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PubMed1985

Microencapsulation of paracetamol using polyacrylate resins (Eudragit Retard), kinetics of drug release and evaluation of kinetic model.

Methacrylate copolymers were used for microencapsulation of paracetamol by phase separation from chloroform with polyisobutylene 6% in cyclohexane. With polyisobutylene as an anti-aggregating agent, high quality microcapsules were obtained. Drug release appeared to fit both first order and Higuchi matrix model kinetics. However, on application of the differential rate treatment, the evidence supported the first order description, which was further supported by computed simulations of the models. Variation of production conditions showed that increasing the proportion of core material raised the microcapsule drug content and the release rate. Reduction of core particle size correlated with reduced coating thickness and faster release rate. The rate constants correlated with the estimated surface areas and wall thicknesses of the various batches. The data were used to estimate an apparent permeability constant for paracetamol in Eudragit RS microcapsules, which was constant and comparable with values found single core, non-aggregated microcapsules containing other similar drugs and different wall materials.

Acetaminophen↗

PubMed2005

A comparison of release kinetics and glutamate receptor properties in shaping rod-cone differences in EPSC kinetics in the salamander retina.

Synaptic transmission from cones is faster than transmission from rods. Using paired simultaneous recordings from photoreceptors and second-order neurones in the salamander retina, we studied the contributions of rod-cone differences in glutamate receptor properties and synaptic release rates to shaping postsynaptic responses. Depolarizing steps evoked sustained calcium currents in rods and cones that in turn produced transient excitatory postsynaptic currents (EPSCs) in horizontal and OFF bipolar cells. Cone-driven EPSCs rose and decayed faster than rod-driven EPSCs, even when comparing inputs from a rod and cone onto the same postsynaptic neurone. Thus, rod-cone differences in EPSCs reflect properties of individual rod and cone synapses. Experiments with selective AMPA and KA agonists and antagonists showed that rods and cones both contact pharmacologically similar AMPA receptors. Spontaneous miniature EPSCs (mEPSCs) exhibited unimodal distributions of amplitude and half-amplitude time width and there were no rod-cone differences in mEPSC properties. To examine how release kinetics shape the EPSC, we convolved mEPSC waveforms with empirically determined release rate functions for rods and cones. The predicted EPSC waveform closely matched the actual EPSC evoked by cones, supporting a quantal release model at the photoreceptor synapse. Convolution with the rod release function also produced a good match in rod-driven cells, although the actual EPSC was often somewhat slower than the predicted EPSC, a discrepancy partly explained by rod-rod coupling. Rod-cone differences in the rates of exocytosis are thus a major factor in producing faster cone-driven responses in second-order retinal neurones.

Animals↗

PubMed1978

Oxidation kinetics and chemostat growth kinetics of Thiobacillus ferrooxidans on tetrathionate and thiosulfate.

Growth of Thiobacillus ferrooxidans in batch culture on 10 mM potassium tetrathionate was optimal at pH 2.5 (specific growth rate, 0.092 h-1). Oxygen electrode studies on resting cell suspensions showed that the apparent Km for tetrathionate oxidation (0.13 to 8.33 mM) was pH dependent, suggesting higher substrate affinity at higher pH. Conversely, oxidation rates were greatest at low pH. High substrate concentrations (7.7 to 77 mM) did not affect maximum oxidation rates at pH 3.0, but produced substrate inhibition at other pH values. Tetrathionate-grown cell suspensions also oxidized thiosulfate at pH 2.0 to 4.0. Apparent Km values (1.2 to 25 mM) were of the same order as for tetrathionate, but kinetics were complex. Continuous culture on growth-limiting tetrathionate at pH 2.5, followed by continuous culture on growth-limiting thiosulfate at pH 2.5, indicated true growth yield values (grams [dry weight] per gram-molecule of substrate) of 12.2 and 7.5, and maintenance coefficient values (millimoles of substrate per gram [dry weight) of organisms per hour) of 1.01 and 0.97 for tetrathionate and thiosulfate, respectively. Yield was increased on both media at low dilution rates by increase in CO2 supply. The apparent maintenance coefficient was lowered without affecting YG, suggesting better energy coupling in CO2-rich environments. Prolonged continuous cultivation on tetrathionate or thiosulfate did not affect the ability of the organism to grow subsequently in ferrous iron medium.

Ferrous Compounds↗

PubMed1987

Alveolar sampling and fast kinetics of tetrachloroethene in man. II. Fast kinetics.

