Comparison of graphical procedures for estimating the intrinsic molar fluorescence of protein-bound drugs for drug-binding studies. A reevaluation of existing plots and introduction of two inverse hyperbolic plots

The effect of glycation on the structure, function and biological fate of human serum albumin as revealed by recombinant mutants

Recombinant wild-type human serum albumin (rHSA), the single-residue mutants K199A, K439A and K525A and the triple-residue mutant K199A/K439A/K525A were produced using a yeast expression system. Portions of the rHSA were glycated to different degrees (2.5-250 mM D-glucose). As detected by far-UV and near-UV CD, intrinsic tryptophan-fluorescence and probed by 1,1'-bis(4-anilino)naphthalene-5,5-disulfonic acid, the single-residue mutations had no effect on albumin conformation, whereas the triple-residue mutation and glycation caused conformational changes. The triple-residue mutation and glycation had comparable increased effects on high-affinity binding of warfarin (site I), but decreased effects on high-affinity binding of dansylsarcosine (site II) and the esterase-like activity of albumin. The relation between plasma half-lives in rats were found to be glycated rHSA (50 mM glucose)<triple-residue mutated rHSA<rHSA. The opposite trend was found for liver and kidney uptakes in mice. Even though the functional and the in vivo properties of rHSA could be effected differently by the minor conformational changes caused by the triple-residue mutation and glycation, the present findings indicate that the effect of glycation can be partly explained by blockage of the positive charges of lysine at positions 199, 439 and 525.

Conformational stability and warfarin-binding properties of human serum albumin studied by recombinant mutants

Correctly folded recombinant wild-type human serum albumin and the single-residue mutants K199A, W214A, R218H and H242Q were produced with the use of a yeast expression system. The changes in R218H resulted in a pronounced decrease in intrinsic fluorescence. Thermodynamic parameters for thermal denaturation of the present mutants and of five additional mutants have been determined, showing small increases in stability for two mutants (R218H and H242Q) and a larger decrease in stability for one (W214A). In the last of these, denaturation was a heterogeneous process starting at physiological temperature. The high-affinity binding constant for warfarin at pH 7.4 was determined by fluorescence spectroscopy: there was a significant increase in affinity for binding of warfarin to H242Q and K199A and a smaller decrease in affinity for W214A and R218H. The findings show that Trp-214 is not as essential for the high-affinity binding of warfarin as has previously been thought.

Exponential hyperbolic sine function fitting of heart rate response to constant load exercise

We attempted to fit heart rate (HR) changes induced by constant exercise loads of different intensities to an exponential hyperbolic sine curve by the least-squares method, and we compared the results with the fitting of the changes to exponential curves. Seven healthy male volunteers performed three different intensities of constant-load exercise on a bicycle ergometer. The exponential hyperbolic sine function adequately fitted the HR responses induced by all three different intensities of loads: low (30 W: correlation coefficient, r = 0.68 +/- 0.13, mean +/- SD), moderate (75 W: r = 0.93 +/- 0.07) and high (125 W: r = 0.97 +/- 0.02). The first-order exponential curve fitted only the moderate load response. Although the second-order exponential equation fitted the HR response for both the moderate and high loads, the equation did not fit the low-load response (r = 0.43 +/- 0.26). In low-load exercise, the sum of the power of the residuals for the exponential hyperbolic sine curve fitting was significantly smaller than that for the first- or second-order exponential curve fitting. In conclusion, the exponential hyperbolic sine function is useful for quantitative analyses of the HR response to exercise loads of various intensities.

Five recombinant fragments of human serum albumin-tools for the characterization of the warfarin binding site

Human serum albumin (HSA) interacts with a vast array of chemically diverse ligands at specific binding sites. To pinpoint the essential structural elements for the formation of the warfarin binding site on human serum albumin, a defined set of five recombinant proteins comprising combinations of domains and/or subdomains of the N-terminal part were prepared and characterized by biochemical standard procedures, tryptophanyl fluorescence, and circular dichroic measurements, indicating well-preserved secondary and tertiary structures. Affinity constants for binding to warfarin were estimated by fluorescence titration experiments and found to be highest for HSA-DOM I-II and HSA, followed by HSA-DOM IB-II, HSA-DOM II, and HSA-DOM I-IIA. In addition, ultraviolet difference spectroscopy and induced circular dichroism experiments were carried out to get an in depth understanding of the binding mechanism of warfarin to the fragments as stand-alone proteins. This systematic study indicates that the primary warfarin binding site is centered in subdomain IIA with indispensable structural contributions of subdomain IIB and domain I, while domain III is not involved in this binding site, underlining the great potential that lies in the use of combinations of recombinant fragments for the study and accurate localization of ligand binding sites on HSA.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.

Analysis of the binding of fluorescent ligands to soluble proteins. Use of simultaneous non-linear least squares regression to obtain estimates of binding parameters

The binding of three fluorescent ligands (warfarin, dansylsarcosine and 1-anilino-8-naphthalene sulphonate) to human albumin was analysed using simultaneous non-linear least squares regression analysis. Both mock and actual fluorescence data were examined and the results indicated that reliable estimates of the binding parameters as well as the molar fluorescence of bound ligand could be obtained. The advantage of this method of analysis is that it makes full use of all the experimental data and it eliminates the need for the graphical procedures usually employed to estimate the molar fluorescence of bound ligand and its binding constants. This type of analysis can be extended to other systems where some physical property of the bound ligand varies with increasing protein concentration.