首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   197篇
  免费   2篇
  国内免费   12篇
  2022年   1篇
  2021年   2篇
  2020年   3篇
  2019年   3篇
  2018年   4篇
  2017年   4篇
  2016年   2篇
  2015年   4篇
  2014年   5篇
  2013年   20篇
  2012年   2篇
  2011年   10篇
  2010年   9篇
  2009年   15篇
  2008年   20篇
  2007年   13篇
  2006年   12篇
  2005年   10篇
  2004年   10篇
  2003年   12篇
  2002年   12篇
  2001年   5篇
  2000年   3篇
  1999年   6篇
  1998年   2篇
  1997年   1篇
  1996年   1篇
  1995年   5篇
  1993年   2篇
  1991年   2篇
  1989年   1篇
  1988年   5篇
  1987年   1篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
排序方式: 共有211条查询结果,搜索用时 31 毫秒
41.
Polyphenol oxidase (PPO) of Thymbra (Thymbra spicata L. var. spicata) was isolated by (NH4)2SO4 precipitation and dialysis. A diphenolase from Thymbra plant, active against 4-methylcatechol, catechol and pyrogallol was characterized in detail in terms of pH and temperature optima, stability, kinetic parameters and inhibition behaviour towards some general PPO inhibitors. 4-Methylcatechol was the most suitable substrate, due to the lowest Km and the biggest Vmax/Km values, followed by catechol and pyrogallol. The Thymbra PPO had maximum activity at pH 5.0, 7.0 and 8.0 with 4-methylcatechol, catechol and pyrogallol substrates, respectively. The optimum temperature of activity for Thymbra PPO was 30, 40 and 50 °C for 4-methylcatechol, catechol and pyrogallol substrates, respectively. It was found that optimum temperature and pH were substrate-dependent studied. The enzyme activity decreased due to heat denaturation of the enzyme with increasing temperature and inactivation time. Inhibition of Thymbra PPO was investigated with inhibitors such as l-cysteine and glutathione using 4-methylcatechol, catechol and pyrogallol as substrates. It was found that l-cysteine was a more effective inhibitor than glutathione owing to lower Ki. The type of inhibition depended on the origin of the PPO studied and also on the substrate used. Furthermore, the IC50 values of inhibitors sudied on PPO were determined by means of activity percentage (I) diagrams.  相似文献   
42.
A different expression pattern of polyphenol oxidases has been observed during storage in cultivars of potato (Solanum tuberosum L.) featuring different length of dormancy: a short-dormant cultivar showed, at the end of the dormancy, both the highest polyphenol oxidase activity and the largest number of enzyme isoforms. An isoform of polyphenol oxidase isolated at the end of the physiological dormancy from a short-dormant cultivar has been purified to homogeneity by means of column chromatography on phenyl Sepharose and on Superdex 200. The purification factor has been determined equal to 88, and the molecular mass of the purified isoform has been estimated to be 69 and 340 kDa by SDS polyacrylamide gel electrophoresis and gel filtration on Superdex 200, respectively, indicating this PPO isoform as a multimer. The corresponding zymogram features a diffused single band at the cathodic region of the gel and the pI of this polyphenol oxidase has been calculated equal to 6.5.  相似文献   
43.
Koussevitzky S  Ne'eman E  Harel E 《Planta》2004,219(3):412-419
Polyphenol oxidase (PPO; EC 1.10.3.2 or EC 1.14.18.1) takes part in the response of tomato plants (Lycopersicon esculentum Mill.) to wounding and herbivore attack, mediated by the octadecanoid wound-signaling pathway. Wounding and methyl jasmonate (MeJA) induce expression of ppo genes and markedly increase the level of the enzyme. We report that pretreatment with MeJA also markedly increased the ability of isolated tomato chloroplasts to import and process PPO precursors (pPPO). Pea (Pisum sativum L.) chloroplasts showed no such response. Wounding or ethylene alone was ineffective but ethylene was synergistic with MeJA. Treatment with MeJA conferred a strong binding of pPPO, or its processing intermediate, to thylakoids and subsequent translocation into the lumen and processing to the mature protein. The effect on PPO import and translocation was evident after 8–16 h exposure to MeJA. Membrane-bound pPPO was cross-linked to a proteinaceous component of the thylakoid translocation apparatus, apparently induced by MeJA. The import and processing of other nuclear-encoded thylakoid proteins were not affected by MeJA in tomato. A 90-kDa protein that co-fractionated with thylakoids was induced along with the increase in competence for PPO import, and was identified as the proteinase-inhibitor multicystatin. It is concluded that the 90-kDa protein observed is part of the MeJA-induced defense response of tomato, not a component of the thylakoid translocation apparatus.Abbreviations Chl Chlorophyll - i and p Prefixes used to denote the intermediate and precursor forms of a protein, respectively - JA Jasmonic acid - LSU Large subunit of Rubisco - MeJA Methyl jasmonate - OE23 and OE33 23- and 33-kDa subunits of the oxygen-evolving complex of PSII - PC Plastocyanin - pPPO (iPPO, PPO) Precursor (intermediate, mature) form of polyphenol oxidase  相似文献   
44.
