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991.
992.
F Kh Sharipov 《Biulleten' eksperimental'no? biologii i meditsiny》1985,100(8):245-247
It has been established in experiments on 280 white randombred male rats weighing 100-120 g that the lifting of the animals from the valley (820 m above the sea level) to the mountains (3379 m above the sea level) brings about within the first day marked suppression of the mitotic activity of the epithelium of involuted renal tubules. This activity increases beginning from the end of the first week, approaches the control value by the 30th day of adaptation and almost completely returns to normal by the 60th day of the animals' stay in the mountains. The circadian rhythms of the mitotic activity appeared undisturbed and was monophasic in nature. 相似文献
993.
994.
995.
Studies with substrate analogues and the pH optimum indicated the involvement of carboxyl group in the active site of goat
carboxypeptidase A. Chemical modification of the enzyme with 1-cyclohexyl-3-(2-morpholinoethyl) carbodiimide methoI -p-toluene sulphonate, a carboxyl specific reagent, led to loss of both esterase and peptidase activities. Protection studies
showed that this carboxyl group was in the active site and was protected by Βp-phenylpropionic acid and glycyl-L-tyrosine. Kinetic studies also confirmed the involvement of carboxylic group because the
enzyme modification with water soluble carbodiimide was a two step reaction which excluded the possibility of tyrosine or
lysine which are known to give a one step reaction with this reagent 相似文献
996.
Daily ingestion of iodide alone is not adequate to sustain production of the thyroid hormones, tri- and tetraiodothyronine. Proper maintenance of iodide in vivo also requires its active transport into the thyroid and its salvage from mono- and diiodotyrosine that are formed in excess during hormone biosynthesis. The enzyme iodotyrosine deiodinase responsible for this salvage is unusual in its ability to catalyze a reductive dehalogenation reaction dependent on a flavin cofactor, FMN. Initial characterization of this enzyme was limited by its membrane association, difficult purification and poor stability. The deiodinase became amenable to detailed analysis only after identification and heterologous expression of its gene. Site-directed mutagenesis recently demonstrated that cysteine residues are not necessary for enzymatic activity in contrast to precedence set by other reductive dehalogenases. Truncation of the N-terminal membrane anchor of the deiodinase has provided a soluble and stable source of enzyme sufficient for crystallographic studies. The structure of an enzyme·substrate co-crystal has become invaluable for understanding the origins of substrate selectivity and the mutations causing thyroid disease in humans. 相似文献
997.
VEGETATION'S FOURTH DIMENSION 总被引:3,自引:1,他引:2
D. WALKER 《The New phytologist》1982,90(3):419-429
998.
999.
Stereospecific binding of 3H-dopamine in neostriatal membrane preparations: inhibitory effects of sodium ascorbate 总被引:2,自引:0,他引:2
It has been pointed out by several different groups of investigators in the past several years that ascorbic acid was a potent inhibitor of the binding of dopamine (DA) agonists including 3H-DA itself and 3H-ADTN, 3H-apomorphine and 3H-norpropylapomorphine to neostriatal membrane preparations. However, the significance of this effect of ascorbic acid has been controversial. For example, it has recently been claimed that the stereospecific binding of DA agonists is facilitated by ascorbic acid and can be measured only in its presence. In the present study in neostriatal membrane preparations in the absence of ascorbic acid, the binding of 3H-DA was very potently inhibited by potent DA agonists (DA, ADTN, apomorphine). Considerably weaker effects were obtained with norepinephrine, isoproterenol, serotonin, catechol and pyrogallol. Stereospecific effects were clearly observed in that the binding of 3H-DA was inhibited to a much greater extent by several biologically active enantiomers than by their less active counterparts. For example, (-)-2-hydroxyapomorphine and (-)-norpropylapomorphine were much more potent inhibitors than their corresponding (+) isomers. This binding of 3H-DA was also very strongly inhibited by sodium ascorbate and several other reducing agents. In control experiments in the neostriatal membrane preparation in the absence of ascorbic acid, there was no detectable decomposition of 3H-DA. The data suggest that 3H-DA can, in the absence of sodium ascorbate, bind stereospecifically to a site that has the properties of a DA receptor. Furthermore, sodium ascorbate is a potent inhibitor of this stereospecific binding. 相似文献
1000.
The role of arctic zooplankton in biogeochemical cycles: respiration and excretion of ammonia and phosphate during summer 总被引:1,自引:0,他引:1
M. Alcaraz R. Almeda A. Calbet E. Saiz C. M. Duarte S. Lasternas S. Agustí R. Santiago J. Movilla A. Alonso 《Polar Biology》2010,33(12):1719-1731
The study of the structural and functional properties of key components of polar marine ecosystems has received increased
attention in order to better understand the ecological consequences of future sea temperature rise and seasonal ice retraction.
Owing to this purpose, during the ATOS-Arctic cruise, held in July 2007 in the framework of the 2007–2008 International Polar
Year, we studied the respiratory carbon demand of mesozooplankton as well as their contribution to the regeneration of inorganic
nitrogen and phosphorus (NH4-N and PO4-P) via excretion. The studied area comprised several stations along a latitudinal gradient in the East Greenland current, plus a
network of stations NW of the Svalbard islands. The specific respiratory carbon losses and phosphorus (PO4-P) excretion rates were similar or slightly higher than some reports for Arctic mesozooplankton, but the nitrogen (NH4-N) excretion rates were higher by a factor of 3 when compared with previous data sets. The mesozooplankton respiratory losses
were equivalent to 23% of primary production, and at turn zooplankton contributed by excretion to more than 50% of the N and
P required by phytoplankton. Although C:N, C:P and N:P metabolic atomic quotients almost coincided with the average Redfield’s
stoichiometric ratios, the low C:N values when compared to previous reports suggested a predominance of protein-related metabolic
substrates. The potential consequences of changes observed in the C:N, N:P and C:P metabolic ratios of mesozooplankton for
Arctic marine ecosystems are discussed. 相似文献