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71.
A spontaneous mutant of the yeast Candida maltosa SBUG 700 was isolated showing pseudohyphal marphology under all growth conditions tested. The C. maltosa PHM mutant takes up glucose with the kinetics of C. maltosa SBUG 700 and starved cells contain the same cyclic AMP concentration. Addition of glucose to the PHM mutant does not result in an increase of the intracellular cyclic AMP level and in catabolite inactivation of fructose-1,6-bisphosphatase, malate dehydrogenase and phosphoenolpyruvate carboxykinase. However, addition of 2,4-dinitrophenol is followed by a rapid, transient increase of the cyclic AMP level in the mutant cells, but not by catabolite inactivation. These results show that a common mechanism might be responsible for catabolite inactivation and glucose-induced cAMP signaling or that glucose-induced cAMP signaling is required for catabolite inactivation in C. maltosa. 相似文献
72.
Effect of short-and long-term exposure to low environmental temperature on brain regional GABA metabolism 总被引:1,自引:0,他引:1
Single exposure of adult male rats to low environmental temperature (LET, 12 ± 0.5°C) for 2 h significantly increased the hypothalamic and striatal GABA levels without affecting those in other regions of brain. The activity of glutamate decarboxylase (GAD) was elevated in hypothalamus (H) and corpus striatum (CS) under these conditions. GABA accumulation rate (measured with ethanolamine-O-sulfate, an inhibitor of GABA-transaminase) was also increased in both H and CS of rats exposed to LET for 2 h. Unlike after a single exposure, the repeated exposure (2 h/day) for 7, 15, and 30 onsecutive days did not change the hypothalamic GABA metabolism. No change in GABA metabolism was observed in CS when rats were repeatedly exposed to LET for 7 consecutive days. Prolongation of repeated exposure to LET (2 h/day) for 15 and 30 consecutive days decreased the striatal GABA level and increased the activity of GABA-transaminase, although GAD activity was not altered under these conditions. These results suggest that single exposure to LET accelerates GABA synthesis and may reduce the GABAergic activity in both H and CS; whereas repeated exposure to LET for 15 or 30 consecutive days enhances GABAergic activity with the stimulation of GABA utilization only in CS without affecting its synthesizing process. Thus, it may be suggested that the hypothalamic and striatal GABA system may play a characteristic role in response to short-and long-term exposure to LET. 相似文献
73.
Danielle L. Laval-Martin Isabelle A. Carr Saverio J. Barbera Leland N. Edmunds 《Chronobiology international》1990,7(2):99-105
-We have previously shown that NAD kinase and NADP phosphatase activities display circadian rhythms, in the soluble (SN) and membrane-bound (P) fractions of crude extracts of the achlorophyllous ZC mutant of the phytoflagellate Euglena gracilis (which displays circadian rhythmicity of cell division). We determined if changes in the affinity of NADP phosphatase and NAD kinase for their substrates, NADP+ and NAD+, were occurring by calculating the ratios 100(velocity found in Km conditions/velocity found in saturating conditions). The rationale was that if the affinity remained unchanged according to circadian time (CI), these values should always equal 50, independently of any changes in enzyme quantity; values greater than 50 should indicate increases in enzyme affinity, and values less than 50 decreases in affinity. Our results indicated that these values calculated for NADP phosphatase exhibited a complex pattern of rhythmicity, while those for NAD kinase displayed circadian variations strongly correlated with the rhythms in enzyme activity. The curves showed troughs at CT 00-04 both in dividing and nondividing cells and peaks at CT 18-20 or at CT 08-14 in cells sampled, respectively, from a dividing or a stationary culture. Such variations are indicative of changes in the kinetic properties of the enzyme, which may reflect modifications in its affinity either for effectors (such as Ca2+-calmodulin) or for its substrate, NAD+. This may be due to (i) the expression of different isoenzymes at different CTs; (ii) different posttranslational modifications of the enzyme; or (iii) concentrations of effectors varying in a circadian manner. 相似文献
74.
