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81.
Yeasts degrade glucose through different metabolic pathways, where the choice of the pathway is dependent on the nature of the limitation in the various substrates. When oxygen is limiting in addition to glucose, yeasts often grow according to a mixture of oxidative and reductive metabolism. Oxygen may be limiting either by supply or by inherent biological restrictions such as the respiratory bottleneck in Saccharomyces cerevisiae or by both. A unified model incorporating both supply and biological limitations is proposed for the quantitative prediction of growth rates, consumption and production rates, as well as key metabolite concentrations during mixed oxidoreductive metabolism occuring as a result of such oxygen limitations. This simple unstructured model can be applied to different yeast strains while at the same time requiring a minimum number of measured parameters. "Estimators" are utilized in order to predict the presence of supply-side or biological limitations. The values of these estimators also characterize the relative importance of oxidative to total metabolism. Results from the aerobic and oxygen-limited chemostat cultures were used to corroborate the model predictions. During these experiments, the heat released by the yeast cultures was also monitored on-line. The model correctly predicted the overall stoichiometry, steady-state concentrations, and rates including heat dissipation rates measured in the various situations of oxygen limitations. Direct continuous measurements such as heat can be used in conjunction with the unified model for on-line proces control. (c) 1992 John Wiley & Sons, Inc. 相似文献
82.
Genotypic differences in aluminium (Al) tolerance hold considerable promise in overcoming an important limitation to plant growth in acid soils. Little is known, however, about the biochemical basis of such differences. Extracellular properties, particularly low root cation-exchange capacity (CEC), have been associated with Al tolerance, since roots of low CEC adsorb less Al than do those of high CEC. A solution culture study was conducted in which 12 plant species (monocots and dicots) were grown in solution culture of low ionic strength (ca 2 mM) for 8 d at four Al concentrations (0, 16, 28 and 55 M). The species differed significantly in Al tolerance as shown by differences in root length. Root length relative to that of the same species grown in the absence of Al varied from 6 to 117% at 16 M Al, and from 6 to 75% at 28 M Al. Species tolerance of Al was not closely associated with differences in root CEC. Although in some species Al sensitivity was associated with high adsorption of Al during a 10- or 40-min exposure to Al (expressed on a fresh mass or root length basis), this was not a good predictor of Al tolerance across all species studied. 相似文献
83.
Redox potentials in microsites of the rhizosphere of flooded rice were continuously measured for several days. Close to the root tips redox potential markedly increased. The highest increase was measured in the rhizosphere of the tips of short lateral roots. Aerobic redox conditions were reached there, except in a very strongly reduced soil. Both the extension of the oxidation zone around the root tips and the maximum redox potential reached were influenced by the reducing capacity of the soil. The radius of the redox rhizosphere varied from less than 1 mm in a strongly reduced soil up to 4 mm in a weakly reduced one. The root-induced oxidation processes in the rhizosphere depended on the atmospheric oxygen supply to the roots. 相似文献
84.
Hainan Gu 《Biochemical genetics》1991,29(7-8):345-354
Variations at three flight-related enzyme loci, -glycerophosphate dehydrogenase (-Gpdh), glucose-6-phosphate dehydrogenase (G6pd), and phosphoglucomutase (Pgm), ofEpiphyas postvittana (Walker) moths were investigated using starch gel electrophoresis. Among the three enzyme loci, -Gpdh andG6pd were found to be monomorphic, butPgm was polymorphic, with a total of seven different genotypes and five alleles identified in this study. Comparisons of allozyme variability at thePgm locus showed significant differentiation among five natural populations sampled from geographically distinct localities in New Zealand and Australia and between laboratory populations differentiated by artificial selection on flight capacity. ThePgm polymorphism was shown to be associated with the variation of flight capacity, but the role of the enzyme locus in the evolution of flight behavior is to be demonstrated in this species.This work was supported by the research scholarship of the University of New England. 相似文献
85.
Henrika G. M. Tiedink Laura H. J. De Haan Wim M. F. Jongen Jan H. Koeman 《Cell biology and toxicology》1991,7(4):371-386
4-chloro-methoxyindole is a naturally occurring compound in Vicia faba which can easily react with nitrite to form a N-nitroso compound. In this in vitro study, the potential genotoxic effects of nitrosated 4-chloro-6-methoxyindole and its structural analogue 4-chloroindole were evaluated for the first time by using both Salmonella and Chinese hamster V79 cells. Additionally, the inhibition of gap junctional intercellular communication in V79 cells by these compounds was determined; this is a validated parameter for tumor-promoting activity. Most assays were also performed with nitrosated indole-3-acetonitrile, a naturally occurring compound in brassicas. Both nitrosated chloroindoles were highly mutagenic to Salmonella typhimurium TA100 without the need of exogenous metabolic activation and were potent inducers of Sister Chromatid Exchanges. Nitrosated indole-3-acetonitrile generated the same effects, although at much higher concentrations. Equivocal results were obtained for the nitrosated chloroindoles in a forward mutation assay using the hypoxanthine guaninephosphoribosyltransferase locus. All nitrosated indole compounds significantly inhibited gap junctional intercellular communication. These results indicate that nitrosated chloroindoles and nitrosated indole-3-acetonitrile should be considered as mutagens and agents with potential tumor-promoting capacity.Abbreviations BrdU
5-bromo-2-deoxyuridine
- 4Cl
4-chloroindole
- 4C6MI
4-chloro-6-methoxy-indole
- DMSO
dimethyl sulfoxide
- EBSS
Earle's balanced salt solution
- EMS
ethyl methanesulfonate
- GJIC
gap junctional intercellular communication
- HBSS
Hanks balanced salt solution
- HGPRT
hypoxanthine guaninephosphoribosyl transferase
- I3A
indole-3-acetonitrile
- MNNG
1-methyl-1-nitroso-3-nitroguanidine
- NOC
N-nitroso compounds
- NQO
4-nitroquinolone-N-oxide
- SCE
sister chromatid exchange
- 6TG
6-thioguanine
- TPA
12-O-tetradecanoylphorbol-13-acetate 相似文献
86.
