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1.
Zalfa A. Abdel Malek Kristie L. Kreutzfeld Mac E. Hadley Marvin D. Bregman Victor J. Hruby Frank L. Meyskens jr. 《In vitro cellular & developmental biology. Plant》1986,22(2):75-81
Summary Cell density is a factor that affects the capacity of Cloudman S91 melanoma cells to respond to melanotropins in monolayer
culture. Continuous exposure of melanoma cells to α-melanotropin or its potent analog [Nle4,D-Phe7]-α-MSH, resulted in maximal stimulation of tyrosinase after 2 d of treatment, but the magnitude of stimulation decreased
thereafter despite the continued presence of the melanotropins. However, when melanoma cells continually exposed to melanotropins
were subcultured to an initial low cell density and maintained in contact with α-MSH or [Nle4,D-Phe7]-α-MSH (long-term culture), tyrosinase activity was rapidly restored and greatly enhanced. Also, when cells were seeded at
initial densities ranging from 0.2 to 3.2×106 cells/flask, and exposed for 24 h to 10−7
M α-MSH, only the cultures seeded at low densities (0.2 and 0.4×106 cells/flask) exhibited maximal tyrosinase activity during the 24 h exposure to the melanotropins. Therefore, tyrosinase activity
was primarily affected by cell density rather than by the duration of time the cells were in culture or by continuous exposure
to melanotropin. Other flasks of various cell densities were treated with 10−7 M α-MSH or [Nle4,D-Phe7]-α-MSH for 24 h, followed byremoval of the melanotropins from the culture medium. The magnitude and duration of theresidual stimulation of melanoma tyrosinase activity by melanotropins were also found to be dependent on the initial cell density.
These results reveal that there is a limited range of optimal cell densities at which melanoma cells can respond to melanotropins
and express increased tyrosinase activity. 相似文献
2.
Marine Bacterial Sulfated Fucoglucuronomannan (SFGM) Lyase Digests Brown Algal SFGM into Trisaccharides 总被引:2,自引:0,他引:2
Sakai T Kimura H Kojima K Shimanaka K Ikai K Kato I 《Marine biotechnology (New York, N.Y.)》2003,5(1):70-78
Abstract
Three kinds of trisaccharides were prepared by digesting fucoidan from the brown alga Kjellmaniella crassifolia, with the extracellular enzymes of the marine bacterium Fucobacter marina. Their structures were determined as Δ4,5GlcpUA1-2(L-Fucp(3-O-sulfate)α1-3)D-Manp, Δ4,5GlcpUA1-2(L-Fucp(3-O-sulfate)α1-3)D-Manp(6-O-sulfate), and Δ4,5GlcpUA1-2(L-Fucp(2,4-O-disulfate)α1-3)D-Manp(6-O-sulfate), which indicated the existence of a novel polysaccharide in the fucoidan and a novel glycosidase in the extracellular
enzymes. In order to determine the complete structure of the polysaccharide and the reaction mechanism of the glycosidase,
the fucoidan was partially hydrolyzed to obtain glucuronomannan, which is the putative backbone of the polysaccharide, and
its sugar sequence was determined as (-4-D-GlcpUAβ1-2D-Manpα1-)n, which disclosed that the main structure of the polysaccharide is (-4-D-GlcpUAβ1-2(L-Fucp(3-O-sulfate)α1-3)D-Manpα1-)n. Consequently, the glycosidase was deduced to be an endo-α-D-mannosidase that eliminatively cleaves the α-D-mannosyl linkage between D-Manp and D-GlcpUA residues in the polysaccharide and produces the above trisaccharides. The novel polysaccharide and glycosidase were tentatively
named as sulfated fucoglucuronomannan (SFGM) and SFGM lyase, respectively. 相似文献
3.