Iron absorption in the presence of varying amounts and sizes of dietary fiber was measured. A method using non-everted rat intestinal segments perfused in oxygen was refined. Neutral detergent fiber (NDF), a component of dietary fiber, was extracted from cooked pinto bean (Phaseolus vulgaris). The NDF did not affect iron absorption in intestinal segments from iron replete rats. However, 4 and 6 mg of NDF/ml significantly decreased iron absorption in the intestinal segments from anemic rats. NDF with a smaller particle size of 0.125 mm increased iron absorption relative to that absorbed with 0.180 mm particles. Histological examination validated using non-everted intestinal segments perfused with oxygen as a method for studying dietary effects on iron absorption. Segments which are not everted are less prone to damage. Perfusion with oxygen maintained metabolic activity in the tissue during the experiment. 相似文献
75.
Summary The toxicity of chromium and tin on growth, photosynthetic carbon-fixation, oxygen evolution, heterocyst differentiation and nitrogenase activity ofAnabaena doliolum and its interaction with bivalent cations has been studied. Some interacting cations, viz. Ca2+, Mg2+ and Mn2+, substantially antagonised the toxic effects of chromium and tin with reference to growth, heterocyst differentiation and nitrogenase activity in the following hierarchal sequence: Ca2+ > Mg2+ > Mn2+. However, the sequence of hierarchy was Mg2+ > Ca2+ > Mn2+ for carbon fixation and Mn2+ > Mg2+ > Ca2+ for photosynthetic oxygen evolution. Synergistically inhibitory patterns were noticed for all the parameters, viz. growth,14CO2 uptake, oxygen evolution, heterocyst differentiation and nitrogenase activity ofA. doliolum when Ni2+, Co2+ and Zn2+ were combined with the test metals in the growth medium. These cations followed the following sequence of synergistic inhibition: Ni2+ > Co2+ > Zn2+. Among all the interacting cations, Ca2+, Mg2+ and Mn2+ exhibited antagonistic effects which relieved the test cyanobacterium from metal toxicity. In contrast to this, Ni2+, CO2+ and Zn2+ showed synergistic inhibition which potentiating the toxicity of test metals in the N2-fixing cyanobacteriumA. doliolum. It is evident from the present study that bivalent cations, viz. Ca2+, Mg2+, Mn2+, Ni2+, Co2+ and Zn2+, may appreciably regulate the toxicity of heavy metals in N2-fixing cyanobacteria if present in aquatic media. 相似文献
76.
Jose L. Vega Vicki L. Holmberg Edgar C. Clausen James L. Gaddy 《Archives of microbiology》1988,151(1):65-70
Intrinsic growth and substrate uptake parameters were obtained for Peptostreptococcus productus, strain U-1, using carbon monoxide as the limiting substrate. A modified Monod model with substrate inhibition was used for modeling. In addition, a product yield of 0.25 mol acetate/mol CO and a cell yield of 0.034 g cells/g CO were obtained. While CO was found to be the primary substrate, P. productus is able to produce acetate from CO2 and H2, although this substrate could not sustain growth. Yeast extract was found to also be a growth substrate. A yield of 0.017 g cell/g yeast extract and a product yield of 0.14 g acetate/g yeast extract were obtained. In the presence of acetate, the maximum specific CO uptake rate was increased by 40% compared to the maximum without acetate present. Cell replication was inhibited at acetate concentrations of 30 g/l. Methionine was found to be an essential nutrient for growth and CO uptake by P. productus. A minimum amount of a complex medium such as yeast extract (0.01%) is, however, required. 相似文献
77.