Wang J  Constabel CP 《Planta》2004,220(1):87-96
In order to functionally analyze the predicted defensive role of leaf polyphenol oxidase (PPO; EC 1.10.3.1) in Populus, transgenic hybrid aspen (Populus tremula × P. alba) plants overexpressing a hybrid poplar (Populus trichocarpa × P. deltoides) PtdPPO1 gene were constructed. Regenerated transgenic plants showed high PPO enzyme activity, PtdPPO1 mRNA levels and PPO protein accumulation. In leaf disk bioassays, forest tent caterpillar (Malacosoma disstria) larvae feeding on PPO-overexpressing transgenics experienced significantly higher mortality and reduced average weight gain compared to larvae feeding on control leaves. However, this effect was observed only when older egg masses were used and the resulting larvae showed reduced growth and vigor. In choice tests, no effect of PPO overexpression was detected. Although PPO in poplar leaves is latent and requires activation with detergents or trypsin for full enzymatic activity, in caterpillar frass the enzyme was extracted in the fully activated form. This activation correlated with partial proteolytic cleavage, suggesting that PPO latency and activation during digestion could be an adaptive and defense-related feature of poplar PPO.  相似文献   
45.
Polyphenol oxidase (PPO) was isolated from the B-serum obtained after repetitive freeze-thawing of the bottom fraction isolated from ultracentrifuged fresh latex. The B-serum was subjected to acetone precipitation and CM-Sepharose chromatography, affording two PPOs, PPO-I and PPO-II, which, upon SDS-PAGE, were 32 and 34 kDa, respectively. Both PPOs possessed the same pI (9.2), optimum pH (7) and optimum temperature (35-45 degrees C). They are stable up to 60 degrees C and active at broad pH ranges from 4-9. The K(m) values of PPO-I for dopamine, L-dopa and catechol as substrates are 2.08, 8.33 and 9.09 mM, while those for PPO-II are 2.12, 4.76 and 7.14 mM, respectively. Among various PPO inhibitors tested, 4-hexylresorcinol was the most potent. Anionic detergents were among the most effective activators of the enzymes, while cationic and nonionic detergents showed little and no effect on the PPO activities, respectively.  相似文献   
46.
Metabolic thiols such as cysteine and glutathione are involved in the biosynthetic pathway of phaeomelanins. They attack the o-quinones generated by polyphenol oxidase in its action on mono and o-diphenols and thus generate adducts. Determination of the molar absorptivities of these adducts is useful for spectrophotometric studies of phaeomelanin biosynthesis, antibrowning reagents in plants, and polyphenol oxidase assay methods. For their calculation, a method based on the depletion of o-diphenol by the action of polyphenol oxidase in the presence of thiol has been proposed. However, the method is slow and presents certain problems, for which reason we propose a new and faster method based on the action of polyphenol oxidase on o-diphenols which are in excess with respect to oxygen. Under these assay conditions there is rapid enzymatic formation of o-quinones, which react stoichiometrically with a thiol giving rise to the corresponding thiol-diphenol adduct. The method has been successfully applied to adducts of cysteine and glutathione with several o-diphenolic substrates of polyphenol oxidase involved in phaeomelanin biosynthesis in skin, neurones, and plants.  相似文献   
47.
It was found that enzyme from a microbial strain, Monocillium spp. ATCC 20621, catalyzed the oxidative reaction of rifamycin B to form rifamycin O. The identification of the reaction products suggested that the reaction proceeded by the oxidative cyclization of rifamycin B to give rifamycin O, which spontaneously hydrolyzed to rifamycin S in neutral aqueous milieu. The characteristic of the enzyme was different as compared with that of other polyphenol oxidases such as laccase. It is proposed that this new type of enzyme be classified into a subgroup EC 1.10.3.6 with a trivial name rifamycin B oxidase.  相似文献   
48.
The extractability of chlorogenic acid from defatted sunflower seed flour in water and salt solutions at different pH values and also in aqueous organic solvents was determined. It increased with increase in pH and at pH 8 in water nearly 70% chlorogenic acid was removed in a single extraction, while NaCl did not increase the extraction, and, MgCl2 and CaCl2 increased it, especially at higher concentrations. Methanol, ethanol, isopropanol and acetone, at 20% concentration in water, caused the maximum extraction of polyphenol. These organic solvents without added water were poor solvents for the extraction of polyphenol from the flour.  相似文献   
49.
Abstract Oolong tea extract (OTE) and the purified polymeric polyphenols from OTE have been found to inhibit glucosyltransferase (GTase) of mutans streptococci. In view of the partial fermentation characteristic of oolong tea, we describe here an in vitro model reaction system to produce partially fermented products of d-(+)-catechin or green tea extract (GTE) using horseradish peroxidase. A dimeric catechin molecule was identified as dehydro-dicatechin A by instrumental analyses. The molecular size of some oligomeric catechins was estimated by the elution profile with HPLC. These catechin oligomers markedly inhibited GTase from Streptococcus sobrinus 6715. As the degree of polymerization of catechin or GTE increased, GTase was inhibited more effectively. These results suggest that polymeric polyphenols found in OTE are synthesized by partial fermentation due to oxidases/peroxidases present in tea leaves.  相似文献   
50.
Decomposition of lignin, holocellulose, polyphenols and soluble carbohydrates was investigated in relation to nitrogen (N) dynamics in leaf litter of 14 tree species. The influence of organic chemical components and N on litter mass loss rate was then evaluated for 14 litter types. The study was carried out over a 3-year period on upper and lower parts of a forest slope in a cool temperate forest in Japan. The decomposition processes were divided into early and late phases based on N immobilization and mobilization. Mass loss rate of whole litter and organic chemical components was similar for the upper and lower sites. Litter mass loss was faster in the immobilization phase than in the mobilization phase in each of 14 litter types, which was ascribed to the decreased mass loss of holocellulose, polyphenols and soluble carbohydrates in the mobilization phase as compared to the immobilization phase. Mass loss rate of lignin was not different between the phases. Litter mass loss rate in the immobilization and mobilization phases was negatively correlated to lignin content and positively correlated to contents of polyphenols and soluble carbohydrates at the start of these phases, but was not correlated to holocellulose and N contents in either phase.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号