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1.
In hansenula polymorpha glycerol is metabolized via glycerol kinase and NAD(P)-independent glycerol-3-phosphate (G3P) dehydrogenase, enzymes which hitherto were reported to be absent in this methylotrophic yeast. Activity of glycerol kinase was readily detectable when cell-free extracts were incubated at pH 7–8 with glycerol/ATP/Mg2+ and a discontinuous assay for G3P formation was used. This glycerol kinase activity could be separated from dihydroxyacetone (DHA) kinase activity by ion exchange chromatography. Glycerol kinase showed relatively low affinities for glycerol (apparent K
m=1.0 mM) and ATP (apparent K
m=0.5 mM) and was not active with other substrates tested. No inhibition by fructose-1,6-bisphosphate (FBP) was observed. Both NAD-dependent and NAD(P)-independent G3P dehydrogenases were present. The latter enzyme could be assayed with PMS/MTT and cosedimented with the mitochondrial fraction. Glucose partly repressed synthesis of glycerol kinase and NAD(P)-independent G3P dehydrogenase, but compared to several other non-repressing carbon sources no clear induction of these enzymes by glycerol was apparent. Amongst glycerolnegative mutants of H. polymorpha strain 17B (a DHA kinase-negative mutant), strains blocked in either glycerol kinase or membrane-bound G3P dehydrogenase were identified. Crosses between representatives of the latter mutants and wild type resulted in the isolation of, amongst others, segregants which had regained DHA kinase but were still blocked in the membrane-bound G3P dehydrogenase. These strains, employing the oxidative pathway, were only able to grow very slowly in glycerol mineral medium.Abbreviations DHA
dihydroxyacetone
- G3P
glycerol-3-phosphate
- EMS
ethyl methanesulphonate
- MTT
3-(4,5-dimethyl-thiazolyl-2)-2,5-diphenyl tetrazolium bromide
- PMS
phenazine methosulphate
- FBP
fructose-1,6-bisphosphate 相似文献
2.
C. Claret A. Bories P. Soucaille 《Journal of industrial microbiology & biotechnology》1993,11(2):105-112
Summary Microbial conversion of glycerol into dihydroxyacetone (DHA) byGluconobacter oxydans was subjected to inhibition by excess substrate. Comparison of cultures containing increasing initial DHA contents (0 to 100 g l–1) demonstrated that DHA also inhibited this fermentation process. The first effect was on bacterial growth (cellular development stopped when DHA concentration reached 67 gl–1), and then on oxidation of glycerol (DHA synthesis only occurred when the DHA concentration in the culture medium was lower than 85 g l–1). Productivity, specific rates and, to a lesser extent, conversion yields decreased as initial concentrations of DHA increased. The changes in the specific parameters according to increasing initial DHA contents were described by general equations. These formulae satisfactorily express the concave aspect of the curves and the reduction in biological activity when the cells were in contact with DHA concentrations of up to 96 g l–1.Abbreviations
X, S, P
biomass, substrate, product concentrations
-
r
x,r
s,r
p
rates of growth, consumption and production
- ,q
s,q
p
specific rates of growth, glycerol consumption and DHA production
-
Y
x/s, Yp/s
conversion yields of substrate into biomass and product
-
K
s
constant of affinity of cells to the substrate
-
K
ip
product inhibition constant
-
P
m
threshold concentration of DHA in substrate 相似文献
3.
