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611.
Dulyaninova NG Podlepa EM Toulokhonova LV Bykhovsky VY 《Biochimica et biophysica acta》2000,1478(2):211-220
As the rate-limiting enzyme, catalyzing the first reaction in NAD salvage synthesis, nicotinate phosphoribosyltransferase (NAPRTase, EC 2.4.2.11) is of important interest for studies of intracellular pyridine nucleotide pool regulation. We have purified NAPRTase 520-fold from Brevibacterium ammoniagenes ATCC 6872 without using an over-expression system by applying acid treatment, salt fractionation, Ca-phosphate gel treatment, anion exchange column chromatography and size-exclusion gel filtration. Unlike this enzyme from other sources, B. ammoniagenes NAPRTase was found to be controlled by the feedback inhibition by the end product NAD with K(i)=0.7+/-0.1 mM. The reaction products, pyrophosphate and nicotinate mononucleotide, also decreased the enzyme activity, as did other intermediates of NAD synthesis, such as AMP, ADP and a NAD direct precursor, nicotinate adenine dinucleotide or deamido NAD. The enzyme was observed to require a nucleoside triphosphate for its activity and showed the maximum affinity for ATP. The specificity, however, turned out to be poor, and ATP could be substituted by other nucleoside triphosphates as well as by sodium triphosphate. The kinetic characteristics of the enzyme are reported. For the first time, our data have experimentally revealed such complicated stimulatory and inhibitory effects by the intermediates of NAD biosynthesis on one of its salvage enzymes, NAPRTase. On the basis of these data, the key role of NAPRTase is discussed in light of the regulation of NAD metabolism in B. ammoniagenes. 相似文献
612.
In order to determine the structure of the fibrillar cell wall, the material isolated from cystocarpic thalli of the red seaweed Iridaea undulosa was fractionated using different media. While classical methods produced a scarce solubilization of material, the use of lithium salts in polar aprotic solvents (dimethylsulfoxide or N, N-dimethylacetamide), had successfully extracted higher amounts of material. The final residue from the Li(+)/DMSO extraction contains cellulose and a mannan, while that from the Li(+)/DMAc extraction contains only cellulose and traces of a galactan. Methylation analyses of both residues confirm the presence of those polysaccharides, and shows that the mannan is (1-->4)-linked. Treatment with proteases suggests that the protein is efficiently shielded from digestion. All the extracts and residues contain major amounts of (glyco)proteins and/or proteins, in agreement with a previous suggestion that they are of major importance in the structure of the cell wall. 相似文献
613.
The localization of hyaluronan has been determined in tailbud stage embryos of Xenopus laevis using a neurocan-alkaline phosphatase fusion protein. This polysaccharide was located between the germ layers and enriched in mesenchyme, the lumen of the neural tube, the embryonic gut, the hepatic cavity and the heart. A full-length cDNA for a hyaluronan synthase, Xhas2 has been cloned. The expression pattern of Xhas1 and 2 is closely similar to the distribution of hyaluronan in the embryo. Xhas1 produces hyaluronan with a molecular mass of around 40–200 kDa, while the product formed by Xhas2 has a molecular mass above 1 million Da. 相似文献
614.
Tli/Tlj=ri/rj 《中国科学:生命科学英文版》2000,43(1):57-67
1H NMR spectra of low-spin cyanide-ligated bacterial hemoglobin from Vitreoscilla (VtHb-CN) are reported. The assignments of the 1H NMR spectra of VtHb-CN have been made through MCOSY, NOESY, 1D TOE and SUPERWEFT experiments. Almost all resonance peaks of heme and ligated His85 are identified. The spin-lattice relaxation time T1's and the variation relationships of chemical shifts of these peaks with temperature have been acquired, from which the distances between the measured protons and Fe3 , and the diamagnetic chemical shifts have been acquired, respectively. The ionization constants of pKa's of ligated His85 are determined through pH titration of chemical shift, which is 4.95 for ligated His85 C2H proton. The lower pKa is attributed to the influence of the Fe3 of carrying positive charge and the coordination of His85 and Fe3 of heme. 相似文献
615.
