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At least 793 records · Page 44Linked to original sources
PubMed2005

Equilibrium and kinetic modelling of Cd(II) biosorption by algae Gelidium and agar extraction algal waste.

In this study an industrial algal waste from agar extraction has been used as an inexpensive and effective biosorbent for cadmium (II) removal from aqueous solutions. This biosorbent was compared with the algae Gelidium itself, which is the raw material for agar extraction. Equilibrium data follow both Langmuir and Redlich-Peterson models. The parameters of Langmuir equilibrium model are q(max)=18.0 mgg(-1), b=0.19 mgl(-1) and q(max)=9.7 mgg(-1), b=0.16 mgl(-1), respectively for Gelidium and the algal waste. Kinetic experiments were conducted at initial Cd(II) concentrations in the range 6-91 mgl(-1). Data were fitted to pseudo-first- and second-order Lagergren models. For an initial Cd(II) concentration of 91 mgl(-1) the parameters of the pseudo-first-order Lagergren model are k(1,ads)=0.17 and 0.87 min(-1); q(eq)=16.3 and 8.7 mgg(-1), respectively, for Gelidium and algal waste. Kinetic constants vary with the initial metal concentration. The adsorptive behaviour of biosorbent particles was modelled using a batch reactor mass transfer kinetic model. The model successfully predicts Cd(II) concentration profiles and provides significant insights on the biosorbents performance. The homogeneous diffusivity, D(h), is in the range 0.5-2.2 x10(-8) and 2.1-10.4 x10(-8)cm(2)s(-1), respectively, for Gelidium and algal waste.

Adsorption↗
PubMedDate not supplied

[Kinetic model of the action of oligomeric enzymes using phosphofructokinase as an example. II. General formulation of the hierarchical model].

A general approach is suggested to describe the steady-state kinetics of the oligomeric enzymes on the base of the generalized statistical Ising model. Detailed analysis is given for the case of a oligomeric enzyme with a hierarchical supramolecular organization. A protomer of this enzyme composed of several equivalent subunits represents the quarternary level of structure. In their turn the finite or infinite number of protomers is associated into a oligomer thus creating a new "quinternary" level of the enzyme organization. The model accounts for the ligand-induced homotrophic cooperative interactions: firstly, between the neighbouring protomers and secondly, between the subunits of the same protomer. The influence of protomer conformation on the subunit state and the cooperativity induction caused by two-ligand binding are also taken into consideration. Monod-Wyman-Changeux's and Koshland's models are shown to be special limit cases of the suggested general theory.

Enzymes↗
PubMed2005

Gompertz kinetics model of fast chemical neurotransmission currents.

At a chemical synapse, transmitter molecules ejected from presynaptic terminal(s) bind reversibly with postsynaptic receptors and trigger an increase in channel conductance to specific ions. This paper describes a simple but accurate predictive model for the time course of the synaptic conductance transient, based on Gompertz kinetics. In the model, two simple exponential decay terms set the rates of development and decline of transmitter action. The first, r, triggering conductance activation, is surrogate for the decelerated rate of growth of conductance, G. The second, r', responsible for Y, deactivation of the conductance, is surrogate for the decelerated rate of decline of transmitter action. Therefore, the differential equation for the net conductance change, g, triggered by the transmitter is dg/dt=g(r-r'). The solution of that equation yields the product of G(t), representing activation, and Y(t), which defines the proportional decline (deactivation) of the current. The model fits, over their full-time course, published records of macroscopic ionic current associated with fast chemical transmission. The Gompertz model is a convenient and accurate method for routine analysis and comparison of records of synaptic current and putative transmitter time course. A Gompertz fit requiring only three independent rate constants plus initial current appears indistinguishable from a Markov fit using seven rate constants.

Algorithms↗
PubMed1990

Characterization of Cl- transport in vacuolar membrane vesicles using a Cl(-)-sensitive fluorescent probe: reaction kinetic models for voltage- and concentration-dependence of Cl- flux.

The effects of Cl- concentration and membrane potential (delta psi) on Cl- influx in isolated vesicles of vacuolar membrane (tonoplast) from red beet (Beta vulgaris L.) storage tissue have been characterized using the Cl(-)-sensitive fluorescent probe, 6-methoxy-1-(3-sulfonatopropyl)quinolinium (SPQ). The initial rate of Cl- transport into the vesicles was enhanced both by the imposition of a positive delta psi and by increases in extravesicular Cl- concentration. The kinetic mechanism underlying these responses was investigated by examining the accuracy with which the data could be described by several transport models. A model based on constant field theory yielded a poor description of the data, but satisfactory fits were generated by pseudo-two-state reaction kinetic models based on classical carrier schemes. Fits were equally good when it was assumed that charge translocation accompanied Cl- entry, or when charge was carried by the unloaded transport system, as long as only a single charge is translocated in each carrier cycle. Expansion of the models to three states enabled description of the Cl- concentration dependence of transport by changes in a single, voltage insensitive rate constant which is tentatively identified with Cl- binding at the external surface of the membrane. The derived value of the dissociation constant between Cl- and the transport system is estimated at between 30 and 52 mM.