Andreia S. Fernandes Manuela M. Pereira Miguel Teixeira 《Journal of bioenergetics and biomembranes》2001,33(4):343-352
The succinate dehydrogenase from the thermohalophilic bacterium Rhodothermus marinus is a member of the succinate:menaquinone oxidoreductases family. It is constituted by three subunits with apparent molecular masses of 70, 32, and 18 kDa. The optimum temperature for succinate dehydrogenase activity is 80°C, higher than the optimum growth temperature of R. marinus, 65°C. The enzyme shows a high affinity for both succinate (K
m = 0.165 mM) and fumarate (K
m = 0.10 mM). It contains the canonical iron–sulfur centers S1, S2, and S3, as well as two B-type hemes. In contrast to other succinate dehydrogenases, the S3 center has an unusually high reduction potential of +130 mV and is present in two different conformations, one of which presents an unusual EPR signal with g values at 2.035, 2.009, and 2.001. The apparent midpoint reduction potentials of the hemes, +75 and –65 mV at pH 7.5, are also higher than those reported for other enzymes. The heme with the lower potential (heme bL) presents a considerable dependence of the reduction potential with pH (redox–Bohr effect), having a pK
a
OX = 6.5 and a pK
a
red = 8.7. This behavior is consistent with the proposal that in these enzymes menaquinone reduction occurs close to heme bL, near to the periplasmic side of the membrane, and involving dissipation of the proton transmembrane gradient. 相似文献
4.
Lens Major Intrinsic Protein (MIP) is a member of a family of membrane transport proteins including the Aquaporins and bacterial
glycerol transporters. When expressed in Xenopus oocytes, MIP increased both glycerol permeability and the activity of glycerol kinase. Glycerol permeability (p
Gly
) was 2.3 ± 0.23 × 10−6 cm sec−1 with MIP vs. 0.92 ± 0.086 × 10−6 cm sec−1 in control oocytes. The p
Gly
of MIP was independent of concentration from 5 × 10−5 to 5 × 10−2
m, had a low temperature dependence, and was inhibited approximately 90%, 80% and 50% by 1.0 mm Hg++, 0.2 mm DIDS (diisothiocyanodisulfonic stilbene), and 0.1 mm Cu++, respectively. MIP-enhanced glycerol phosphorylation, resulting in increased incorporation of glycerol into lipids. This
could arise from an increase in the total activity of glycerol kinase, or from an increase in its affinity for glycerol. Based
on methods we present to distinguish these mechanisms, MIP increased the maximum rate of phosphorylation by glycerol kinase
(0.12 ± 0.03 vs. 0.06 ± 0.01 pmol min−1 cell−1) without changing the binding of glycerol to the kinase (K
M
∼ 10 μm).
Received: 23 May 1997/Revised: 4 August 1997 相似文献
5.
Erko Stackebrandt Valerie J. Fowler Wolfgang Schubert Johannes F. Imhoff 《Archives of microbiology》1984,139(4):366-370
While most strains of heterofermentative lactobacilli and strains of Leuconostoc species contained only traces of a dehydratase reacting with glycerol or propanediol-1,2, three strains of Lactobacillus brevis and one strain of L. buchneri that metabolized glycerol readily in the presence of glucose, contained propanediol-1,2 dehydratase (EC 4.2.1.28). This cobamide requiring enzyme from L. brevis B 18 was partially purified. It reacts with the substrates propanediol-1,2, glycerol and ethanediol-1,2 with the relative activities of about 3:2:1. This ratio remained unchanged throughout the purification procedure. The substrate affinities were measured: propanediol-1,2 K
m=0.6 mM, glycerol K
m=4 mM, ethanediol-1,2 K
m=5.3 mM coenzyme B12 (substrate glycerol) K
m=0.007 mM. The activity of the dehydratase was promoted by potassium or ammonium ions and inhibited by sodium, lithium, magnesium or specially manganese. The apparent molecular weight of propanediol-1,2 dehydratase was determined as Mr=180,000.Dedicated to Prof. Dr. H. G. Schlegel on behalf of his 60th birthday 相似文献
6.
