The location of drug binding sites in human serum albumin

The excretion of cephem antibiotics into saliva is inversely associated with their plasma protein-binding activities

BACKGROUND

The excretion of medicated drugs into saliva may disturb the oral environment and antibiotic excretion into saliva appears to be regulated by many factors that have not been fully explored.

METHODS

Excretion of four cephem antibiotics into saliva was examined in healthy volunteers and rats, using high-performance liquid chromatography, and the relationship between excretion levels and plasma protein-binding activities of the antibiotics was investigated.

RESULTS

Following addition of 50 microgram/ml of each antibiotic to human plasma, protein binding rates (PBRs) of cefuzonam (CZON, molecular weight (MW): 535.58), cefotaxime (CTX, MW: 477.45), flomoxef (FMOX, MW: 518.45) and cefozopran (CZOP, MW: 551.99) were 87.8 +/- 1.2, 70.8 +/- 0.8, 36.2 +/- 0.5 and 8.3 +/- 0.3%, respectively. In rat plasma, PBRs of the four antibiotics were 94.0 +/- 0.5, 62.1 +/- 1.4, 54.0 +/- 0.8 and 6.0 +/- 0.8%, respectively. Similar PBRs were observed when the antibiotic concentration was increased to 100 and 200 microgram/ml. CZOP was most rapidly excreted into saliva and had the highest concentration in saliva among the tested antibiotics, while the plateau level of CZON was the lowest. The excreted levels of each antibiotic in saliva, when locally perfused through the rat facial artery, were inversely associated with each PBR. Similarly, the ratios of antibiotic concentration in saliva to rat plasma were almost constant for each antibiotic, revealing an inverse relationship with PBRs.

CONCLUSION

These results appear to indicate that low molecular weight antibiotics are excreted into saliva through passive diffusion, inversely relating to their PBRs, and that high concentrations of antibiotics in the saliva have the potential to change the oral ecological environment.

Human serum albumin binding of novel antiretroviral nucleoside derivatives of AZT

The binding of novel nucleoside derivatives (2-7) to the Human Serum Albumin (HSA) was studied using zidovudine (AZT), as standard compound. The applicability of two different techniques to separate unbound drug from drug-protein complex was analyzed: the gel filtration and ultrafiltration methods. Ultrafiltration was found to be an adequate procedure for the separation of unbounded drug from the drug-protein complex. Incubation temperature ranging from 0 to 37 degrees C did not modify considerably the bound fractions. The same effects were observed as HSA concentration was modified. Binding assays of studied compounds to purified 1% (w/v) HSA at 0 degrees C, indicate that bound fraction of 2-7 ranges from 13 to 47%, exhibiting a higher affinity to HSA than AZT (12%), which would introduce some interesting improvements in their pharmacokinetic properties. In addition, by means of displacement studies using HSA site specific drugs such as diazepam and salicylate, it was determined that AZT binds to site I of the HSA molecule, by a mainly entropy driven process (DeltaS = 10.834 cal/mol degrees K), being these observations extensive to 2-7. Some structural basis to explain enhanced affinity of these novel derivatives was also established.

Data plotting of warfarin binding to human serum albumin

The binding of warfarin to human serum albumin was studied by equilibrium dialysis at pH 7.4 in a 67 mM sodium phosphate buffer at 37 degrees C. The equilibrium data were analysed using a computer program for curve fitting. The analysis was made fitting the data to equations for one, two and three classes of binding sites with one, two and three sites at the primary binding site (n(1)=1, 2 or 3). The data fitting was acceptable for two and three classes of binding sites but the best fit was obtained with the equation for two classes of binding sites, allowing us to define the binding by a model with two independent classes of binding sites on the serum albumin molecule.

Binding properties of human albumin modified by covalent binding of penicillin

Derivatisation of lysine residues in human albumin was performed in vitro by reaction with penicillin G. This modification reaction has been reported to occur in patients treated with high dosages of the antibiotic. The structure of the modified protein was characterised by mass spectrometry and circular dichroism. The number of the lysine residues involved depends on the time of incubation and on the drug/protein molar ratio. The secondary structure of the modified protein does not change significantly with respect to the native protein. Furthermore, the binding properties of the modified albumin were characterised by CD spectroscopy. Phenylbutazone, diazepam and bilirubin, known to bind to specific binding areas, were used as markers. A decrease of the affinity to the high-affinity binding sites was observed after the modification.

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.

Reversible binding of ethacrynic acid to human serum albumin: difference circular dichroism study

The reversible binding of ethacrynic acid was characterized by a difference circular dichroism method. A 2/1 stoichiometry was determined for the [drug]/[HSA] (human serum albumin) complex. The reversible binding of ethacrynic acid to HSA determines direct competition with ligands that selectivity bind to site II and to the fatty acid site. Furthermore, indirect competition was shown for ligands for site I (anti-cooperative) and to site III (cooperative).

