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At least 505 records · Page 28Linked to original sources

PubMed2005

[Initial 111In-platelet kinetics: indicator of platelet sequestration/destruction site or quality control of platelet labelling?].

Taking into consideration the existing disagreement in the literature, the aim of this paper was to estimate the value of the initial kinetics of autologous platelets labelled with 111In-oxinate, performed during the first 20 minutes after their intravenous injection. Two hypothesis were tested: 1. Initial 111In-platelet kinetics indicates the platelet sequestration site (in patients with normal mean platelet life span)/destruction site (in patients with shortened mean platelet life span), 2. Initial 111In-platelet kinetics indicates the quality of platelet separation and labelling procedure. We performed initial labelled platelet kinetics in thrombocytopenic patients (in order to test the first hypothesis) as well as in control (healthy) subjects (in order to test the second hypothesis). Thirty-nine persons were investigated: 33 with thrombocytopenia: 25 with shortened mean platelet life span, caused by chronic im mune thrombocytopenic purpura (ITP), eight with normal platelet life span and thrombocytopenia caused by myelodysplastic syndrome (MDS), six healthy, control subjects (C). In all 39 persons platelet blood count on the day of platelet labelling was determined, autologous platelet labelling with 111In-oxinate was performed, general and differential yields of platelet labelling (GYL and DYL), as well as mean labelled platelets life span were determined. Besides that, initial labelled platelets kinetics was performed with initial 111In-platelets accumulation in the liver (IPAL) calculation, as well as the late platelet kinetics for platelet sequestration index and platelet sequestration/destruction site determination. We obtained two types of initial labelled platelets kinetics (not only in the patients with shortened platelet life span, but also in the subjects with normal labelled platelets life span), which differed in the IPAL value and in the ratio of the liver and the heart radioactivity: IPAL < 20% and IPAL>20%. We found statistically significant difference in GYL and DYL between the two groups: IPAL<20% and IPAL > 20%. Both yields were higher in IPAL<20% group. There was no significant difference between the two IPAL groups in the platelet blood count, labelled platelet life span, sequestration index and sequestration site. No correlation could be found between IPAL on one side and platelet blood count, sequestration index, and sequestration site on another. We concluded that initial labelled platelet kinetics could not indicate the platelet sequestration/destructon site (which is accomplished by the late labelled platelets kinetics), but nevertheless, it is very sensitive and useful method of platelet separation and labelling quality control. While in vitro quality control parameters (GYL and DYL) indicate the quality of only one part of this procedure, initial labelled platelet kinetics reflects discrete platelet function disturbance that might happen from the moment of blood sample collection till the labelled platelets intravenous injection.

Adult↗

PubMed1999

Incorporating qualitative knowledge in enzyme kinetic models using fuzzy logic.

Modeling of metabolic pathway dynamics requires detailed kinetic equations at the enzyme level. In particular, the kinetic equations must account for metabolite effectors that contribute significantly to the pathway regulation in vivo. Unfortunately, most kinetic rate laws available in the literature do not consider all the effectors simultaneously, and much kinetic information exists in a qualitative or semiquantitative form. In this article, we present a strategy to incorporate such information into the kinetic equation. This strategy uses fuzzy logic-based factors to modify algebraic rate laws that account for partial kinetic characteristics. The parameters introduced by the fuzzy factors are then optimized by use of a hybrid of simplex and genetic algorithms. The resulting model provides a flexible form that can simulate various kinetic behaviors. Such kinetic models are suitable for pathway modeling without complete enzyme mechanisms. Three enzymes in Escherichia coli central metabolism are used as examples: phosphoenolpyruvate carboxylase; phosphoenolpyruvate carboxykinase; and pyruvate kinase I. Results show that, with fuzzy logic-augmented models, the kinetic data can be much better described. In particular, complex behavior, such as allosteric inhibition, can be captured using fuzzy rules. The resulting models, even though they do not provide additional physical meaning in enzyme mechanisms, allow the model to incorporate semiquantitative information in metabolic pathway models.

Bacterial Proteins↗

PubMed2007

Kinetics of biodegradation of binary and ternary mixtures of PAHs.