Gert-Jan W. M. van Alebeek Gaalda Tafazzul Michael J. J. Kreuwels Jan T. Keltjens Godfried D. Vogels 《Archives of microbiology》1994,162(3):193-198
The levels of cyclic 2,3-diphosphoglycerate (cDPG) in methanogenic bacteria are governed by the antagonistic activities of cDPG synthetase and cDPG hydrolase. In this paper we focus on the synthetase from Methanobacterium thermoautotrophicum. The cytoplasmic 150 kDa enzyme catalyzed cDPG synthesis from 2,3-diphosphoglycerate (apparent Km=21 mM), Mg2+ (Km=3.1 mM) and ATP (Km=1–2 mM). In batch-fed cultures, the enzyme was constitutively present (6–6.5 nmol per min per mg protein) during the different growth phases. In continuous cultures, activity decreased in response to phosphate limitation. The synthetase reaction proceeded with maximal rate at pH 6 and at 65° C and was specifically dependent on high (>0.3M) K+ concentrations. The reaction conditions remarkably contrasted to those of cDPG degradation catalyzed by the previously described membrane-bound cDPG hydrolase.Abbreviations
cDPG
Cyclic 2,3-diphosphoglycerate
-
2,3-DPG
2,3-Diphosphoglycerate
-
2-PG
2-Phosphoglycerate
-
3-PG
3-Phosphoglycerate 相似文献
7.
Cell-free extracts of crotonate-grown cells of the syntrophic butyrate-oxidizing bacteriumSyntrophospora bryantii contained high hydrogenase activities (8.5–75.8 µmol · min–1 mg–1 protein) and relatively low formate dehydrogenase activities (0.04–0.07 µmol · min–1 mg–1 protein). The K
M
value and threshold value of the hydrogenase for H2 were 0.21 mM and 18 µM, respectively, whereas the K
M
value and threshold value of the formate dehydrogenase for formate were 0.22 mM and 10 µM, respectively. Hydrogenase, butyryl-CoA dehydrogenase and 3-OH-butyryl-CoA dehydrogenase were detected in the cytoplasmic fraction. Formate dehydrogenase and CO2 reductase were membrane-bound, likely located at the outer aspect of the cytoplasmic membrane. Results suggest that during syntrophic butyrate oxidation H2 is formed intracellularly while formate is formed at the outside of the cell. 相似文献
8.
M. Marjanovic J. S. Willis 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1995,165(6):428-432
Red cells of hibernating species have a higher relative rate of Na+–K+ pump activity at low temperature than the red cells of a mammal with a typical sensitivity to cold. The kinetics of ATP stimulation of the Na+–K+ pump were determined in guinea pig and ground squirrel red cells at different temperatures between 5 and 37°C by measuring ouabain-sensitive K+ influx at different levels of ATP. In guinea pig cells, elevation of intracellular free Mg2+ to 2 mmol·l-1 by use of the divalent cation ionophore A23187 caused the apparent affinity of the pump for ATP to increase with cooling to 20°C, rather than to decrease, as occurs in cells not loaded with Mg2+. In ground squirrel cells raising intracellular free Mg2+ had little effect on apparent affinity of the pump for ATP at 20°C. ATP affinity rose slightly with cooling both in Mg2+-enriched and in control ground squirrel cells. Increased intracellular free Mg2+ in guinea pig cells stimulated Na+–K+ pump activity so that at 20°C the pump rate was the same in the Mg2+-enriched guinea pig and control ground squirrel cells. Pump activity in Mg2+-enriched guinea pig cells at 5°C was significantly improved but still lower than pump activity in control cells from ground squirrel. Thus, loss of affinity of the Na+–K+ pump for ATP that occurs with cooling in cold-sensitive guinea pig red cells can be, at least partially, prevented by elevating cytoplasmic free Mg2+. Conversely, in ground squirrel red cells natural rise of free Mg2+ may in part account for the preservation of the ATP affinity of their Na+–K+ pump with cooling.Abbreviations
K
m
Michaelis-Menten constant for apparent affinity
-
MOPS
3-(N-morpholino)-propanesulphonic acid
- [Mg2+]i
intracellular concentration of free Mg2+
-
OD
optical density
-
RBC
red blood cell(s)
-
T
b
body temperature 相似文献
9.
