冬青属植物化学成分研究进展(综述)

近5年来,从冬青属植物中分离得到51种新化合物,绝大多数被鉴定为三萜及其皂甙,并显示其独特的生物活性。本文总结了这些化合物的结构和^13C-NMR数据。     

安普霉素的生物合成: 辛二糖C7′-N上甲基的甘氨酸来源

安普霉素是一类结构特殊的氨基糖苷类抗生素, 其分子中含一个稀有的辛二糖结构单元. 迄今为止, 尚无关于安普霉素生物合成途径及其前体化合物的完整报道. 研究发现, 向发酵培养基中添加甘氨酸和丝氨酸均可以出现在菌体生长保持基本不变的情况下促进抗生素产量的现象, 表明甘氨酸和/或丝氨酸可能参与了安普霉素的合成. [2-¹³C]甘氨酸示踪实验的核磁共振(NMR)检测结果表明, 甘氨酸专一地掺入安普霉素辛二糖环C7′-N甲基. 同时发现, 菌体胞内的S腺苷甲硫氨酸(SAM)水平上升与外加甘氨酸的量呈正比, 但是甲硫氨酸的加入会抑制安普霉素的合成. 据报道, 甲硫氨酸对结构中含有甲基取代基的抗生素, 如对rapamycin的 生物合成中转甲基过程有抑制作用. 由此推测, 尽管甲硫氨酸本身对抗生素产量呈抑制作用, 甘氨酸仍然可能通过甲硫氨酸循环提供甲基. 此外, [2-¹³C,¹⁵N]丝氨酸的示踪实验结果表明丝氨酸也可能作为安普霉素的限制性前体物参与了NH₂的合成.     

南蛇藤化学成分研究进展(综述)

近几十年来,从南蛇藤植物中已分离得到60余种化合物,其中大部分是倍半萜、三萜和黄酮。本文概述从南蛇藤中得到的数十种化合物的结构和波谱数据。     

微藻油脂组成的核磁共振碳谱表征

微藻油脂由于富含二十二碳六烯酸(DHA) 等多不饱和脂肪酸,且在培养过程中能吸收二氧化碳,因此在人类营养学和环境保护两个层面受到了前所未有的重视.为了深入考察微藻油脂的培养和提取过程,建立一种不破坏油脂就能区分油脂来源和特征的方法,是必不可少的.本文介绍了一种核磁共振碳谱表征方法.因为不必酯化,所以不仅可以测定双键数目和位置不同的脂肪酰基的摩尔比,而且测定了不同酰基在甘油烷基上的分布状况.本文分析了富含多不饱和脂肪酸的市售微藻油、富含饱和脂肪酸的棕榈油和富含单不饱和脂肪酸的橄榄油的组成,讨论了区分微藻微藻油中饱和脂肪酰基和不饱和脂肪酰基的测定方法和两种酰基的排列方式.     

The use of 13C-NMR for studies of wheat straw decomposition

13C-NMR was used to study the field decomposition of surface retained and incorporated wheat straw. Results showed decreasing proportions of straw carbon as carbohydrate and increasing proportions of aromatic compounds during straw decomposition. These changes were greater for the surface retained straw, however greater relative microbial contamination of incorporated straw may have affected results. The cost of 13C-NMR may lessen its role in studies of this nature.     

Determination of membrane lipid phase transition temperature from C-NMR intensities

A new and simple approach for the determination of the temperature of gel-to-liquid crystalline phase transitions (T_C) of biological (chloroplast) membrane lipids from ¹³C-NMR resonance intensities is proposed. The variation of intensity of a temperature-sensitive NMR resonance is monitored by recording the spectra of the sample at a range of temperatures. From such a series of spectra recorded at different temperatures, a temperature-insensitive resonance is located. Then the ratio of the intensity of the temperature-sensitive to the intensity of the temperature-insensitive resonance is calculated from each spectrum to even out the procedural error, if any. The values of this ratio at different temperatures, when plotted against sample temperature, shows a break at T_C as confirmed by spin label ESR studies.     

