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排序方式: 共有258条查询结果,搜索用时 15 毫秒
31.
32.
Vedasto G. Ndibalema Simon Mduma Sigbjørn Stokke Eivin Røskaft 《African Journal of Ecology》2008,46(4):547-555
We measured the responses of ungulate grazers to roads by recording the density of dust on grasses and compared distance sampling of ungulates in Serengeti National Park (SNP), Tanzania. Data were collected on the east and west side of the Ngorongoro–Seronera main gravel road to test if road traffic and dust were important factors determining distribution patterns amongst grazers. Results indicate that dust increased progressively with traffic speed and volume during dry season. More dust was intercepted on the west than on the east side of the road mainly because of wind effects. Dust deposition (measured as density g grass g?1 dust) was higher on short grasses than on long grasses during the dry and late‐dry seasons than during the wet season, when paired perpendicular distances up to 300 m were compared. Mean number of observed grazer species indicated that most fed further from the west side of the road than from the east perhaps to minimize higher density of dust commonly spread on foliage up to 200 m away from the road. Despite that most grazers avoided road side grass shoulders, supporting the ‘dust aversion hypothesis’, the test predictions from the ‘road disturbance’ and the ‘road attraction’ hypotheses did not support the responsive behaviours of grazers toward roads. 相似文献
33.
A simple method for preparation of methyl-labelled (-) carnitine 总被引:2,自引:0,他引:2
34.
K T Stokke 《Biochimica et biophysica acta》1972,280(2):329-335
35.
Peroxisomal disorders (Zellweger's syndrome, neonatal adrenoleukodystrophy, infantile Refsum's syndrome, rhizomelic chondrodysplasia) show a series of enzymatic defects related to peroxisomal dysfunctions. Accumulation of phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) has been found in several of these patients, caused by a defect in the alpha-oxidation mechanism of this acid. The fact that the alpha-oxidation of phytanic acid is defective in the peroxisomal disorders as well as in classical Refsum's disease makes it likely that this oxidation normally takes place in the peroxisomes. A series of experiments preformed to localize the phytanic acid oxidase in subcellular fractions of rat liver show, however, that the alpha-oxidation of phytanic acid is a mitochondrial process. Free phytanic acid is the substrate, and the only cofactors necessary are ATP and Mg2+. 相似文献
36.
Øystein Åmellem Trond Stokke Joe A. Sandvik Lars Smedshammer Erik O. Pettersen 《Experimental cell research》1997,232(2):361
We have studied hypoxia-induced inactivation of cells from three established human cell lines with different p53 status. Hypoxia was found to induce apoptosis in cells expressing wild-type p53 (MCF-7 cells), but not in cells where p53 is either mutated (T-47D cells), or abrogated by expression of the HPV18 E6 oncoprotein (NHIK 3025 cells). Apoptosis was demonstrated by DNA fragmentation, using agarose gel electrophoresis of DNA and DNA nick end labeling (TUNEL). We demonstrate that extremely hypoxic conditions (<4 ppm O2) do not cause any change of expression in the p53 protein level in these three cell lines. In addition, the localization of p53 in MCF-7 cells was found exclusively in the nucleus in only some of the cells both under aerobic and hypoxic conditions. Furthermore, no correlation was found between the p53-expression level and whether or not a cell underwent apoptosis. Flow cytometric TUNEL analysis of MCF-7 cells revealed that initiation of apoptosis occurred in all phases of the cell cycle, although predominantly for cells in S phase. Apoptosis was observed only during a limited time window (i.e., ≈10 to ≈24 h) after the onset of extreme hypoxia. While 66% of the MCF-7 cells lost their ability to form visible colonies following 15 h exposure to extreme hypoxia, only ∼28% were induced to apoptosis, suggesting that ∼38% were inactivated by other death processes. Commitment to apoptotic cell death was observed in MCF-7 cells even for oxygen concentrations as high as 5000 ppm. Our present results indicate that the p53 status in these three tumor cell lines does not have any major influence on cell's survival following exposure to extremely hypoxic conditions, whereas following moderate hypoxia, cells expressing functional p53 enhanced their susceptibility to cell death. Taken together, although these results suggest that functional p53 might play a role in the induction of apoptosis during hypoxia, other factors seem to be equally important. 相似文献
37.