Absorption of gas from alveoli is examined in a simplified model of the respiratory system during a stylized single breath consisting of constant inspiratory flow, constant expiratory flow, and breathholding. The equations describing gas behavior are general since they are based upon conservation of mass. The equations simplify considerably when gases that are not soluble in pulmonary tissue and/or blood are utilized. In a three-compartment model, diffusing capacity of the lung for carbon monoxide (D
CO
) will be underestimated except when both uneven distribution of lung volume andD
CO
are present; under most circumstances, the standard clinical 10-s method [9] is at least as accurate as any other. When pulmonary capillary blood flow
is calculated by the one point method [2] in a one-compartment lung, it is underestimated; in the three-compartment model, it is underestimated except when both uneven distribution of
. and lung volume are present. The multiple single breath method [2] accurately measuresD
CO
and
. Measurement of pulmonary tissue volume is improved by correcting the value of the intercept of acetylene absorption to the time when carbon monoxide apparently began rather than utilizing the beginning of inspiration.Nomenclature
D
CO
diffusing capacity of the lung for CO (ml CO, STPD/min/mm Hg)
-
pulmonary capillary blood flow rate (L/min)
-
V
t
pulmonary tissue volume (L)
-
V
A
alveolar compartment volume (L)
-
V
Ao
alveolar compartment volume at conclusion of inspiratory flow (L)
-
inspiratory flow rate (L/sec)
-
expiratory flow rate (L/sec)
-
Bunsen coefficient of pulmonary tissue for test gas (ml test gas/ml tissue/atm)
-
Bunsen coefficient of pulmonary tissue for test gas (ml test gas/ml blood/atm)
-
F
A
fractional pressure of test gas in the alveolar compartment (atm) 相似文献
87.
Specific protection of nucleotides in the lac operator from DMS methylation and DNase I nicking by crude bacterial cell extracts 总被引:1,自引:0,他引:1
Crude bacterial cell extracts prepared from an Escherichia coli lacIq strain were shown to protect specific nucleotides in the lac operator from methylation by dimethyl sulfate (DMS) or digestion by DNase I, whereas no protection was observed using extracts prepared from a nearly isogenic lacI- strain. These experiments show that it is not necessary to use purified regulatory proteins in experiments designed to localize sequences on DNA which interact with proteins. Therefore, crude cell extracts should be useful in DNA "footprinting" experiments to define regions of DNA which bind to unknown regulatory proteins. 相似文献
88.
An immobilized cyclic multi-step enzyme system -- the urea cycle 总被引:1,自引:0,他引:1
89.
90.
Until now, additions of lime have been used to restore the buffering capacity of acidified lakes, but an alternative method
which is more effective in the treatment of lakes with organogenic sediments has recently been applied in a full-scale experiment.
The method, called CONTRACID, is based on the cation exchange properties of lake sediment. A sodium carbonate (soda ash) solution
is injected into the sediment (by a harrow), so that the sediment becomes sodium stocked. A reverse exchange occurs during
subsequent acidification. Liming has a limited effect on humic lakes, since Ca-humates have a reduced reverse exchange ability
and also the lime, which remains undissolved, is rendered inactive.
Ionic exchange processes and nutrient transport were studied in water/sediment cores andin situ enclosures after additions of soda ash-, lye- and lime solutions with subsequent re-acidification. Sodium carbonate additions
in laboratory systems resulted in a sorption to the sediment of 42–62% of the added sodium ions (5 eq m−2) and a release of 14–78 mg Pm−2 sediment. Similar results were obtained in the enclosures where phosphorus release stimulated algal growth. Sediment pH,
elevated by the sodium base addition, was lowered by re-acidification. Limed systems released no phosphorus and only about
25% of the added lime remained active for future neutralization. With the injection of the sodium carbonate solution into
the sediment, only about 12% of the added sodium was recovered in lake water by spring circulation. Lake water alkalinity
was then 0.12 meq l−1 and pH 6.7. Total phosphorus had been raised by 0.007 mg P l−1 causing an increase in phytoplankton biomass.
Observations indicate that manipulations of acidic lake sediment according to the CONTRACID method create a long-lasting neutralizing
capacity and a biological stimulation (through phosphorus release), which makes the method an attractive alternative to frequent
liming. 相似文献