P Dantigny 《Journal of industrial microbiology & biotechnology》1998,21(4-5):215-218
The influence of sub-optimal temperatures (T) on the microbial growth rate (μ) has been assessed by means of dimensionless
variables: Tdim = [T−Tmin]/[Topt−Tmin] and μdim = μ/μopt. Tmin represents the temperature at which there is no growth, Topt the optimum temperature at which the growth rate, μopt, is maximum. Data sets, growth rate vs temperature, have been taken from the literature for 12 organisms (psychrotrophs, mesophiles and thermophiles). In order
to compare these organisms, the power law function has been used: [μdim] = [Tdim]α. The parameters μopt and Topt are determined from direct readings whereas Tmin and αare estimated by means of a non-linear regression. An accurate estimation of Tmin is obtained providing low growth rate data are available. A wide range of optimal temperatures where the growth rate almost
equals μopt prevents one from obtaining a narrow confidence interval forα. On the basis of the analysis hereafter developed, thermophiles
are characterized by values of the power α less than mesophiles and psychrotrophs. Almost all of these values are significantly
different from two, previously determined for Staphylococcus xylosus and widely used for predicting the microbial growth in foods.
Received 15 May 1998/ Accepted in revised form 25 September 1998 相似文献
4.
Summary A Monte Carlo simulation is proposed to study the dynamics of helper T-cells (N
H) and viral (N
V) populations in an immune response model relevant to HIV. Cellular states are binary variables and the interactions are described
by logical expressions. Viral population shows a nonmonotonic growth before reaching a constant value while helper T-cells
grow to a constant after a relaxation/reaction time. Initially, the population of helper cells grows with time with a power-law,
N
H ∼t
β, before reaching the steady-state; the growth exponent β increases systematically (β ≈ 1 – 2) with the mutation rate (P
mut≈0.1–0.4). The critical recovery time (t
c) increases exponentially with the viral mutation, t
c≈Ae
αP
mut
, with α=4.52±0.29 in low mutation regime and α=15.21±1.41 in high mutation regime. The equilibrium population of helper T-cell
declines slowly with P
mut and collapses at ∼ 0.40; the viral population exhibits a reverse trend, i.e., a slow increase before the burst around the
same mutation regime. 相似文献
5.
Thomas A. Gorr Barbara K. Mable Traute Kleinschmidt 《Journal of molecular evolution》1998,47(4):471-485
Phylogenetic relationships among reptiles were examined using previously published and newly determined hemoglobin sequences.
Trees reconstructed from these sequences using maximum-parsimony, neighbor-joining, and maximum-likelihood algorithms were
compared with a phylogenetic tree of Amniota, which was assembled on the basis of published morphological data. All analyses differentiated α chains into αA and αD types, which are present in all reptiles except crocodiles, where only αA chains are expressed. The occurrence of the αD chain in squamates (lizards and snakes only in this study) appears to be a general characteristic of these species. Lizards
and snakes also express two types of β chains (βI and βII), while only one type of β chain is present in birds and crocodiles.
Reconstructed hemoglobin trees for both α and β sequences did not yield the monophyletic Archosauria (i.e., crocodilians + birds) and Lepidosauria (i.e., Sphenodon+ squamates) groups defined by the morphology tree. This discrepancy, as well as some other poorly resolved nodes, might be
due to substantial heterogeneity in evolutionary rates among single hemoglobin lineages. Estimation of branch lengths based
on uncorrected amino acid substitutions and on distances corrected for multiple substitutions (PAM distances) revealed that
relative rates for squamate αA and αD chains and crocodilian β chains are at least twice as high as those of the rest of the chains considered. In contrast to
these rate inequalities between reptilian orders, little variation was found within squamates, which allowed determination
of absolute evolutionary rates for this subset of hemoglobins. Rate estimates for hemoglobins of lizards and snakes yielded
1.7 (αA) and 3.3 (β) million years/PAM when calibrated with published divergence time vs. PAM distance correlates for several speciation
events within snakes and for the squamate ↔ sphenodontid split. This suggests that hemoglobin chains of squamate reptiles
evolved ∼3.5 (αA) or ∼1.7 times (β) faster than their mammalian equivalents. These data also were used to obtain a first estimate of some
intrasquamate divergence times.
Received: 15 September 1997 / Accepted: 4 February 1998 相似文献
6.
The thermodynamic parameters for six p53 carboxy-terminus peptide fragments as determined by analytical ultracentrifugal analysis
were compared over the experimental temperature range of 275–310 K to evaluate the Gibbs free energy change as a function
of temperature, ΔG
o
(T), from 0 to 400 K using our general linear third-order fitting function, ΔG
o
(T) = α + βT
2 + γT
3. Data obtained at the typical experimental temperature range are not sufficient to accurately describe the variations observed
in the oligomerization of these p53 fragments. It is necessary to determine a number of thermodynamic parameters, all of which
can be precisely assessed using this general third-order linear fitting function. These are the heat of reaction, innate temperature-invariant
enthalpy, compensatory temperatures and the thermodynamic molecular switch occurring at the thermal set point. This methodology
can be used to distinguish the characteristic structure and stability of p53 carboxy-terminal fragments or other p53 mutants.
