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L H Janssen

Publications and source records attributed to L H Janssen.

At least 73 records · Page 4Linked to original sources
PubMed1983

Allosteric properties of the oxyphenbutazone--human serum albumin complex.

The conformational change of albumin which occurs around physiological pH, the so-called N-B transition, has been studied by measuring the induced circular dichroic signal of the oxyphenbutazone-albumin complex. This N-B transition has been characterized by a set of parameters according to the two-state model of Monod, Wyman and Changeux. The influence of calcium ions on the N-B transition has been interpreted in terms of a change in some of the parameters describing the two-state model, viz. a decrease of the apparent pK value of the histidines and of the apparent allosteric constant of the oxyphenbutazone-albumin complex. This apparent pK change increases with increasing Ca2+ concentration, whereas the apparent allosteric constant approaches a final value at 5 mM Ca2+. From acid-base titration curves of albumin in the presence and in the absence of Ca2+ it could be concluded that in the presence of Ca2+ less histidines are titratable than in the absence of Ca2+. Assuming that these histidines are not involved in the N-B transition it follows that at least four to five histidines are involved in the N-B transition.

Allosteric Regulation↗
PubMed1982

A comparative study of some physico-chemical properties of human serum albumin samples from different sources--I. Some physico-chemical properties of isoionic human serum albumin solutions.

Human serum albumin samples from different sources were investigated. The fatty acid content of the albumin before and after deionization on a mixed bed ion-exchange column varied from sample to sample. When an albumin sample from one source was deionized under standard conditions the amount of fatty acid bound by the albumin was reduced to a reproducible amount. In samples from different sources, however, the amount bound varied considerably. Also the isoionic pH of the albumin varied from sample to sample. This variation could be attributed to the difference in the fatty acid content and the different number of titratable histidines and acid amino-acid residues in the albumin from different sources. It can be concluded from the specific conductance of these isoionic solutions that ions other than H-, OH and protein are effectively removed by a mixed bed ion-exchange column. The specific conductance of the albumin samples is directly related to the isoionic pH. Therefore, the isoionic pH and the specific conductance of the albumins reflect the heterogeneity of the albumin samples with respect to their primary structure and fatty acid content.

Chemical Phenomena↗
PubMed1982

A comparative study of some physico-chemical properties of human serum albumin samples from different sources--II. The characteristics of the N-B transition and the binding behaviour with regard to warfarin and diazepam.

A comparative study of the N-B transition and the drug binding properties of human serum albumin samples from various sources has been carried out with the help of circular dichroism and equilibrium dialysis. It was found that when warfarin was used as a marker the midpoint pH and the cooperative nature of the N-B transition that occurs in the albumin around physiological pH varied with the albumin sample. The midpoint pH was found to be related to the cooperative nature of the albumin samples. A similar relationship has been found for allosteric proteins. However, when diazepam was used as a marker molecule for the N-B transition, variations in the midpoint pH and cooperative nature of the N-B transition disappeared. This is attributed to the strong allosteric effect of diazepam on binding. The affinity of warfarin for albumin depends strongly on the sample, but this is not the case with diazepam. The cooperative binding properties found for the albumin samples are compared with those found for the albumin in serum. After a discussion it is concluded that the cooperative binding properties of a particular albumin sample should be taken into account when that sample is used in binding studies.

Binding Sites↗
PubMed1981

Human serum albumin as an allosteric two-state protein. Evidence from effects of calcium and warfarin on proton binding behaviour.

The proton binding of human serum albumin, and the influence on it of Ca2+ and warfarin (3-(alpha-acetonylbenzyl)-4-hydroxycoumarin), has been studied in the pH region 6 to 9, in order to get more information on the conformational change occurring in serum albumin around neutral pH, the so-called N-B transition. Some of the results for human serum albumin are compared with bovine serum albumin. A two-state model describing this transition is presented. In this model the two states are assumed to be the N state (found at low pH) and the B state (found at high pH). The ligand to be considered is the proton, having the highest affinity for the N conformation. An allosteric constant, L, governs the equilibrium between the two states. Both Ca2+ and warfarin can act as allosteric effectors, thereby increasing L. The model is used to describe results such as: (a) the cooperativity in proton binding, most clearly observable when Ca2+ is present, and the difficulty of measuring experimentally this cooperativity; (b) the different number of protons bound when Ca2+ is present or absent; (c) the fraction of protein found in one of the two conformations; (d) the correspondence between the increase of L due to addition of Ca2+ or warfarin, as predicted from model calculations, and the experimentally found increase of L.

