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821.
The reactions of aliphatic and aromatic amines with reducing sugars are important in both drug stability and synthesis. The
formation of glycosylamines in solution, the first step in the Maillard reaction, does not typically cause browning but results
in decreased potency and is hence significant from the aspect of drug instability. The purpose of this research was to present
(1) unreported ionic equilibria of model reactant (kynurenine), (2) the analytical methods used to characterize and measure
reaction products, (3) the kinetic scheme used to measure reaction rates and (4) relevant properties of various reducing sugars
that impact the reaction rate in solution. The methods used to identify the reversible formation of two products from the
reaction of kynurenine and monosaccharides included LC mass spectrometry, UV spectroscopy, and 1-D and 2-D 1H–1H COSY NMR spectroscopy. Kinetics was studied using a stability-indicating HPLC method. The results indicated the formation
of α and β glycosylamines by a pseudo first-order reversible reaction scheme in the pH range of 1–6. The forward reaction
was a function of initial glucose concentration but not the reverse reaction. It was concluded that the reaction kinetics
and equilibrium concentrations of the glycosylamines were pH-dependent and also a function of the acyclic content of the reacting
glucose isomer. 相似文献
822.
Distinguishing between cryptic species is a perennial problem for biologists. Bombus ruderatus and Bombus hortorum are two species of bumblebee, which can be indistinguishable from their morphology. The former species is in decline, whereas the latter is ubiquitous. In the UK, isolated records of B. ruderatus occur amongst many for B. hortorum. For ecological studies of B. ruderatus to be feasible, the two species need to be reliably distinguishable. We present a diagnostic tool for quick and reliable identification of problematic individuals based on a restriction enzyme digest of the cytochrome b region of mitochondrial DNA. 相似文献
823.
A RAPD marker specific for the G genome of wheat was identified. The corresponding 1171-bp DNA sequence was cloned and analyzed. Screening of the database did not reveal any homologies with the known plant DNA sequences. Using the primers specific to the flanking regions of the marker sequence, PCR analysis of the polyploid wheat species and the diploid species of the section Sitopsis was carried out. In addition, using the cloned sequence as a molecular hybridization probe, RFLP analysis of the genomic DNA of these species was performed. 相似文献
824.
K R Chintalacharuvu J F Piskurich M E Lamm C S Kaetzel 《Journal of cellular physiology》1991,148(1):35-47
The HT-29 human colon carcinoma cell line differentiates in glucose-free medium to an enterocytic phenotype. We previously isolated a series of HT-29 subclones selected for high levels of expression of secretory component (SC), the epithelial receptor for polymeric immunoglobulins. To develop a model system for studying effects of cell polarity on SC expression and release from the cell surface, the HT-29.74 subclone was induced to differentiate in glucose-free medium. Expression of SC was induced by glucose deprivation in both the parental HT-29 cell line and, to an even greater extent, in the HT-29.74 subclone. Prolonged glucose deprivation of HT-29.74 cells resulted in morphological changes consistent with enterocytic differentiation. Metabolic radiolabeling of SC in differentiated HT-29.74 cells indicated that proteolytic cleavage of membrane-bound to free SC occurred both on the cell surface and intracellularly, possibly in a vacuolar apical compartment or intrapeithelial lumen. To study effects of cell polarity on SC release, differentiated HT-29.74 cells were depolarized by culturing in low calcium medium. Within 2 hours after transfer of the cells into low calcium medium, a burst of SC release was observed concomitant with cell depolarization. Subsequently, release of SC declined significantly and remained low as long as cells were maintained in a depolarized state. The extent of cell depolarization could be controlled by varying the extracellular calcium concentration or by substituting the divalent cation Sr++, which partially prevents depolarization, for Ca++. In either case, the magnitude of the initial burst and subsequent decline in release of SC was proportional to the extent of cell depolarization. We conclude that cell polarity plays an important role in controlling the release of SC in intestinal epithelial cells, most likely by regulating the distribution of membrane-bound SC and SC protease, which are on the basolateral and apical cell surfaces, respectively, in differentiated cells. 相似文献
825.
826.
827.
828.