The kinetics of biodegradation of mixtures of polycyclic aromatic hydrocarbons (PAHs) by Sphingomonas paucimobilis strain EPA505 were investigated. The investigation focused on three- and four-ring PAHs, specifically 2-methylphenanthrene, fluoranthene, and pyrene. Uptake rates in aerobic batch suspended cultivations were measured for the individual PAHs and their binary and ternary mixtures. It was observed that kinetics were influenced by the mixture composition and the kinetic properties of the components. A material balance equation containing the Monod model was numerically fitted to uptake data to determine extant kinetic parameters for the individual PAHs. Similarly, equations containing kinetic interaction models derived from enzyme kinetics were fitted to the uptake data obtained from experiments with binary and ternary mixtures. The investigation considered the following interaction types: no-interaction (Monod), pure competitive interaction, noncompetitive or mixed-type interaction, uncompetitive inhibition, and nonspecific interaction based on pure competition (SKIP). Model fit was evaluated based on probabilistic and statistical criteria and inferences were reached about underlying interaction mechanisms based on model fit. Mixture kinetics were most adequately simulated by the pure competitive interaction model with mutual substrate exclusivity. This model is fully predictive, relying only on parameters determined in the sole-PAH experiments. It was shown that for low percent inhibition values and with limited data, pure competitive interaction kinetics may not be evident, resembling no-interaction kinetics. This study is a reasonable starting point for understanding and modeling biodegradation of complex PAH mixtures in engineered and natural systems.

Biodegradation, Environmental↗

PubMed2004

Squeezed exponential kinetics to describe a nonglassy downhill folding as observed in a lattice protein model.

We previously studied the so-called strange kinetics in the two-dimensional lattice HP model. To further study the strange kinetics, folding processes of a 27-mer cubic lattice protein model with Gō potential were investigated by simulating how the bundle of folding trajectories, consisting of a number of independent Monte Carlo simulations, evolves as the folding reaction proceeds, covering a wide range of temperature. Three realms of folding kinetics were observed depending on temperature. Although at temperatures where folding was two-state-like, the kinetics was conventional single exponential, we found that the time course data were well represented by a squeezed (or "shrunken") exponential function, exp [-(t/tau)beta] with beta > 1, at temperatures lower than the folding temperature, where folding was fastest and of a nonglassy downhill type. The squeezed exponential kinetics was found to pertain to the subdiffusion on the nonglassy downhill free energy surface and presents a marked contrast both to the single exponential kinetics and to the stretched exponential kinetics that was observed at lower temperatures where folding was also downhill but topological frustration came into effect. The observed temperature dependence of the folding kinetics suggests that some small single-domain proteins may follow the squeezed exponential kinetics at about the room temperature.

Kinetics↗

PubMed1983

Elimination kinetics of plasma exchange.

Interest in the therapeutic use of plasma exchange for various diseases is growing. The two different effects of plasma exchange are elimination and activation. The kinetics are linear for elimination by plasma exchange, but not for activation. Plasma exchange is performed intermittently and can be described by intermittent kinetics. According to intermittent kinetics, plasma exchange removes 50% to 75% of a substance in plasma within 1-2 h, corresponding to an elimination half-life of 30-40 min. Hybrid kinetics, a mixture of actually intermittent but theoretically continuous elimination by plasma exchange, can however also be applied. Hybrid kinetics are more convenient and more reliable than intermittent kinetics. This is because hybrid kinetics are based solely on the concentrations before each plasma exchange; hybrid kinetics also reflect removal from the entire body and not just from the plasma compartment. According to hybrid kinetics, the amount of a substance in the body removed within 3-4 days is 50% of the difference between the initial and the final plasma concentration, depending on the intensity of plasma exchange. The intensity may well contribute at least in part to the beneficial effect of plasma exchange in various diseases.

Antibodies↗

PubMed1997

Burst kinetics of co-expressed Kir6.2/SUR1 clones: comparison of recombinant with native ATP-sensitive K+ channel behavior.

