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Stitt M 《Plant physiology》1987,84(2):201-204
The control of the fructose 2,6-bisphosphate (Fru2,6P2) concentration and its possible role in controlling carbohydrate synthesis and degradation are discussed. This regulator metabolite is involved in the fine tuning of photosynthetic metabolism, and in controlling photosynthetic partitioning, and may also be involved in the response to hormones, wounding, and changing water relations. Study of the mechanisms controlling Fru2,6P2 concentrations could reveal insights into how these responses are mediated. However, the detailed action of Fru2,6P2 requires more attention, especially in respiratory metabolism where the background information about the compartmentation of metabolism between the plastid and cytosol is still inadequate, and the potential role of pyrophosphate has to be clarified. 相似文献
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The febrile responses of Sprague-Dawley rats to semi-purified human endogenous pyrogen were studied at a thermoneutral ambient temperature (26 degrees C) and in the cold (3 degrees C). It was found that while rats developed typical monophasic febrile responses at thermoneutrality, febrile responses were absent in the cold-exposed rats. Experiments were conducted to determine whether this lack of febrile responses in cold-exposed rats was due to an inability of these animals to generate or retain heat in the cold. Thermogenesis and vasoconstriction were stimulated in cold-exposed rats by selectively cooling the hypothalamus, using chronically implanted thermodes. It was shown that, using this stimulus, metabolic rate could be increased by more than 50 percent and body temperature could be driven up at a rate of 5 degrees C/hour in rats exposed to the cold. Therefore, it was concluded that the lack of febrile responses of cold-exposed rats to pyrogen is in no way due to a physical or physiological inability to retain heat. Instead, it appears that in some manner cold exposure suppresses the sensitivity or responsiveness of the rat to pyrogenic stimuli. 相似文献
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The febrile responses of male Sprague-Dawley rats to a semi-purified endogenous pyrogen (EP) derived from human monocytes are markedly enhanced 3 days after the animals are intravenously injected with a variety of immunoadjuvants. The present study was designed to investigate the site within the body at which these substances act to produce this febrile-enhancing phenomenon. Stainless steel microinjection cannula guide tubes were implanted within the region of the organum vasculosum lamina terminalis (OVLT) of the rats and control febrile dose-response curves to EP were established. Minute quantities of the immunoadjuvants zymosan, lipopolysaccharide endotoxin, and the synthetic adjuvant peptide, muramyl dipeptide, were microinjected into the OVLT region and 3 days later, the febrile responses of the animals were retested. In each case the febrile response elicited by a standard dose of EP was more than doubled, the slope of the fever dose-response curve was tripled, and the dose threshold was lowered by a factor of four to five. These responses are identical with those produced when much larger amounts of these immunoadjuvants are injected intravenously, and, thus, we conclude that the site of action of these substances in enhancing fever in response to EP resides in or near the OVLT region. It is proposed that EP stimulates a type of reticuloendothelial cell residing within the OVLT to release prostaglandin E, which in turn crosses the blood-brain barrier to effect the changes in the thermoregulatory neurons of the preoptic anterior hypothalamic area that result in fever. 相似文献
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Substrate specificity and product inhibition of different forms of fructokinases and hexokinases in developing potato tubers 总被引:10,自引:0,他引:10
The substrate dependence and product inhibition of three different fructokinases and three different hexokinases from growing potato (Solanum tuberosum L.) tubers was investigated. The tubers contained three specific fructokinases (FK1, FK2, FK3) which had a high affinity for fructose K
m=64, 90 and 100 (M) and effectively no activity with glucose or other hexose sugars. The affinity for ATP (K
m=26, 25 and 240 M) was at least tenfold higher than for other nucleoside triphosphates. All three fructokinases showed product inhibition by high fructose (K
i=5.7, 6.0 and 21 mM) and were also inhibited by ADP competitively to ATP. Sensitivity to ADP was increased in the presence of high fructose, or fructose-6-phosphate. In certain conditions, the K
i (ADP) was about threefold below the K
m (ATP). All three fructokinase were also inhibited by fructose-6-phosphate acting non-competitively to fructose (K
i=1.3 mM for FK2). FK1 and FK2 showed very similar kinetic properties whereas FK3, which is only present at low activities in the tuber but high activities in the leaf, had a generally lower affinity for ATP, and lower sensitivity to inhibition by ADP and fructose. The tuber also contained three hexokinases (HK1, HK2, HK3) which had a high affinity for glucose (K
m=41, 130 and 35 M) and mannose but a poor affinity for fructose (K
m=11, 22 and 9 mM). All three hexokinases had a tenfold higher affinity for ATP (K
m=90, 280 and 560 M) than for other nucleoside triphosphates. HK1 and HK2 were both inhibited by ADP (K
i=40 and 108 M) acting competitively to ATP. HK1, but not HK2, was inhibited by glucose-6-phosphate, which acted non-competitively to glucose (K
i=4.1 mM). HK1 and HK2 differed, in that HK1 had a narrower pH optimum, a higher affinity for its substrate, and showed inhibition by glucose-6-phosphate. The relevance of these properties for the regulation of hexose metabolism in vivo is discussed.Abbreviations FK
fructokinase
- Fru6P
fructose-6-phosphate
- Glc6P
glucose-6-phosphate
- HK
hexokinase
- NTP
nucleoside triphosphate
- Pi
inorganic phosphate
- UDPGlc
uridine-5-diphosphoglucose
This work was supported by the Deutsche Froschungsgemeinschaft (SFB 137). We are grateful to Professor E. Beck (Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth, FRG) for providing laboratory facilities. 相似文献
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Control of Photosynthetic Sucrose Synthesis by Fructose 2,6-Bisphosphate : III. Properties of the Cytosolic Fructose 1,6-Bisphosphatase 总被引:12,自引:10,他引:2 下载免费PDF全文
The cytosolic fructose 1,6-bisphosphatase from spinach (Spinacia oleracea U.S. hybrid 424) leaves has been partially purified and its response to fructose 2,6-bisphosphate, AMP, and fructose 1,6-bisphosphate studied, using concentrations present in the cytosol during photosynthesis. In the presence of fructose 2,6-bisphosphate, the substrate saturation kinetics for fructose 1,6-bisphosphate are sigmoidal, with half-maximal activity being attained in 0.1 to 1 millimolar concentration range. The inhibition is enhanced by AMP. Using these results, and information published elsewhere on metabolite concentrations, it is discussed how fructose 1,6-bisphosphatase activity will vary in vivo in response to alterations in the availability of triose phosphate and AMP, and the accumulation of the product, fructose 6-phosphate. 相似文献
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