Kimio HIRABAYASHI Goro KIMURA Yachiyo FUKUNAGA Kumiko TSUKADA Makoto ANDO Atsushi HATTORI 《水生生物学报》2006,12(6):1-659
Adults of aquatic insects emerge fromstreamslive inthe nearby riparian zone where they can select streamsidevegetations as preferred sites in order to(a)completemetamorphosis,(b)rest while awaiting proper swarmingtime,(c)feedin order to produce eggs,or(d)mate toreproduce[1—3].Most adult aquatic insect dispersal studies have fo-cused on quantifying the degree of upstream move-ment[4,5].Some researchers have shown distinct patternsof upstreammovement related to post-mating ovipositionalbehavior… 相似文献
616.
城市化是生物多样性快速丧失的主要原因之一。鸟类作为城市生态系统的重要组成部分, 其生物多样性格局和物种保护已成为城市生态学研究的热点。为揭示城市化过程中城区和郊区破碎化林地中鸟类群落的多样性差异和嵌套格局, 本研究于2021年春、夏季鸟类繁殖期采用样线法对海口和三亚市的城区、郊区共13个林地斑块中的鸟类群落进行调查。使用NODF (nestedness metric based on overlap and decreasing fill)和WNODF (weighted nestedness metric based on overlap and decreasing fill)方法进行嵌套格局分析。研究发现: (1)共记录到林鸟100种, 隶属于11目39科, 其中三亚郊区的鸟类丰富度最高, 共8目29科68种。记录到国家重点保护鸟类共18种, 其中两城市郊区的国家重点保护鸟类物种数均多于城区, 海口郊区还记录到国家I级重点保护鸟类黄胸鹀(Emberiza aureola)。(2)鸟类群落多度、物种丰富度、Pielou均匀度指数和Shannon-Wiener多样性指数在不同区域中均存在差异。海口城区的鸟类群落多度显著高于海口郊区(P < 0.05), 三亚郊区的鸟类群落物种丰富度、Pielou均匀度指数和Shannon-Wiener多样性指数均显著高于三亚城区和海口郊区(P < 0.05)。(3)嵌套分析结果表明, 海口和三亚市的城区、郊区林地鸟类群落均呈现反嵌套分布格局。线性回归分析显示, 三亚市城区和郊区的斑块面积与鸟类物种丰富度呈显著正相关, 而物种丰富度与斑块距最近大面积林地的距离之间无显著相关性。研究表明, 两城市鸟类群落多样性都表现出郊区高于城区的特点, 少数优势种(如白头鹎 Pycnonotus sinensis)占据了城市中的主要生态位。受城市化的影响, 海口郊区与城区鸟类群落有同质化的趋势。鸟类在城区和郊区斑块间的高流动性、种间竞争和斑块中资源的可利用性等因素可能导致斑块间鸟类群落的反嵌套分布格局。我们建议应加强城区和郊区鸟类的保护, 减少对林地的破坏, 提高城市鸟类多样性。 相似文献
617.
618.
619.
Primary structure and configuration of tea polysaccharide 总被引:7,自引:0,他引:7
ZHOU Peng XIE Mingyong NIE Shaoping & WANG Xiaoru . Key Laboratory of Food Science of Ministry of Education Nanchang University Nanchang China . Key Laboratory of Analytical Science of Ministry of Education Xiamen University Xiamen China 《中国科学:生命科学英文版》2004,47(5):416-424
Polysaccharide is a class of natural macromole-cules of which many species have been found to carry significant biological activities. Although the research on activities of saccharide has been at a lower level in the past comparing to those of proteins and nucleic acids, much progress has been made in recent years because of accelerated activities worldwide[1]. Such progress has been made mostly in areas of structural analysis, and researches on structure-activity relation-ships. The biologic… 相似文献
620.