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891.
Formation of tight junctions in epithelial cells. I. Induction by proteases in a human colon carcinoma cell line 总被引:2,自引:0,他引:2
The experimental modulation of tight junctions (TJ) was studied in the human adenocarcinoma cell line HT 29 by freeze-fracture electron microscopy. The cell line has virtually no TJ when grown in culture. TJ could be induced by mild treatment with a variety of endopeptidases (trypsin, chymotrypsin, collagenase, elastase, plasmin, thrombin, papain, and pronase). Pronase induced the formation of TJ at low (but not at high) concentrations. All exopeptidases studied were unable to induce the formation of TJ. At 0 degree C the trypsin-induced formation of TJ was greatly slowed down although not entirely inhibited. However, when cells were briefly treated with trypsin at 0 degree C and subsequently transferred to 37 degrees C in the presence of protease inhibitors, TJ were rapidly assembled. Thus an induction phase at low temperature and an assembly phase at high temperature could be experimentally separated. When cells were briefly trypsinized at 0 degrees and subsequently kept at 0 degree C without trypsin for several hours, TJ still formed abundantly upon incubation at 37 degrees C. It appears therefore that the effect produced by the protease is retained for long periods in the cold. 相似文献
892.
893.
After the urea cycle was proposed, considerable efforts were put forth to identify critical intermediates. This was then followed by studies of dietary and nutritional control of urea cycle enzyme activity and allosteric effectors of urea cycle enzymes. Correlation of urea cycle enzyme activity with isolated cell experiments indicated conditions where enzyme activity would be rate limiting. At physiological levels of ammonia the activation of carbamoyl-phosphate synthetase (EC 6.3.4.16) by N-acetylglutamate (NAG) is important. Various levels of NAG corresponded well with changes in the rate of citrulline and urea synthesis. Arginine was found to be an allosteric activator of N-acetylglutamate synthetase (EC 2.3.1.1). Therefore, it was possible that the rate of carbamoyl phosphate synthesis was dependent on the level of urea cycle intermediates, particularly arginine. Evidence for arginine in the regulation of NAG synthesis is not as clear as for NAG on carbamoyl phosphate synthetase I. The concentration of hepatic arginine is not necessarily an indication of the mitochondrial concentration. Only mitochondrial arginine stimulates the N-acetylglutamate synthetase. Recent studies indicate that the mitochondrial concentration of arginine is higher than the cytosolic concentration and is well above the Ka for N-acetylglutamate synthetase. Therefore, it appears that changes in arginine concentration are not physiologically important in regulating levels of NAG. However, it is possible that responses to the effector may vary with time after eating, and it may be this responsiveness that controls the level of NAG and thereby urea synthesis. 相似文献
894.
A semi-natural Drosophila melanogaster population was twice forced through a genetic bottleneck and allowed to recover naturally. In one case additional variation was introduced to the recovering population. The percentage of lethal chromosomes, the level of allelism between these lethals, and the effective population size calculated from the allelism of these lethals all rose sharply in the few generations following each bottleneck, though this was not the case in the very first generation. Thereafter this rise decelerated rapidly and never returned to pre-bottleneck levels. Additional introduced variation had little effect. The reasons for and implications of this pattern have been considered. 相似文献
895.
A. M. Northover 《The Histochemical journal》1985,17(4):443-452
Summary The addition of oxalate to a suspension of rabbit peritoneal neutrophils before fixation with glutaraldehyde and postfixation with osmium tetroxide-antimonate greatly enhanced the amount of calcium antimonate precipitate subsequently detectable with the electron microscope. Using chlortetracycline as a fluorescent probe for membrane-associated calcium, it was found that both glutaraldehyde and osmium tetroxide release calcium from membrane-associated stores in suspensions of living neutrophils. These findings suggest that some of the calcium released from cellular stores during fixation with glutaraldehyde is trapped within the neutrophil by oxalate which then reacts with potassium antimonate. This produces a more copious precipitate of calcium antimonate than fixation without oxalate. It is suggested, therefore, that the histochemical localization of calcium by antimonate techniques may not always represent thein vivo situation. The use of oxalate during fixation, however, may give a better indication of the amount of calcium stored within a cell. 相似文献
896.