It should be used for the thermodynamic characterization of any interacting biological system. 相似文献
7.
S M Kotwal M M Gote M I Khan J M Khire 《Journal of industrial microbiology & biotechnology》1999,23(1):661-667
The thermophilic fungus Humicola sp constitutively produces intracellular α-galactosidase (1.33 U mg−1 protein) within 48 h at 45°C in shaken flasks, when grown in a medium containing 7% wheat bran extract as a carbon source
and 0.5% yeast extract as a nitrogen source. The enzyme has been purified to homogeneity by ultrafiltration, ethanol precipitation,
DEAE cellulose and Sephacryl S-300 chromatography with a 124-fold increase in specific activity and 29.5% recovery. The molecular
weight of the enzyme is 371.5 kDa by gel filtration on Sephacryl S-300 and 87.1 kDa by SDS-polyacrylamide gel electrophoresis.
The enzyme has an optimum temperature of 65°C and an optimum pH of 5.0. Humicola α-galactosidase is a glycoprotein with 8.3% carbohydrate content and is acidic in nature with a pI of 4.0. The K
m
S for p-nitrophenyl-α-D-galactopyranoside, O-nitrophenyl-α-D-galactopyranoside, raffinose and stachyose are 0.279, 0.40, 1.45 and 1.42 mM respectively. The enzyme activity was strongly
inhibited by Ag+ and Hg2+. D-Galactose inhibited α-galactosidase competitively and the inhibition constant (K
i) for galactose was 11 mM.
Received 28 January 1999/ Accepted in revised form 07 April 1999 相似文献
8.
An extracellular raw-starch-digesting α-amylase was isolated from Geobacillus thermodenitrificans HRO10. The culture conditions for the production of α-amylase by G. thermodenitrificans HRO10 was optimized in 1.2–l bioreactor using full 24 and 32 factorial designs. From the optimal reaction conditions, a model (Y = − 594.206 − 0.178T2 − 8.448pH2 + 6.020TpH − 0.005T2pH2) was predicted, which was then used for α-amylase production. In the bioreactor studies, the enzyme yield under optimized
conditions (pH 7.1, 49°C) was 30.20 U/ml, a 51% improvement over the results (19.97 U/ml) obtained when the traditional one-factor-at-a-time
method was employed. This α-amylase does not require extraneous calcium ions for activity, which may be a commercially important
observation. 相似文献
9.
Comparison of the effects of temperature and water activity on growth rate of food spoilage moulds 总被引:1,自引:0,他引:1
Sautour M Soares Mansur C Divies C Bensoussan M Dantigny P 《Journal of industrial microbiology & biotechnology》2002,28(6):311-315
The influence of temperature (T) and water activity (a
w) on the growth rate (μ) of seven moulds (Alternaria alternata, Aspergillus flavus, Cladosporium cladosporioides, Mucor racemosus, Penicillium chrysogenum, Rhizopus oryzae and Trichoderma harzianum) was assessed in suboptimal conditions. Firstly, the dependence of fungal growth on temperature, at a
w 0.99, was modelled through an approach described previously for bacteria. A dimensionless growth rate variable: μ
dimα=μ/μ
optα depended on the following normalised temperature: T
dim=(T−T
min)/(T
opt− T
min) according to a power function: μ
dimα=[T
dim]
α
, where α was an exponent to be estimated. Secondly, the same approach was used to describe the influence of a
w on fungal growth, at the respective optimum temperatures for each mould. Similarly, μ
dimβ=μ/μ
optβ depended on the following normalised water activity: a
wdim=(a
w−a
wmin)/(a
wopt−a
wmin) according to a power function: μ
dimβ=[a
wdim]β. Results show: (i) for each mould, the α-value is significantly less than the β-value, confirming that water activity has a greater influence than temperature on fungal development; (ii) the α-values and the β-values depend on the mould; (iii) the α-value is less than 1 for the mesophilic mould A. flavus, whereas the other moulds are characterised by higher α-values ranging from 1.10 to 1.54; (iv) the mesophilic A. flavus exhibits a low β-value, 1.50, compared to the hydrophilic T. harzianum, β=2.44, while β-values are within the range (1.71–2.37) for the other moulds. Journal of Industrial Microbiology & Biotechnology (2002) 28, 311–315 DOI: 10.1038/sj/jim/7000248
Received 27 June 2001/ Accepted in revised form 04 February 2002 相似文献
10.