Co-expression of clones encoding Kir6.2, a K+ inward rectifier, and SUR1, a sulfonylurea receptor, reconstitutes elementary features of ATP-sensitive K+ (KATP) channels. However, the precise kinetic properties of Kir6.2/SUR1 clones remain unknown. Herein, intraburst kinetics of Kir6.2/SUR1 channel activity, heterologously co-expressed in COS cells, displayed mean closed times from 0.7 +/- 0.1 to 0.4 +/- 0.03 msec, and from 0.4 +/- 0.1 to 2.0 +/- 0.2 msec, and mean open times from 1.9 +/- 0.4 to 4.5 +/- 0.8 msec, and from 12.1 +/- 2.4 to 5.0 +/- 0.2 msec between -100 and -20 mV, and +20 to +80 mV, respectively. Burst duration for Kir6.2/SUR1 activity was 17. 9 +/- 1.8 msec with 5.6 +/- 1.5 closings per burst. Burst kinetics of the Kir6.2/SUR1 activity could be fitted by a four-state kinetic model defining transitions between one open and three closed states with forward and backward rate constants of 1905 +/- 77 and 322 +/- 27 sec-1 for intraburst, 61.8 +/- 6.6 and 23.9 +/- 5.8 sec-1 for interburst, 12.4 +/- 6.0 and 13.6 +/- 2.9 sec-1 for intercluster events, respectively. Intraburst kinetic properties of Kir6.2/SUR1 clones were essentially indistinguishable from pancreatic or cardiac KATP channel phenotypes, indicating that intraburst kinetics per se were insufficient to classify recombinant Kir6.2/SUR1 amongst native KATP channels. Yet, burst kinetic behavior of Kir6.2/SUR1 although similar to pancreatic, was different from that of cardiac KATP channels. Thus, expression of Kir6.2/SUR1 proteins away from the pancreatic micro-environment, confers the burst kinetic identity of pancreatic, but not cardiac KATP channels. This study reports the kinetic properties of Kir6.2/SUR1 clones which could serve in the further characterization of novel KATP channel clones.

ATP-Binding Cassette Transporters↗

PubMed2005

Kinetics of proton diffusion in the regimes of fast and slow exchange between the membrane surface and the bulk solution.

The phenomenological model developed in our recent publications [9,10] is used to investigate the kinetics of proton diffusion from a source to a detector on the membrane surface. In most cases the observed kinetics shows a single diffusional maximum with the exponential ascending front and the power-law descending tail. The kinetics depends on the distance between the source and the detector. If the detector is located inside the proton collecting antenna, the kinetics corresponds to the surface diffusion at the times near the maximum and shortly thereafter, and it turns into the bulk diffusion kinetics at longer times, after the equilibrium is established between the membrane surface and the bulk solution. If the detector is located outside the antenna, the kinetics corresponds to the bulk diffusion at all times where the signal is nonvanishing. What is seen at locations near the antenna radius depends on the exchange regime. In the regime of fast exchange between the surface and the bulk as compared to the bulk diffusion, the kinetics shows a single peak whose location is intermediate between the peaks for the surface and bulk diffusion. In the regime of slow exchange there are two maxima corresponding to the surface and bulk diffusion. In buffered solutions the antenna radius decreases with increasing buffer concentration, which changes the kinetics from the surface to bulk diffusion. The theory is applied to interprete recent experiments on a phospholipid membrane [25]. It is found that (i) the fast exchange regime is operating since only a single maximum is observed; (ii) the shift of the maximum toward longer times with increasing buffer concentration is a manifestation of the transition from the surface to bulk diffusion kinetics.

Algorithms↗

PubMed1988

Sigmoid kinetics of human erythrocyte glucose-6-phosphate dehydrogenase.

Several disagreements and inconsistencies have appeared regarding whether human erythrocyte glucose-6-phosphate dehydrogenase exhibits sigmoid or classical kinetics with respect to NADP+ binding. The latest report is that the purified enzyme exhibits classical kinetics while the intracellular enzyme exhibits sigmoid kinetics (H. N. Kirkman, and G. F. Gaetani (1986) J. Biol. Chem. 261, 4033-4038). The various investigations were carried out at fixed pH, ionic strength, and temperature. The steady-state kinetics of crude and purified erythrocyte glucose-6-phosphate dehydrogenase are reported here at various temperatures, ionic strengths, and pH values and as a function of glucose 6-phosphate concentration. Sigmoid kinetics were observed for both purified and crude enzyme samples at high pH, temperature, ionic strength, and concentration of glucose 6-phosphate with Hill coefficients varying between 1.40 and 1.90. In contrast, at low pH, temperature, and ionic strength, the crude enzyme samples exhibit sigmoid kinetics while the purified samples exhibit classical kinetics despite the high concentration of glucose 6-phosphate. High concentrations of glucose 6-phosphate and factors favoring the enzyme in the dimeric form are necessary conditions for the observation of sigmoid kinetics in human erythrocyte glucose-6-phosphate dehydrogenase. These factors are high pH, ionic strength, and temperature. The observed sigmoid kinetics in this enzyme is explained as arising from tetramer-dimer transitions.

