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81.
Reidar Grundström 《Hydrobiologia》1987,147(1):215-219
The population dynamics of phytoplankton and zooplankton in a fertilized enclosure were studied from April 1977 to June 1978. During the first spring period, the rotifers Keratella cochlearis, Kellicottia longispina, and Polyarthra vulgaris were scarce. During the following spring, all three species were abundant. An attempt is made to explain these differences. Food resource competition from cladocerans, and the low food quality and possible inhibitory effects of Coccomyxa sp. were found to be the most likely factors limiting the growth of these rotifers during the first spring period. 相似文献
82.
NADH diferric transferrin reductase in liver plasma membrane 总被引:6,自引:0,他引:6
I L Sun P Navas F L Crane D J Morré H L?w 《The Journal of biological chemistry》1987,262(33):15915-15921
Evidence is presented that rat liver plasma membranes contain a distinct NADH diferric transferrin reductase. Three different assay procedures for demonstration of the activity are described. The enzyme activity is highest in isolated plasma membrane, and activity in other internal membranes is one-eighth or less than in plasma membrane. The activity is inhibited by apotransferrin and antitransferrin antibodies. Trypsin treatment of the membranes leads to rapid loss of the transferrin reductase activity as compared with NADH ferricyanide reductase activity. Erythrocyte plasma membranes, which lack transferrin receptors, show no diferric transferrin reductase activity, although NADH ferricyanide reductase is present. The transferrin reductase is inhibited by agents that inhibit diferric transferrin reduction by intact cells and is activated by CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfate) detergent. Inhibitors of mitochondrial electron transport have no effect on the activity. We propose that the NADH diferric transferrin reductase in plasma membranes measures the activity of the enzyme that causes the reduction of diferric transferrin by intact cells. This transmembrane electron transport system requires the transferrin receptor for diferric transferrin reduction. Because the transmembrane electron transport has been shown to stimulate cell growth, the reduction of diferric transferrin at the cell surface may be an important function for diferric transferrin in stimulation of cell growth, in addition to its role in iron transport. 相似文献
83.
Summary Voltage-dependent K channels could be identified in on-cell and excised patch-clamp records on membranes of isolated plant cell vacuoles. The current through a membrane patch is dominated by a channel population with a conductance of about 121 pS in symmetrical 250mm KCl solution. The single channel adopts at least two conducting levels the 121-pS state being most frequently observed. The channel shows outward rectification, representing a cation flux into the vacuoles. The rectification appears to be caused by a vanishing open probability and a short channel lifetime at hyperpolarizing voltages. A selectivity ratio of potassium over sodium of about 6 was derived as an estimate. Occasionally, an additional population of K channels with a single-channel conductance of approximately 18 pS is observed. This channel type exhibits outward rectification as well. 相似文献
84.
Summary An analysis was made of the effects of different sampling and extraction techniques on the amounts and pattern of monoterpenoids isolated from needles of Norway spruce. The following isolation and analysis procedure was finally adopted: liquid nitrogen-cooled needles were pulverized by a microdismembrator, extracted with pentane overnight at 2°–3°C and concentrated to a volume not less than 3 ml/g fresh weight on a Vigreux column. The crude extract was injected splitless (with solvent split) onto a cold programmed temperature vaporized (PTV) precolumn of a gas chromatograph and the vaporizable compounds heated to a capillary column. This method was tested for production of artefacts and quantitative extraction and applied to needles of eleven 80-year-old spruce trees. 相似文献
85.
The amounts of free sterols, steryl esters and lipid phosphorus were determined in the sapwood and heartwood of mature, and in the outer and inner sapwood of young Pinus sylvestris trees. In the mature trees (up to 70 years old) the heartwood contains significantly higher amounts of free sterols than the sapwood. No radial gradient can be demonstrated in the amounts of steryl esters. Lipids extracted from the sapwood contain higher amounts of phosphorus than those from the heartwood. Stems of young Pinus sylvestris trees (up to 13 years old) show in the inner sapwood higher amounts of both free sterols and steryl esters than the peripheral younger wood zone. The inner sapwood of the young stems shows slightly higher amounts of lipid phosphorus than the outer sapwood. The results indicate that Pinus sylvestris accumulates both free sterols and steryl esters in the stems at a very early stage of the life cycle. Sterol accumulation in the innermost parts of the stems seems not to depend on heartwood formation. 相似文献
86.