Physiological aspects of continuous lactic acid fermentations at high dilution rates 总被引:1,自引:0,他引:1
U. Kulozik 《Applied microbiology and biotechnology》1998,49(5):506-510
The effects of the substrate conditions on the volumetric productivity of Lactobacillus helveticus at different cell densities up to 60 g l−1 in a continuous stirred-tank reactor with microfiltration to retain the biomass were investigated. At low dilution rates,
D, the steady-state volumetric productivity, r
p, gradually increased to a maximum at D = 1.2–1.5 h−1, because of reduced product inhibition. At higher D values, r
p unexpectedly decreased, although the substrate conditions further improved. The maxima of r
p at different cell densities coincided with a critical specific substrate utilization rate beyond which the cell metabolism
seems to be controlled through a catabolic modulator factor, and r
p decreases.
Received: 8 September 1997 / Received last revision: 31 December 1997 / Accepted: 2 January 1998 相似文献
11.
Basharov MA 《European biophysics journal : EBJ》2012,41(1):53-61
In many studies on the protein folding problem it is assumed that the internal rotational barriers about NCα and CαC backbone bonds in unfolded polypeptides are quite small, around 0.7 kcal/mol, of an order comparable to the energy of kT at normal temperature (where k is Boltzmann’s constant and T is the temperature in K) and hence that rotations about these bonds occur almost freely. Here it is highlighted that such
consideration is an unfortunate mistake. Approximate values for the rotational barriers of NCα and CαC bonds are suggested from computations of U(f \phi , ψ) potential energy surface (PES) maps of a number of oligopeptides by a semiempirical method for conformational analysis.
The proposed values are about 16 kcal/mol for NCα bonds and 6 kcal/mol for CαC bonds. The values of the same barriers estimated from some ab initio quantum-mechanical PES maps for several dipeptides
available in literature are also highlighted. 相似文献
12.
N. V. Potekhina G. M. Streshinskaya Yu. I. Kozlova E. B. Kudryashova S. N. Senchenkova A. S. Shashkov L. N. Anan’ina 《Biochemistry. Biokhimii?a》2009,74(12):1368-1374
Cell walls of Bacillus subtilis VKM B-760 and VKM B-764 are characterized by heterogeneous composition of teichoic acids. Polymer I with structure -6)-β-D-Galp-(1→1)-sn-Gro-(3-P-, polymer II with structure -6)-α-D-Glcp-(1→1)-sn-Gro-(3-P-, and a small amount of unsubstituted 1,3-poly(glycerol phosphate) were detected in strain VKM B-760. Strain VKM
B-764 contains an analogous set of teichoic acids, but a characteristic feature of polymer II is the presence of disubstituted
glycerol residue with α-glucopyranose localization in the integral chain at C-1 hydroxyl and β-glucopyranose as a side branch
at C-2 hydroxyl (polymer III): -6)-α-D-Glcp-(1→1)-[β-D-Glcp-(1→2)]-sn-Gro-(3-P-. The structures of polymer I in bacilli and polymer III in Gram-positive bacteria are described for the first time.
Teichoic acids were studied by chemical methods and on the basis of combined analysis of one-dimensional 1H-, 13C-, and 31P-NMR spectra, homonuclear two-dimensional 1H/1H COSY, TOCSY, and ROESY, and heteronuclear two-dimensional 1H/13C gHSQC- and HMQC-TOCSY experiments. Simultaneous presence of several different structure teichoic acids in the bacillus cell
walls as well as chemotaxonomical perspectives of the application of these polymers as species-specific markers for members
of the Bacillus genus is discussed. 相似文献
13.