Erythrocytes↗

PubMed1998

An algorithmic method for determining the kinetic system of receptor-channel complexes.

The mathematical study of receptor-channel kinetics involving numerous sites and conformations of the channel calls for specific analytic methods generally based on stochastic formulation in terms of Markov processes. These methods allow the determination of the number of states from the experimental data. When the number of states is known, it is necessary to try numerous kinetic diagrams to find the best one. The construction of the kinetic diagram and the corresponding kinetic system are based on physiological hypotheses. When the number of states is large, the kinetic schema becomes difficult to establish. We present a method that uses an algorithmic scheme to deduce a kinetic system directly from physiological hypotheses. This method takes into account any number of ligands and sites. The set of all the states given by the combination of site occupation and channel conformations is reduced by using two types of hypothesis: (1) molecular constraints that specify the transitions physically possible between states and (2) kinetic considerations related to the assumed physiology of the system, which gives the conditions necessary for a transition between two states. These hypotheses are expressed in terms of rules operating on the initial states of transitions. The expression of rules does not ensure their coherence (i.e., the fact that each kinetic transition is defined by one and only one rule). A mathematical condition has been found that ensures the coherence of rules. When coherence has been established, the corresponding dynamic system can be automatically generated. Because the rules are established in a systematic way and their coherence can be mathematically established, the computer implementation of this method makes it easy to test various kinetic hypotheses for problems where the number of states is large.

Algorithms↗

PubMed2000

A geometric approach to determine adsorption and desorption kinetic constants.

A geometric method based on Langmuir kinetics has been derived to determine adsorption and desorption kinetic constants. In the conventional procedure, either the adsorption kinetic constant (k(a)c) or desorption kinetic constant (k(d)c) is found from kinetic experiments and the other is calculated by their correlation with the equilibrium constant, i.e, k(d)c = Kcon/k(a)c, where Kcon has been known from equilibrium studies. The determined constants (Kcon, k(a)c, k(d)c), if based only on the conventional procedure, may not be accurate due to their mathematical dependence. Therefore, the objectives of this study are applying a geometric approach to directly determine Langmuir kinetic constants and describe adsorption behavior. In this approach, both adsorption kinetic constant (k(a)g) and desorption kinetic constant (k(d)g) are obtained only from data of kinetic experiments, and a geometric equilibrium constant (Kgeo) is calculated by Kgeo = k(a)g/k(d)g. The deviation between Kgeo and Kcon can prove the accuracy of k(a)g and k(d)g which were determined by this method. This approach was applicable to selenate, selenite and Mg2+ adsorption onto SiO2 regardless of whether the adsorbate formed inner- or outer-sphere complexes. However, this method showed some deviation between Kcon and Kgeo for Mn2+ adsorption because of the formation of surface Mn(II)-hydroxide clusters, which was inconsistent with the basic assumption of this method of monolayer adsorption.

Adsorption↗

PubMed1987

Catalytic and allosteric mechanism of AMP nucleosidase from primary, beta-secondary, and multiple heavy atom kinetic isotope effects.

