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81.
EPR spectroscopy is a powerful tool to identify at a molecular level, the different steps of catalyst preparation, and of catalytic reactions: - the distribution among the different sites in zeolites can be determined; - the dispersion of the active phase may be appreciated; - the unsaturation degree of the active site may be evaluated using probe molecules such as water or13C enriched carbon monoxide.
- Deposition of paramagnetic transition metal ions onto a support is monitored, and the coordination sphere of the metallic center is characterized by EPR.
- The catalyst is also characterized after activation (thermal oxidation or reduction):
- The catalytic mechanisms can be investigated by studying the elementary steps of the catalytic reaction, as illustrated for methanol oxidation over Mo/SiO2 catalysts whose EPR results have extended the reaction mechanism proposed on the basis of kinetic data. In addition, reaction intermediates may be isolated inquasi-in situ conditions as in the case of olefin oligomerization catalyzed by Ni/SiO2 systems.
82.
Summary An enzyme-immobilized microplate for determination of linamarin was prepared by covalently linking cassava leaf linamarase
to the microplate. For linamarin determination, cassava roots were homogenised in 0.1 Mo-phosphoric acid and the filtrate adjusted to pH 6 with NaOH prior to adding into the wells. The cyanide released was then
determined spectrophotometrically. One nmol linamarin can be detected. The microplate method is suitable for analysis of large
number of samples and is useful for screening purposes. 相似文献
83.
Summary We used in vitro growth inhibition assays to demonstrate that synthetic cecropin protein has potent activity against a range of plant pathogenic bacteria. We then prepared transgenic tobacco plants which express cecropin mRNA and protein. We have used Pseudomonas syringae pv tabaci infection of these transgenic tobacco as a model system to evaluate whether the plants which express cecropin protein also have increased tolerance to infection. We found no dramatic difference in disease response between plants which are expressing cecropin protein and control plants which were derived from the transformation with a binary vector which did not carry the gene encoding cecropin protein. 相似文献
84.
Citrate metabolism was studied in non-growing cells of Leuconostoc mesenteroides subsp. mesenteroides and subsp. dextranicum with respect to energetics, formation of degradation products and stoichiometry. The use of selective ionophores and uncoupler showed that citrate utilization was coupled to the proton motive force generated by ATP hydrolysis. Differences in citrate metabolism observed in 20 Leuconostoc strains were related to strains but not to the species or subspecies studied. Citrate metabolism was stimulated by glucose up to a concentration of 25 mmol 1-1 and decreased at higher concentrations. The main degradation products resulting from the co-metabolism of citrate (10 mmol 1-1 ) and glucose (2 mmol 1-1 ) were acetate, lactate and pyruvate. Only four Leuconostoc strains produced low levels of acetoin and diacetyl. No strains produced ethanol or acetaldehyde. Citrate degradation ability was stable for at least 130 generations in 81% of the Leuconostoc strains. 相似文献
85.
The epithelial lining of the gastrointestinal (GI) tract is in a state of continuous cell renewal, and the proliferating and differentiating/differentiated cell populations are spatially clearly demarcated. Members of the epidermal growth factor (EGF) family of peptides, the trefoil peptides and enteroglucagon appear to be the most important enterotrophic molecules for both normal cell renewal and healing after cell damage. Transforming growth factor-a (TGF-a) appears to be the primary physiological ligand for the EGF receptor (EGFR), promoting normal cell renewal, and TGF-a/EGFR are part of an autocrine loop in many intestinal cancers. In response to damage, a differentiating cell lineage arises from adjacent epithelium secreting EGF, TGF-a and trefoil peptides; this may be viewed as part of a ‘repair kit’ in damaged endodermally-derived tissue. 相似文献
86.