Inactivation of artemisinin by thalassemic erythrocytes

Plasmodium falciparum infecting alpha-thalassemic erythrocytes (Hb H or Hb H/Hb Constant Spring) is resistant to artemisinin derivatives. Similar resistance, albeit at a much lower level, is shown by the parasite infecting beta-thalassemia/Hb E erythrocytes. The resistance is due to host-specific factors, one of which is the higher uptake of the drugs by thalassemic erythrocytes than normal erythrocytes, due to binding with Hb H. In addition to higher drug binding, incubation of artemisinin with alpha-thalassemic erythrocytes resulted in preferential inactivation of the drug. Both thalassemic and normal erythrocytes have the capability to inactivate the drug. Addition of serum can protect against inactivation by normal erythrocytes, but not by thalassemic erythrocytes. Incubation with either the hemolysate or the membrane fraction from these erythrocytes also resulted in preferential inactivation of the drug. The drug was also inactivated by purified Hb H. It is concluded that the ineffectiveness of artemisinin derivatives against P. falciparum infecting thalassemic erythrocytes is due partly to competition of the host cell components for binding with the drugs, and partly to inactivation of the drugs by the cell components.

Circular dichroism simulation shows a site-II-to-site-I displacement of human serum albumin-bound diclofenac by ibuprofen

PURPOSE

The purpose of this study was to confirm the hypothesis that a site-II-to-site-I displacement takes place when some nonsteroidal anti-inflammatory drugs are displaced by another drug from their high-affinity binding site to a site of lower affinity on human serum albumin (HSA).

METHODS

Diclofenac, sodium salt, was used as a representative example because of its prominent reversal of the Cotton effect. Effects of site-specific drugs on the free fraction of diclofenac were determined by equilibrium dialysis, and effects on induced circular dichroism (CD) of diclofenac bound to HSA were studied by CD and CD simulation techniques.

RESULTS

Ibuprofen, a site-II-specific drug, altered the CD spectrum of the diclofenac-HSA complex at a molar ratio of 0.5 : 1 to that obtained at a higher ratio (5:1) without ibuprofen. The induced CD spectrum obtained in the presence of ibuprofen was very similar to one that assumed that all diclofenac displaced from its high-affinity binding site (site II) became rebound to a lower-affinity site (site I). The rebinding could be influenced by a free energy linkage between the two sites which would make site I (or parts thereof) more suitable for diclofenac binding.

CONCLUSION

We have confirmed the existence of a site II-to-site I displacement, which is very striking and pharmacologically important, because the concentration of unbound drug being displaced is much lower than expected for a competitive mechanism.

Study of heterocycle rings binding to human serum albumin

The binding of drugs to human serum albumin determines the drug distribution through the systemic circulation and its pharmacological effects on the organism. Then, with the aim of obtaining information concerning the drug structural features which favour their binding on seroalbumin we have studied the seroalbumin binding to the heterocyclic drugs such as warfarin, propylthiouracil and cromoglycate and to similar compounds such as 4-hydroxy-coumarin, 3-acetylcoumarin, coumarin, benzylthiouracil, propyluracil, thiouracil, chromone and chromanol. These compounds were competitively displaced by warfarin at their primary binding sites on seroalbumin. The comparative analysis of the binding data showed that heterocyclic compounds such as benzopyranes (coumarins and chromanol) and benzyl pyrimidines with 4-hydroxyl groups bind specifically in the warfarin binding site. Then, 4-hydroxyl-bencene heterocycles will displace other ligands from the subdomain IIA of the seroalbumin molecule. Therefore, we can predict that the administration concomitant of warfarin, cromoglycate, propylthiouracil and analogous heterocyclic drugs involves the displacement of the drug without 4-hydroxyl and benzyl groups, increasing their free fraction in serum and the amount of active drug.

Interaction of Schiff base with bovine serum albumin: site-specific photocleavage

A Schiff-base ligand with donor/acceptor substituents viz. 2, 3-bis¿[(2-hydroxy-4-diethylamino) (phenyl) (methylene)]amino¿-2-butenedinitrile was synthesized, its binding properties with bovine serum albumin (BSA) and its site-specific photocleavage in the presence of cobaltous chloride have been evaluated. The Schiff-base ligand showed increase in absorption with a 5-nm red shift in the absorption maximum consistent with the binding of Schiff-base ligand to hydrophobic sites on the protein. The binding plot obtained from the absorption titration gives a binding constant of 6.4 +/- 0.3 x 10(4) M(-1). The CD spectrum of BSA in presence of the ligand shows that binding of the ligand leads to a change in the helicity of the protein. This ligand has been found to induce site-specific photocleavage of the protein in the presence of cobaltous chloride. The gel electrophoresis pattern of a photolyzed sample of BSA/Schiff-base ligand/cobaltous chloride shows that protein is cleaved into two polypeptide fragments, indicating site-specific binding for the ligand to the protein.

Site I on human albumin: differences in the binding of (R)- and (S)-warfarin

The binding of drugs known to interact with area I on human serum albumin (HSA) was investigated using a chiral stationary phase obtained by anchoring HSA to a silica matrix. In particular, this high-pressure affinity chromatography selector was employed to study the binding properties of the individual enantiomers of warfarin. The pH and composition of the mobile phase modulate the enantioselective binding of warfarin. Displacement chromatography experiments evidenced significant differences in the binding of the warfarin enantiomers to site I. The (S)-enantiomer was shown to be a direct competitor for (R)-warfarin, while (R)-warfarin was an indirect competitor for the (S)-enantiomer. Salicylate directly competed with (R)-warfarin and indirectly with (S)-warfarin. This behavior was confirmed by difference CD experiments, carried out with the same [HSA]/[drug] system in solution.