Adenosine 5'-phosphate was synthesized with specific heavy atom substitutions to permit measurement of V/K kinetic isotope effects for the N-glycohydrolase activity of the allosteric AMP nucleosidase and the acid-catalyzed solvolysis of these compounds. The effects of allosteric activation on the kinetic isotope effects together with the kinetic mechanism of AMP nucleosidase [DeWolf, W. E., Jr., Emig, F. A., & Schramm, V. L. (1986) Biochemistry 25, 4132-4140] indicate that the kinetic isotope effects are fully expressed. Comparison of individual primary and secondary kinetic isotope effects with combined isotope effects and the isotope effect of the reverse reaction indicated that kinetic isotope effects in AMP nucleosidase arise from a single step in the reaction mechanism. Under these conditions, kinetic isotope effects can be used to interpret transition-state structure for AMP nucleosidase. Changes in kinetic isotope effects occurred as a function of allosteric activator, demonstrating that allosteric activation alters transition-state structure for AMP nucleosidase. Kinetic isotope effects, expressed as [V/K(normal isotope]/[V/K(heavy isotope)], were observed with [2'-2H]AMP (1.061 +/- 0.002), [9-15N]AMP (1.030 +/- 0.003), [1'-2H]AMP (1.045 +/- 0.002), and [1'-14C]AMP (1.035 +/- 0.002) when hydrolyzed by AMP nucleosidase in the absence of MgATP. Addition of MgATP altered the [2'-2H]AMP effect (1.043 +/- 0.002) and the [1'-2H]AMP effect (1.030 +/- 0.003) and caused a smaller decrease of the 14C and 15N effects. Multiple heavy atom substitutions into AMP caused an increase in observed isotope effects to 1.084 +/- 0.004 for [1'-2H,1'-14C]AMP and to 1.058 +/- 0.002 for [9-15N,1'-14C]AMP with the enzyme in the absence of ATP.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Monophosphate↗

PubMed2002

Structure-based kinetic modeling of excited-state transfer and trapping in histidine-tagged photosystem II core complexes from synechocystis.

Chlorophyll fluorescence decay kinetics in photosynthesis are dependent on processes of excitation energy transfer, charge separation, and electron transfer in photosystem II (PSII). The interpretation of fluorescence decay kinetics and their accurate simulation by an appropriate kinetic model is highly dependent upon assumptions made concerning the homogeneity and activity of PSII preparations. While relatively simple kinetic models assuming sample heterogeneity have been used to model fluorescence decay in oxygen-evolving PSII core complexes, more complex models have been applied to the electron transport impaired but more highly purified D1-D2-cyt b(559) preparations. To gain more insight into the excited-state dynamics of PSII and to characterize the origins of multicomponent fluorescence decay, we modeled the emission kinetics of purified highly active His-tagged PSII core complexes with structure-based kinetic models. The fluorescence decay kinetics of PSII complexes contained a minimum of three exponential decay components at F(0) and four components at F(m). These kinetics were not described well with the single radical pair energy level model, and the introduction of either static disorder or a dynamic relaxation of the radical pair energy level was required to simulate the fluorescence decay adequately. An unreasonably low yield of charge stabilization and wide distribution of energy levels was required for the static disorder model, and we found the assumption of dynamic relaxation of the primary radical pair to be more suitable. Comparison modeling of the fluorescence decay kinetics from PSII core complexes and D1-D2-cyt b(559) reaction centers indicated that the rates of charge separation and relaxation of the radical pair are likely altered in isolated reaction centers.

Cyanobacteria↗

PubMed1997

Kinetic evidence for folding and unfolding intermediates in staphylococcal nuclease.

The complex kinetic behavior commonly observed in protein folding studies suggests that a heterogeneous population of molecules exists in solution and that a number of discrete steps are involved in the conversion of unfolded molecules to the fully native form. A central issue in protein folding is whether any of these kinetic events represent conformational steps important for efficient folding rather than side reactions caused by slow steps such as proline isomerization or misfolding of the polypeptide chain. In order to address this question, we used stopped-flow fluorescence techniques to characterize the kinetic mechanism of folding and unfolding for a Pro- variant of SNase in which all six proline residues were replaced by glycines or alanines. Compared to the wild-type protein, which exhibits a series of proline-dependent slow folding phases, the folding kinetics of Pro- SNase were much simpler, which made quantitative kinetic analysis possible. Despite the absence of prolines or other complicating factors, the folding kinetics still contain several phases and exhibit a complex denaturant dependence. The GuHCl dependence of the major observable folding phase and a distinct lag in the appearance of the native state provide clear evidence for an early folding intermediate. The fluorescence of Trp140 in the alpha-helical domain is insensitive to the formation of this early intermediate, which is consistent with a partially folded state with a stable beta-domain and a largely disordered alpha-helical region. A second intermediate is required to model the kinetics of unfolding for the Pro- variant, which shows evidence for a denaturant-induced change in the rate-limiting unfolding step. With the inclusion of these two intermediates, we are able to completely model the major phase(s) in both folding and unfolding across a wide range of denaturant concentrations using a sequential four-state folding mechanism. In order to model the minor slow phase observed for the Pro- mutant, a six-state scheme containing a parallel pathway originating from a distinct unfolded state was required. The properties of this alternate unfolded conformation are consistent with those expected due to the presence of a non-prolyl cis peptide bond. To test the kinetic model, we used simulations based on the six-state scheme and were able to completely reproduce the folding kinetics for Pro- SNase across a range of denaturant concentrations.

