Multiple cobinding of two ligands to serum albumin: a stoichiometric description of binding equilibria

Predicting plasma protein binding of drugs: a new approach

In spite of the large amount of plasma protein binding data for drugs, it is not obvious and there is no clear consensus among different disciplines how to deal with this parameter in multidimensional lead optimization strategies. In this work, we have made a comprehensive study on the importance of plasma protein binding and the influencing factors in order to get new insights for this molecular property. Our analysis of the distribution of percentage plasma protein binding among therapeutic drugs showed that no general rules for protein binding can be derived, except for the class of chemotherapeutics, where a clear trend towards lower binding could be observed. For the majority of indication areas, however, empirical rules are missing. We present here an extensive list of multiply determined primary association constants for binding to human serum albumin (HSA) for 138 compounds from the literature. Correlating these binding constants with the percentage fraction of protein bound showed that the percentage data above 90%, corresponding to a binding constant below 6 microM, are of insufficient accuracy. Furthermore, it could be demonstrated that the lipophilicity of drugs, traditionally felt to dominate binding to HSA, is not the only relevant descriptor. Here, we report a generic model for the prediction of drug association constants to HSA, which uses a pharmacophoric similarity concept and partial least square analysis (PLS) to construct a quantitative structure-activity relationship. It is able to single out the submicromolar to nanomolar binders, i.e. to differentiate between 99.0 and 99.99% plasma protein binding. Depending on the system, this can be important in medicinal chemistry programs and may together with other computed physicochemical and ADME properties assist in the prioritization of synthetic strategies.

Cobinding of bilirubin and laurate to human serum albumin: spectroscopic characterization of stoichiometric complexes

Light absorption and CD spectra of bound bilirubin and albumin fluorescence spectra have been recorded from mixtures containing albumin, A, bilirubin, B, and laurate, L, in Tris-NaCl buffer at pH 8.2, 25 degrees C. Concentrations of the corresponding stoichiometric complexes, ABiLj, for i = 0/3 and j = 0/3, have been calculated from previously determined stoichiometric cobinding constants (H. Sato et al. (1988) Arch. Biochem. Biophys. 260, 811-821). Spectral data of the complexes have finally been found by iterative computer fitting using the principle of several acceptable solutions (R. Brodersen et al. (1987) Eur. J. Biochem. 169, 487-495). The results were utilized at the microscopic level to investigate ligand-induced conformational changes. When laurate was bound to AB, a decrease of the distance between Trp-214 and the bound bilirubin occurred, as measured according to Förster's principle. The distances were 21.9 +/- 0.3 A in AB, 19.7 +/- 0.3 A in ABL, and 17.9 +/- 0.2 A in ABL2.

Effect of protein concentration on the binding of gold(I) to human serum albumin

The binding of aurothiosulphate, gold(I), by human serum albumin has been studied by equilibrium dialysis at four different albumin concentrations, 37 degrees, pH 7.2-7.4 and ionic strength 0.15 M. The results show that the interaction of aurothiosulphate with albumin depends on albumin concentration. This observation is linked with the previous observation that the usual independent site description cannot be used to represent the clinically important low concentration data. All the observed dependences are satisfactorily accounted for by assuming that gold(I) competes with a highly bound contaminant for the high affinity (Cys(34)-SH) site. This description is supported by the experimental observation that a fraction of this site is originally blocked both in vivo and in in vitro. The present interpretation yields a high affinity binding constant 100 times larger than found previously and provides an explanation for the lack of correlation between dose and therapeutic and toxic effects in chrysotherapy.

Characterization of the aflatoxin B1-binding site of rat albumin

A fluorescence-enhancement method was used to investigate the non-covalent interaction between aflatoxin B1 and rat albumin. Solvent-induced shifts in the emission spectrum of aflatoxin B1 provided evidence that the aflatoxin B1-binding site of rat albumin is a highly nonpolar environment. A dissociation constant of 20 microM was determined at 20 degrees C. The possibility that aflatoxin B1 binds one of the three major drug sites of albumin was investigated by ligand-displacement experiments. Mechanisms whereby marker ligands displace aflatoxin B1 were further investigated by comparing the experimental binding parameters with those derived theoretically, assuming competitive binding. The results indicate that: aflatoxin B1 and phenylbutazone compete for a common high-affinity site on rat albumin; high-affinity binding of aflatoxin B1 and site-II marker ligands takes place independently; aflatoxin B1 does not compete with either cholate or warfarin for the same high-affinity site, but the simultaneous binding of warfarin or cholate negatively modulates the binding of aflatoxin B1 to albumin. Fluorescence energy-transfer studies show that the lone tryptophan residue, Trp-214, is not associated with the aflatoxin B1-binding site.

Cobinding of bilirubin and sulfonamide and of two bilirubin molecules to human serum albumin: a site model

Differential light absorption spectra of the bilirubin-albumin 1:1 complex, obtained on addition of 20 different sulfonamides, differ with respect to shape and amplitude. This finding seems to indicate that the sulfonamide molecule is bound in direct touch with the bilirubin. The light absorption spectrum of bilirubin-albumin 1:1 undergoes changes on cobinding of a fatty acid anion, laurate, and on variation of pH, previously explained by a change of dihedral angle between the two chromophores of the bilirubin molecule. In bilirubin-albumin 2:1, binding of laurate and variation of pH cause little change of the spectrum. This is best explained by binding of the two bilirubin molecules in close proximity, preventing conformational changes in the complex. From measurements of fluorescence of the lone tryptophan group in albumin and quenching on binding of bilirubin, we calculated the distance of 22 A from tryptophan to the first bound bilirubin molecule, and of 18 A to the second. Mutual quenching of the bilirubin fluorescence from two bound bilirubin molecules seemed to indicate that the two are bound closely together. A model of bilirubin-albumin with a binding site capable of accommodating one bilirubin and one sulfonamide molecule, or two molecules of bilirubin, is compatible with our findings.

Aflatoxin B1 transport in rat blood plasma. Binding to albumin in vivo and in vitro and spectrofluorimetric studies into the nature of the interaction

Binding of [3H]aflatoxin B1 to rat plasma was investigated in vivo and in vitro. Column chromatographic and polyacrylamide gel electrophoretic analyses clearly demonstrated that aflatoxin B1 bound primarily plasma albumin. Very little binding activity was shown by other plasma proteins. Spectrofluorimetric studies were undertaken to gain some insight into the nature of the aflatoxin-albumin interaction. Quenching of the lone tryptophan fluorescence intensity upon aflatoxin binding was due, at least in part, to a ligand-induced conformational change in the albumin molecule. Aflatoxin B1 binds an apolar site with an association constant of 30 mM-1 at pH 7.4 and 20 degrees C. Neither charcoal treatment of rat albumin nor the presence of 0.15 M NaCl had any significant effect on the interaction. The association constant was pH-dependent, increasing about 1.7-fold as the pH increased from 6.1 to 8.4. This pH dependence is ascribed to a pH-induced conformational change in the albumin molecule. Thermodynamic studies indicated that the aflatoxin-albumin interaction was exothermic (delta H = -29.3 kJ X mol-1), with a delta S value of -13.8 J X mol-1 X K-1.