Repression of lignin biosynthesis promotes cellulose accumulation and growth in transgenic trees. 总被引:40,自引:0,他引:40
W J Hu S A Harding J Lung J L Popko J Ralph D D Stokke C J Tsai V L Chiang 《Nature biotechnology》1999,17(8):808-812
Because lignin limits the use of wood for fiber, chemical, and energy production, strategies for its downregulation are of considerable interest. We have produced transgenic aspen (Populus tremuloides Michx.) trees in which expression of a lignin biosynthetic pathway gene Pt4CL1 encoding 4-coumarate:coenzyme A ligase (4CL) has been downregulated by antisense inhibition. Trees with suppressed Pt4CL1 expression exhibited up to a 45% reduction of lignin, but this was compensated for by a 15% increase in cellulose. As a result, the total lignin-cellulose mass remained essentially unchanged. Leaf, root, and stem growth were substantially enhanced, and structural integrity was maintained both at the cellular and whole-plant levels in the transgenic lines. Our results indicate that lignin and cellulose deposition could be regulated in a compensatory fashion, which may contribute to metabolic flexibility and a growth advantage to sustain the long-term structural integrity of woody perennials. 相似文献
38.
Guro Valen Anders Sondn Jarle Vaage Elisabeth Malm B. Thomas Kjellstrm 《Free radical biology & medicine》1999,26(11-12):1480-1488
Reactive oxygen intermediates induce cell injury in a variety of pathophysiological conditions. Human umbilical cord vein endothelial cell (HUVEC) cultures were exposed to 1 or 200 microM H2O2 for 15 min, and observed after 15 min, or 1, 4, 24, or 120 h. Factor VIII and the cytoskeletal proteins vimentin and tubulin were visualized immunocytochemically. Release of lactate dehydrogenase (indices of cell membrane injury) did not increase after H2O2 exposure; nor was cellular expression of factor VIII affected. 200 microM H2O2 induced cell contraction after 15 min which disappeared after 1 and 4 h, but was evident again after 24 h. Immediately after exposure, the filamentous structure of vimentin and tubulin disappeared, but normalized after 1 h. After 120 h, the cytoskeleton filaments were coarsened and disorganized, and an abundance of multinucleated giant cells were observed. Catalase (150 U/ml) abolished all effects of H2O2. One microM H2O2 did not induce any changes in HUVEC. Thus, the present concentrations of H2O2 did not induce cell necrosis or altered expression of factor VIII. Early, reversible cell contraction and depolymerization of cytoskeletal proteins were observed, followed by a delayed contraction and cell atypia after 200 microM H2O2. 相似文献
39.
Flora J. Magige Bård G. Stokke Ragna Sortland Eivin Røskaft 《African Journal of Ecology》2009,47(3):400-408
Ostrich breeding behaviour in the Serengeti ecosystem, Tanzania was investigated for differences in laying dates between low altitude western area (WA) and high altitude eastern area (EA) populations. Ostriches in WA laid eggs significantly earlier than in EA. The differences could be attributed to topography and rainfall pattern. Reliable rains in lower altitudes ensure availability of food that in turn influences the whole process of the reproductive cycle. Clutches were contributed by several females with a nest having up to 38 eggs. We also compared the frequency of observation of predators, ostriches, nests, 'singletons' (single eggs laid randomly) and broods between the two areas. There was no significant difference between WA and EA in 1) ostrich/nest ratio, indicating similar breeding densities; 2) ostrich/predator and predator/nest ratios, indicating that predation pressure was equally high; 3) nest/singleton and predator/singleton ratios, indicating that loss of nests did not vary between areas. However, there were significantly more predators, nests and ostriches compared to broods in EA than in WA, indicating a significantly lower reproductive success in EA. Using metapopulation terminology, ostriches in EA could be regarded as a 'sink' population and those in WA as a 'source' population, but investigations over longer time-periods are needed to further resolve if this is the case. 相似文献
40.
Nordstrand LM Svärd J Larsen E Nilsen A Ougland R Furu K Lien GF Rognes T Namekawa SH Lee JT Klungland A 《PloS one》2010,5(11):e13827