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991.
Galactose-1-phosphate uridylyltransferase (GalT) catalyzes the reversible transformation of uridine 5'-diphosphate glucose (UDPGlc) and galactose-1-phosphate into uridine 5'-diphosphate galactose (UDPGal) and glucose-1-phosphate through a double displacement mechanism, with the intermediate formation of a covalent uridylyl-enzyme (UMP-enzyme). The covalent linkage is a phosphoramidate formed between the UMP moiety and the His 166 N(epsilon)(2) of GalT, with His 166 N(delta1) retaining a proton throughout the catalytic cycle. Cys 160 and Ser 161 in Escherichia coli GalT are engaged in hydrogen bonding with the peripheral phosphoryl oxygen atoms of the substrate in the crystalline UMP-enzyme and in the crystalline complex of H166G-GalT with UDPGlc [Wedekind, J. E., Frey, P. A., and Rayment, I. (1996) Biochemistry 35, 11560-11569; Thoden, J. B., Ruzicka, F. J., Frey, P. A., Rayment, I., and Holden, H. M. (1997) Biochemistry 36, 1212-1222]. Site-directed mutagenesis, thermodynamic, transient kinetic, and steady-state kinetic studies have been performed to investigate the roles of Cys 160 and Ser 161 in catalysis. The absence of the thiol group of Cys 160 in the variants C160S and C160A did not seriously alter the enzymatic activity. However, the variant S161A displayed 7000-fold less activity than wild-type GalT. The low activity of S161A was directly related to impaired uridylylation rate constant (3.7 x 10(-)(2) s(-)(1)) and de-uridylylation rate constant (0.5 x 10(-)(2) s(-)(1)) resulting from a higher kinetic barrier for uridylyl-group transfer by the variant S161A as compared with the wild-type GalT. Equilibrium uridylylation studies showed that neither Cys 160 nor Ser 161 was involved in stabilizing the uridylyl-enzyme intermediate. The results lead to the conclusion that the conserved Cys 160 does not play a critical role in catalysis. Ser 161 is most likely involved in donating a hydrogen bond to the beta-phosphoryl group of a substrate, thereby providing proper orientation for nucleophilic catalysis. 相似文献
992.
Orson FM Kinsey BM Hua PJ Bhogal BS Densmore CL Barry MA 《Journal of immunology (Baltimore, Md. : 1950)》2000,164(12):6313-6321
Genetic immunization is a novel form of vaccination in which transgenes are delivered into the host to produce the foreign protein within host cells. Although systemic immune responses have been relatively easy to induce by genetic immunization, the induction of regional and mucosal immunity has often been more challenging. To address the problem of eliciting mucosal immunity in the lung, we utilized macroaggregated albumin to target plasmid DNA to the lung. Macroaggregated albumin is trapped in the lung after i. v. injection, and it is routinely used in radiolabeled form as an imaging modality to evaluate pulmonary blood flow. To couple DNA to this targeting agent, polyethyleneimine (a polycation that binds DNA and enhances transfection) was conjugated to serum albumin, and the conjugate was aggregated by heating to produce particles of 25-100 microm. The resulting particles bound plasmid DNA avidly, and when injected i.v. in mice, the particles distributed in the peripheral lung tissue in the alveolar interstitium. Particle-bound luciferase plasmid transfected a variety of cell lines in vitro, and after i.v. injection, gene expression was detected exclusively in the lung. Using human growth hormone as the encoded foreign Ag for immunization, i.v. injection of the particle-bound plasmid elicited both pulmonary mucosal and systemic immune responses, whereas naked DNA injected either i.v. or i.m. elicited only systemic responses. Thus, particle-bound plasmid DNA may have utility for genetic immunization by intravascular delivery to the lung and potentially to other organs and tissues. 相似文献
993.
In order to characterize the contributions of respiratory and photosynthetic actions to energy conversions, the mixotrophic cells of Marchantia polymorpha were cultivated in the medium containing 10kg/m(3) glucose as an organic carbon source. The cultures were conducted with the supply of ordinary air (0.03% CO(2)) at constant incident light intensities of 50 and 180W/m(2). From the results of metabolic analysis, it was found that the cell yield based on ATP synthesis was estimated to be 6.3x10(-3)kg-dry cells/mol-ATP in these cultures. Under the examined conditions, energy conversion efficiency through respiration was larger than that through photosynthesis, and efficiency of overall energy conversion to ATP was maximized when the sum of energies from glucose and light captured by the cells was approximately 7.2x10(5)J/(hkg-dry cells). Taking into account the efficiency of overall energy conversion, a batch culture of M. polymorpha in a bioreactor was carried out by regulating incident light intensity ranging from 9 to 58W/m(2). In the culture with light regulation, the cell yield of 6.2x10(-9)kg-dry cells/J was achieved on the basis of energy provided to the system throughout the culture, and this value was 2.3 and 9.3 times as large as those obtained in the cultures under constant incident light intensities of 50 and 180W/m(2), respectively. 相似文献
994.