Fast kinetic phenomena were studied in human subjects exposed to tetrachloroethene (perchloroethylene, PER). The duration of exposure ranged from one to 60 minutes and the concentration of PER in inhaled air ranged from 0.02 to 0.40 mmol/m3. The PER concentration in mixed venous blood (pulmonary artery) was estimated by alveolar concentration (CAlv) measured after a residence time of 10 s. During exposure, stoppage of intake (breath holding up to 50 s) caused a decrease of CAlv down to about 60% of the CAlv at a residence time of 10 s. At the end of exposure, stoppage of intake (breathing fresh air) caused a decrease of CAlv with t1/2 = 15-25 s; after two to four minutes, the decrease slowed down abruptly and the concentration remained more or less constant for about one to three minutes. After this stationary level, the decrease of CAlv continued but at a slower rate. During and after exposure, the decrease of CAlv seems to be caused by large differences in the circulation times of blood flowing through rapid, well perfused tissues and slower, well perfused tissues which may explain the stationary level. From this point of view, the vessel rich group in a compartment model must be split up in order to predict tissue and organ concentrations during peak environmental concentrations.

Blood Volume↗

PubMed1986

Kinetics of methaemoglobin production. (1). Kinetics of methaemoglobinaemia induced by the cyanide antidotes, p-aminopropiophenone, p-hydroxyaminopropiophenone or p-dimethylaminophenol after intravenous administration.

Methaemoglobin profiles have been studied by using ISIS, a simulation package, and NONLIN, a non-linear least-squares analysis regression program. A simple kinetic model which satisfactorily describes methaemoglobin profiles after p-aminopropiophenone (PAPP) administration and 4-dimethylaminophenol (DMAP) administration has been developed. The two compounds differed mainly in their effective rates of elimination. The model less satisfactorily described methaemoglobin profiles after p-hydroxyaminopropiophenone (PHAPP) administration.

Aminophenols↗

PubMedDate not supplied

Dynamics and kinetics of enzymes. Kinetic equilibrium of forces in biochemistry.

To explain the high specificity, high reaction rate, and high thermodynamic efficiency in enzymatic processes, cooperation of the enzyme with a molecular transfer unit is assumed. A "kinetic equilibrium of forces" is suggested, which enables high reaction rates to occur under equilibrium conditions and a thorough examination of the substrate to be made without consumption of free energy. In case of ATPases, ion-binding proteins are the most probable transfer units. By analyzing the elementary effect in muscle contraction it is shown that the new theorem may be of substantial value in elucidating biochemical processes.

Actins↗

PubMed1986

Kinetic studies on the major form of aldehyde reductase in ox kidney: a general kinetic mechanism to explain substrate-dependent mechanisms and the inhibition by anticonvulsants.

The inhibition of the major form of ox kidney aldehyde reductase (AR 1) by sodium barbitone revealed linear mixed kinetics. This behaviour is distinct from the non-linear intercept effect we reported for valproate [Daly and Mantle (1982) Biochem. J. 205, 381]. 4-Carboxybenzaldehyde exhibits partial uncompetitive substrate inhibition. These results are discussed in terms of a model that involves nucleotide-induced isomerization and an additional flux (with some substrates and inhibitors) through an enzyme.nucleotide.substrate/inhibitor ternary complex.

Aldehyde Dehydrogenase↗

PubMed1990

ATP inhibition of Phycomyces pyruvate kinase: a kinetic study of the inhibitory effects on the allosteric kinetics shown by the enzyme.

Studies on ATP effects on the allosteric kinetics shown by pyruvate kinase from Phycomyces blakesleeanus NRRL 1555 (-) are reported. Phosphoenolpyruvate showed an allosteric ATP-dependent substrate inhibition. The results supported the existence of spatially distinct catalytic binding sites and the inhibitory binding sites for phosphoenolpyruvate, and ATP showed opposite heterotropic effects with respect to these two types of binding site. With respect to Mg2+ ions, ATP caused a negative heterotropic effect. The global inhibitory effect of ATP was in agreement with the predictions postulated by the two-state concerted-symmetry model of Monod, Wyman and Changeux.

Adenosine Triphosphate↗

PubMed2005

Transient absorption spectra and kinetics of folic acid (vitamin B11) and some kinetic data of folinic acid and methotrexate.

