全文获取类型
收费全文 | 6867篇 |
免费 | 398篇 |
国内免费 | 228篇 |
出版年
2023年 | 36篇 |
2022年 | 75篇 |
2021年 | 93篇 |
2020年 | 107篇 |
2019年 | 158篇 |
2018年 | 186篇 |
2017年 | 142篇 |
2016年 | 155篇 |
2015年 | 157篇 |
2014年 | 341篇 |
2013年 | 587篇 |
2012年 | 270篇 |
2011年 | 444篇 |
2010年 | 309篇 |
2009年 | 348篇 |
2008年 | 362篇 |
2007年 | 369篇 |
2006年 | 350篇 |
2005年 | 334篇 |
2004年 | 254篇 |
2003年 | 261篇 |
2002年 | 221篇 |
2001年 | 177篇 |
2000年 | 112篇 |
1999年 | 129篇 |
1998年 | 126篇 |
1997年 | 124篇 |
1996年 | 130篇 |
1995年 | 110篇 |
1994年 | 102篇 |
1993年 | 86篇 |
1992年 | 66篇 |
1991年 | 57篇 |
1990年 | 61篇 |
1989年 | 69篇 |
1988年 | 55篇 |
1987年 | 41篇 |
1986年 | 33篇 |
1985年 | 56篇 |
1984年 | 71篇 |
1983年 | 40篇 |
1982年 | 55篇 |
1981年 | 44篇 |
1980年 | 30篇 |
1979年 | 38篇 |
1978年 | 32篇 |
1977年 | 22篇 |
1976年 | 20篇 |
1975年 | 13篇 |
1973年 | 15篇 |
排序方式: 共有7493条查询结果,搜索用时 15 毫秒
31.
Glutathione synthetase in tobacco suspension cultures: catalytic properties and localization 总被引:5,自引:0,他引:5
Glutathione synthetase activity (EC 6.3.2.3) was analysed in ammonium sulfate precipitates of extracts l'rom photohetevotrophically grown cells of Nicotiana tabactm L. cv. Samsun by determination of glutathione as its monobromobimane derivative. Maximal enzyme activity was obtained at pH 8.0–9.0 in Tris-HCl and CHES as buffer systems. The enzyme showed an absolute requirement for Mg2+ and was slightly stimulated by K+. When Mg2+ was replaced by Mn2+ less synthetase activity was observed, and above 30 m M Mn2+ no activity was found. The enzyme was specific for glycine (KM = 0.308 m M ). No product formation was observed with ß -alanine and γ y-aminobutyrate using substrate conccntrations of 10 m M . The apparent KM values for γ -glutamylcysteine and γ -glutamyl- l -α-aminobutyrate were, respectively, 0.022 and 0.033 m M . By chloroplast Isolation ca 24% of the total glutathione synthetase activity of the cells could be shown to be localized in the chloroplasts, the rest being attributed to the cytoplasm of the tobacco cells. 相似文献
32.
Twisted ribbons made of polystyrene were used as a packing material for the cultivation of anchorage dependent cells. Normal human fibroblast cells grown on this support in a laboratory scale reactor reached densities of about 5–7×105 cells/ml. The cells adhered strongly to the carrier and no cell detachment was observed upon transfer to serum free medium. The properties of this packing material and its potential use are discussed. 相似文献
33.
M. Rizzardini A. Graziani C. Carugo L. Cantoni 《Journal of biochemical and molecular toxicology》1988,3(1):33-45
Male C57BY/10 mice were chronically fed hexachlorobenzene (HCB) (0.02% of the diet) alone or in combination with a single subcutaneous dose of iron (12.5 mg iron per mouse). After eight weeks the group of mice pretreated with the iron overload was highly sensitized to the porphyrogenic effect of HCB, as shown by liver porphyrin accumulation. A synergistic effect of iron was evident on other parameters too, such as HCB-induced hepatic damage, activation of type O of xanthine oxidase, and decreased activity of copper zinc superoxide dismutase and glutathione peroxidase(s). None of these parameters was affected by iron alone. Iron alone and in association with HCB markedly raised the level of lipid peroxides, the increase in the HCB group being smaller. The combined treatment resulted in a significant reduction of HCB's inductive effects on microsomal heme and cytochromes P-450 and b5 and on the activity of aryl hydrocarbon hydroxylase. The content of nonprotein sulfhydryl groups was reduced to the same extent in mice treated with HCB or HCB plus iron. The results suggest that reactive intermediates such as are formed by lipid peroxidation are not sufficient on their own to create the conditions for uroporphyrinogen decarboxylase impairment, as evident in the group of mice receiving iron overload alone. Conversely, HCB administration induced a specific condition of imbalance in the liver between formation and inactivation of reactive intermediates which was associated with hepatic porphyrin accumulation and was potentiated by concomitant administration of iron. 相似文献
34.