Allosteric Regulation↗
PubMed1980

The effect of albumin conformation on the binding of warfarin to human serum albumin. The dependence of the binding of warfarin to human serum albumin on the hydrogen, calcium, and chloride ion concentrations as studied by circular dichroism, fluorescence, and equilibrium dialysis.

The pH dependence of the binding of warfarin to human serum albumin has been studies by circular dichroism, fluorescence, and equilibrium dialysis. The pH dependences of the induced ellipticity and the fluorescent intensity of the warfarin . albumin complexes at low drug to protein ratios parallel the neutral to base transition, occurring in the protein over the pH range 6 to 9. Dialysis data show albumin to have a single high affinity site for warfarin over this pH range. This affinity is higher for the basic than for the neutral conformation. Calcium ions increase the high affinity constant of warfarin for albumin over the same pH region, by affecting the neutral to base transition. On the other hand, chloride ions reduce the induced ellipticity and the fluorescent intensity of the complexes at all pH investigated, the mechanism being primarily that of displacement of the drug from albumin.

Binding Sites↗
PubMed1979

Influence of hydrogen and chloride ions on the interaction between sulfaethidole and bovine serum albumin studied by microcalorimetric and acid-base titrimetric methods.

From earlier studies it is known that bovine serum albumin has one high affinity binding site and several lower affinity sites for the sulfa drug N1-(5-ethyl-1,3,4-thiadiazol-2-yl)sulfanilamide (sulfaethidole) (Kostenbauder, H.B., Jawad, M.J., Perrin, J.H., and Averhart, V. (1971) J. Pharm. Sci. 60, 1658-1660). This binding has been further studied using equilibrium dialysis, microcalorimetry, and pH titration technique. Results of these studies show that the binding of sulfaethidole to the first (high affinity) site may be accompanied by an uptake of protons. Proton uptake is found to be zero at pH 7.4 and approximately 0.6 at pH 8.5 for each sulfaethidole molecule bound. The other binding sites for sulfaethidole are not proton linked. The first, and probably the other binding sites, are also Cl- ion linked; for example, the binding of sulfaethidole to the first binding site is accompanied by the displacement of (on average) one Cl- ion at pH 7.4 in 0.1 M NaCl. This explains the observation that the heat of binding of sulfaethidole to the high affinity site is -33.0 kJ.mol-1 in the absence of chloride ions, but only -22.8 kJ.mol-1 in the presence of 0.1 M Cl- (at pH 7.4).

Calorimetry↗
PubMed1975

The effect of potassium chloride on the Bohr effect of human hemoglobin.

The normal and differential titration curves of liganded and unliganded hemoglobin were measured at various KCl concentrations (0.1 to 2.0 M). In this range of KCl concentrations, the curves for deoxyhemoglobin showed no salt-induced pK changes of titratable groups. In the same salt concentration range oxyhemoglobin showed a marked change in titration behavior which could only be accounted for by a salt-induced increase in pK of some titratable groups. These results show that the suppression of the alkaline Bohr effect by high concentrations of neutral univalent salt is not caused by a weakening of the salt bridges in deoxyhemoglobin but is due to an interaction of chloride ions with oxyhemoglobin. Measurements of the Bohr effect at various KCl concentrations showed that at low chloride ion concentration (5 times 10-3 M) the alkaline Bohr effect is smaller than at a concentration of 0.1 M. This observation indicates that at a chloride ion concentration of 0.1 M, part of the alkaline Bohr effect is due to an interaction of chloride ions with hemoglobin. Furthermore, at low concentrations of chloride ions the acid Bohr effect has almost vanished. This result suggests that part of the acid Bohr effect arises from an interaction of chloride ions with oxyhemoglobin. The dependence of the Bohr effect upon the chloride ion concentration can be explained by assuming specific binding of chloride ions to both oxy- and deoxyhemoglobin, with deoxyhemoglobin having the highest affinity.

Binding Sites↗