Enzyme Stability↗

PubMedDate not supplied

Kinetic chain lengths in highly cross-linked networks formed by the photoinitiated polymerization of divinyl monomers: a gel permeation chromatography investigation.

Highly cross-linked networks formed by the photoinitiated polymerization of multifunctional monomers are finding application in the field of biomaterials because of their chemical versatility, reaction control, and ability to polymerize under physiological conditions. Typically, degradation is introduced into these networks via the cross-links and leads to the release of nondegradable but water-soluble kinetic chains formed during the chain polymerization process. In this study, gel permeation chromatography (GPC) was used to characterize kinetic chain length distributions in highly cross-linked systems that are being developed for orthopedic applications. By polymerizing divinyl monomers to various conversions and subsequently degrading them, we investigated the aspects of network structural evolution related to kinetic chain formation. In general, the average kinetic chain length increased with conversion until the onset of autodeceleration, when the kinetic chains decreased in length as the propagation reaction became diffusion-controlled. The distribution of kinetic chains also changed when different initiation conditions (i.e., initiator concentration and incident light intensity) were used, and a decrease in the kinetic chain lengths was observed at higher initiation rates. Finally, kinetic chain lengths were examined as a function of depth in thick samples polymerized with different light intensities and with a photobleaching initiator. Light attenuation through the sample led to different initiation rates as a function of depth and, consequently, spatial heterogeneity in the network structure as measured by the distributions of kinetic chains.

Butadienes↗

PubMed2002

Kinetics of protein aggregation. Quantitative estimation of the chaperone-like activity in test-systems based on suppression of protein aggregation.

The experimental data on the kinetics of irreversible aggregation of proteins caused by exposure to elevated temperatures or the action of denaturing agents (guanidine hydrochloride, urea) have been analyzed. It was shown that the terminal phase of aggregation followed, as a rule, first order kinetics. For the kinetic curves registered by an increase in the apparent absorbance (A) in time (t) the methods of estimation of the corresponding kinetic parameters A(lim) and kI (A(lim) is the limiting value of A at t --> infinity and kI is the rate constant of the first order) have been proposed. Cases are revealed when the reaction rate constant kI calculated from the kinetic curve of aggregation of the enzymes coincides with the rate constant for enzyme inactivation. Such a situation is interpreted as a case when the rate of aggregation is limited by the stage of denaturation of the enzyme. A conclusion has been made that, in order to establish the mechanism of protein aggregation, the kinetic investigations of aggregation should be carried out over a wide range of protein concentrations. The refolding experiments after denaturation of proteins by guanidine hydrochloride or urea have been also analyzed. It was shown that aggregation accompanying refolding follows first order kinetics at the final phase of the process. The model of protein refolding explaining such a kinetic regularity has been proposed. When aggregation of protein substrate follows first order kinetics, parameters A(lim) and kI may be used for the quantitative characterization of the chaperone-like activity in the test-systems based on suppression of protein aggregation.

Animals↗

PubMed2004

General kinetic laws of monomolecular-bimolecular reaction A+B <= => C in solutions.

Non-Markovian kinetic equations of the reversible monomolecular-bimolecular reactions of the type A+B right arrow over left arrow C (at arbitrary ratio between A and B concentrations) derived earlier are used in the calculation of kinetics on macroscopic space-time scales. It is found that the kinetics of the systems with different structure of reactants is universal, and it is the direct generalization of the kinetic law of mass action of formal chemical kinetics. The analysis of the kinetics allows one to establish the time range of the applicability of the law of mass action. It is shown that beyond these limits the usual kinetic law of mass action becomes invalid, and correct description of the kinetics even in the most rough approximation calls for the non-Markovian corrections to usual kinetic laws.

Biochemistry↗

PubMed2005

Sorption kinetics of As(V) with iron-oxide-coated cement-a new adsorbent and its application in the removal of arsenic from real-life groundwater samples.