Biological Transport↗
PubMed1978

Synergism of substrate binding with enzymes, as observed by equilibrium isotope exchange kinetics: model patterns.

The effects of synergistic binding among co-substrates on the kinetic saturation patterns for equilibrium isotope exchange rates have been derived. Models considered for a two reactant-two product system were those in which one substrate may either decrease the dissociation rate or increase the association rate for its co-substrate. These studies help define limits of this versatile kinetic probe technique. The most sensitive indicator for synergistic substrate binding appears to be the different apparent Km values for different exchange rate curves; e.g,, A in equilibrium P and B in equilibrium Q. Model patterns for mutual synergism effects are also presented; the slowest exchange reactions were found the most sensitive to preferred orders of substrate binding. Conditions that can prevent detection of preferred or even compulsory order binding are discussed.

Enzymes↗
PubMed2000

A program for the optimization of cyclosporine therapy using population kinetics modeling.

Cyclosporine is one of the most widely used immunosuppressive agents in organ transplantation. Due to large inter- and intra-individual variations, its behavior in the specific patient is still difficult to predict. Dosage optimization is thus mainly performed on a trial-and-error basis. In this paper, we present a new program based on the population kinetics approach, which was designed to help physicians in the difficult task of adjusting patient specific cyclosporine dosing regimens. Dose optimization is carried out by model simulation, using a two-compartment mathematical model of cyclosporine kinetics to predict the drug behavior in the patient. Two of the model parameters are assumed from the literature, the other two are estimated from the patient data through a Bayesian estimation procedure. Previous information needed by the Bayesian algorithm is derived by a population analysis, performed beforehand and based on a nonlinear mixed effect model. A user-friendly graphical interface written in Delphi under Windows makes the program easily accessible to physicians. A preliminary retrospective validation of the program, performed on data from 18 renal transplanted patients, yielded very satisfactory results.

Algorithms↗
PubMedDate not supplied

Mobility of sulfate in forest soils: kinetic modeling.

Understanding sulfate transport and retention dynamics in forest soils is a prerequisite in predicting SO4 concentration in the soil solution and in lake and stream waters. In this study forest soil samples from the Gårdsjön catchments, Sweden, were used to study SO4 transport in soil columns from the upper three soil horizons (E, Bs, and BC). The columns were leached using a sequential leaching technique. The input solutions were CaSO4 equilibrated with forest floor material. Leaching behavior of SO4 and concentration in the effluent were measured from columns from individual horizons. Sulfate was always retained in the Bs and BC horizons, while the pattern for the E horizon varied. Attempts were also made to model SO4 breakthrough results based on miscible displacement approaches and solute convection-dispersion equation (CDE) in porous media. Several retention mechanisms were incorporated into the CDE to account for possible reversible and irreversible SO4 reactions in individual soil layers. Our modeling efforts were inadequate in describing the mobility of SO4 in the top (E) horizon. Moreover, a linear equilibrium approach was generally inadequate for describing SO4 sorption during transport in the Bs and BC horizons. In contrast, we found that the model provided good descriptions of all breakthrough results when SO4 reactivity was accounted for based on nonlinear equilibrium or first-order kinetic processes. Moreover, based on model parameter estimates, the reactivity or retention of SO4 during transport is concentration dependent. We conclude that sulfate retention during transport in this forest soil is most likely controlled by kinetic reactivity of SO4 of the reversible and irreversible mechanisms.

Adsorption↗
PubMed2005

Kinetic modeling of acetophenone reduction catalyzed by alcohol dehydrogenase from Thermoanaerobacter sp.

NADPH-dependent alcohol dehydrogenase (ADH) from Thermoanaerobacter sp. was kinetically characterized using reduction of acetophenone as a model. To achieve 98% conversion of acetophenone, cofactor regeneration by oxidation of 2-propanol with the same enzyme was used. The enzyme was stable in the batch reactor. It was enantioselective towards (S)-1-phenylethanol (ee>99.5%). Due to its high deactivation in continuously operated stirred tank reactor (kd=0.0141 min-1) there was no way to keep high conversion of acetophenone at 98%. The deactivation occurred in the repetitive batch as well. A mathematical model for the acetophenone reduction with cofactor regeneration describing the behaviour in a batch, repetitive-batch and continuously stirred tank reactor was developed.