Serum antioxidant enzyme activity in Parkinson's disease 总被引:2,自引:0,他引:2
Jawahar Kalra Ali H. Rajput Subrahmanyam V. Mantha Kailash Prasad 《Molecular and cellular biochemistry》1992,110(2):165-168
Summary The activities of superoxide dismutase (SOD; EC 1.15.1.1) and glutathione peroxidase (GSHPx; EC 1.11.1.9.), the enzymes that metabolize the superoxide anion and hydrogen peroxide, respectively, were measured in serum
from healthy subjects and patients with Parkinson's disease (PD). The activities of SOD and GSHPx in patients with PD were higher than those in normal healthy individuals. These results suggest that the increased activities
of these enzymes could be due to oxidative stress in the initial stages of this disease. 相似文献
35.
Glutathione reductase fromSaccharomyces cerevisiae undergoes redox interconversionin situ andin vivo
José Peinado Javier Florindo J. López-Barea 《Molecular and cellular biochemistry》1992,110(2):135-143
Redox interconversion of glutathione reductase was studiedin situ withS. cerevisiae. The enzyme was more sensitive to redox inactivation in 24 hour-starved cells than in freshly-grown ones. While 5 μM NADPH
or 100 μM NADH caused 50% inactivation in normal cells in 30 min, 0.75 μM NADPH or 50 μM NADH promoted a similar effect in
starved cells. GSSG reactivated the enzyme previously inactivated by NADPH, ascertaining that the enzyme was subjected to
redox interconversion. Low EDTA concentrations fully protected the enzyme from NADPH inactivation, thus confirming the participation
of metals in such a process. Extensive inactivation was obtained in permeabilized cells incubated with glucose-6-phosphate
or 6-phosphogluconate, in agreement with the very high specific activities of the corresponding dehydrogenases. Some inactivation
was also observed with malate, L-lactate, gluconate or isocitrate in the presence of low NADP+ concentrations.
The inactivation of yeast glutathione reductase has also been studiedin vivo. The activity decreased to 75% after 2 hours of growth with glucono-δ-lactone as carbon source, while NADPH rose to 144%
and NADP+ fell to 86% of their initial values. Greater changes were observed in the presence of 1.5 μM rotenone: enzymatic activity
descended to 23% of the control value, while the NADH/NAD+ and NADPH/NADP+ ratios rose to 171% and 262% of their initial values, respectively. Such results indicate that the lowered redox potential
of the pyridine nucleotide pool existing when glucono-δ-lactone is oxidized promotesin vivo inactivation of glutathione reductase. 相似文献
36.
M. Cifuentes P. Fernández-LLebrez J. Pérez J. M. Pérez-Fígares E. M. Rodríguez 《Cell and tissue research》1992,270(3):485-494
Summary The circulation of the cerebrospinal fluid along the central canal and its access to the parenchyma of the spinal cord of the rat have been analyzed by injection of horseradish peroxidase (HRP) into the lateral ventricle. Peroxidase was found throughout the central canal 13 min after injection, suggesting a rapid circulation of cerebrospinal fluid along the central canal of the rat spinal cord. It was cleared from the central canal within 2 h, in contrast with the situation in the brain tissue, where it remained in the periventricular areas for 4 h. In the central canal, HRP bound to Reissner's fiber and the luminal surface of the ependymal cells; it penetrated through the intercellular space of the ependymal lining, reached the subependymal neuropil, the basement membrane of local capillaries, and appeared in the lumen of endothelial pinocytotic vesicles. Furthermore, it accumulated in the labyrinths of the basement membrane contacting the basolateral aspect of the ependymal cells. In ependymocytes, HRP was found in single pinocytotic vesicles. The blood vessels supplying the spinal cord were classified into two types. Type-A vessels penetrated the spinal cord laterally and dorsally and displayed the tracer along their external wall as far as the gray matter. Type-B vessels intruded into the spinal cord from the medial ventral sulcus and occupied the anterior commissure of the gray matter, approaching the central canal. They represented the only vessels marked by HRP along their course through the gray matter. HRP spread from the wall of type-B vessels, labeling the labyrinths, the intercellular space of the ependymal lining, and the lumen of the central canal. This suggests a communication between the central canal and the outer cerebrospinal fluid space, at the level of the medial ventral sulcus, via the intercellular spaces, the perivascular basement membrane and its labyrinthine extensions. 相似文献
37.
