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  • kinetics
  • Springer  (176)
  • PANGAEA
  • 1990-1994  (176)
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  • 1
    Electronic Resource
    Electronic Resource
    Initial steps of αand β-d-glucose binding to intact red cell membrane (1993)
    Springer
    The journal of membrane biology 132 (1993), S. 167-178 
    Details
    ISSN: 1432-1424
    Keywords: red cell ; glucose transport protein ; GLUT1 ; kinetics ; rapid reactions ; tryptophan
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The kinetics of the initial phases of d-glucose binding to the glucose transport protein (GLUT1) of the human red cell can be followed by stopped-flow measurements of the time course of tryptophan (trp) fluorescence enhancement. A number of control experiments have shown that the trp fluorescence kinetics are the result of conformational changes in GLUT1. One shows that nontransportable l-glucose has no kinetic response, in contrast to d-glucose kinetics. Other controls show that d-glucose binding is inhibited by cytochalasin B and by extracellular d-maltose. A typical time course for a transportable sugar, such as d-glucose, consists of a zero-time displacement, too fast for us to measure, followed by three rapid reactions whose exponential time courses have rate constants of0.5–100 sec+−1 at 20°C. It is suggested that the zero-time displacement represents the initial bimolecular ligand/GLUT1 association. Exponential 1 appears to be located at, or near, the external membrane face where it is involved in discriminating among the sugars. Exponential 3 is apparently controlled by events at the cytosolic face. Trp kinetics distinguish the K d of the epimer, d-galactose, from the K dfor d-glucose, with results in agreement with determinations by other methods. Trp kinetics distinguish between the binding of the α- and β-d-glucose anomers. The exponential 1 activation energy of the β-anomer, 13.6 ± 1.4 kcal mol+−1, is less than that of α-d-glucose, 18.4 ± 0.8 kcal mol+−1, and the two Arrhenius lines cross at ≈23.5°C. The temperature dependence of the kinetic response following α-d-glucose binding illustrates the interplay among the exponentials and the increasing dominance of exponential 2 as the temperature increases from 22.3 to 36.6°C. The existence of these interrelations means that previously acceptable approximations in simplified reaction schemes for sugar transport will now have to be justified on a point-to-point basis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00239006
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  • 2
    Electronic Resource
    Electronic Resource
    Inhibition of inward rectifying tonoplast channels by a vacuolar factor: Physiological and kinetic implications (1991)
    Springer
    The journal of membrane biology 122 (1991), S. 251-258 
    Details
    ISSN: 1432-1424
    Keywords: patch-clamp ; plant vacuole ; single-channel inhibition ; kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Regulation of ion-channel activity must take place in order to regulate ion transport. In case of tonoplast ion channels, this is possible on both the cytoplasmic and the vacuolar side. Isolated vacuoles of youngVigna unguiculata seedlings show no or hardly any channel activity at tonoplast potentials 〉80 mV, in the vacuole-attached configuration. When the configuration is changed to an excised patch or whole vacuole, a fast (excised patch) or slow (whole vacuole) increase of inward rectifying channel activity is seen. This increase is accompanied by a shift in the voltage-dependent gating to less hyperpolarized potentials. In the whole vacuole configuration the level of inward current increases and also the activation kinetics changes. Induction of channel activity takes up to 20 min depending on the age of the plants used and the diameter of the vacuole. On the basis of the estimated diffusion velocities, it is hypothesized that a compound with a mol wt of 20,000 to 200,000 is present in vacuoles of young seedlings, which shifts the population of channels to a less voltage-sensitive state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01871425
