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排序方式: 共有220条查询结果,搜索用时 31 毫秒
1.
Demonstration of Both a Photosynthetic and a Nonphotosynthetic CO(2) Requirement for NH(4) Assimilation in the Green Alga Selenastrum minutum 总被引:1,自引:1,他引:0 下载免费PDF全文
Nitrogen-limited and nitrogen-sufficient cell cultures of Selenastrum minutum (Naeg.) Collins (Chlorophyta) were used to investigate the dependence of NH4+ assimilation on exogenous CO2. N-sufficient cells were only able to assimilate NH4+ maximally in the presence of CO2 and light. Inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron also inhibited NH4+ assimilation. These results indicate that NH4+ assimilation by N-sufficient cells exhibited a strict requirement for photosynthetic CO2 fixation. N-limited cells assimilated NH4+ both in the dark and in the light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, indicating that photosynthetic CO2 fixation was not required for NH4+ assimilation. Using CO2 removal techniques reported previously in the literature, we were unable to demonstrate CO2-dependent NH4+ assimilation in N-limited cells. However, employing more stringent CO2 removal techniques we were able to show a CO2 dependence of NH4+ assimilation in both the light and dark, which was independent of photosynthesis. The results indicate two independent CO2 requirements for NH4+ assimilation. The first is as a substrate for photosynthetic CO2 fixation, whereas the second is a nonphoto-synthetic requirement, presumably as a substrate for the anaplerotic reaction catalyzed by phosphoenolpyruvate carboxylase. 相似文献
2.
Tomato fruit ripening and ethylene production were inhibited following treatment with methyl bromide (MB). Methyl bromide significantly delayed ripening initiation in mature-green (MG) fruit and retarded the rate of ripening of turning (T) fruit as measured by color development and flesh softening. Treatment with MB caused an initial transient burst of ethylene production, but the subsequent ripening-associated increase in ethylene was delayed. Ethylene treatment partially overcame MB inhibition in MG fruit but had no affect on T fruit. The inhibition of ethylene production by MB appears to be due to lack of formation of 1-aminocycloprone-1-carboxylic acid (ACC) in MG fruit, whereas in T fruit lack of conversion of ACC to ethylene is indicated. A key feature of MB inhibition of ripening in tomato appears to be reduced sensitivity to ethylene. 相似文献
3.
Chylomicron-chylomicron remnant clearance by liver and bone marrow in rabbits. Factors that modify tissue-specific uptake 总被引:2,自引:0,他引:2
M M Hussain R W Mahley J K Boyles M Fainaru W J Brecht P A Lindquist 《The Journal of biological chemistry》1989,264(16):9571-9582
The metabolism of [14C]cholesterol- and [3H]retinol-labeled chylomicrons obtained from canine thoracic duct or rabbit mesenteric lymph was investigated in normal fasted rabbits. Typically, 70-80% of the chylomicrons injected into the rabbits were cleared from the plasma in 20 min, and their uptake was accounted for principally by the liver and the bone marrow. Surprisingly, the bone marrow was a major site of uptake; the uptake ranged from about half that of the liver to a nearly equal amount. The importance and specificity of chylomicron-chylomicron remnant uptake by the bone marrow were established by demonstrating that (a) bone marrow throughout the body accumulated these lipoproteins, (b) the level of uptake was consistent regardless of how the values were calculated or how the chylomicrons were prepared, (c) the uptake represented specific binding, and (d) radiolabeled intestinal lipoproteins induced in vivo delivered cholesterol and retinol to the marrow. Electron microscopic examination of the rabbit bone marrow established that perisinusoidal macrophages uniquely accounted for the uptake of the chylomicrons. Whereas liver cleared a variety of both triglyceride-rich lipoproteins (chylomicrons, chylomicron remnants, and very low density lipoproteins) and cholesterol-rich lipoproteins (beta-very low density lipoproteins and high density lipoproteins containing apolipoprotein E), bone marrow uptake appeared to be restricted to the triglyceride-rich lipoproteins. More chylomicron remnants (generated in a hepatectomized rabbit) were cleared by the liver than by the bone marrow, and the addition of excess apolipoprotein E to chylomicrons resulted in their preferential uptake by the liver. The role of chylomicron-chylomicron remnant delivery of lipids or lipid-soluble vitamins to rabbit bone marrow is open to speculation, and whether triglyceride-rich lipoprotein uptake occurs to a significant extent in the bone marrow of humans remains to be determined. 相似文献
4.