V. A. Bouryi 《Neurophysiology》1998,30(4-5):301-304
Barium currents through ion channels formed by α1-subunit of L-type Ca2+ channel (I
α1) were recorded from cultured chinese hamster ovary (CHO) cells. The cells were stably transfected with either a cardiac or
a smooth muscle (SM) variant of α1-subunit. TheI
α1 in both cases exhibited similar fast voltage-dependent activation kinetics and slow apparent inactivation kinetics. With
10 mM Ba2+ in the bath solution,I
α1 was activated at potentials more positive than −40 mV, peaked between 0 and +10 mV, and reversed at about +50 mV. In addition
to slow apparent inactivation of inward current, both subunits provided an extremely slow voltage-dependent inactivation at
potentials more positive than −100 mV, with half-maximum inactivation at −43.4 mV for cardiac and −41.4 mV for SM α1-subunits.
The onset of inactivation as well as recovery from this process were within a time range of minutes. The voltage dependence
of steady-state inactivation could be fitted by the sum of two Boltzmann's equations with slope factors of about 12 mV and
5 mV. A less sloped component has its midpoints at −75.6 and −63.7 mV, and a steeper component has its midpoints at −42.8
and −37.7 mV for cardiac and SM α1-subunits, respectively. Relative contribution of the steeper component was higher in both
subunits (0.86 and 0.66 for cardiac and SM subunits, respectively). For comparison, the inactivation curves for 5-sec-long
conditioning prepulses could be fitted by single Boltzmann's distribution with a 20 mV more positive midpoint and a slope
factor of about 13 mV. In contrast to the steady-state inactivation curves, they showed considerable overlap with the steady-state
activation curve. Our results reflect functional consequences of known sequence differences between α1-subunits of the cardiac
and SM L-type Ca2+ channels and could be used in structural modeling of Ca2+ channel gating. In addition, they show that depolarization-induced window current has a transient nature and decays with
the development of extremely slow inactivation. This is the first demonstration that slow inactivation of the L-type Ca2+ channel is an intrinsic property of its α1-subunits. 相似文献
14.
Gerald Rosen 《Bulletin of mathematical biology》1980,42(1):95-106
For precise boundary conditions of biological relevance, it is proved that the steadily propagating plane-wave solution to
the Fisher equation requires the unique (eigenvalue) velocity of advance 2(Df)1/2, whereD is the diffusivity of the mutant species andf is the frequency of selection in favor of the mutant. This rigorous result shows that a so-called “wrong equation”, i.e.
one which differs from Fisher's by a term that is seemingly inconsequential for certain initial conditions, cannot be employed
readily to obtain approximate solutions to Fisher's, for the two equations will often have qualitatively different manifolds
of exact solutions. It is noted that the Fisher equation itself may be inappropriate in certain biological contexts owing
to the manifest instability of the lowerconcentration uniform equilibrium state (UES). Depicting the persistence of a mutantdeficient
spatial pocket, an exact steady-state solution to the Fisher equation is presented. As an alternative and perhaps more faithful
model equation for the propagation of certain species properties through a homogeneous population, we consider a reaction-diffusion
equation that features a cubic-polynomial rate expression in the species concentration, with two stable UES and one intermediate
unstable UES. This equation admits a remarkably simple exact analytical solution to the steadily propagating plane-wave eigenvalue
problem. In the latter solution, the sign of the eigenvelocity is such that the wave propagates to yield the “preferred” stable
UES (namely, the one further removed from the unstable intermediate UES) at all spatial points ast→∞. The cubic-polynomial equation also admits an exact steady-state solution for a mutant-deficient or mutant-isolated spatial
pocket. Finally, the perpetuating growth of a mutant population from an arbitrary localized initial distribution, a mathematical
problem analogous to that for ignition in laminar flame theory, is studied by applying differential inequality analysis, and
rigorous sufficient conditions for extinction are derived here. 相似文献
15.