Folic acid (FA; vitamin B11) exhibits antitumor properties. Its transients, resulting from reaction with OH radicals, are of special interest and were studied by pulse radiolysis. The complex composition of FA offers the formation of numerous transients having different electronic structure stability (lifetime). Investigating their kinetic profiles of decay at various wavelengths, it was established that transients with very unstable electron structure (very short-lived radicals) tend to convert into more stable electronic structures (longer lived) by intramolecular electron transfer process. The biological importance of a FA radical depends on its concentration and on its specific reaction rate constant (k value) for a given process. The rate constant for the OH attack is k(OH + FA) = 1.1 x 10(10) L mol(-1) s(-1). Superimposed absorption spectra of FA radicals as well as formation and decay rate constants of total processes are presented. The primary spectrum is characterized by lambda = 425 nm, epsilon = 1.64 x 10(4) L mol(-1) s(-1). The chemical structure of FA is similar to that of folinic acid (FNA) and of methotrexate (MTX), but their biological properties are different. Therefore, their rate constants for the reactions with e-aq and OH were also determined for comparison: k(FNA + e-aq) = 3.1 x 10(10) L mol(-1) s(-1), k(FNA + OH) = 0.8 x 10(10) L mol(-1) s(-1), k(MTX + e-aq) = 1 x 10(10) L mol(-1) s(-1); k(MTX + OH) = 2.3 x 10(10) L mol(-1) s(-1), and k(FA + e-aq) = 1.9 x 10(10) L mol(-1) s(-1).

Folic Acid↗

PubMedDate not supplied

[Kinetics of reactions between hemoglobin and carbon monoxide. II. Analysis of kinetic schemes in linear approximation].

The kinetics of hemoglobin (Hb) reactions with carbon monoxide were calculated for a number of classical schemes: with one, two, four binding sites and an allosteric scheme with T- and R-conformers. The developed theory predicts the existence of only one relaxation time for every scheme the values of which depends on the velocity constants, saturation degree and the Hb concentration.

Binding Sites↗

PubMedDate not supplied

Kinetics of drug decomposition. Part XXXVI. Stability of 10-(1'-methyl-4'-piperazinylpropyl)-phenothiazine derivatives on the grounds of kinetics of thermal degradation and Hammett equation.

Thermal degradation of aqueous and buffered solutions of perazine, prochlorperazine, trifluoperazine, thioproperazine, thiethylperazine and butaperazine salts was examined by kinetic method using an accelerated testing of pharmaceutical preparations. The order, rate constants and activation parameters (Q100, E, delta H not equal to, delta S not equal to, delta G not equal to ) of the reaction given were discussed. The predicted stability of the examined derivatives was compared on the grounds of a calculated time t10% and K293 kappa. A dependence between the stability and kind of substituent in the C2 positions was discussed in terms of the Hammett equation.

Chemical Phenomena↗

PubMed1975

[Enzyme kinetic glucose determination by the glucose dehydrogenase method. Enzyme kinetic substrate determination using competitive inhibitors, II (author's transl)].

The sensitivity of enzyme kinetic substrate determinations can be improved with the aid of competitive inhibitors. As an example, the determination of glucose dehydrogenase in the presence of potassium thiocyanate is described. The method has the advantage of rapid operation with satisfactory precision.

Alcohol Oxidoreductases↗

PubMedDate not supplied

Kinetics of drug decomposition. Part 37. Kinetics of autoxidation of narcotine in aqueous solutions.

The overall apparent first-order rate constants for the reaction of autoxidation of Narcotine (NC) - k1 and Cotarnine (CT) - k2 and the rate constants for autoxidation of the protonated (ks) and undissociated (kb) forms have been determined. The kinetic equation for the reaction of autoxidation at constant concentration of NC, CT and the atmospheric oxygen depending on the pH of the reaction medium has been established as: k1 = ks([H+]/([H+]+K'a))+kb(K'a/([H+]+K'a), k2=(ks[H+]+K'a))+kb (K'a/([H+]+K'a) (1+1+1/kt+1K)). The following thermodynamic parameters for the rate constants ks and kb have been calculated: deltaH not equal to, log A and deltaS not equal to.

Drug Stability↗

PubMed1986

Kinetics of mitochondrial calcium transport. II. A kinetic description of the sodium-dependent calcium efflux mechanism of liver mitochondria and inhibition by ruthenium red and by tetraphenylphosphonium.

Sodium-dependent calcium efflux from rat liver mitochondria has been studied as a function of mitochondrial calcium loads (2 to 40 nmol/mg) and extramitochondrial sodium concentrations (5 to 40 mM). The resulting data can be fit to a terreactant model which exhibits simultaneous kinetics (i.e. both sodium and calcium must be bound simultaneously for transport to occur). The Hill coefficients for the calcium and sodium dependences were 1.0 +/- 0.1 and 2.0 +/- 0.2, respectively. The cooperativity of the sodium dependence allows the terreactant model to be reduced to a bireactant model in which the sodium concentration only appears mathematically as the square of the sodium concentration. The data then fit the relationship (Formula: see text) The experimentally determined value of Vmax is found to be 2.6 +/- 0.5 nmol/mg/min, and the load of calcium (KCa) and concentration of sodium (KNa) necessary to stimulate the efflux to half its maximal calcium-dependent activity and sodium-dependent activity, respectively, were 8.1 +/- 1.4 nmol of Ca2+/mg and 9.4 +/- 0.6 mM Na+. This sodium-dependent calcium efflux from liver mitochondria was inhibited by magnesium, by ruthenium red, and by tetraphenylphosphonium. Fifty percent inhibition was obtained at 1.0-1.5 mM magnesium, at 12 nmol of ruthenium red/mg of protein, and at 0.2 microM tetraphenylphosphonium.