Thomas Jahns Alexandra Zobel Diethelm Kleiner Heinrich Kaltwasser 《Archives of microbiology》1988,149(5):377-383
Evidence for the existence of an energy-dependent urea permease was found for Alcaligenes eutrophus H16 and Klebsiella pneumoniae M5a1 by studying uptake of 14C-urea. Since intracellular urea was metabolized immediately, uptake did not result in formation of an urea pool. Evidence is based on observations that the in vivo urea uptake and in vitro urease activity differ significantly with respect to kinetic parameters, temperature optimum, pH optimum, response towards inhibitors and regulation. The K
m for urea uptake was 15–20 times lower (38 M and 13 M urea for A. eutrophus and K. pneumoniae, respectively) than the K
m of urease for urea (650 M and 280 M urea), the activity optimum for A. eutrophus was at pH 6.0 and 35°C for the uptake and pH 9.0 and 65°C for urease. Uptake but not urease activity in both organisms strongly decreased upon addition of inhibitors of energy metabolism, while in K. pneumoniae, potent inhibitors of urease (thiourea and hydroxyurea) did not affect the uptake process. Significant differences in the uptake rates were observed during growth with different nitrogen sources (ammonia, nitrate, urea) or in the absence of a nitrogen source; this suggested that a carrier is involved which is subject to nitrogen control. Some evidence for the presence of an energy-dependent uptake of urea was also obtained in Pseudomonas aeruginosa DSM 50071 and Providencia rettgeri DSM 1131, but not in Proteus vulgaris DSM 30118 and Bacillus pasteurii DSM 33.Non-standard abbreviations CCCP
Carbonylcyanide-m-chlorphenylhydrazone
- DCCD
dicyclohexylcarbodiimide
- DNP
2,4-dinitrophenole 相似文献
78.
Jui H. Wang 《Journal of bioenergetics and biomembranes》1988,20(4):407-422
Recent studies of chemically modified F1-ATPases have provided new information that requires a revision of our thinking on their catalytic mechanism. One of the subunits in F1-ATPase is distinguishable from the other two both structurally and functionally. The catalytic site and regulatory site of the same subunit are probably sufficiently close to each other, and the interaction between the various catalytic and regulatory sites are probably sufficiently strong to raise the uni-site rate of ATP hydrolysis by several orders of magnitude to that of promoted (multi-site) ATP hydrolysis. Although all three subunits in F1 possess weak uni-site ATPase activity, only one of them () catalyzes promoted ATP hydrolysis. But all three subunits catalyze ATP synthesis driven by the proton flux. Internal rotation of the 33 or 3 moiety relative to the remainder of the F0F1 complex did not occur during oxidative phosphorylation by reconstituted submitochondrial particles. 相似文献
79.
80.
J.-J. Bonnet S. Benmansour J.-M. Vaugeois J. Costentin 《Journal of neurochemistry》1988,50(3):759-765
At 0°C, when Na+ was the only cation present in the incubation medium, increasing the Na+ concentration from 3 to 10 mM enhanced the affinity of [3H]l-[2-(di-phenylmethoxy)ethyl]-4-(3-phenyl-2-propenyl)piperazine ([3H]GBR 12783) for the specific binding site present in rat striatal membranes without affecting the 5max. For higher Na+ concentrations, specific binding values plateaued and then slightly decreased at 130 mM Na+. In a 10 mM Na+ medium, the KD and the Bmax were, respectively, 0.23 nM and 12.9 pmol/mg of protein. In the presence of 0.4 nM [3H]GBR 12783, the half-maximal specific binding occurred at 5 mM Na+. A similar Na+ dependence was observed at 20°C. Scatchard plots indicated that K+, Ca2+, Mg2+, and Tris+ acted like competitive inhibitors of the specific binding of [3H]GBR 12783. The inhibitory potency of various cations (K+, Ca2+, Mg2+, Tris+, Li+ and choline) was enhanced when the Na+ concentration was decreased from 130 to 10 mM. In a 10 mM Na+ medium, the rank order of inhibitory potency was Ca2+ (0.13 mM) > Mg2+ > Tris+ > K+ (15 mM). The requirement for Na+ was rather specific, because none of the other cations acted as a substitute for Na+. No anionic requirement was found: Cl-, Br-, and F- were equipotent. These results suggest that low Na+ concentrations are required for maximal binding; higher Na+ concentrations protect the specific binding site against the inhibitory effect of other cations. 相似文献