Catherine Jumarie Christiane Malo 《In vitro cellular & developmental biology. Animal》1994,30(11):753-760
Summary Caco-2 cell human colon adenocarcinoma cell line was used to study the hormonal regulation of small intestinal epithelial
cell differentiation. We had previously shown that insulin-transferrin-selenium and triiodothyronine (5 × 10−8
M)-supplemented medium can best replace serum after 2 days of culture for both the maintenance and differentiation of Caco-2
cells. The present study demonstrates that precoating petri dishes with complete serum allows the growth and differentiation
of Caco-2 cells seeded directly in serum-free medium. On the other hand, precoating with dialyzed serum inhibits alkaline
phosphatase and dipeptidyl-dipeptidase IV activities by more than 50%. The results obtained with complete serum-precoated
culture plates indicate that there is no synergy between insulin and triiodothyronine because cells maintained in transferrin-selenium
and triiodothyronine-supplemented medium, with or without insulin, express comparable enzyme activities. Moreover, large increases
in alkaline phosphatase and dipeptidyl-dipeptidase IV activities were observed when triiodothyronine was added to the culture
medium by the time confluency was reached. In contrast, γ-glutamyltransferase was lowered to a greater extent when triiodothyronine
was present from the beginning of culture. These findings show that triiodothyronine preferentially stimulates alkaline phosphatase
and dipeptidyl-dipeptidase IV activities during the differentiation period whereas it selectively inhibits γ-glutamyltransferase
during the proliferation phase. Triiodothyronine acts in a dose-dependent manner. 相似文献
87.
Chris E. Cooper Emma S. R. Green Catherine A. Rice-evans Michael J. Davies John M. Wriggleswort 《Free radical research》1994,20(4):219-227
The addition of 25μM hydrogen peroxide to 20μM metmyoglobin produces ferryl (FeIV = O) myoglobin. Optical spectroscopy shows that the ferryl species reaches a maximum concentration (60-70% of total haem) after 10 minutes and decays slowly (hours). Low temperature EPR spectroscopy of the high spin metmyoglobin (g = 6) signal is consistent with these findings. At this low peroxide concentration there is no evidence for iron release from the haem. At least two free radicals are detectable by EPR immediately after H2O2 addition, but decay completely after ten minutes. However, a longer-lived radical is observed at lower concentrations that is still present after 90 minutes. The monohydroxamate N-methylbutyro-hydroxamic acid (NMBH) increases the rate of decay of the fenyl species. In the presence of NMBH, none of the protein-bound free radicals are detectable; instead nitroxide radicals produced by oxidation of the hydroxamate group are observed. Similar results are observed with the trihydroxamate, desferoxamine. “Ferryl myoglobin” is still able to initiate lipid peroxidation even after the short-lived protein free radicals are no longer detectable (E.S.R. Newman, C.A. Rice-Evans and M.J. Davies (1991) Biochemical and Biophysical Research Communications 179, 1414-1419). It is suggested that the longer-lived protein radicals described here may be partly responsible for this effect. The mechanism of inhibition of initiation of lipid peroxidation by hydroxamate drugs, such as NMBH, may therefore be due to reduction of the protein-derived radicals, rather than reduction of ferryl haem. 相似文献
88.
The complex [Eu(tpy)3](ClO4)3 where TPY=2,2′; 6,2″-terpyridine, has been prepared and reexamined. The complex appears to be stable in acetonitrile solution with respect to decomplexation of the ligands but the addition of water does cause partial replacement of tpy. Analogous complexes have been prepared with 3,3′; 5,3″-polymethylene bridged derivatives of tpy having two or three carbons in the bridge. The bridging enforces a cisoid geometry of the ligand and prohibits its replacement by added water. An X-ray determination was carried out for [Eu(3b)3](ClO4)3, where 3b=3,3′; 5,3″-dimethylene tpy, which crystallizes in the monoclinic space group P21/c with a=11.908(4), b=15.768(5), c=29.513(9) Å, β=93.60(2)°, μ=13.5 cm−1 and Z=4. The complex forms a tricapped trigonal prism with each of the ligands adopting the same dl conformation. Variable temperature NMR analysis of the bridged ligand complexes indicates that conformational inversion of the bound ligand is not a concerted process and barriers for inversion of individual methylene units can be estimated from coalescence of the signals from the geminal methylene protons. The luminescence properties of the bridged tpy complexes are similar to the parent unbridged system. 相似文献
89.