In the search of alternative adsorbents for the removal of arsenic, a new adsorbent (iron-oxide coated cement) was examined under various conditions to evaluate its suitability in removing As(V). A removal of more than 99% was obtained within 2 h for an initial As(V) concentration of 1 mg/L. Kinetic studies showed a removal which became somewhat constant after 2 h. Thorough investigations to understand the mechanism of solute adsorption onto the new sorbent with the help of four kinetic models, viz. approximately first-order reversible kinetics model, pseudo-first-order kinetic model, second-order kinetic model, and pseudo-second-order kinetic model showed that the sorption kinetics is consistent with the pseudo-second-order model from which it can be inferred that the mechanism of adsorption is chemical interaction or chemisorption. The equilibrium data followed the Langmuir isotherm at low concentrations of As(V) (<or=4 mg/L), but followed the Freundlich isotherm over a wide range of concentrations (0.5-10 mg/L) which can be due to solid solution precipitation. Studies on the effect of pH on As(V) removal revealed somewhat constant ( approximately 95%) As(V) removal over a wide pH range of 4-10. The efficiency of this adsorbent was also checked for the removal of arsenic from three real ground water samples containing both As(III) and As(V) with total arsenic in the range of 0.33-1.2 mg/L. The total arsenic removal was 71%, 87% and 75% for the samples containing 0.33, 0.88 and 1.2 mg/L total arsenic, respectively at the same adsorbent dose of 3 g/L. However, increasing the adsorbent dose will enhance removal capacity. Investigations of these real samples with the four kinetic models revealed most reliable fit with the pseudo-second order model. However, good fit was also obtained with the other three models. Values of the diffusion coefficients calculated from the first order kinetics data, indicated that, film diffusion is the rate limiting step for each of the three real life groundwater samples.

Adsorption↗

PubMed2001

Influence of exercise intensity on the on- and off-transient kinetics of pulmonary oxygen uptake in humans.

1. The maximal oxygen uptake (V(O(2),peak)) during dynamic muscular exercise is commonly taken as a crucial determinant of the ability to sustain high-intensity exercise. Considerably less attention, however, has been given to the rate at which V(O(2)) increases to attain this maximum (or to its submaximal requirement), and even less to the kinetic features of the response following exercise. 2. Six, healthy, male volunteers (aged 22 to 58 years), each performed 13 exercise tests: initial ramp-incremental cycle ergometry to the limit of tolerance and subsequently, on different days, three bouts of square-wave exercise each at moderate, heavy, very heavy and severe intensities. Pulmonary gas exchange variables were determined breath by breath throughout exercise and recovery from the continuous monitoring of respired volumes (turbine) and gas concentrations (mass spectrometer). 3. For moderate exercise, the V(O(2)) kinetics were well described by a simple mono-exponential function, following a short cardiodynamic phase, with the on- and off-transients having similar time constants (tau(1)); i.e. tau(1,on) averaged 33 +/- 16 s (+/- S.D.) and tau(1,off) 29 +/- 6 s. 4. The on-transient V(O(2)) kinetics were more complex for heavy exercise. The inclusion of a second slow and delayed exponential component provided an adequate description of the response; i.e. tau(1,on) = 32 +/- 17 s and tau(2,on) = 170 +/- 49 s. The off-transient V(O(2)) kinetics, however, remained mono-exponential (tau(1,off) = 42 +/- 11 s). 5. For very heavy exercise, the on-transient V(O(2)) kinetics were also well described by a double exponential function (tau(1,on) = 34 +/- 11 s and tau(2,on) = 163 +/- 46 s). However, a double exponential, with no delay, was required to characterise the off-transient kinetics (i.e. tau(1,off) = 33 +/- 5 s and tau(2,off) = 460 +/- 123 s). 6. At the highest intensity (severe), the on-transient V(O(2)) kinetics reverted to a mono-exponential profile (tau(1,on) = 34 +/- 7 s), while the off-transient kinetics retained a two-component form (tau(1,off) = 35 +/- 11 s and tau(2,off) = 539 +/- 379 s). 7. We therefore conclude that the kinetics of V(O(2)) during dynamic muscular exercise are strikingly influenced by the exercise intensity, both with respect to model order and to dynamic asymmetries between the on- and off-transient responses.

Adult↗