Several denitrifying Pseudomonas strains contained an NADP+-specific 2-oxoglutarate dehydrogenase, in contrast to an NAD+-specific pyruvate dehydrogenase, if the cells were grown anaerobically with aromatic compounds. With non-aromatic substrates or after aerobic growth the coenzyme specificity of 2-oxoglutarate dehydrogenase changed to NAD+-specificity. The reaction stoichiometry and the apparent K
m-values of the enriched enzymes were determined: pyruvate 0.5 mM, coenzyme A 0.05 mM, NAD+ 0.25 mM; 2-oxoglutarate 0.6 mM, coenzyme A 0.05 mM, NADP+ 0.03 mM. Isocitrate dehydrogenase was NADP+-specific. The findings suggest that these strains contained at least two lipoamide dehydrogenases, one NAD+-specific, the other NADP+-specific. 相似文献
10.
A hyper-thermostable, alkaline lipase from Pseudomonas sp. with the property of thermal activation 总被引:1,自引:0,他引:1
A hyper-thermostable, alkaline lipase from a newly-isolated, mesophilic Pseudomonas sp. was optimal at pH 11 and at 90 °C. It had a half-life of more than 13 h at 90 °C. It was activated by 30% when heated at 90 °C for 2 h. The enzyme had a greater affinity for mustard oil (K
m=40 mg ml–1) than for olive oil (K
m=140 mg ml–1). 相似文献
11.
During growth on glycerol two marine Desulfovibrio strains that can grow on an unusually broad range of substrates contained high activities of glycerol kinase, NAD(P)-independent glycerol 3-phosphate dehydrogenase and the other enzymes necessary for the conversion of dihydroxyacetone phosphate to pyruvate. Glycerol dehydrogenase and a specific dihydroxyacetone kinase were absent. During growth on dihydroxyacetone, glycerol kinase is involved in the initial conversion of this compound to dihydroxyacetone phosphate which is then further metabolized. Some kinetic properties of the partially purified glycerol kinase were determined. The role of NAD as electron carrier in the energy metabolism during growth of these strains on glycerol and dihydroxyacetone is discussed.Glycerol also supported growth of three out of four classical Desulfovibrio strains tested. D. vulgaris strain Hildenborough grew slowly on glycerol and contained glycerol kinase, glycerol 3-phosphate dehydrogenase and enzymes for the dissimilation of dihydroxyacetone phosphate. In D. gigas which did not grow on glycerol the enzymes glycerol kinase and glycerol 3-phosphate dehydrogenase were absent in lactate-grown cells.Abbreviations DHA
dihydroxyacetone
- DHAP
dihydroxyacetone phosphate
- G3P
glycerol 3-phosphate
- GAP
glyceraldehyde 3-phosphate
- 3-PGA
3-phosphoglycerate
- 2-PGA
2-phosphoglycerate
- 2,3-DPGA
2,3-diphosphoglycerate
- PEP
phosphoenolpyruvate
- DH
dehydrogenase
- GK
glycerol kinase
- DHAK
dihydroxyacetone kinase
- TIM
triosephosphate isomerase
- PGK
3-phosphoglycerate kinase
- PK
pyruvate kinase
- LDH
lactate dehydrogenase
- DTT
dithiotreitol
- HEPES
4-(2-hydroxyethyl)-1-piperazine ethane sulfonic acid
- PIPES
piperazine-1,1-bis(2-ethane sulfonic acid)
- BV2+/BV+
oxidized/reduced benzylviologen
- PMS
phenazine methosulfate
- DCPIP
2,6-dichlorophenolindophenol
- MTT
3-(4,5-dimethylthiazol-2-yl)-2,4-diphenyltetrazolium bromide 相似文献
12.