Characterization of solute binding at human serum albumin site II and its geometry using a biochromatographic approach

Chiral recognition mechanism relationships for binding at site II on human serum albumin (HSA) were investigated using D, L dansyl amino acids. Sodium phosphate salt was used as a solute-HSA interaction modifier. A new model was developed using a biochromatographic approach to describe the variation in the transfer equilibrium constant with the salt concentration, i.e., the nature of the interactions. The solute binding was divided into two salt concentration ranges c. For the low c values, below 0.03 M, the nonstereoselective interactions constituted the preponderant contribution to the variation in the solute binding with the salt concentration. For the high c values, above 0.03 M, the solute binding was governed by the hydrophobic effect and the stereoselective interactions. The different contributions implied in the binding process provided an estimation of both the surface charge density (sigma/F) and the surface area of the site II binding cavity accessible to solvent, which were found to be equal to around 10.10(-7) mol/m(2) and 2 nm(2). As well, the excess of sodium ions excluded by the solute transfer from the surface area of the pocket were about(-0.7) for dansyl norvaline and (-0.8) for dansyl tryptophan.

Characterization of minor site probes for human serum albumin by high-performance affinity chromatography

This study used high-performance affinity chromatography (HPAC) and immobilized human serum albumin (HSA) columns to examine the specificity and cross-reactivity of various compounds that have been proposed as markers for the minor binding sites of HSA. These agents included acetyldigitoxin and digitoxin as probes for the digitoxin site, phenol red as a probe for the bilirubin site, and cisor trans-clomiphene as markers for the tamoxifen site. None of these probes showed any significant binding at HSA's indole-benzodiazepine site. However, phenol red did bind at the warfarin-azapropazone site of HSA, and cis/trans-clomiphene gave positive allosteric effects caused by the binding of warfarin to HSA. Digitoxin and acetyldigitoxin were found to bind to a common, unique region on HSA; cis- and trans-clomiphene also appeared to interact at a unique site, although trans-clomiphene displayed additional direct competition with phenol red. From these results it was possible to develop a model that described the general relationship between these binding regions on HSA. This information should be useful in future studies that employ HPAC for characterizing the binding of HSA to other drugs or clinical agents.

Enantioselective binding sites on bovine serum albumin to dansyl amino acids

The enantioselective binding sites on bovine serum albumin were examined by HPLC using 19 racemic 5-N, N-dimethylamino-1-naphthalenesulfonyl derivatives of alpha-amino acids (dansyl amino acids) as chiral probes. On a bovine serum albumin bonded chiral stationary phase, seven L-forms eluted faster than their D-forms, while ten D-forms eluted before their L-forms. It was speculated that either two classes or two different binding sites exist on bovine serum albumin which can be distinguished by N-dansyl-L-proline and N-dansyl-D-norvaline. This was confirmed by fluorometric experiments where non-fluorescent 1-naphthalenesulfonyl derivatives were synthesized and competitive adsorption experiments were performed.

Crystal structure of human serum albumin at 2.5 A resolution

A new triclinic crystal form of human serum albumin (HSA), derived either from pool plasma (pHSA) or from a Pichia pastoris expression system (rHSA), was obtained from polyethylene glycol 4000 solution. Three-dimensional structures of pHSA and rHSA were determined at 2.5 A resolution from the new triclinic crystal form by molecular replacement, using atomic coordinates derived from a multiple isomorphous replacement work with a known tetragonal crystal form. The structures of pHSA and rHSA are virtually identical, with an r.m. s. deviation of 0.24 A for all Calpha atoms. The two HSA molecules involved in the asymmetric unit are related by a strict local twofold symmetry such that the Calpha atoms of the two molecules can be superimposed with an r.m.s. deviation of 0.28 A in pHSA. Cys34 is the only cysteine with a free sulfhydryl group which does not participate in a disulfide linkage with any external ligand. Domains II and III both have a pocket formed mostly of hydrophobic and positively charged residues and in which a very wide range of compounds may be accommodated. Three tentative binding sites for long-chain fatty acids, each with different surroundings, are located at the surface of each domain.

