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排序方式: 共有841条查询结果,搜索用时 15 毫秒
1.
A computer simulation model was developed to investigate nitrogenfluxes associated with microbial interactions in plankton communities.A short time scale was used, appropriate to the build-up anddecline of phytoplankton blooms in temperate shelf waters aftera mixing or upwelling event. The model depicts a continuum ofevents, many of which have been observed in coastal, upwellingand oceanic systems, including two phytoplankton peaks correspondingto new production and regenerated production.It predicts that nitrogen loss through sedimentation of phytoplanktonand faeces may result in a smaller bloom with a delayed onsetand prolonged duration. Microbial regeneration of nitrogen wasfound to be important in sustaining the middle stages of a phytoplanktonbloom, whereas micro- and meso-zooplankton regeneration occurredtowards the end of the bloom. 相似文献
2.
Using the imidate procedure, 2,3,4,6-tetra-O-benzyl-1-O-(N-methylacetimidoyl)-β-d-galactopyranose was condensed with various monosaccharides to provide, in good yield and with high stereoselectivity, α-linked disaccharides. 相似文献
3.
Investigation of the structure and localization of the urease of Helicobacter pylori using monoclonal antibodies 总被引:20,自引:0,他引:20
The urease of Helicobacter pylori (formerly Campylobacter pylori) has been partly purified by fast protein liquid chromatography. This material contained 10 nm doughnut-like structures when examined by electron microscopy and comprised three major polypeptides (61 kDa, 56 kDa and 28 kDa). Only two of these polypeptides (61 kDa and 28 kDa) were observed in urease-containing material isolated by preparative non-denatured PAGE. Monoclonal antibodies (mAbs) were produced which were directed against two of these polypeptides (56 kDa and 28 kDa). Only mAbs directed against the 28 kDa polypeptide inhibited or captured urease activity. These results suggest that the 56 kDa polypeptide is not essential for enzyme activity. Anti-urease mAbs were used in an indirect immunogold technique to localize the enzyme at the ultrastructural level. In both prefixed bacteria and ultrathin cryosectioned bacteria the enzyme was located on the cell surface and in material apparently shed from that surface. 相似文献
4.
P C Newell G Liu 《BioEssays : news and reviews in molecular, cellular and developmental biology》1992,14(7):473-479
Streamer F mutants have been found to be useful tools for studying the pathway of signal transduction leading to chemotactic cell movement. The primary defect in these mutants is in the structural gene for the cyclic GMP specific phosphodiesterase. This defect allows a larger and prolonged peak of cyclic GMP to be formed in response to the chemotactic stimulus, cyclic AMP. This characteristic aberrant pattern of cyclic GMP accumulation in the streamer F mutants has been correlated with similar patterns of changes in the influx of calcium from the medium, myosin II association with the cytoskeleton, myosin phosphorylation and a decrease in speed of movement of the amoebae. From these studies a sequence of events can be deduced that leads from cell surface cyclic AMP stimulation to cell polarization prior to movement of the amoebae in response to the chemotactic stimulus. 相似文献
5.
Two NADP-isocitrate dehydrogenase isoenzymes designated as NADP-IDH1 and NADP-IDH2 (EC 1.1.1.42) were identified in pea (Pisum sativum) leaf extracts by diethylaminoethylcellulose chromatography. The predominant form was found to be NADP-IDH1 while NADP-IDH2 represented only about 4% of the total leaf enzyme activity. These enzymes share few common epitopes as NADP-IDH2 was poorly recognized by the specific polyclonal antibodies raised against NADP-IDH1, and as a consequence NADP-IDH2 does not result from a post-translational modification of NADP-IDH1. Subcellular fractionation and isolation of chloroplasts through a Percoll gradient, followed by the identification of the associated enzymes, showed that NADP-IDH1 is restricted to the cytosol and NADP-IDH2 to the chloroplasts. Compared with the cytosolic isoenzyme, NADP-IDH2 was more thermolabile and exhibited a lower optimum pH. The data reported in this paper constitute the first report that the chloroplastic NADP-IDH and the cytosolic NADP-IDH are two distinct isoenzymes. The possible functions of the two isoenzymes are discussed.Abbreviations BSA
bovine serum albumin
- DEAE
diethylaminoethyl
- NADP-IDH
NADP-isocitrate dehydrogenase
- NADP-IDH1
cytosolic NADP-IDH
- NADP-IDH2
chloroplastic NADP-IDH 相似文献
6.