G. P. Findlay 《The Journal of membrane biology》1982,68(1):179-189
Summary In cells of the freshwater algaHydrodictyon africanum, in solutions where [K+]0=0.1mm and pH0>7.0, the membrane in the light is hyperpolarized. The membrane potential difference {ie179-1} has values from –180 to –275 mV, more negative than any ion diffusion potential difference, and is predominantly a function of pH0, and independent of [K+]0. The hyperpolarization of the membrane appears to arise from an electrogenic efflux of H+, estimated from voltage-clamp data to be about 8 nmol m–2 sec–1 when pH0=8.5. In the light the membrane conductanceg
m is about 0.084 S m–2. At light-off, {ie179-2} becomes less negative, with a halftime for change of 15 to 30 sec andg
m decreases by about 0.052 S m–2. After dark periods of up to 300 sec, {ie179-3} is largely independent of pH0 for values greater than 6.0 and usually behaves as a combined K+ and Na+ diffusion potential with permeability ratioP
Na/P
K=0.05 to 0.2. The membrane potassium conductanceg
K has either a low value of 2–6×10–2 Sm–2, or a high value of up to 18×10–2 S m–2 depending on [K+]0, the transition from low to high values occurring when {ie179-4} moves over a threshold value that is more negative than {ie179-5}, the electrochemical equilibrium potential for K+. The time for half-change of the transition is about 30 sec. The results are consistent with a model of the membrane in which the pump electromotive force and conductance are in parallel with diffusive electromotive forces and conductances. When the pump is operating its properties determine membrane properties, and when it is inoperative, or running at a diminished rate, the membrane properties are determined more by the diffusive pathways. Changes in both pump rate andg
K can account for a variety of characteristic changes in membrane PD and conductance occurring in response to ligh-dark changes, changes in light intensity, pasage of externally applied electric current across the membrane and changes in ionic constituents of the external medium. 相似文献
13.
AxenicTrentepohlia odorata was cultured at three different NH4Cl levels (3.5 × 10–2, 3.5 × 10–3, 3.5 × 10–4 M) and three different light intensities (48, 76, 122 µmol m–2 s–1). Chloride had no effect on growth over this range of concentration. High light intensity and high NH4Cl concentration enhanced the specific growth rate. The carotenoid content increased under a combination of high light intensity and low N concentration. WhenD. bardawil was exposed to the same combination of growth conditions, there was an increase in its carotenoid content. The light saturation and the light inhibition constants (K
s andK
i, respectively) for growth, and the saturation constant (K
m) for NH4Cl were determined. TheK
s andK
i values were higher inT. odorata (66.7 and> 122 mol m–2 s–1, respectively) than inD. bardawil (5.1 and 14.7 µmol m–2 s–1, respectively). TheK
m value determined at 122 µmol m–2 s–1, however, was lower inT. odorata (0.048 µM) than inD. bardawil (0.062 µM).Author for correspondence 相似文献
14.
J. L. Albi J. Planas J. Sánchez 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(2):118-122
Glucose metabolism in peripheral blood lymphocytes from the brown trout Salmo trutta has been studied. Glucose is taken up by means of a sodium-independent saturable process (K
m=10.8 mmol·l-1), as well as by simple diffusion. Once within the cell, most of glucose is directed to lactate production through either the Embden-Meyerhof pathway or the hexose-monophosphate shunt. Rates of lactate formation are higher than rates of CO2 formation. Glutamine does not exert an effect on either glucose uptake or glucose metabolism. The present study provides information regarding the nature of energy sources for different cell types in salmonids.Abbreviations 3-OMG
3-O-methyl glucose
- EM
Embden-Meyerhoff pathway
- G6D
glucose-6-phosphate dehydrogenase
- HK
hexokinase
- HMS
hexose monophosphate shunt
- ICDH
isocitrate dehydrogenase
-
K
m
apparent Michaelis constant
- LDH
lactate dehydrogenase
- MCB
modified Cortland buffer
- PBL
peripheral blood lymphocytes
- PFK
fructose-6-phosphate kinase
- PK
pyruvate kinase
- RBC
red blood cells
-
V
max
maximal rate of uptake 相似文献
15.
