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31.
The N-terminal pentapeptide of the lipoprotein from the outer membrane of Escherichia coli was obtained by coupling S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-cysteine to O-tert-butylseryl-O-tert-butyl-seryl-asparaginyl-alanine tert-butyl ester followed by deprotection with trifluoroacetic acid. The tetrapeptide was built up from alanine tert-butyl ester with N-9-fluorenylmethyloxycarbonyl protected amino acids. S-[2,3-Bis(palmitoyloxy)propyl]-N-palmitoylcysteine was obtained from N,N'-dipalmitoylcystine di-tert-butyl ester via reduction to the thiol, and S-alkylation with racemic 3-bromo-1,2-propanediol followed by esterification with palmitic acid in the presence of dicyclohexylcarbodiimide/dimethylaminopyridine and deprotection with trifluoroacetic acid. The compounds were characterized unequivocally by 13C-NMR and mass spectra. The diastereomers of S-[2,3-bis(palmitoyloxy)propyl]-N-palmitoylcysteine tert-butyl ester with opposite configuration at the propyl-C-2 atom could be separated on a silica-gel column.  相似文献   
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The bee venom constituent, melittin, is structurally and functionally related to alamethicin. By forming solvent-free planar bilayers of small area (approx. 100 microns 2) on the tip of fire-polished glass pipettes we could observe single melittin pores in these membranes. An increase in the applied voltage induced further non-integral conductance levels. This indicates that melittin forms multi-level pores similar to those formed by alamethicin. Trichotoxin A40, an antibiotic analogue of alamethicin, also induces a voltage-dependent bilayer conductivity, but no stable pore states are resolved. However, chemical modification of the C-terminal molecule part by introduction of a dansyl group leads to a steeper voltage-dependence and pore state stabilization. Comparing structure and activity of several natural and synthetic amphiphilic polypeptides, we conclude that a lipophilic, N-terminal alpha-helical part of adequate length (dipole moment) and a large enough hydrophilic, C-terminal region are sufficient prerequisites for voltage-dependent formation of multi-state pores.  相似文献   
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Summary The Avena geo-curvature test is a bioassay for auxin-type growth regulators. Etiolated oat coleoptiles are used and the test is conducted in special perspex trays under diffuse daylight. The coleoptiles, with the primary leaves intact, are arranged in grooves on the trays in 4 series of 6 coleoptiles each, and cut to a size of 10 mm. The test substance is applied on an agar strip covering only the lower halves of the apical cut surfaces of each series. After a 4-hour stay in the dark in moist Petri dishes the coleoptile cylinders are shadowgraphed on a 35-mm photographic film. The curvatures are measured from an enlarged projection (10 times natural size) of the shadowgraphs.The lowest IAA concentration which can be determined is around 30 to 60 g/l, i.e. about 1 to 2 ng IAA. The concentration response curves follow a logarithmic course up to 1,000 g/l. The standard error of the mean in a test series comprising 6 coleoptiles is on an average ± 7%. The simple and quick procedures make it possible for a worker to test 60 to 80 fractions a day.Dedicated to Professor Hans Söding on the occasion of his seventieth birthday. The senior author expresses his special gratitude for having been initiated by Professor Söding in the study of auxins.present address: Dpt. of Biology Princeton Univ. Princeton, N. J. 08540, U.S.A.  相似文献   
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T1 phage, BU-T1 phage, infectious DNA extracted from phage phiX 174, and chromatographically purified ribonuclease were exposed to thermal hydrogen atoms, and the loss of plaque-forming ability, infectivity, or enzymatic activity was determined after various exposure times. Atomic hydrogen was generated by two different methods: (1) by a high-frequency discharge in hydrogen gas and (2) by irradiating a foil of polyethyleneter-ephthalate with 2-MeV protons. With increasing exposure time the surviving fraction of all objects tested approaches a constant level. After subtracting this constant "indestructible" fraction in either system, all objects were inactivated according to exponential curves. Furthermore, no BU sensitization was found to occur in BU-T1 phage exposed to atomic hydrogen, whereas gamma irradiation of samples from the same batches revealed a BU effect of a factor of 2.2. These experiments demonstrate hydrogen atoms to be efficient in causing biological damage. Consequently the terminology of "direct" and "indirect" radiation effect may have to be redefined.  相似文献   
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Lymphocyte entry into lymph nodes and Peyer's patches is initiated by the adhesion of the lymphocytes to specialized postcapillary high endothelial venules (HEV). The binding of lymphocytes to lymph node HEV is mediated by the cell surface receptor gp90MEL-14 (gp90). Previous work has shown that gp90 is down-regulated over a period of days after mitogenic or mixed lymphocyte reaction stimulation of T lymphocytes. In our study, it is shown that stimulation of lymphocytes with activators of protein kinase C (PKC), such as PMA or 1-oleoyl 2-acetyl-glycerol, results in the nearly complete loss of surface expression of gp90 within 1 h. Pretreatment of the cells with H-7 or staurosporine, PKC inhibitors, but not HA1004, a general protein kinase inhibitor, prevents the loss of gp90MEL-14. Within 15 min of stimulation of PKC, a novel form of gp90 can be immunoprecipitated from the supernatant of stimulated cells. Upon deglycosylation, this soluble gp90 polypeptide is shown to be 12 kDa smaller than the cell surface protein. Peptide mapping showed identical patterns for surface and soluble receptor, confirming that the soluble Ag is related to the cell membrane protein. Together, these experiments suggest that activation of PKC results in the proteolytic cleavage of gp90MEL-14, resulting in receptor shedding and the inability of the lymphocytes to adhere to HEV endothelium. Furthermore, because supernatant from unstimulated, normal lymphocytes also contains a small amount of the low Mr form of gp90, cell surface proteolysis may be part of the normal turnover of this receptor glycoprotein. These experiments suggest that PKC may play a role in the regulation of lymphocyte traffic to lymphoid tissues.  相似文献   
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Regional Reductions of Transketolase in Thiamine-Deficient Rat Brain   总被引:1,自引:0,他引:1  
Abstract: Thiamine deficiency impairs oxidative metabolism and causes metabolic encephalopathy. An early reduction in transketolase (TK) activity may be an important pathogenic event. To assess the role of TK, we have delineated the regional/cellular distribution of TK protein and mRNA in adult rat brain in pyrithiamine-induced thiamine deficiency. TK activity declined in both vulnerable and spared regions. Immunoblots showed a parallel reduction of TK protein. With a few exceptions, immunocytochemistry indicated an overall decline of TK immunoreactivity and the decrease was not specific to vulnerable areas. In contrast to the pronounced, general decline of TK protein, in situ hybridization revealed a regional decrease of 0–25% of TK mRNA in thiamine deficiency. Northern blots indicated a similar level of TK mRNA in whole brain in thiamine deficiency. These results show that the decline of TK activity results from a proportional decrease of TK protein, and the deficiency may be due to an instability of TK protein or an inhibition of TK mRNA translation. The lack of correlation of the distribution, and the absence of specific alteration, of TK in affected regions suggest that the reduced TK may not be linked directly to selective vulnerability in thiamine deficiency.  相似文献   
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