K K Caldwell M K Newell J C Cambier K N Prasad J M Masserano W Schlegel D M Cooper 《Analytical biochemistry》1988,175(1):177-190
The GTP-dependence for stimulatory and inhibitory regulation of plasma membrane adenylate cyclase activity was measured in plasma membrane fractions isolated from a variety of cell types (platelets, lymphocytes, PC12 cells, GH3 cells, NBP2 cells, and hepatocytes). This report shows that the isolation of plasma membranes for the study of GTP-dependent adenylate cyclase activity was, for some cells, enhanced by the exposure of the cells to glycerol prior to cell lysis. The isolation of plasma membranes from other cells, which did not appear to be sensitive to glycerol pretreatment, was enhanced by the removal of heavy particulate matter prior to fractionation of the cell lysate. The regulation of enzyme activity by various agents was found to be dependent upon the presence of (exogenous) GTP to varying degrees, indicating variable contamination of membrane preparations with GTP. It is concluded that (i) exposure of platelets and lymphocytes to glycerol prior to cell lysis decreases subsequent contamination of the plasma membrane preparation with GTP, and (ii) although glycerol pretreatment of other cells does not ensure the subsequent isolation of plasma membrane adenylate cyclase activity displaying high requirements for (exogenous) GTP, it is a reasonable first approach to be used during the development of procedures for the isolation of plasma membranes. 相似文献
7.
C-Glycosylation of 5,7-dihydroxy-3′,4′,5′-trimethoxyflavone was carried out with acetobromo-α-d-glucose, -α-d-galactose, -α-d-xylose, -β-l-arabinose and -αt-L-rhamnose. The respective 6-C-glycosides and 6, 8-di-C-glycosides (excepted for galactose) were isolated and permethylated. MS and TLC comparison confirmed the proposed structures of five natural tricetin-derived C-glycosides. 相似文献
8.
A fluorescent pigment was isolated from the culture fluid of Methanobacterium thermoautotrophicum strain H. This pigment was shown to be 7,8-didemethyl-8-hydroxy-5-deazariboflavin by various spectroscopic and chromatographic techniques. This compound was previously described as the FO acid hydrolysis fragment of coenzyme F420. On the basis of the time of appearance of the pigment in the course of fermentation, it is suggested that this substance may be an over-produced biosynthetic precursor of F420. 相似文献
9.
Determination of the specific radioactivity of fatty acids separated as their methyl esters by gas-liquid chromatography 总被引:1,自引:0,他引:1
Free or combined (3)H-labeled fatty acids are converted to their methyl-(14)C esters or, if labeled with (14)C, to their methyl-(3)H esters. For a given specific radioactivity of the methyl group, the nuclide ratio in the esters separated by GLC is a direct measure of the specific radioactivity of the fatty acids, and quantitative collection is unnecessary. Methods of methylation with minimum quantities of labeled methanol, and of deriving nuclide ratios from channel ratios in a scintillation spectrometer, are given. 相似文献
10.
Alternative Developmental Pathways Determined by Environmental Conditions in the Cellular Slime Mold Dictyostelium discoideum 总被引:19,自引:3,他引:16 下载免费PDF全文
The cellular slime mold Dictyostelium discoideum grows in the soil as a population of independent, uninucleate amoebae. Upon entrance to the stationary phase, the amoebae collect in multicellular aggregates to form organized fruiting bodies composed of spores and stalk cells. Depending upon environmental conditions, the developing aggregate either constructs the fruiting body at the site of aggregation or transforms into a structure that can migrate to a more favorable location. Environmental conditions that favor migration are (i) the accumulation of metabolite(s) produced by the aggregate and (ii) a low ionic strength in the substratum. Conditions that prevent migration or that stop a migrating slug are (i) the presence of buffer and (ii) illumination by overhead light. 相似文献