Summary Traditional protocols for the immunohistochemical localization of peptides and tyrosine hydroxylase (TH) in nerve fibers in cryostat sections require the tissue to be thoroughly fixed and rinsed and to be processed for the cryostat sectioning and the immunohistochemical staining more or less directly after freezing. In the present study it was tested whether also unfixed, rapidly frozen tissue, conforming to guinea pig and bovine heart specimens, can be used for the visualization of neuropeptides [neuropeptide Y (NPY) and substance P (S P)] and TH in cryostat sections. The following observations were made: 1) NPY-immunoreactive (IR) and S P-IR nerve fibers could be clearly identified in both fixed and unfixed sections of this type of tissue. 2) TH-IR nerve fibers could be detected in unfixed tissue if the sections were post-fixed with aldehydes by the use of a two-step fixation process related to a sudden change of pH. However, the outlines of the nerve fibers were sometimes diffuse. 3) Storage of unfixed tissue for periods of up to 2.5 yeart at –80° C did not lead to a decrease in immunoreactivity. 4) Somewhat higher concentrations of primary antibodies had to be used for sections of unfixed tissue than for sections of fixed tissue when the FITC method was used. This waste of antibodies was partly overcome by use of the biotin-streptavidin method. The glyoxylic acid induced catecholamine(CA)-fluorescence method for demonstration of sympathetic nerve fibers was also applied and was found to give optimal results after storage of tissue for up to 2.5 years. The study shows that the use of unfixed rapidly frozen tissue represents a fast and realistic method for the demonstration of neuropeptide immunoreactivity, that it to some extent can be used for the visualization of TH-containing nerve fibers and that it is a suitable method to maintain longterm neuropeptide and TH immunoreactivity as well as long-term CA-fluorescence reaction. 相似文献
87.
Håkan Tegelström 《Biochemical genetics》1987,25(1-2):95-110
Genetic variation of mitochondrial DNA (mtDNA) in 18 great tits (Parus major) from three neighboring localities in Sweden was investigated with eight tetranucleotide restriction endonucleases. The 18 individuals could be separated into 13 different maternal lineages. The high number of female lineages present in this regional population contrasts with a low level of sequence divergence between the different mtDNA clones, with a mean of 0.19% sequence divergence between all individuals. There was no obvious spatial structuring of mtDNA clones among the three localities. The presence of a high number of different clones with a low degree of sequence divergence could be explained by the effects of a large long-term effective population size, with the mtDNA clones having diverged about 25,000–200,000 years ago.This study was supported by the Swedish Natural Science Research Council, the Erik Philip-Sörensen Foundation, and the Nilsson-Ehle Foundation. 相似文献
88.
The title disaccharide glycoside was synthesized by halide ion-promoted glycosidation, using methanol and the disaccharide bromide derived from methyl 2-azido-3-O-(2,3,4,6-tetra-O-benzoyl--d-galactopyranosyl)-4,6-O-benzylidene-2-deoxy-1-thio--d-galactopyranoside. This derivative in turn was prepared by silver triflate-promoted condensation of monosaccharide derivatives. 相似文献
89.
K99 Fimbriae from enterotoxigenicEscherichia coli (ETEC) were found to bind specifically to sialic acid, as measured in a haemagglutination inhibition assay using the intact bacteria and human erythrocytes. The affinity forN-glycolylneuraminic acid was about twice that ofN-acetylneuraminic acid (NeuAc), and other monosaccharides were found to be at least ten-fold less effective as inhibitors. The specificity was found to depend on electrostatic interaction where the carboxyl group and its orientation plays an important role. 2--Benzyl-NeuAc was a better inhibitor than 2--methyl-NeuAc suggesting a hydrophobic patch near the binding site on the protein. Axially oriented hydroxyl groups as in 4-epi-NeuAc and 3-hydroxy-NeuAc seemed to participate in binding since these derivatives were better inhibitors thanN-acetylneuraminic acid. K99 was found to have a higher affinity for 4-O-acetyl-NeuAc and lower affinity forN-acetylneuraminic acid withO-substituents at C7-C9 as compared toN-acetylneuraminic acid. Hence, the degree ofO-acetylation of sialic acid in the mucosa of the small intestine may influence colonization and determine susceptibility to infection. 相似文献
90.
The effect of monensin on the secretion of thyroglobulin was studied in open follicles isolated from pig thyroid tissue; in this system, thyroglobulin is secreted into the incubation medium. When monensin was present during a 4-h chase incubation after pulse-labelling with 3H-leucine, the secretion of labelled thyroglobulin was reduced by about 85%; in electron-microscopic autoradiographs of rat thyroid lobes labelled and chase-incubated under similar conditions the relative number of grains over follicle lumina was strongly reduced when monensin was present during the chase. These observations are in agreement with the consensus that monensin arrests transport of secretory proteins in the Golgi complex. In other experiments, pulse-labelled follicles were chase-incubated for 1.5 h whereby labelled thyroglobulin was transported from the RER to exocytic vesicles. Monensin present during a subsequent chase of 0.5 h caused only a moderate decrease of labelled thyroglobulin secretion. TSH present during the second chase-stimulated secretion in both control and monensin-exposed follicles. TSH also caused a drastic reduction of exocytic vesicles in rat thyroid lobes, and the number of vesicles remaining in the cells was the same in controls and lobes exposed to the ionophore. The observations are interpreted to show that monensin does not inhibit the basal or TSH-stimulated transport of thyroglobulin from the site of monensin-induced arrest in the Golgi complex to the apical cell surface or the exocytosis of thyroglobulin. 相似文献