Interaction of human serum albumin with furosemide glucuronide: a role of albumin in isomerization, hydrolysis, reversible binding and irreversible binding of a 1-O-acyl glucuronide metabolite

Furosemide 1-O-acyl glucuronide (Fgnd) was reversibly bound to a single class of binding sites on human serum albumin (HSA), and the binding of Fgnd decreased with increasing F concentrations, suggesting that Fgnd binds to the same warfarin binding sites on HSA as F binds. The rate of Fgnd degradation (hydrolysis and acyl migration) decreased in the presence of HSA. Although the formation of acyl migration isomers of Fgnd was slower in the presence of HSA than in its absence, hydrolysis of Fgnd to F was faster in the presence of HSA. Rapid minor irreversible binding of Fgnd to HSA within 30 min was followed by slow major irreversible binding. Slow irreversible binding of Fgnd to HSA was decreased by F, though not significantly. This suggests that major irreversible binding may proceed via reversible binding. It has been reported that acyl migration is a prerequisite for irreversible binding. Therefore, these results indicate that HSA decreases irreversible binding of Fgnd to protein by suppressing acyl migration. Furthermore, these results suggest that HSA may prevent irreversible binding of Fgnd to other proteins in the body by decreasing the concentration of reactive Fgnd in the unbound form. HSA eliminates reactive Fgnd by hydrolysis to F. Therefore, it is concluded that HSA works as a scavenger to decrease reactive compounds by reversible binding or eliminates reactive compounds by irreversible binding.

Structural heterogeneity of the binding sites of HSA for phenyl-groups and medium-chain fatty acids. Demonstration of equilibrium between different binding conformations

A new facet of the very heterogeneous albumin molecule is described. Chromatography at pH 6-9 of human serum albumin (HSA) on a phenyl-sepharose column separates it into two nonconvertible conformations that are, in turn, in equilibrium with its binding and nonbinding forms. The hydrophobic interaction of HSA with phenyl-sepharose depends on ionic strength, pH, and time of contact with the immobilized ligand. Binding as a function of pH shows a minimum at pH 6.5, and the binding profile at pH 7-9 fits the titration of a weak monoprotic acid with a pKa of 7.3. There was no observable difference in the CD spectra or the masses of the two forms. The equilibrium between the albumin forms was examined under defined conditions and cannot be explained by a simple two-state model. Thus rechromatography of the nonbinding fraction derived from a sample in which 50% of the protein was originally retained resulted only in 10-20% bound protein. Correspondingly only 70-80% of the binding form was retained. A model explaining the observations can be derived if two species, I and II, exist in the solution, both being in an equilibrium with a binding and a nonbinding form, but in which I is not in equilibrium with II. The rate of conversion between the binding and nonbinding conformations was determined to be faster than 15 s at room temperature.

HSA-solute interactions, enantioselectivity, and binding site geometrical characteristics

Recently, a theoretical model was proposed to study the existence of pockets of acetonitrile (ACN) called clusters in a hydroorganic mixture. The proposal used ACN as an interaction organic modifier between D,L-dansyl amino acids and their binding site in human serum albumin at site II. This solute binding is governed by primary and secondary interactions. The primary interactions are under the dependence of the solute solvation by ACN clusters and electrostatic interactions. Following this first step, the solute engages strong short-range interactions with the residues of site II. Using a biochromatographic approach, the solute binding, i.e., the solute retention, was divided into two dielectric constant (epsilon) ranges. In the first range, epsilon > epsilon c (epsilon c is the critical dielectric constant); the primary and secondary nonstereoselective electrostatic interactions were the major contributions to the variation in the solute binding with the ACN fraction in the mixture. In the second range, epsilon < epsilon c, the solute retention variation with the ACN fraction was governed by its solvation by the ACN clusters and also by the secondary hydrophobic stereoselective interaction. The mathematical model developed provided the determination of the surface charge density of site II as well as the cluster number that solvates each solute.

Chemical modification of human albumin at cys34 by ethacrynic acid: structural characterisation and binding properties

Derivatization of the free cys3,4 in human albumin, which is reported to occur under physiological conditions, has been performed in vitro by reaction of the protein with ethacrynic acid. This modification has been investigated by mass spectrometry and circular dichroism. Ethacrynic acid has been proven to bind human albumin either covalently and non-covalently. This post-translational modification does not determine significant changes in the secondary structure of the protein, as shown by the comparable circular dichroism spectra of the native and the modified proteins. Furthermore, the binding properties of the human albumin samples have been investigated by circular dichroism and equilibrium dialysis. The affinity to the higher affinity binding sites does not change either for drugs binding to site I, like phenylbutazone, or to site II, like diazepam, while a small but significant increase has been observed for bilirubin, known to bind to site III. Nevertheless significant decreases of the affinity at the lower affinity binding sites of the modified protein were observed for both drugs binding to site I or to site II.

Development of tryptophan-modified human serum albumin columns for site-specific studies of drug-protein interactions by high-performance affinity chromatography

Human serum albumin (HSA) is one of the main proteins involved in the binding of drugs and small solutes in blood or serum. This study examined the changes in chromatographic properties that occur for immobilized HSA following the chemical modification of HSA's lone tryptophan residue (Trp-214). Trp-214 was reacted with o-nitrophenylsulfenyl chloride, followed by immobilization of the modified protein and normal HSA onto separate silica-based HPLC supports. The binding properties of the modified and normal HSA were then analyzed and compared by using frontal analysis and zonal elution experiments employing R/S-warfarin and L-tryptophan as probe compounds for the warfarin and indole binding regions of HSA. The modified HSA was found to have the same number of binding sites as normal HSA for R-warfarin and L-tryptophan but lower association equilibrium constants for these test solutes. Zonal elution studies with R- and S-warfarin on the modified HSA column demonstrated the importance of Trp-214 in determining the stereoselective binding of HSA for these agents. These studies also indicated that tryptophan modification can alter HSA-based separations for chiral solutes.