Summary We first present two simple dimeric models of cotransport that may account for all of the kinetics of Na++-d-glucose cotransport published so far in the small intestine. Both the sigmoidicity in the Na++ activation of transport (positive cooperativity) and the upward deviations from linearity in the Eadie-Hofstee plots relative to glucose concentrations (negative cooperativity) can be rationalized within the concept of allosteric kinetic mechanisms corresponding to either of two models involving sequential or mixed concerted and sequential conformational changes. Such models also allow for 2 Na++ 1 S and 1 Na++ 1 S stoichiometries of cotransport at low and high substrate concentrations, respectively, and for partial inhibition by inhibitors or substrate analogues. Moreover, it is shown that the dimeric models may present physiological advantages over the seemingly admitted hypothesis of two different cotransporters in that tissue. We next address the reevaluation of Na++-d-glucose cotransport kinetics in rabbit intestinal brush border membrane vesicles using stable membrane preparations, a dynamic approach with the Fast Sampling Rapid Filtration Apparatus (FSRFA), and both nonlinear regression and statistical analyses. Under different conditions of temperatures, Na++ concentrations, and membrane potentials clamped using two different techniques, we demonstrate that our data can be fully accounted for by the presence of only one carrier in rabbit jejunal brush border membranes since transport kinetics relative to glucose concentrations satisfy simple Michaelis-Menten kinetics. Although supporting a monomeric structure of the cotransporter, such a conclusion would conflict with previous kinetic data and more recent studies implying a polymeric structure of the carrier protein. We thus consider a number of alternatives trying to reconcile the observation of Michaelis-Menten kinetics with allosteric mechanisms of cotransport associated with both positive and negative cooperativities for Na++ and glucose binding, respectively. Such models, implying energy storage and release steps through conformational changes associated with ligand binding to an allosteric protein, provide a rational hypothesis to understand the long-time debated question of energy transduction from the Na++ electrochemical gradient to the transporter.This research was supported by grant MT-7607 from the Medical Research Council of Canada. One of the authors (A.B.) was supported by a scholarship from the Fonds de la Recherche en Santé du Québec and C. C. was supported by a fellowship from the GRTM. The technical assistance of Mrs. C. Leroy has been greatly appreciated. The authors also thank D.D. Maenz and C. Malo for insightful discussions and C. Gauthier for the art work. 相似文献
90.
Involvement of Calcium Channels in Depolarization-Evoked Release of Adenosine from Spinal Cord Synaptosomes 总被引:3,自引:1,他引:2
Abstract: The potential involvement of L- and N-type voltage-sensitive calcium (Ca2+ ) channels and a voltage-independent receptor-operated Ca2+ channel in the release of adenosine from dorsal spinal cord synaptosomes induced by depolarization with K+ and capsaicin was examined. Bay K 8644 (10 n M ) augmented release of adenosine in the presence of a partial depolarization with K+ (addition of 6 m M ) but not capsaicin (1 and 10 μ M ). This augmentation was dose dependent from 1 to 10 n M and was followed by inhibition of release from 30 to 100 n M . Nifedipine and nitrendipine inhibited the augmenting effect of Bay K 8644 in a dose-dependent manner, but neither antagonist had any effect on release of adenosine produced by K+ (24 m M ) or capsaicin (1 and 10 μ M ) ω-Conotoxin inhibited K+ -evoked release of adenosine in a dose-dependent manner but had no effect on capsaicin-evoked release. Ruthenium red blocked capsaicin-induced release of adenosine but had no effect on K+ -evoked release. Although L-type voltage-sensitive Ca2+ channels can modulate release of adenosine when synaptosomes are partially depolarized with K+ , N-type voltage-sensitive Ca2+ channels are primarily involved in K+ -evoked release of adenosine. Capsaicin-evoked release of adenosine does not involve either L- or N-type Ca2+ channels, but is dependent on a mechanism that is sensitive to ruthenium red. 相似文献