concentration. Similarly, at the among-tree scale, analysis revealed that trees with a later seed
production schedule produced on average larger acorns. However, no clear tendency was observed between seed production schedule
and mean tannin concentration. The observed variation in seed production schedule is likely to affect seed fate per se, and
also via changes in seed characteristics. 相似文献
166.
Westereng B Ishida T Vaaje-Kolstad G Wu M Eijsink VG Igarashi K Samejima M Ståhlberg J Horn SJ Sandgren M 《PloS one》2011,6(11):e27807
Many fungi growing on plant biomass produce proteins currently classified as glycoside hydrolase family 61 (GH61), some of which are known to act synergistically with cellulases. In this study we show that PcGH61D, the gene product of an open reading frame in the genome of Phanerochaete chrysosporium, is an enzyme that cleaves cellulose using a metal-dependent oxidative mechanism that leads to generation of aldonic acids. The activity of this enzyme and its beneficial effect on the efficiency of classical cellulases are stimulated by the presence of electron donors. Experiments with reduced cellulose confirmed the oxidative nature of the reaction catalyzed by PcGH61D and indicated that the enzyme may be capable of penetrating into the substrate. Considering the abundance of GH61-encoding genes in fungi and genes encoding their functional bacterial homologues currently classified as carbohydrate binding modules family 33 (CBM33), this enzyme activity is likely to turn out as a major determinant of microbial biomass-degrading efficiency. 相似文献
167.
168.
Brown and Wanger [Brown, R.T., Wanger, A.R., 1964. Resistance to punishment and extinction following training with shock or nonreinforcement. J. Exp. Psychol. 68, 503-507] investigated rat behaviors with the following features: (1) rats were exposed to reward and punishment at the same time, (2) environment changed and rats relearned, and (3) rats were stochastically exposed to reward and punishment. The results are that exposure to nonreinforcement produces resistance to the decremental effects of behavior after stochastic reward schedule and that exposure to both punishment and reinforcement produces resistance to the decremental effects of behavior after stochastic punishment schedule. This paper aims to simulate the rat behaviors by a reinforcement learning algorithm in consideration of appearance probabilities of reinforcement signals. The former algorithms of reinforcement learning were unable to simulate the behavior of the feature (3). We improve the former reinforcement learning algorithms by controlling learning parameters in consideration of the acquisition probabilities of reinforcement signals. The proposed algorithm qualitatively simulates the result of the animal experiment of Brown and Wanger. 相似文献
169.
Effects of a 50 Hz electric field on plasma lipid peroxide level and antioxidant activity in rats 总被引:5,自引:0,他引:5
Harakawa S Inoue N Hori T Tochio K Kariya T Takahashi K Doge F Suzuki H Nagasawa H 《Bioelectromagnetics》2005,26(7):589-594
The effects of exposure to extremely low frequency electric fields (ELF EFs) on plasma lipid peroxide levels and antioxidant activity (AOA) in Sprague-Dawley rats were studied. The test was based on comparisons among rats treated with a combination of the oxidizing agent, 2,2'-azobis(2-aminopropane) dihydrochloride (AAPH) and 50 Hz EF of 17.5 kV/m intensity for 15 min per day for 7 days, AAPH alone, EF alone or no treatment. EF significantly decreased the plasma peroxide level in rats treated with AAPH, similar to treatment by ascorbic acid or the superoxide dismutase. Ascorbic acid increased AOA; however, EF and superoxide dismutase did not change AOA compared with sham exposure in stressed rats. No influence on the lipid peroxide level and AOA in unstressed rats was observed with EF exposure alone. Although the administration of AAPH decreased AOA, this decrease did not change when EF was added. These data indicate that the ELF EF used in this study influenced the lipid peroxide level in an oxidatively stressed rat. 相似文献
170.
Ueta K Ishihara T Matsumoto Y Oku A Nawano M Fujita T Saito A Arakawa K 《Life sciences》2005,76(23):2655-2668
We examined the effects of T-1095, an orally active inhibitor of Na(+)-glucose cotransporter (SGLT), on the development and severity of diabetes in Goto-Kakizaki (GK) rat, a spontaneous, non-obese model of type 2 diabetes. T-1095 was administered as dietary admixture (0.1% w/w) beginning at 7 weeks of age for 32 weeks. Untreated male GK rats were hyperglycemic compared with Wistar rats. Throughout the study, T-1095 treatment significantly decreased both blood glucose and hemoglobin A(1C) levels in the GK rats. The concomitant increase of urinary glucose excretion indicated that the hypoglycemic action of T-1095 is derived from the enhancement of urinary glucose disposal. Although food intake was not changed in the T-1095-treated rats, the body weight gain was retarded. T-1095 treatment partially ameliorated oral glucose tolerance but not the impaired glucose-induced insulin secretion. Homeostasis model assessment (HOMA) indicated the existence of insulin resistance in GK rats and a significant restoration by T-1095-treatment. There was a reduction of the thermal response in tail-flick testing following long-term hyperglycemia (diabetic neuropathy). Treatment of T-1095 significantly prevented the development of diabetic neuropathy in male GK rats. Sustained improvement of hyperglycemia and prevention of diabetic neuropathy by the T-1095-treatment provide further support the use of SGLT inhibitors for the treatment of diabetes. 相似文献