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  • 3
    Electronic Resource
    Electronic Resource
    Na activation delays and their relation to inactivation in frog skeletal muscle (1990)
    Springer
    The journal of membrane biology 118 (1990), S. 233-242 
    Details
    ISSN: 1432-1424
    Keywords: Na channels ; skeletal muscle ; kinetics ; chloramine-T ; electrophysiology ; current inactivation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Delays in the development of activation of Na currents were studied using voltage-clamped frog skeletal muscle fibers. Na currents elicited by a depolarizing voltage step from a hyperpolarized membrane potential were delayed in their activation when compared to Na currents elicited from the resting potential. The magnitude of the delay increased with larger hyperpolarizing potentials and decreased with larger depolarizing test potentials. Delays in activation observed following chloramine-T treatment that partially removes inactivation did not differ from delays observed before treatment. Longer exposures of the muscle fiber to chloramine-T led to a complete loss of inactivation, coincident with an elimination of the hyperpolarization-induced delays in activation. Steady-state slow inactivation was virtually unaffected by prolonged exposures of the fibers to chloramine-T that eliminated fast inactivation. The results show that chloramine-T acts at a number of sites to alter both activation and inactivation. Markov model simulations of the results show that chloramine-T alters fundamental time constants of the system by altering both activation and inactivation rate constants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01868607
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  • 4
    Electronic Resource
    Electronic Resource
    Characterization of ion channels from Acetabularia plasma membrane in planar lipid bilayers (1993)
    Springer
    The journal of membrane biology 133 (1993), S. 145-160 
    Details
    ISSN: 1432-1424
    Keywords: Acetabularia ; K+ channels ; kinetics ; planar lipid bilayers ; voltage dependence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Plasma membrane from Acetabularia acetabulum was prepared by aqueous-polymer two-phase partitioning and incorporated into planar 1-palmitoyl-2-oleoyl phosphatidylethanolamine bilayers by stirring in the presence of a (cis∶trans) 325∶100 mm KCl gradient. Under these conditions five distinct K+-selective channels were observed which had unitary chord-conductances (determined between 30 mV either side of the reversal potential) and frequencies of incorporation (in parentheses) of 1,600 pS (26%), 485 pS (21%), 259 pS (53%), 140 pS (37%) and 27 pS (37%). Two Cl−-selective channels were also observed, which had unitary chord-conductances of 8 and 48 pS and were present in 21 and 16% of bilayers, respectively. The voltage dependencies of channel open probability (P o ), open-state time constant (τ o) and closed-state time constant (τ c) were determined for the 259, 140 and 27 pS K+ channels. The P o of all three channels increased with increasingly positive membrane potentials. Thus, since these channels were oriented with their extracellular face adjacent to the cis chamber, which was grounded, all would exhibit outward rectification in vivo. Changes in P o were effected by modulation of τ c in all channels, which shortened as membrane potentials became more positive, and also of τ o in the 140 and 27pS channels, which increased as membrane potentials became more positive. Extracellular (cis) KCl concentration (and/or the KCl gradient across the bilayer) affected the P o of all three K+ channels, shifting the P o /membrane potential relationship in the direction of the change in the potassium reversal potential. In all channels this was achieved largely by changes in τ c .