Transport and metabolism of exogenous fumarate and 3-phosphoglycerate in vascular smooth muscle

The keto (linear) form of exogenous fructose 1,6-bisphosphate, a highly charged glycolytic intermediate, may utilize a dicarboxylate transporter to cross the cell membrane, support glycolysis, and produce ATP anaerobically. We tested the hypothesis that fumarate, a dicarboxylate, and 3-phosphoglycerate (3-PG), an intermediate structurally similar to a dicarboxylate, could support contraction in vascular smooth muscle during hypoxia. To assess ATP production during hypoxia we measured isometric force maintenance in hog carotid arteries during hypoxia in the presence or absence of 20 mM fumarate or 3-PG. 3-PG improved maintenance of force (p < 0.05) during the 30-80 min period of hypoxia. Fumarate decreased peak isometric force development by 9.5% (p = 0.008) but modestly improved maintenance of force (p < 0.05) throughout the first 80 min of hypoxia. 13C-NMR on tissue extracts and superfusates revealed 1,2,3,4-13C-fumarate (5 mM) metabolism to 1,2,3,4-13C-malate under oxygenated and hypoxic conditions suggesting uptake and metabolism of fumarate. In conclusion, exogenous fumarate and 3-PG readily enter vascular smooth muscle cells, presumably by a dicarboxylate transporter, and support energetically important pathways.     

Direct determination of total soil carbohydrate content

A direct procedure for the determination of total soil carbohydrate content and a classic determination after acid hydrolysis, both employing the phenol-sulphuric acid method, are compared. The direct procedure enables simultaneous determination of mono-, oligo- and polysaccharides without prior hydrolysis. This procedure is reproducible and takes only a short period of time. The correlation between the proposed method and the classic one with hydrolysis was high (r²=0.9843; n=11; p=0.05).     

[1-13C]Glucose Metabolism in the Tumoral and Nontumoral Cerebral Tissue of a Glioma-Bearing Rat

C6 cells were used to establish a glioma-bearing rat model by stereotaxic injection in the left caudate nucleus. The tumor status was evaluated by magnetic resonance imaging and conventional histology. The glioma-bearing rats were infused for 1 h with a [1-(13)C]glucose solution. Perchloric acid extracts of the tumor and the ipsilateral and contralateral hemispheres were analyzed by 13C-NMR spectroscopy. The 13C-labeling patterns in compounds, mainly amino acids, indicated no drastic modification of carbon metabolism in both ipsilateral and contralateral hemispheres, as compared with control rats, whereas profound metabolic differences between brain tissue and tumor were observed. Glutamine C4 enrichment was lower in the glioma than in the brain [mean +/- SD values, 5.4 +/- 2.3 (n = 5) and 15.0 +/- 0.8% (n = 10), respectively] and also lower than the glutamate C4 enrichment in the glioma (mean +/- SD value, 22.6 +/- 4.2%; n = 5), indicating that tumor glutamine was neither synthesized inside the glioma nor taken up from the surrounding brain. The glutamine C4 enrichment in the serum (6.7 +/- 0.5%; n = 10) suggested that the glioma imported glutamine from the blood, a process probably connected with angiogenesis.     

In vivo nuclear magnetic resonance studies of the metabolism of methanol and pyruvate by Methanosarcina barkeri

Abstract The metabolism of methanol and pyruvate by cells of Methanosarcina barkeri was probed in vivo by NMR taking advantage of the non-invasive characteristics of this technique. Upon administration of substrates, the kinetics of substrate consumption, the product formation and the energetic state of the cells was monitored using carbon-13, phosphorus-31 or proton NMR. The effects of several inhibitors and uncouplers were investigated. Cells supplied with pyruvate developed considerable levels of nucleotide triphosphate; methane production was monitored, as well as CO₂ and H₂ formation. Most of the pyruvate was utilized for the synthesis of valine or intermediates of the valine pathway. The origin of the carbon atom in methane was elucidated using ¹³C-labelled pyruvate.     