Small-angle neutron scattering (SANS) on the unilamellar vesicle (ULV) populations (diameter 500 and 1,000 Å) in D2O was used to characterize lipid vesicles from dimyristoylphosphatidylcholine (DMPC) at three phases: gel Lβ′, ripple Pβ′ and liquid Lα. Parameters of vesicle populations and internal structure of the DMPC bilayer were characterized on the basis of the separated form factor (SFF) model. Vesicle shape changes from nearly spherical in the Lα phase to elliptical in the Pβ′ and Lβ′ phases. This is true for vesicles prepared via extrusion through pores with the diameter 500 Å. Parameters of the internal bilayer structure (thickness of the membrane and the hydrophobic core, hydration and the surface area of the lipid molecule) were determined on the basis of the hydrophobic–hydrophilic (HH) approximation of neutron scattering length density across the bilayer ρ(x) and of the step function (SF) approximation of ρ(x). DMPC membrane thickness in the Lα phase (T=30°C) demonstrates a dependence on the membrane curvature for extruded vesicles. Prepared via extrusion through 500 Å diameter pores, vesicle population in the Lα phase has the following characteristics: average value of minor semi-axis 266±2 Å, ellipse eccentricity 1.11±0.02, polydispersity 26%, thickness of the membrane 48.9±0.2 Å and of the hydrophobic core 19.9±0.4 Å, surface area 60.7±0.5 Å2 and number of water molecules 12.8±0.3 per DMPC molecule. Vesicles prepared via extrusion through pores with the diameter 1,000 Å have polydispersity of 48% and membrane thickness of 45.5±0.6 Å in the Lα phase. SF approximation was used to describe the DMPC membrane structure in Lβ′ (T=10°C) and Pβ′ (T=20°C) phases. Extruded DMPC vesicles in D2O have membrane thickness of 49.6±0.5 Å in the Lβ′ phase and 48.3±0.6 Å in the Pβ′ phase. The dependence of the DMPC membrane thickness on temperature was restored from the SANS experiment. 相似文献
16.
Angelo Filosa A. A. Ismail A. M. English 《Journal of biological inorganic chemistry》1999,4(6):717-726
Fourier transform infrared (FTIR) spectroscopy is used to compare the thermally induced conformational changes in horse,
bovine and tuna ferricytochromes c in 50 mM phosphate/0.2 M KCl. Thermal titration in D2O at pD 7.0 of the amide II intensity of the buried peptide NH protons reveals tertiary structural transitions at 54 °C in horse and at 57 °C in bovine c. These transitions, which occur well before loss of secondary structure, are associated with the alkaline isomerization involving
Met80 heme-ligand exchange. In tuna c, the amide-II-monitored alkaline isomerization occurs at 35 °C, followed by a second amide II transition at 50 °C revealing
a hitherto unreported conformational change in this cytochrome. Amide II transitions at 50 °C (tuna) and 54 °C (horse) are
also observed during the thermal titration of the CN–-ligated cytochromes (where CN– displaces the Met80 ligand), but a well-defined 35 °C amide II transition is absent from the titration curve of the CN–adduct of tuna c. The different mechanisms suggested by the FTIR data for the alkaline isomerization of tuna and the mammalian cytochromes c are discussed. After the alkaline isomerization, loss of secondary structure and protein aggregation occur within a 5 °C
range with T
m values at 74 °C (bovine c), 70 °C (horse c) and 65 °C (tuna c), as monitored by changes in the amide I′ bands. The FTIR spectra were also used to compare the secondary structures of the
ferricytochromes c at 25 °C. Curve fitting of the amide I (H2O) and amide I′ (D2O) bands reveals essentially identical secondary structure in horse and bovine c, whereas splitting of the α-helical absorption of tuna c indicates the presence of less-stable helical structures. CN– adduct formation results in no FTIR-detectable changes in the secondary structures of either tuna or horse c, indicating that Met80 ligation does not influence the secondary structural elements in these cytochromes. The data provided
here demonstrate for the first time that the selective thermal titration of the amide II intensity of buried peptide NH protons in D2O is a powerful tool in protein conformational analysis.
Received: 1 April 1999 / Accepted: 24 August 1999 相似文献
17.
By the use of the Immobiline technique at pH ranges 7.0–7.6 and 6.9–7.9, 16 different hemoglobin (Hb) phenotypes were observed
in 61 English Saanen goats. They are explained in this breed by a genetic theory of five β-globin genes (A
4,A
6,A
8,E, andD) and two closely linked α-globin loci (′α and ″α) of which the ″α has a variant allele, provisionally called ″α
X
. Family data together with observed and expected Hb frequencies were in agreement with the genetic theory. Among six Barbary
sheep there were three Hb phenotypes explained by the occurrence of the β-chain allelesB andC
na. 相似文献
18.