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00233795
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  • 5
    Electronic Resource
    Electronic Resource
    A comparison of pyridoxal 5′-phosphate dependent decarboxylase and transaminase enzymes at a molecular level (1991)
    Springer
    Cellular and molecular life sciences 47 (1991), S. 1104-1118 
    Details
    ISSN: 1420-9071
    Keywords: Transaminase ; decarboxylase ; serine hydroxymethyltransferase ; pyridoxal 5′-phosphate ; enzyme mechanism ; stereochemistry ; kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract Pyridoxal 5′-phosphate is a coenzyme for a number of enzymes which catalyse reactions at Cα of amino acid substrates including transaminases, decarboxylases and serine hydroxymethyltransferase. Using the X-ray coordinates for a transaminase, aspartate aminotransferase, and the results of stereochemical and mechanistic studies for decarboxylases and serine hydroxymethyltransferase, an active-site structure for the decarboxylase group is constructed. The structure of the active-site is further refined through active-site pyridoxyllysine peptide sequence comparison and a 3-D catalytic mechanism for the L-α-amino acid decarboxylases is proposed. The chemistry of serine hydroxymethyltransferase is re-examined in the light of the proposed decarboxylase mechanism.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01918374
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  • 6
    Electronic Resource
    Electronic Resource
    The polymerization of sickle hemoglobin in solutions and cells (1993)
    Springer
    Cellular and molecular life sciences 49 (1993), S. 110-117 
    Details
    ISSN: 1420-9071
    Keywords: Polymerization ; sickle hemoglobin ; sickle cell disease ; kinetics ; thermodynamics ; polymer domains ; nucleation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The polymerization of sickle hemoglobin occurs by the same mechanisms in solutions and in cells, and involves the formation of 14 stranded fibers from hemoglobin molecules which have assumed a deoxy quaternary structure. The fibers form via two types of highly concentration-dependent nucleation processes: homogeneous nucleation in solutions with hemoglobin activity above a critical activity, and heterogeneous nucleation in similarly supersaturated solutions which also contain hemoglobin polymers. The latter pathway is dominant, and creates polymer arrays called domains. The individual polymers bend, but also cross-link, and the resulting mass behaves as a solid. The concentration of polymerized hemoglobin increases exponentially unless clamped by rate limiting effects such as oxygen delivery.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01989414
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  • 7
    Electronic Resource
    Electronic Resource
    Absorption of intramuscular phenobarbitone in children with severe falciparum malaria (1992)
    Springer
    European journal of clinical pharmacology 42 (1992), S. 107-110 
    Details
    ISSN: 1432-1041
    Keywords: Phenobarbitone ; Cerebral malaria ; P.falciparum ; kinetics ; drug absorption ; children
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Medicine
    Notes: Summary The absorption of intramuscular phenobarbitone 7 mg·kg−1 was studied in 11 Karen children aged between 1.7 and 11 y with severe falciparum malaria. Eight of the children were comatose. Clinical findings were compared with those in 9 further children with severe malaria of similar age range (four of whom were unconscious), who received an identical placebo. One child, who had received placebo, had repeated convulsions and died 1 h after admission to hospital. The remainder made an uncomplicated recovery. There were no convulsions subsequent to treatment, although the study was too small to assess anticonvulsant efficacy. There was no observable toxicity, but phenobarbitone recipients had a significant tendency to deepen in their level of coma or to become sleepy within the 4 h after drug administration. Phenobarbitone was rapidly absorbed, reaching a mean (range) peak concentration of 34.2 [29.3–42.6] μmol·l−1 in a median (range) of 4 (2.5–12) h. These values are comparable to those previously reported in healthy children and in children with febrile convulsions. Intramuscular phenobarbitone is well absorbed in children with severe malaria; the optimum prophylactic anticonvulsant dose remains to be determined.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00314929
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  • 8
    Electronic Resource
    Electronic Resource
    Kinetics of atrial natriuretic peptide in young and elderly subjects (1992)
    Springer
    European journal of clinical pharmacology 42 (1992), S. 449-452 
    Details
    ISSN: 1432-1041
    Keywords: Atrial natriuretic peptide ; kinetics ; ageing
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Medicine