N E Reiner 《Journal of immunology (Baltimore, Md. : 1950)》1987,138(6):1919-1925
Interleukin 1 (IL 1) is a principal mediator of the host immune response to microbial challenge. Accessory cells of the monocyte-macrophage series are a major source of this cytokine and are also chronically parasitized by protozoa of the genus Leishmania. This suggests that characterization of the macrophage IL 1 response to Leishmania would increase our understanding of the regulation of host immunity to these organisms. In the present study, the macrophage IL 1 response to Leishmania donovani was examined because infections with this organism have findings consistent with parasite-specific T cell unresponsiveness. Cytokine activity was measured either by direct stimulation or by co-stimulation of thymocytes. Conditioned media from BALB/c resident peritoneal macrophages infected with amastigotes of L. donovani contained no more IL 1 than did supernatant fluids of control cells. In contrast, supernatants from cells stimulated with lipopolysaccharide or heat-killed Listeria monocytogenes had significantly increased cytokine content. Resident cells infected with L. donovani for 4 hr before being stimulated with Listeria demonstrated a suppressed IL 1 response (approximately 40% of Listeria alone) to this secondary particulate stimulus. In contrast, the secondary response of leishmania-preinfected cells to lipopolysaccharide was not affected. To examine whether accessory cell nonresponsiveness to L. donovani (with respect to IL 1) was related to the state of macrophage activation, elicited peritoneal macrophages obtained by injection of proteose peptone were also studied. These cells responded to stimulation with lipopolysaccharide and fixed Staphylococcus aureus with increases in intracellular, membrane, and secreted cytokine activities. In contrast, L. donovani failed to induce any of these activities. This was found to be the case irrespective of whether amastigotes were alive or killed or opsonized with specific antibodies. Elicited cells preinfected with Leishmania responded normally to secondary stimulation with lipopolysaccharide, but not S. aureus (64% of Staphylococcus alone). In addition, attachment and penetration of L. donovani promastigotes and their subsequent conversion to amastigotes within macrophages failed to induce IL 1 synthesis. The findings of this study indicate that L. donovani has the ability to both evade and suppress the macrophage IL 1 response. Because this monokine provides an obligatory signal during macrophage-dependent T cell activation, evasion of signal transduction for IL 1 synthesis may be related to defects in cell-mediated immunity which occur during infections with this organism. 相似文献
829.
E Emmanouilidou A G Teschemacher A E Pouli L I Nicholls E P Seward G A Rutter 《Current biology : CB》1999,9(16):915-918
Regulated exocytosis involves the Ca(2+)-triggered fusion of secretory vesicles with the plasma membrane, by activation of vesicle membrane Ca(2+)-binding proteins [1]. The Ca(2+)-binding sites of these proteins are likely to lie within 30 nm of the vesicle surface, a domain in which changes in Ca2+ concentration cannot be resolved by conventional fluorescence microscopy. A fluorescent indicator for Ca2+ called a yellow 'cameleon' (Ycam2) - comprising a fusion between a cyan-emitting mutant of the green fluorescent protein (GFP), calmodulin, the calmodulin-binding peptide M13 and an enhanced yellow-emitting GFP - which is targetable to specific intracellular locations, has been described [2]. Here, we generated a fusion between phogrin, a protein that is localised to secretory granule membranes [3], and Ycam2 (phogrin-Ycam2) to monitor changes in Ca2+ concentration ([Ca2+]) at the secretory vesicle surface ([Ca2+]gd) through alterations in fluorescence resonance energy transfer (FRET) between the linked cyan and yellow fluorescent proteins (CFP and YFP, respectively) in Ycam2. In both neuroendocrine PC12 and MIN6 pancreatic beta cells, apparent resting values of cytosolic [Ca2+] and [Ca2+](gd) were similar throughout the cell. In MIN6 cells following the activation of Ca2+ influx, the minority of vesicles that were within approximately 1 microm of the plasma membrane underwent increases in [Ca2+](gd) that were significantly greater than those experienced by deeper vesicles, and greater than the apparent cytosolic [Ca2+] change. The ability to image both global and compartmentalised [Ca2+] changes with recombinant targeted cameleons should extend the usefulness of these new Ca2+ probes. 相似文献
830.
Mechanics and modeling of plant cell growth 总被引:1,自引:0,他引:1