Influence of glycosylation on the drug binding of human serum albumin

The influence of glycosylation on the drug binding of human serum albumin (HSA) was studied using HSA containing different amounts and degrees of glycosylated HSA. The drugs used were furosemide, naproxen, procaine, phenylbutazone, salicylic acid, sulphamethoxazole, tolbutamide and warfarin. The drug-HSA parameters (lognK) were measured by the ultrafiltration method, frontal analysis and a modified Hummel-Dreyer method. The modified Hummel-Dreyer method was the simplest method with high precision and required the smallest amounts of proteins. The lognK values were well correlated with the octanol-water partition coefficients; the correlation coefficients were over 0.95. The results suggested that hydrophobic interaction is the predominant force for the drug binding. The early stage of glycosylation of HSA did not significantly affect the drug-binding capacity. Generally, the binding affinity of HSA decreased, perhaps due to a conformational change or steric hindrance (except naproxen) when further glycosylation occurred.

Effects of uremic toxins and fatty acids on serum protein binding of furosemide: possible mechanism of the binding defect in uremia

To elucidate the mechanism of impaired serum binding of furosemide observed in patients with renal dysfunction, we examined in vitro the serum protein binding of furosemide in the absence and presence of uremic toxins that are endogenously retained solutes in uremic serum and act as inhibitors of drug binding. Analysis of the binding data of furosemide at its therapeutic concentration (6.6 mg/L) indicated that, among the four uremic toxins studied, 3-carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF) showed the greatest inhibitory potency for the binding of furosemide to serum; moreover, the inhibition was competitive. CMPF thus most likely represents the primary determinant for the serum binding defect of furosemide in uremia. However, CMPF and oleate appear to exert a synergistic effect on the inhibition of furosemide serum binding—perhaps through a cascade effect on furosemide-binding inhibition in the oleate–CMPF–furosemide system, in which the binding of oleate to its low-affinity sites indirectly displaces furosemide from albumin and thus increases the transiently liberated CMPF molecules. Similar cascade effects on furosemide binding in the presence of CMPF were also originated by other long-chain (C18) fatty acids, linoleate and stearate, although to a lesser extent. Because CMPF is not effectively removed by ordinary hemodialysis treatment, the combined direct and cascade effects of CMPF and fatty acids appear to contribute to the increase in the free fraction of furosemide during hemodialysis.

Direct resolution, characterization, and stereospecific binding properties of an atropisomeric 1,4-benzodiazepine

The chromatographic resolution of 7-chloro-1,3-dihydro-1-(1,1-dimethylethyl)-5- (2-fluorophenyl)-2H-1,4-benzodiazepin-2-on (7), the 2'-fluoro, N1-tertbutyl analogue of diazepam, was attained on both analytical and preparative (mgs) scales, by using several chiral stationary phases (CSPs). The stereochemistry of this compound was characterized by means of 1H-NMR Nuclear Overhauser Effect (NOE) analysis. The single enantiomers of 7 were tested for their configuration and stereochemical stability by circular dichroism (CD), and their interaction with the central nervous system (CNS) benzodiazepine receptor was assayed, showing a significant difference in their binding affinities. Protein binding studies with human serum albumin (HSA, the main benzodiazepine carrier in human plasma) immobilized on a silica stationary phase revealed that HSA also preferentially binds one stereoisomer of 7. However, both on line CD detection and stereospecific interaction with other common drugs clearly demonstrated that the stereoselectivity of immobilized HSA for 7 is opposite to that for all the other studied benzodiazepines. In addition, HSA stereoselectivity for 7 is opposite to CNS receptor binding stereoselectivity for the same compound. Such HSA anomalous stereoselectivity for 7 was also confirmed in aqueous buffer solution by competitive displacement studies. Compared to other chiral 1,4-benzodiazepines, compound 7 thus shows several anomalous binding properties: HSA and the CNS receptor demonstrated opposite enantioselective discrimination; HSA has reversed enantioselectivity for compound 7; and HSA stereospecifically binds the low-affinity enantiomer.

Relationship between lipophilicity and binding to human serum albumin of arylpropionic acid non-steroidal anti-inflammatory drugs

A possible relationship between lipophilicity and binding to human serum albumin was investigated for 11 arylpropionate non-steroidal anti-inflammatory drugs. The lipophilic parameter was determined by a reversed-phase high-performance liquid chromatographic procedure as the capacity factor (k'). The binding of arylpropionic acids to human serum albumin was studied in vitro by equilibrium dialysis. For each compound, a Scatchard analysis was performed considering two classes of binding sites characterized by high- and low-affinity constants, K1 and K2, respectively. A linear relationship was found between lipophilicity and binding parameters, n1K1 (r = 0.88, P < 0.0005) and n2K2 (r = 0.96, P < 0.0002). These results suggest the role of hydrophobic interactions in the binding of arylpropionic acids to human serum albumin.