Rainer Voigt Jelle Atema 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1992,171(5):673-683
1. | We determined the spectral tuning properties of 47 chemoreceptor cells of the antenna of Homarus americanus to amino acids and other compounds. Tests with 17 single compounds at 10-4 M showed 40 of 47 cells responded best to hydroxyproline, 4 cells to taurine and 3 cells to betaine. Mean tuning breadth (H-metric) doubled with 10 fold increase in concentration. |
2. | In hydroxyproline-best cells the mean threshold for hydroxyproline (Hyp) was found between 10-7 M and 10-8 M. An equimolar mixture of the 17 compounds generated a shallower stimulus-response function with thresholds similar to Hyp function (mixture suppression). Hyp-best cells were relatively narrowly tuned, often with arginine or leucine as second best stimuli. |
3. | Thus, physiologically the second antenna of H. americanus is a major chemoreceptor organ. It is more than any of the 5 chemoreceptor organs studied so far dominated by a single best-cell type (Hyp). Receptor cell composition of antennae resembles that of antennules more than legs or maxillipeds. Hyp-best cells in antennae and lateral antennules have similar tuning spectra. |
4. | Our cell tuning studies argue for independent receptors for all amino acids tested. We conclude that diversity of receptor cell tuning is created by cell-specific blends of receptors. At the organ level, differences in organ tuning result from different blends of receptor cells. |
38.
A comparative study on tyrosine oxidation was made with a pure cationic and anionic peroxidase from peanut cell culture medium. The results showed that both isozymes possessed almost identical capacity to oxidize tyrosine to dityrosine, isodityrosine and polytyrosine with the main difference being the pH optimum (pH 4 for the anionic and pH 7 for the cationic isozyme). Variation of reaction time after 1.5 h incubation had little effect on the quantity and quality of the oxidation products. On the other hand, increase of enzyme units correspondingly increased tyrosine-oxidation. The removal of heme and carbohydrate moieties from the holoenzyme arrested the reaction thereby suggesting the role played by these moieties in stabilizing the active site of peroxidase isoenzymes. Isolated cell wall extracts catalyzed the tyrosine-oxidation equally well as the purified peroxidase. Even though polyclonal antibodies against anionic peroxidase inhibited the in vitro tyrosine reaction they did not affect the tyrosine oxidation by the cell walls, while the cationic antibodies did.Abbreviations A.PRX
anionic peanut peroxidase
- C.PRX
cationic peanut peroxidase
- PcAb
polyclonal antibodies
- ELISA
enzyme-linked-immuno-sorbent-assay
- TFMS
trifluoromethane sulfonic acid 相似文献
39.
40.
活性氧对巨噬细胞呼吸爆发影响及云芝多糖的保护作用 总被引:2,自引:0,他引:2
用化学发光法观察到叔丁基氢过氧化物对培养的小鼠腹腔巨噬细胞呼吸爆发有强烈的抑制作用。云芝多糖经腹腔注射后,能增强巨噬细胞呼吸爆发功能对叔丁基氢过氧化物损伤的抵抗力。云芝多糖处理的巨噬细胞谷胱甘肽过氧化物酶基础活力显著提高,在叔丁基氢过氧化物作用下,云芝多糖处理的巨噬细胞仍有较高的谷胱甘肽过氧化物酶活力。说明巨噬细胞的免疫功能与谷胱甘肽过氧化物酶活力有关,非特异性免疫多糖可提高细胞抗氧化能力,减轻活性氧损伤作用。 相似文献