W. Liebrich K. H. Hoffmann 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,161(1):93-99
Summary Ecdysone 20-monooxygenase, the enzyme system which converts ecdysone into 20-hydroxyecdysone, was characterized in the midgut of 4-day-old female adult Gryllus bimaculatus using an in vitro radioassay. Differential centrifugation and sucrose gradient centrifugation revealed that ecdysone 20-monooxygenase activity is associated with the microsomal fractions. The 20-monooxygenase was found to be most active in potassium phosphate buffer, pH 7.8, at an osmolarity of 100 mOsm and at 39 °C assay temperature. The conversion of ecdysone into 20-hydroxyecdysone was linear over an incubation period of 12 min and with respect to a protein concentration of 3 mg·ml–1. K+ and Na+ (10–3–10–1
M), Ca2+ (2.3 mM), and EDTA (1–5 mM) did not affect monooxygenase activity, whereas Mg2+ (2.3–10 mM) slightly inhibited enzyme activity. The enzyme complex has an apparent Km for ecdysone of 3.7·10–7
M and is competitively inhibited by its product, 20-hydroxyecdysone, with an apparent Ki of 4·10–6
M. The cytochrome P-450 nature of the steroid hydroxylase was shown by its obligate requirement for NADPH and its inhibition by carbon monoxide, metyrapone, and p-chloromercuribenzoate, but not by cyanide. The insect systemic growth disruptor, azadirachtin, was found to inhibit ecdysone 20-monooxygenase activity with a I50 of 8·10–4
M. From the CO-difference spectrum, a cytochrome P-450 content of 285 pmol·mg protein–1 was calculated for midgut microsomes of 4-day-old females.Abbreviations
GO
carbon monoxide
-
EDTA
ethylenediamine tetraacetic acid
-
HPLC
high performance liquid chromatography
-
I
50
concentration for 50% inhibition
-
KCN
potassium cyanide
-
K
1
inhibition constant
-
K
m
Michaelis-Menten constant
-
MOPS
3-morpholinopropanesulfonic acid
-
NADH/NAD
+
nicotinamide adenine dinucleotide reduced/oxidized
-
NADPH/NADP
+
nicotinamide adenine dinucleotide phosphate reduced/oxidized
-
Na
2
S
2
O
4
sodium dithionite
-
SEM
Standard error of mean
-
TLC
thin-layer chromatography
-
TRIS
2-amino 2-hydroxymethyl-1,3-propanediol (trishydroxymethyl aminomethane)
-
V
max
maximal reaction velocity 相似文献
16.
Coello M.N. Hadjsassi A. Lebeault J.M. 《World journal of microbiology & biotechnology》2001,17(4):337-342
The activity of 6-phosphogluconate dehydrogenase, aspartate kinase and phosphoenolpyruvate carboxylase has been studied at different dilution rates in aerobic continuous culture of Corynebacterium glutamicum. 6-Phosphogluconate dehydrogenase and aspartate kinase reached their maximum values at the lower dilution rates (0.02–0.06 h–1), when L-lysine was produced. The phosphoenolpyruvate carboxylase activity seemed to be independent of metabolite synthesis. The production of L-lysine was also studied in non-growing cells in batch cultures. In these conditions, statistical analysis revealed significant differences in L-lysine titres when glucose or gluconic acid were used as carbon sources. Higher L-lysine concentration obtained with gluconic acid was found to be associated with a high 6-phosphogluconate dehydrogenase activity. 相似文献
17.