Binding of indole-3-acetic acid to human serum albumin and competition with L-tryptophan

Indole-3-acetic acid (IAA) is a product of tryptophan (Trp) metabolism and is found to be markedly increased in uremic sera. IAA binding to defatted human serum albumin at 37 degrees C and pH 5, 7.4, and 8.5 was studied by equilibrium dialysis, and data were analyzed assuming two independent high affinity binding sites plus a class of low affinity sites. The estimated values of the association constant of dominant site were: 7.96 x 10(3) M-1 at pH 5, 11.57 x 10(3) M-1 at pH 7.4, and 6.30 x 10(3) M-1 at pH 8.5. The competition between IAA and Trp for albumin binding at pH 7.4 was investigated. The results suggest that one specific albumin site is common for IAA and Trp, but the data were not adequately predicted by a purely competitive scheme. A better prediction was achieved assuming that the binding of IAA to a site different from the common site inhibits Trp binding.

Multivariate analysis of the association of bromocresol purple anion with bovine serum albumin

The interaction of bromocresol purple (BCP) anions with bovine serum albumin (BSA) was investigated by principal factor analysis method, and reaction model, the number of molecular species and spectra of each component present in the reaction mixture were determined. The number of molecular species concerning the absorption intensity was three, including free BCP anion. Most part of the spectral change could be explained by the monomer binding of bromocresol purple anion (D) to serum albumin (P), a simple one step equilibrium, P + D = PD. A second type of association of BCP anions with serum albumin was also present, though in a small amount. Of six models tested which consisted of three or four molecular species, the sequential two step reaction model, P = PD = PD2, was the best model to explain the spectral data, and an existence of BCP anions as a dimer on the serum albumin was demonstrated. The dissociation constants were estimated at K1 = 1.6 x 10(-6) M for the first step and K2 = 1.2 x 10(-5) M for the second step.

Pharmacological importance of stereochemical resolution of enantiomeric drugs

Drug enantiomers have identical properties in an achiral environment, but should be considered as different chemical compounds. This is because they often differ considerably in potency, pharmacological activity and pharmacokinetic profile, since the modules with which they interact in biological systems are also optically active. Within biological systems, the metabolism of one isomer may be via a different pathway or occur at a different rate from that of the other isomer. Preferential binding of one isomer to plasma proteins may cause differences in circulating free drug and hence alter concentrations at active sites. Interactions of both isomers may differ at the active sites through which pharmacological action is mediated. Actions and levels of activity of the stereoisomers in vivo may also differ. All the pharmacological activity may reside in a single enantiomer, whereas several possibilities exist for the other enantiomer-- it may be inactive, have a qualitatively different effect, an antagonistic effect or produce greater toxicity. Two isomers may have nearly identical qualitative pharmacological activity, qualitatively similar pharmacological activity but quantitatively different potency, or qualitatively different pharmacological activity. To avoid adverse effects and optimise the therapeutic value of enantiomeric drugs, it is necessary that methods for the resolution of racemates be evolved and devolved to determine isomeric purity, establish the effectiveness of isomers of the drug, and detect the presence of an enantiomer with lower therapeutic activity and undesirable adverse effects. Even if a drug is given as a pure enantiomer, methods to discriminate between enantiomers are required because racemisation can occur both in vitro and in vivo. Methods developed for resolution of drug enantiomers should facilitate routine testing of single isomers and their metabolites, studies of pharmacological, toxicological and clinical effectiveness, routine analysis of racemates, pure enantiomers or intermediates in manufacturing processes, and investigation of the potential for inversion of an enantiopure drug substance during the early stages of drug development and therapeutic drug monitoring.

Structural specificity requirements in the binding of beta lactam antibiotics to human serum albumin

The binding of some cephalosporins of pharmacological interest, to human serum albumin was studied using ultrafiltration method. The identification of the binding sites in albumin was also performed using probes for the so-called sites I, II, bilirubin and fatty acids binding sites. Cephalosporins were classified into three groups according to their affinity for albumin: low affinity (K = 10-10(2) M-1), medium affinity (K = 10(3) M-1) and high affinity (K = 10(4) M-1). Cephalosporin binding to albumin produced a perturbation of several basic amino acids of the protein such as histidine and lysine. It was found that only cefuroxime, ceftazidime and cefoperazone interact slightly with site I on serum albumin, while site II possesses capacity to bind: cephradine, cephalexin, ceftazidime, ceftriaxone, cefoperazone, cefaclor and cefsulodin. The bilirubin binding site showed capacity to interact with a great number of cephalosporins: ceftriaxone, cefazolin, cephaloglycin, cefamandole, cefotaxime, cefoxitin, cefuroxime, cefoperazone and cefadroxil. Ceftriaxone showed capacity to bind to the fatty acid binding site on HSA. No relation was found between the displacement of the marker and the chemical nature of the substituents at R1 and R2. Cephalosporins interact with HSA at the binding region that involves: tyrosyl 411, histidyl 146 and lysyls 195, 199, 225, 240 and 525 residues. The chemical modification of specific amino acids showed that the interaction of these amino acids with beta lactam antibiotics is not carried out to the same extent for all the cephalosporins tested. The results obtained revealed that the binding sites for cephalosporins on albumin are structurally heterogeneous, having different amino acids in the vicinity of the ligand molecule.