2-Propanol↗
PubMed2000

Body weight-specific zinc compartmental masses in girls significantly exceed those reported in adults: a stable isotope study using a kinetic model.

Maintaining optimal zinc status is important for normal growth and development in children, but minimal data are available regarding zinc metabolism in this age group. Our objectives were to utilize stable isotope-based compartmental modeling techniques to investigate zinc metabolism in healthy children; to expand a current stable isotope-based model to include red blood cell data; and to compare kinetic parameters in children with those previously reported in adults. Seven healthy girls, age 9.94 +/- 0.79 y, received 1.1 mg of a (67)zinc-enriched tracer orally and 0.5 mg of a (70)zinc-enriched tracer intravenously. Blood, urine and fecal samples were collected for 6 d. Stable isotope enrichments were measured by thermal ionization magnetic sector mass spectrometry. A six-compartment model based on a model previously reported in adults was used; the model excluded red blood cell data. Body weight-corrected masses of the body zinc compartments derived using this model were significantly greater in children than those reported in adults. Modification of the model to include a red blood cell compartment increased the total identifiable zinc mass of the nongastrointestinal compartments by approximately 2.5%. We conclude that compartmental modeling can be used to describe zinc kinetics in children, and that the body weight-corrected zinc pool masses are significantly greater in children than in adults.

Adult↗
PubMed2005

Voltage-clamp predictions by gompertz kinetics model relating squid-axon Na+-gating and ionic currents.

Gompertz kinetics is a simple, realistic, accurate, and computationally parsimonious alternative for prediction of macroscopic changes in Na+ conductance during voltage clamp. Conductance delay and time course depend on initial amplitudes and decay rates of surrogates for the macroscopic gating currents. The model is tested by the fit to published data of other authors. The proposed physical basis for the model is that membrane potential perturbation triggers motion of charged "gating" components of the axon membrane at rapid (activating) and at slow (inactivating) rates. The resulting distortion increases and more slowly diminishes the probability that conduction channels will be open.

Animals↗
PubMed2002

Analysis of the glow curves obtained from LiF:Mg,Cu,Na,Si TL material using the general order kinetics model.

Three-dimensional thermoluminescence (TL) spectra based on temperature, wavelength and intensity for newly developed LiF:Mg,Cu,Na,Si TL material at the Korea Atomic Energy Research Institute (KAERI) were measured and analysed. The glow curves were obtained by integration of luminescence intensity over all wavelengths at each temperature, and various trapping parameters related to the traps were determined by analysing these curves. A computerised glow curve deconvolution (CGCD) method which was based on the general order kinetics (GOK) model was used for the glow curve analysis. The glow curves of LiF:Mg,Cu,Na,Si TL material were deconvoluted to six isolated glow curves which have peak temperatures at 333, 374, 426, 466, 483 and 516 K. The main glow peak of peak temperature at 466 K had activation energy of 2.06 eV and a kinetic order of 1.05. This TL material was also found to have three recombination centres, 1.80 eV, 2.88 eV and 3.27 eV by analysis of the TL spectra.

Copper↗
PubMed2004

Probabilistic kinetic model of slow oxidation of low-density lipoprotein: I. Theory.

The microscopic probabilistic model has been introduced to explain the kinetics of very slow oxidation of low-density lipoprotein (LDL) from human plasma. The LDL oxidation, carried out in very unfavorable conditions, is assumed to be initiated by the traces of the transition-metal ions associated with the lipoprotein. The substrates for the metal-ion attack are alpha-tocopherol and the pre-formed lipid hydroperoxide. The theory assumes oscillation of the metal ions and alpha-tocopherol from the oxidized to the reduced states. In this model alpha-tocopherol acts as a pro-oxidant. The entire oxidation process consists of rare bursts of events in individual LDL particles. The reactions within the particles are treated in terms of probabilities of individual active species to participate in a specified reaction. The circular flow of the radical reactions could be visualized as circular flow of microscopic probabilities. The empirical, macroscopic quantities are quantitatively related with the microscopic probabilities, determined by a set of five adjustable parameters. The differential equations describing the initial radical generation rate and the rates of change of concentration of oxygen, hydroperoxide, co-antioxidant and trapped radicals in an LDL system are numerically solved in a finite difference approach.

Free Radicals↗