V. Prakash 《Journal of biosciences》1985,9(3-4):165-175
The protein α-globulin fromSesamum indicum L. has been characterised for its size and shape using αarious chemical, physico-chemical and hydrodynamic properties. The
protein has an S20,w
0 of 12.8, D20,w °f 4.9 × 10-7 cm2/sec and a partial specific αolume of 0.725 ml/g in the natiαe state. The intrinsic αiscosity of the protein was determined
to be 3 0 ml/g indicating it to be globular in shape. The molecular weight of the protein as determined by αarious approaches
in analytical ultracentrifugation αaries from 2.6–2.74 × 105. The molecular weight from sedimentation equilibrium yields a αalue of 2.74 × 105 in the natiαe state and a αalue of 19000 in the dissociated and denatured state in 6 M guanidine hydrochloride. The eαaluation
of frictional ratios using Stokes radius and results from electron microscopy confirms the protein to be globular in shape.
The protein consists of at least 12–14 subunits. The eαaluation of hydrophobic parameters and energetics of interaction of
subunits indicate that the protein is stabilized predominantly by hydrophobic interactions. 相似文献
19.
A flow cytometry analysis and in vitro enzyme activity study is carried out on the methylotrophic yeast, Hansenula polymorpha, during both (a) batch growth and (b) continuous cultures subjected to single perturbations in either system dilution rate or influent carbon substrate composition. Flow cytometry of yeasts growing diauxically on a glucose: methanol mixture during exponential growth, exhibit DNA and RNA distributions indicative of the S-synthesis-phase of the cell cycle. Cells at the stationary growth stage exhibit DNA and RNA distributions that indicate one portion of the population in the G
0/G1 resting phase and another in the M-mitosis-phase.Yeast cells grown at a steady-state of D=0.2 h1, then shifted to D=0.35 h–1, at a constant influent substrate mixture, are also examined with both flow cytometry and in vitro enzyme assays. Distributions of DNA, RNA, and total protein at either steady state and during the shift between dilution rates did not resemble any observed in batch culture. Flow cytometry indicates significant changes in cell composition within 20 min of the imposed dilution rate shift. In vitro enzyme assays show a response time in decreasing methanol oxidase activity of 2.5–3 h upon a dilution rate shift-up, while hexokinase activity increases to its steady-state level in less than 3 h. Similar cell compositional changes are reported for shifts in influent substrate methanol: glucose ratio at a constant dilution rate of D=0.35 h –1. Results suggest that an unsteady-state regime, oscillating between conditions that promote maximum enzyme activity of either glucose- or methanol-metabolizing enzymes, may allow simultaneous enhanced time-averaged production of both sets of enzymes. 相似文献
20.
Rapid light-response curves (RLC) of variable chlorophyll fluorescence were measured on estuarine benthic microalgae with
the purpose of characterising its response to changes in ambient light, and of investigating the relationship to steady-state
light-response curves (LC). The response of RLCs to changes in ambient light (E, defined as the irradiance level to which a sample is acclimated to prior to the start of the RLC) was characterised by constructing
light-response curves for the RLC parameters α
RLC, the initial slope, ETRm,RLC, the maximum relative electron transport rate, and E
k,RLC, the light-saturation parameter. Measurements were carried out on diatom-dominated suspensions of benthic microalgae and
RLC and LC parameters were compared for a wide range of ambient light conditions, time of day, season and sample taxonomic
composition. The photoresponse of RLC parameters was typically bi-phasic, consisting of an initial increase of all parameters
under low ambient light (E < 21–181 μmol m−2 s−1), and of a phase during which α
RLC decreased significantly with E, and the increase of ETRm,RLC and E
k,RLC was attenuated. The relationship between RLC and LC parameters was dependent on ambient irradiance, with significant correlations
being found between α
RLC and α, and between ETRm,RLC and ETRm, for samples acclimated to low and to high ambient irradiances, respectively. The decline of α
RLC under high light (Δα
RLC) was strongly correlated (P < 0.001 in all cases) with the level of non-photochemical quenching (NPQ) measured before each RLC. These results indicate
the possibility of using RLCs to characterise the steady-state photoacclimation status of a sample, by estimating the LC parameter
E
k, and to trace short-term changes in NPQ levels without dark incubation. 相似文献