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51.
Acid phosphatase purified from maize scutellum, upon acylation with succinic anhydride, still shows negative co-operativity for the hydrolysis of glucose-6-phosphate at pH 5.4. This phenomenon is abolished by glucose, for both native and succinylated enzymes, through stimulation of the initial velocities at sub-optimal substrate concentrations. However, negative co-operativity for the enzymatic hydrolysis of p-nitrophenylphosphate at pH 5.4 is suppressed only at high concentrations of glucose. Furthermore, the hydrolysis of p-nitrophenylphosphate is noncompetitively inhibited (low affinity form of the enzyme molecule) by glucose, which suggests the existence of different substrate binding sites. 相似文献
52.
Monolayer black membranes from bipolar lipids of archaebacteria and their temperature-induced structural changes 总被引:1,自引:0,他引:1
Alessandra Gliozzi Ranieri Rolandi Mario De Rosa Agata Gambacorta 《The Journal of membrane biology》1983,75(1):45-56
The membrane of Caldariella acidophila, an extreme thermophilic archaebacterium, is characterized by unusual bipolar complex lipids. They consist of two nonequivalent polar heads, linked by a C40 alkylic component. The molecular organization of these lipids in the plasma membrane is still a matter of study. In this paper, we present current-voltage measurements on artificial bipolar lipid membranes, indicating that molecules are indeed organized as a covalently bound bilayer, in which each molecule is completely stretched and spans its entire thickness. Furthermore, conformational transitions of these artificial membranes (which could be formed only above 70 degrees C from a lipid/squalene dispersion) are analyzed in the 80 to 15 degrees C temperature range. Abrupt variations in capacitance and valinomycin-induced conductance seem to indicate the occurrence of at least two structural changes. Measurements are also extended to different solvent systems. Results are consistent with the picture of a monolayer bipolar lipid membrane in which few solvent molecules align themselves parallel to the lipophilic chains. The amount of solvent as well as the temperature at which conformational transitions occur, depend on the solvent system in which the lipid is dispersed. 相似文献
53.
G Biasi L Saggin F Dazzi A De Rossi D Collavo 《Journal of immunology (Baltimore, Md. : 1950)》1983,130(3):1447-1450
The role of Ia-positive accessory cells in the generation of a secondary cytotoxic response to tumor-associated antigens induced by Moloney murine sarcoma virus (M-MSV) was evaluated. Spleen cells from M-MSV-immune A.TL mice, depleted of accessory cells by anti-Iak serum plus C treatment and stimulated in secondary mixed leukocyte tumor cell culture (MLTC) with syngeneic Ia-negative A6ATL Moloney leukemic cells, failed to generate virus-specific cytotoxic T lymphocytes (CTL). CTL generation in Ia-depleted MLTC may be reconstituted by the addition of nonimmune Ia-positive spleen or peritoneal cells obtained not only from syngeneic A.TL but also from I-incompatible A.TH mice. This lack of restriction observed in accessory cell function is explained in terms of a nonspecific mechanism of CTL triggering mediated by soluble factors. In fact, IL 2 as well as supernatants obtained from I region-incompatible cultures consisting of M-MSV-immune, Ia-depleted A.TL spleen cells and A.TH Ia-positive cells, reconstituted secondary virus-specific CTL generation. 相似文献
54.
A ouabain p-aminobenzenediazonium derivative with a high specific radioactivity has been synthesized from ouabain and used as a photolabel for the (sodium plus potassium)-activated adenosinetriphosphatase from Electrophorus electricus electric organ and from dog kidney. In the dark it binds reversibly to the digitalis receptor site, with binding characteristics comparable to those of ouabain. The photoactivation of the ouabain derivative to produced covalent labeling of the receptor was obtained by energy transfer from a tryptophan residue in the (Na+,K+)ATPase to the ouabain p-aminobenzenediazonium molecule bound at the active site. The great advantage of this procedure compared to previous methods is that free molecules of the photoactivatable derivative are not photodecomposed. Analysis of the photolabeled polypeptides on sodium dodecyl sulfate gel electrophoresis showed that over 90% of the total radioactivity incorporated was found in the large molecular weight alpha-chain of the kidney enzyme (Mr 93 000). The same specific labeling of the alpha-subunit was obtained with a crude microsomal fraction from Electrophorus electricus. A mild tryptic fragmentation of the subunit into two peptide fragments of Mr 58 000 and 41 000, respectively, shows that the digitalis receptor is located in the N-terminal 41 000 fragment. 相似文献
55.
Purification and properties of two lectins from the latex of the euphorbiaceous plants Hura crepitans L. (sand-box tree) and Euphorbia characias L. (Mediterranean spurge). 下载免费PDF全文
L Barbieri A Falasca C Franceschi F Licastro C A Rossi F Stirpe 《The Biochemical journal》1983,215(3):433-439
1. From the latex of two members of the plant family Euphorbiaceae, Hura crepitans L. (sand-box tree) and Euphorbia characias L. (Mediterranean spurge), two lectins were purified by affinity chromatography on acid-treated Sepharose 6B followed by elution with D-galactose. 2. The lectin from E. characias is a single molecular species with Mr 80 000, made up of two identical subunits with Mr 40 000, and is a glycoprotein containing 11% carbohydrate. 3. The lectin from H. creptians appears as a mixture of three isolectins with Mr 140 000, consisting of four different subunits with Mr values 37 500, 35 500, 31 000, and 29 000. 4. Both lectins have haemagglutinating activity, with no specificity for human blood groups. The haemagglutinating activity is inhibited by D-galactose and by galactose-containing oligosaccharides. 5. The lectin from H. crepitans is mitogenic to human T-, but not to B-, lymphocytes. The latex of E. characias is mitogenic to T- and, to a lesser extent, to B-, lymphocytes, but the purified E. characias lectin has no mitogenic activity. 6. The lectin from H. crepitans, but not that from E. characias, inhibits protein synthesis by a rabbit reticulocyte lysate. 相似文献
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Differential Recovery of Auxotrophs After Penicillin Enrichment in Escherichia coli 总被引:6,自引:3,他引:3 下载免费PDF全文
Various auxotrophs are recovered from a penicillin enrichment cycle with differing efficiencies. Reconstruction experiments indicate that, under starvation conditions in the presence of penicillin, most auxotrophs undergo some death, whereas prolineless mutants are virtually immune to penicillin-induced killing. 相似文献
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