Interactions of aldosterone antagonist diuretics with human serum proteins

PURPOSE

The purpose of this study was to investigate the binding mechanism of aldosterone antagonist diuretics with human serum proteins, human serum albumin (HSA) and alpha 1-acid glycoprotein (AGP), as well as to identify the binding sites of the drugs on these proteins.

METHODS

Binding activities of spironolactone (SP) and its pharmacologically active metabolite canrenone (CR) to serum and serum protein were examined by ultrafiltration and spectroscopic techniques. The data for the binding of these drugs to HSA were analyzed on the basis of a theoretical model of simultaneous binding of the ligands.

RESULTS

The binding percentages of antagonist diuretics SP and CR to human serum proteins were 88.0% and 99.2%, respectively, at therapeutic concentrations. SP bound strongly only to HSA. In addition, the displacement results found using fluorescent probes and ultrafiltration methods demonstrated that SP bound to site I, particularly to the warfarin region on HSA, and to the basic binding site on AGP, while CR bound to the warfarin region on HSA.

CONCLUSIONS

The limited results presented here stress the need for caution on coadministration of acidic drugs which bind to the warfarin region on HSA and basic drugs which bind to AGP with SP and its metabolite CR.

Protein-binding high-performance frontal analysis of (R)- and (S)-warfarin on HSA with and without phenylbutazone

Applicability of high-performance frontal analysis (HPFA) to the stereoselective study of drug-drug interaction upon plasma protein binding has been investigated. Racemic warfarin and phenylbutazone were used as model drugs. An on-line HPFA/HPLC system consisting of a HPFA column (diol-silica column), an extraction column, and a chiral separation column was developed, and human serum albumin solution containing racemic warfarin and/or phenylbutazone was injected directly to the HPFA column. When the injection volume was large enough, the binding equilibrium in the sample solution was reproduced in the column, and consequently a plateau region appeared on the chromatogram. This plateau region contains unbound drug(s). A given volume of eluent in the plateau part was transferred into the extraction column by column-switching. The concentrated drug(s) was then transferred to the chiral separation column to determine the unbound concentrations of the enantiomers and/or the competitor. The results agreed with those obtained by a conventional ultrafiltration-HPLC method. The influence of phenylbutazone upon the protein binding of warfarin is enantioselective. In warfarin and human serum albumin mixed solution, the unbound concentration of (R)-warfarin was 1.22 times higher than that of the S-isomer. By addition of phenylbutazone, the unbound concentration of (S)-warfarin increased more than that of (R)-warfarin, resulting in the reversed enantioselectivity, i.e., the unbound concentration of (S)-warfarin became 1.19 times larger than that of (R)-warfarin. The present method was also applicable to human plasma samples.

Characterization of site I on human serum albumin: concept about the structure of a drug binding site

Human serum albumin (HSA) possesses at least three sites or areas for high-affinity binding of drugs. Of these sites, site I was investigated by series of ultrafiltration and equilibrium dialysis experiments. Three ligands, acenocoumarol, dansyl-L-asparagine (DNSA) and n-butyl p-aminobenzoate (n-butyl p-ABE) were employed as marker ligands. Each ligand binds to a single high-affinity site on HSA, and binding studies with different pairs of the ligands revealed independent high-affinity binding. Preliminary displacement studies performed with the typical site I binding drugs warfarin, phenylbutazone and iodipamide showed different displacement patterns of the three marker ligands. These studies were followed by stringent competition experiments involving all possible combinations of the three test ligands themselves and of these and the three marker ligands. On the basis of the results obtained it seems that the acenocoumarol and DNSA binding regions correspond to the warfarin and azapropazone binding regions, respectively, of site I reported by others (Fehske, Schläfer, Wollert and Müller (1982) Mol. Pharmacol. 21, 387-393). The new binding region, represented by n-butyl p-ABE, is probably located adjacent to the acenocoumarol binding region but apart from that of DNSA. We have elaborated a model for binding site I in which we propose novel nomenclatures, region Ia, Ib, and Ic for the acenocoumarol, DNSA and n-butyl p-ABE binding regions, respectively. Furthermore, the relation between these regions and the high-affinity binding sites for other drugs have been discussed.

Mode of interaction of loop diuretics with human serum albumin and characterization of binding site

PURPOSE

The purpose of this study was to investigate the binding mechanism of loop diuretics with HSA and to characterize the binding site on HSA.

METHODS

Quantitative analysis of potential interaction between ligands bound to HSA was performed by equilibrium dialysis and data for binding of the two ligands to HSA were analyzed on the basis of a theoretical model of simultaneous binding of two ligands.

RESULTS

The binding of loop diuretics is dependent upon the N-B transition, conformational change of albumin. Furthermore, from the results of binding of the drugs to modified HSA, the lysine residue seems to be involved in the binding of loop diuretics to HSA.

CONCLUSIONS

Analysis using models describing independent, competitive, cooperative and anti-cooperative binding led to the conclusion that loop diuretics bind to site I, particularly to the warfarin region on HSA.