Isomerization between n-butyrate and isobutyrate in enrichment cultures

Abstract An isobutyrate-degrading methanogenic enrichment was obtained from a mesophilic anaerobic digester. Studies with growing cells and cell suspensions showed a reversible isomerization between butyrate and isobutyrate, suggesting that butyrate is an intermediate in the anaerobic degradation of isobutyrate. NMR experiments with ¹³C-labelled butyrate demonstrated that this isomerization resulted from the migration of the carboxyl group.     

The metabolism of [3-(13)C]lactate in the rat brain is specific of a pyruvate carboxylase-deprived compartment

Lactate metabolism in the adult rat brain was investigated in relation with the concept of lactate trafficking between astrocytes and neurons. Wistar rats were infused intravenously with a solution containing either [3-(13)C]lactate (534 mM) or both glucose (750 mM) and [3-(13)C]lactate (534 mM). The time courses of both the concentration and (13)C enrichment of blood glucose and lactate were determined. The data indicated the occurrence of [3-(13)C]lactate recycling through liver gluconeogenesis. The yield of glucose labeling was, however, reduced when using the glucose-containing infusate. After a 20-min or 1-h infusion, perchloric acid extracts of the brain tissue were prepared and subsequently analyzed by (13)C- and (1)H-observed/(13)C-edited NMR spectroscopy. The (13)C labeling of amino acids indicated that [3-(13)C]lactate was metabolized in the brain. Based on the alanine C3 enrichment, lactate contribution to brain metabolism amounted to 35% under the most favorable conditions used. By contrast with what happens with [1-(13)C]glucose metabolism, no difference in glutamine C2 and C3 labeling was evidenced, indicating that lactate was metabolized in a compartment deprived of pyruvate carboxylase activity. This result confirms, for the first time from an in vivo study, that lactate is more specifically a neuronal substrate.     

氟代糖在糖苷酶研究中的应用

近年来,氟代糖应用于糖苷酶反应研究,显示出越来越重要的作用。氟代糖可以作为糖苷酶及其突变酶的水解底物研究酶学性质;氟代糖抑制剂可以标记糖苷酶催化中心,鉴定亲核体氨基酸。尤为重要的是,氟代糖可作为糖苷酶的糖基供体来合成糖类。糖苷酶突变后,可生成糖苷合成酶和硫代糖苷合成酶,可以用与正常底物构型相反的氟代糖作为糖基供体高效合成糖类,收率一般为60%~90%,有的可达100%。糖苷酶及其突变酶以氟代糖为底物高效合成糖类的研究,必将促进生物学、糖生物学和纳米生物材料的发展。     

Acetogenesis from H2 and CO2 by methane- and non-methane-producing human colonic bacterial communities

Abstract: The purpose of the study was to define the potential for reductive acetogenesis of colonic microflora from six non-methane- and four methane-excreting human subjects in relation to numbers of the different H₂-utilizing microorganisms. Faecal bacterial suspensions were incubated in the presence of NaH¹³CO₃ and under a gas phase composed of either 100% N₂ (control) or 80% H₂–20% N₂. The effects of a specific methanogenesis inhibitor or of sulfate supplementation were also determined. Quantitative nuclear magnetic resonance showed the presence of both single- and double-labelled acetate in all incubations under hydrogen. H₂/CO₂-acetogenesis appears to be a quantitatively important activity only in the presence of very low numbers of methanogens. Inhibition of methanogenesis induced a large increase in ¹³CO₂ incorporation into acetate in CH₄-producing samples. These results showed that methanogens can efficiently outcompete acetogens in human colonic contents. In contrast, no clear-cut competition for H₂ between acetogenesis and dissimilatory sulfate-reduction could be demonstrated. A slight reduction of the acetogenic activity was only observed at the highest sulfate addition (100 mM).