995.
Kurihara Y Kurihara H Morita H Cao WH Ling GY Kumada M Kimura S Nagai R Yazaki Y Kuwaki T 《American journal of physiology. Regulatory, integrative and comparative physiology》2000,279(2):R515-R521
Endothelin (ET)-1 is a 21-amino acid peptide that induces a variety of biological activities, including vasoconstriction and cell proliferation, and its likely involvement in cardiovascular and other diseases has recently led to broad clinical trials of ET receptor antagonists. ET-1 is widely distributed in the central nervous system (CNS), where it is thought to regulate hormone and neurotransmitter release. Here we show that CNS responses to emotional and physical stressors are differentially affected in heterozygous ET-1-knockout mice, which exhibited diminished aggressive and autonomic responses toward intruders (emotional stressors) but responded to restraint-induced (physical) stress more intensely than wild-type mice. This suggests differing roles of ET-1 in the central pathways mediating responses to different types of stress. Hypothalamic levels of ET-1 and the catecholamine metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) were both increased in wild-type mice subjected to intruder stress, whereas MHPG levels were not significantly affected in ET-1-knockout mice. Furthermore, immunohistochemical analysis showed that ET-1 and tyrosine hydroxylase, an enzyme in the catecholamine synthesis pathway, were colocalized within certain neurons of the hypothalamus and amygdala. Our findings suggest that ET-1 modulates central coordination of stress responses in close association with catecholamine metabolism. 相似文献
996.
Zhang F Zhang W Liu L Fisher CL Hui D Childs S Dorovini-Zis K Ling V 《The Journal of biological chemistry》2000,275(30):23287-23294
We have cloned full-length human and mouse cDNAs of ABCB9, which encodes a predicted multiple-spanning transmembrane domain and a nucleotide-binding domain with Walker motifs. It is therefore designated as a "half" ATP binding cassette (ABC) transporter. Northern analysis shows that the ABCB9 mRNA is expressed at a high level in testes and moderate levels in brain and spinal cord. A splice variant mRNA deleted in the last pair of predicted transmembrane segments was shown to be expressed in human tissues. Phylogenetic analysis indicates that ABCB9 is closely related to TAP1 and TAP2, two "half" ABC proteins found in endoplasmic reticulum. ABCB9 protein colocalized with the lysosomal markers, LAMP1 and LAMP2, in transfected cells. ABCB9 protein appears to be most highly expressed in the Sertoli cells of the seminiferous tubules in mouse and rat testes. These cells have high levels of phagocytosis and secretory activities. These findings pave the way for further investigation into the potential novel function of ABCB9 in lysosomes. 相似文献
997.
998.
新疆土地荒漠化及其防治对策 总被引:3,自引:0,他引:3
1 荒漠化土地分布现状1.1 分 布北倚阿尔泰山南麓,南亘昆仑山北麓,东至甘、新和中蒙边界,西达哈萨克斯坦荒漠。新疆荒漠化土地地理垂直分布相差悬殊,准噶尔盆地海拔179~1000m,塔里木盆地海拔780~1500m,昆仑山、阿尔金山、天山南坡海拔1500~3100m,阿尔金山库木库里盆地海拔3900~5000m,而吐鲁番盆地为-154m。在昆仑山和阿尔金山北坡、天山南北坡、阿尔泰山南坡的山前洪积平原上分布着大面积卵砾粗岩片碎石戈壁。在罗布泊、乌尔禾谷地、吐鲁番盆地、英吉沙、库车、莎车等县的洪积细土平原和湖积平原高台地上,分布着风蚀残丘(雅丹地… 相似文献
999.
Rumen ciliate species composition was surveyed in domestic yaks kept in Tibet, Sichuan, and Inner Mongolia, China. Twelve genera including 36 species with 18 formae were identified. The species compositions were slightly different among the three areas: yaks in Tibet had the simplest fauna, in contrast, the fauna of yaks in Inner Mongolia were the most abundant and similar to those found in the cattle kept in the same area. This suggests that the rumen ciliate composition of yaks is affected by that of cattle kept together or in proximity. A new species belonging to the genus Entodinium, Entodinium monuo n. sp., was recognized from the yaks in all areas. This new species was common in the yaks but was not detected in the cattle fed near yaks in Inner Mongolia. There was a similar generic ciliate composition in yaks kept in respective areas. Entodinium was the most predominate ciliate (51.9-61.0%) with total ciliate densities estimated as 10(5)/ml per yak. 相似文献
1000.