The only species of fission yeasts capable of growing on glycerol or dihydroxyacetone were Schizosaccharomyces pombe and S. malidevorans. When growing on glycerol or grown on glucose until it was exhausted, these species contained glycerol:NAD+ 2-oxidoreductase and dihydroxyacetone kinase but no glycerol kinase, consistent with utilization of glycerol via dihydroxyacetone. When grown to exhaustion of glucose, S. octosporus, S. slooffiae and S. japonicus contained dihydroxyacetone kinase but no glycerol:NAD+ 2-oxidoreductase or glycerol kinase. Prior to exhaustion of glucose in the medium, all species contained dihydroxyacetone kinase, all species except S. japonicus contained glycerol:NADP+ 2-oxidoreductase, and only S. pombe and S. malidevorans contained glycerol:NAD+ 2-oxidoreductase. Possible roles for the glycerol:NAD+ 2-oxidoreductase, glycerol:NADP+ 2-oxidoreductase and dihydroxyacetone kinase in metabolism of glycerol and dihydroxyacetone are discussed.Non-standard abbreviations DHA
dihydroxyacetone
- DHAK
dihydroxyacetone kinase
- DHAP
dihydroxyacetone phosphate
- GK
glycerol kinase
- G2DH-NAD
glycerol
- NAD+
2-oxidoreductase
- G2DH-NADP
glycerol
- NADP+
2-oxidoreductase
- MEA
malt extract agar
- YEP
yeast extract phosphate medium 相似文献
18.
Mutants of Myxococcus xanthus FBt unable to form myxospores in response to 0.5 M glycerol arise spontaneously with a frequency of 1–3×10–5. These mutants are designated glc. Ultraviolet mutagenesis increases the frequency to a maximum of 7% of the survivors. The reversion frequency following ultraviolet irradiation of spontaneous glc mutants is less than 10–3. Of four glc mutants examined, none form myxospores in response to the alternative inducers, ethylene glycol and dimethyl sulphoxide. One glc mutant is induced by 1.5 M glycerol; strain FBt responds to this glycerol concentration with low efficiency myxospore formation. Strain FBt and glc mutants all produce myxospores with low efficiency in response to phenyl ethanol. Of 117 glc mutants tested, 109 form fruiting bodies containing mature myxospores; thus, mutations to the glc phenotype do not normally block myxospore formation within the fruiting cycle of the organism. 相似文献
19.
Kinetics of lactose transport inKluyveromyces fragilis grown in a chemostat on diluted whey permeate
Lactose transport was studied inKluyveromyces fragilis grown in lactose-limited chemostat cultures. Kinetic parameters were determined using a method based on genetic population evolution. Lactose transport was carried out via three carriers characterized respectively byK
m of 0.1 mM, 3 mM and 15.5 mM. The synthesis of these lactose carriers and their capacity (V
max) are dependent on the dilution rate (D). At D=0.12 h–1, the high affinity transporter is prominent. For intermediate dilution rate, only the high and the medium affinity systems are present. In cells growing at D=0.4 h–1, these carriers are absent but instead, the low affinity transporter is present. The effect on lactose transport of such metabolic inhibitors as CCCP, a proton ionophore, and Antimycin A, an energy inhibitor, were also investigated. The high affinity system is the most sensitive to the effect of these inhibitors. Lactose transport through this carrier is probably a mechanism dependent on the proton motive force. 相似文献
20.
The optimal conditions for opening of stomata in detached epidermis of the Crassulacean Acid Metabolism (CAM) plant Kalanchoe daigremontiana were determined. Stomatal opening in CO2–free air was unaffected by light so subsequently all epidermal strips were incubated in the dark and in CO2–free air. Apertures were maximal after 3 h incubation and were significantly greater at 15° C than 25° C. Thus stomata in isolated epidermis of this species can respond directly to temperature. Stomatal opening was greatest when the incubating buffer contained 17.6 mol m–3 K+, but decreased linearly with increasing K+ concentrations between 17.6 and 300 mol m–3; the decrease in aperture was shown to be associated with increasing osmotic potentials of the solutions. Reasons for this behaviour, which differs from that of many C3 and C4 species, are discussed. Stomatal apertures declined linearly upon incubation of epidermis on buffer solutions containing between 10–11 and 10–5 mol m–3 abscisic acid (ABA). Hence stomata on isolated epidermis of K. daigremontiana respond to lower concentrations of ABA than those of any species reported previously. 相似文献