Summary An immunohistochemical method was used to demonstrate the presence of gonadotrophins in isolated ovarian interstitial cells. The cells were obtained by collagenase digestion of large ovarian follicles after removal of the yolk and the granulosa layer. Using a peroxidase-labelled anti-rabbit serum with anti-chicken follicle stimulating hormone (FSH) serum raised in rabbits, a strong positive reaction was obtained. Anti-human FSH serum also produced a positive result but the reaction was weaker. There was no apparent difference in the staining reaction of cells which had been preincubated with ovine FSH serum. Treatment with anti-ovine luteinizing hormone (LH) resulted in a faintly positive reaction.The viability of the cells was tested by the Trypan Blue method and they were identified as steroid-producing cells by the histochemical demonstration of their 3-hydroxysteroid dehydrogenase activity. 相似文献
897.
More than 3000 spontaneous and induced lys2 mutants were obtained in haploid and diploid strains of yeast Saccharomyces. The ability to utilize alpha-aminoadipate was used for lys2 mutant screening. The spontaneous and induced mutation rates were measured in haploid and diploid strains. Mitotic segregation of pho1 marker linked to LYS2 was studied in lys2 mutants obtained in diploid strains. Fertility of diploid lys2 mutants was tested. The conclusion to be drawn from the data presented is that mutations appeared in one of two homologous chromosomes and then segregated by mitotic homozygotization. 相似文献
898.
Bacterial cells lysogenic for D3112, a transposable Pseudomonas aeruginosa phage restrict the growth of a related heteroimmune B39 phage. The lysogens are divided into two different types PAO(D3112). In the lysogens of the type I the efficiency of B39 growth only decreases slightly, the lysogens of the type II restricting completely the growth of this phage (e.o.p. is less than 10(-7). As shown by the results of Southern hybridization experiments, lysogens of the type I are monolysogens, while those of the type II are double or polylysogens. Restriction of B39 in PAO(D3112) is caused by expression of a locus in the D3112 genome. The locus has been termed as cip (control of interaction of phages). The cip locus was mapped at the interval 1.3-2.45 kb of the D3112 physical map using different deletion derivatives of D3112. Expression of cip only takes place in the prophage state and not during the phage lytic development. When expressed, cip affects the early steps in the growth of B39 lowering the level of integration and transposition processes; the effect is not dependent on the way of initiation of the lytic cycle (through prophage induction or infection). 相似文献
899.
The ability of para-aminobenzoic acid--vitamin (PABA) to influence the sensitivity of mice to alkylating mutagens was studied. PABA had no influence on the cytogenetic effect of thio-TEPA. It was determined that PABA altered the effect of N-ethyl nitrosourea (ENU). The direction of modification depends on animal genotype: pre-treatment with PABA decreases the frequency of chromosome aberrations in bone marrow cells of CBA/LacY and C57BL/JY mice, but significantly increases it in 101/HY mice. The PABA influence on the frequency of gene mutations induced by ENU in melanocytes of mice and revealed by "spot-test" was not determined. 相似文献
900.
Some features of integration of temperature-sensitive pE194, pGG10 and pGG20 plasmids into the Bacillus subtilis chromosome were studied. Several auxotrophic mutations were obtained using insertion of these plasmids into the chromosome. The sites of plasmids for illegitimate recombination were determined. It was shown that the integration into the Bac. subtilis chromosome is characteristic not only for the plasmid pE194 but is the property of Staphylococcus aureus plasmid pC194 and Escherichia coli pBR322 plasmid. The influence of different Bac. subtilis rec mutations on the frequency of integration was studied. 相似文献