Animals↗

PubMed1989

[An experimental analysis of the interrelation of the kinetics of spontaneous blood serum chemiluminescence and of the kinetics of the tumor process].

On the material of 5 strains of transplanted rat tumours (sarcoma 45, carcinoma Guérin, carcinosarcoma Walker, lymphosarcoma Pliss, erythromyelosis Shvets) and also mice tumours induced by Moloney and Rauscher viruses and on the models of chemical carcinogenesis of rats: lung (intratracheal injection of 3,4-benz(a)pyrene), breast (DMBA) and liver DENA) was shown that tumorous processes of various origin are accompanied by either elevation or inhibition of blood serum spontaneous chemiluminescence (SChL). In all investigated cases the extreme points of SChL curve coincide with the moment of change of tumour growth kinetics.

Animals↗

PubMed1979

Studies of skeletal tracer kinetics: II. evaluation of a five-compartment model of [18F]fluoride kinetics in rats.

We have evaluated a five-compartment model of [18F]fluoride kinetics in rats. The initial fluoride distribution was found to be similar to that of [77Br]bromide, a known extracellular-fluid (ECF) tracer, in agreement with the hypothesis underlying the model, and the measured uptake rate in rat bones compared well with the digital computer solution. Simpler models did not give a better fit. In dead rats, fluoride movement was found within the skeleton, presumably from bone ECF to bone substance, although not as rapidly as predicted or found in the live animal. Evaluation of the rate constants permitted estimates to be made of cardiac output, skeletal blood flow, and bone ECF volume, all in accord with independent measurements. It is suggested that skeletal blood flow at rest is a constant fraction of body weight, and probably subserves a hematopoietic as well as a mineralization function.

Animals↗

PubMed1984

The kinetic mechanism of salicylate hydroxylase as studied by initial rate measurement, rapid reaction kinetics, and isotope effects.

The kinetic mechanism of Pseudomonas cepacia salicylate hydroxylase has been examined by steady state initial rate measurements, and stopped flow and equilibrium studies. Results indicate that salicylate and NADH bind to the hydroxylase randomly. The enzyme is reduced and NAD+ is released. Oxygen subsequently binds to the reduced enzyme . substrate complex, leading to the production of hydroxylated product, CO2, and water. Based on results of anaerobic rapid mixing experiments, the rate of enzyme reduction by NADH is enhanced 290- and 240-fold when the hydroxylase is complexed with salicylate and benzoate (a nonsubstrate effector), respectively. Salicylate enhances, whereas benzoate slightly weakens, the NADH binding to the enzyme. Primary isotope effects were observed with (4R)-[4-2H]- and (4R)-[4-3H]NADH but not with the (4S)-[4-2H]NADH. Using varying concentrations of benzoate, the pattern of tritium isotope effect on Vm/Km, T(V/K), also indicates that benzoate and NADH bind to the enzyme randomly. The intrinsic isotope effects, Dk, of (4R)-[4-2H]NADH on the reduction of enzyme . salicylate and enzyme . benzoate complexes were found to be 5.57 and 5.96, respectively. The former is much repressed but the latter is only slightly so in the expression of their corresponding deuterium isotope effects on Vm, DV. Furthermore, values of DV (1.69 to 5.07) show a rough correlation with the extents of uncoupling of substrate hydroxylation and H2O2 formation activities for a series of benzenoid effectors. These results indicate that relative to the step of enzyme reduction, the subsequent reaction(s) leading to H2O2 formation must be fast whereas that for substrate hydroxylation contains at least one slow step.

Anaerobiosis↗

PubMed1983

Kinetics of elementary steps in the cytochrome P-450 reaction sequence. V. Laser temperature-jump investigation of the spin relaxation kinetics of cytochrome P-450 LM2.

The cytochrome P-450 LM2 spin state relaxation kinetics has been resolved by means of laser temperature-jump techniques. The first order rate constants amount to about 10(6) S-1 in the substrate-free and the substrate-bound protein, respectively. Evidence is provided that the spin equilibrium preequilibrates the P-450 reduction but is not rate-limiting. Additional capacitor discharge temperature-jump studies elucidate substrate dependent perturbations.

Animals↗