    Notes: Summary To study the influence of age on the kinetics of atrial natriuretic peptide (ANP) in man, human (99–126) ANP 2.0 μg·min−1 was infused IV for 60 min in 8 healthy young (18 to 25 y) and 9 healthy elderly (71 to 84 y) subjects. Both baseline ANP values and the levels at the end of infusion were higher in the elderly subjects. The mean residence time of ANP in the two age groups was not significantly different, whereas total body clearance (CL) was markedly diminished in the elderly as compared to the young subjects (mean±SD 3.1±1.0 l·min−1 and 6.2±4.1 l·min−1, respectively). The apparent volume of distribution at steady state was lower in the elderly than in the young, but the difference was not significant (mean±SD 44±19 and 103±111, respectively. The decrease in CL largely explained the higher ANP levels found in the elderly subjects. The MRT and the plasma half-life of the terminal phase did not differ between the two groups. In the elderly but not in the young subjects the calculated endogenous creatinine clearance was closely correlated with the CL (r=0.90, P〈0.001), thereby emphasizing the importance of the kidney in the metabolic clearance of ANP in the elderly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00280134
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  • 9
    Electronic Resource
    Electronic Resource
    Interaction of talinolol and sulfasalazine in the human gastrointestinal tract (1992)
    Springer
    European journal of clinical pharmacology 42 (1992), S. 461-462 
    Details
    ISSN: 1432-1041
    Keywords: Talinolol ; Sulfasalazine ; β-adrenoceptor-blocking drugs ; absorption ; drug interaction ; kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Medicine
    Notes: Summary The absorption of talinolol (TA) 50 mg was investigated without and together with the co-administration of sulfasalazine (SASP) 4 g in 11 healthy young volunteers, in order to clarify gastrointestinal transit of TA. Without SASP, the tmax of TA was 2.8 h, Cmax was 112 ng·ml−1 and the half life was 12 h; the AUCo-t was 958 ng·ml−1·h. In the case of concomitant administration of SASP, TA was found only in serum from 3 individuals, with a Cmax of 23 ng·ml−1 and a mean AUCo-t of 84 ng·ml−1·h. TA was not detectable in 5 subjects and it was at the limit of detection (2 ng·ml−1) in 3 subjects. Pharmacokinetic analysis was not possible in any of those individuals. The reason for the interaction appears to be the adsorption of TA by SASP. An interval of 2–3 h should elapse between giving SASP and other drugs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00280137
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  • 10
    Electronic Resource
    Electronic Resource
    Bioavailability of controlled release carbamazepine estimated by mixed effect modelling (1993)
    Springer
    European journal of clinical pharmacology 44 (1993), S. 231-235 
    Details
    ISSN: 1432-1041
    Keywords: Carbamazepine ; kinetics ; population pharmacokinetics ; bioavailability ; controlled release ; non-linear model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Medicine
    Notes: Summary The absorption properties of a conventional tablet of carbamazepine (T) and a controlled release form of carbamazepine (TCR) have been compared using a nonlinear mixed effect model (NONMEM). Plasma carbamazepine concentration data were obtained from an open, steady-state, crossover bioavailability study in which 494 measurements were obtained from 13 patients, with an equal number of samples per patient for each dosage form. The pharmacokinetic model used was a one-compartment open model with first-order absorption and elimination. The objective function was used as a measure of the goodness of fit of the model to the data. Body weight was an important determinant of carbamazepine clearance (CL) but not volume of distribution (V). Accounting for the interindividual variability in volume of distribution did not significantly influence the objective function. Including different rates of absorption (ka) for the two dosage forms resulted in a significant improvement in the objective function, as well as reducing the interindividual variability in the rate of absorption. Adding a parameter for relative bioavailability (f) of TCR improved the objective function statistically, but an unrealistic value for V was obtained, and the absorption and elimination rates appeared to be transposed in the classical “flip-flop” manner. Fixing V to the value obtained before introducing f did not change the objective function and permitted estimation of f without the confounding influence of excessive parameters. The final population parameter estimates (standard error of estimate) were: CL, 0.0522 (0.0019) l·h−1·kg−1; V, 63.7 (FIXED)l; kaT, 0.312 (0.064) h−1; kaTCR, 0.149 (0.016) h−1; f, 1.01 (0.0326); variance (additive) in CL, 0.291 (0.083) (l·h−1·kg−1)2; residual intrasubject error variance (additive), 0.572 (0.082) (mg·l−1)2. The 95% confidence interval of the extent of absorption (f) of 93.6%–107.4% was well within the generally accepted range of ±20%, while the rate of absorption of Tegretol CR was significantly slower than that of Tegretol, as expected for a controlled release product.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00271363
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