G Stenbeck C Harter A Brecht D Herrmann F Lottspeich L Orci F T Wieland 《The EMBO journal》1993,12(7):2841-2845
Several lines of evidence favour the hypothesis that intracellular biosynthetic protein transport in eukaryotes is mediated by non-clathrin-coated vesicles (for a review see Rothman and Orci, 1992). The vesicles have been isolated and a set of their surface proteins has been characterized as coat proteins (COPs). These COPs exist in the cytosol as a preformed complex, the coatomer, which was prior to this study known to contain six subunits: four (alpha-, beta-, gamma- and delta-COP) with molecular weights between 160 and 58 kDa, and two additional proteins of approximately 36 and 20 kDa, epsilon- and xi-COP. Here we describe a novel subunit of the coatomer complex, beta'-COP. This subunit occurs in amounts stoichiometric to the established COPs both in the coatomer and in nonclathrin-coated vesicles and shows homology to the beta-subunits of trimeric G proteins. 相似文献
5.
A major difference between the divergence patterns within the lines-1 families in mice and voles 总被引:3,自引:0,他引:3
Vanlerberghe F; Bonhomme F; Hutchison CA d; Edgell MH 《Molecular biology and evolution》1993,10(4):719-731
L1 retroposons are represented in mice by subfamilies of interspersed
sequences of varied abundance. Previous analyses have indicated that
subfamilies are generated by duplicative transposition of a small number of
members of the L1 family, the progeny of which then become a major
component of the murine L1 population, and are not due to any active
processes generating homology within preexisting groups of elements in a
particular species. In mice, more than a third of the L1 elements belong to
a clade that became active approximately 5 Mya and whose elements are >
or = 95% identical. We have collected sequence information from 13 L1
elements isolated from two species of voles (Rodentia: Microtinae: Microtus
and Arvicola) and have found that divergence within the vole L1 population
is quite different from that in mice, in that there is no abundant
subfamily of homologous elements. Individual L1 elements from voles are
very divergent from one another and belong to a clade that began a period
of elevated duplicative transposition approximately 13 Mya. Sequence
analyses of portions of these divergent L1 elements (approximately 250 bp
each) gave no evidence for concerted evolution having acted on the vole L1
elements since the split of the two vole lineages approximately 3.5 Mya;
that is, the observed interspecific divergence (6.7%-24.7%) is not larger
than the intraspecific divergence (7.9%-27.2%), and phylogenetic analyses
showed no clustering into Arvicola and Microtus clades.
相似文献
6.
Alternative Oxidase Activity in Tobacco Leaf Mitochondria (Dependence on Tricarboxylic Acid Cycle-Mediated Redox Regulation and Pyruvate Activation) 总被引:12,自引:6,他引:6 下载免费PDF全文
Transgenic Nicotiana tabacum (cv Petit Havana SR1) containing high levels of mitochondrial alternative oxidase (AOX) protein due to the introduction of a sense transgene(s) of Aox1, the nuclear gene encoding AOX, were used to investigate mechanisms regulating AOX activity. After purification of leaf mitochondria, a large proportion of the AOX protein was present as the oxidized (covalently associated and less active) dimer. High AOX activity in these mitochondria was dependent on both reduction of the protein by DTT (to the noncovalently associated and more active dimer) and its subsequent activation by certain [alpha]-keto acids, particularly pyruvate. Reduction of AOX to its more active form could also be mediated by intramitochondrial reducing power generated by the oxidation of certain tricarboxylic acid cycle substrates, most notably isocitrate and malate. Our evidence suggests that NADPH may be specifically required for AOX reduction. All of the above regulatory mechanisms applied to AOX in wild-type mitochondria as well. Transgenic leaves lacking AOX due to the introduction of an Aox1 antisense transgene or multiple sense transgenes were used to investigate the potential physiological significance of the AOX-regulatory mechanisms. Under conditions in which respiratory carbon metabolism is restricted by the capacity of mitochondrial electron transport, feed-forward activation of AOX by mitochondrial reducing power and pyruvate may act to prevent redirection of carbon metabolism, such as to fermentative pathways. 相似文献
7.
Evidence for Activation of the Oxidative Pentose Phosphate Pathway during Photosynthetic Assimilation of NO(3) but Not NH(4) by a Green Alga 下载免费PDF全文
Addition of NO3− to N-limited Selenastrum minutum during photosynthesis resulted in an immediate drop in the NADPH/NADP ratio and a slower increase of the NADH/NAD ratio. These changes were accompanied by a rapid decrease in glucose-6-phosphate and increase in 6-phosphogluconate, indicating activation of glucose-6-phosphate dehydrogenase and a role for the oxidation pentose phosphate pathway during photosynthetic NO3− assimilation. In contrast, the short-term changes in pyridine nucleotides and metabolites during photosynthetic assimilation of NH4+ were not consistent with a stimulation of the oxidative pentose phosphate pathway. 相似文献
8.
The consideration of electric and volume transport additionally to diffusion leads to the dependence of concentration profiles in heterogeneous enzyme catalysis on additional parameters (e. g. electrical field strength, convection velocity) which are valuable for influencing and optimizing these systems. For autocatalysis the local periodic concentration profiles are influenced essentially by these phenomena (change of onset and wavelength). When diffusion is negligible equations are given to determine the kinetic parameters vmax and KM from transport measurements. 相似文献
9.