The effect of octanoic acid on the binding of the enantiomers of ibuprofen and naproxen to human serum albumin: a chromatographic implication

PURPOSE

The heats of reaction between the enantiomers and racemates of ibuprofen and naproxen and human serum albumin (HSA) are to be measured with and without the addition of octanoic acid. The effects of octanoic acid on the free energies of interaction between the drugs and HSA is to be determined and compared to that estimated from theoretical equations.

METHODS

The heats of reaction have been measured directly by flow microcalorimetry.

RESULTS

The data showed that octanoic acid lowered the 1:1 binding constants for all the drug-HSA interactions investigated. The effect of octanoic acid was greater on the R than on the S forms of the drugs as shown by the differences in free energies of interaction in the presence and absence of octanoic acid.

CONCLUSIONS

The increased free energy differences for the binding of the enantiomers of both drugs to HSA in the presence of octanoic acid is closer to the value deemed to be necessary for the separation of enantiomers by Davenkov, and shows the importance of the addition of octanoic acid to the mobile phase in the separation of these enantiomers on immobilized albumin columns.

Stereoselective binding of carbenicillin epimers to human serum albumin

Binding of carbenicillin (CBPC) epimers to human serum albumin (HSA) was found to be stereoselective. Epimer-epimer interaction was also observed in the binding to HSA. There were at least three binding sites on HSA for CBPC epimers, one of which (stereoselective site) was more in favor of S-CBPC than R-CBPC. At the stereoselective site, the binding constant of S-CBPC was approximately 4-fold greater than that of R-CBPC. The affinities to other binding sites (non-stereoselective sites) were similar between the epimers, and the affinity of S-CBPC of the non-stereoselective sites was much smaller than that for the stereoselective site. R-CBPC and S-CBPC appeared to displace each other at all the binding sites, i.e., the binding of the epimers was competitive at the non-stereoselective sites as well as at the stereoselective site. By using site marker ligands, it was revealed that CBPC epimers may bind to Site I (warfarin binding site), but not to Site II (diazepam binding site). A binding model with an assumption of competitive interactions at all the binding sites simulated the binding characteristics of CBPC epimers fairly well.

[Role of substitution albumin therapy in drugs, hormones, electrolytes and miscellanous substances transport]

Human serum albumin (HSA) is quantitatively the most important non specific transport protein. HSA binds a wide variety of both endogenous and exogenous ligands. Hypoalbuminaemia may lead to a decreased plasma binding capability of some compounds. Biological/pharmacological consequences depend on the ligand and the target tissue. Many experimental studies suggest that hypoalbuminaemia may influence the metabolism and toxicity of endogenous ligands (bilirubin, metallic ions, oxygen radicals) and the pharmacological effect of some drugs (among others: furosemide, phenytoin, warfarin). The relevance of such information for human surgical situations remains unclear. Clinical studies are scarce and inconclusive. There is a lack of pertinent data supporting the necessity of HSA infusions in order to maintain a minimal plasma concentration and a convenient plasma transport. However, experimental data indicate that major hypoalbuminaemia should be considered with caution.

The effect of co-administered drugs on oxaprozin binding to human serum albumin

The binding of the non-steroidal anti-inflammatory drug oxaprozin to human serum albumin was studied by bioaffinity high-performance liquid chromatography using a column based on immobilized human serum albumin. Displacement studies using marker compounds for the major drug binding sites showed that oxaprozin has a high affinity for the benzodiazepine/indole site and binds to the warfarin site but with a significantly lower affinity. Biochromatography and ultrafiltration techniques were used to screen for possible competition and allosteric interactions between oxaprozin and potential co-administered drugs including non-steroidal anti-inflammatory drugs, antipyretics, hypoglycaemics, inhibitors of angiotensin-converting enzyme, anaesthetics, metal ions and anticancer agents. Competition occurred mainly with drugs bound at the benzodiazepine site (benzodiazepines, various non-steroidal anti-inflammatories).

The effects of pasteurisation on albumin: an EPR binding assay for polymeric albumin

The ability of a nitroxyl fatty acid (NFA) to bind specifically to albumin is abolished when, in the absence of stabilizers, a 4% solution of this protein is heated above a critical temperature of 60 degrees C. This treatment leads to the formation of "albumin polymers" as classically evidenced by GPC. Since the bound fraction is evidenced in EPR spectroscopy by a large anisotropic component, the presence of this anisotropy can be used in the assessment of the quality of the pharmaceutical preparations of albumin, which are usually pasteurized in order to inactivate viruses. Moreover, in sharp contrast with the behavior of albumin dispersions, lyophilised albumin subjected to heat treatment at 70 degrees C for 24 h left the protein untouched regarding its NFA binding and GPC profile.

Location of binding sites on immobilized human serum albumin for some nonsteroidal anti-inflammatory drugs

Anti-inflammatory drugs are widely used therapeutic agents and are very often administered with various other drugs. Because they are highly bound to human serum albumin (HSA), interferences between nonsteroidal anti-inflammatory drugs (NSAIDs) and coadministered drugs may arise from their interactions at a binding site on HSA. Although the percentage of binding to HSA is generally accurately determined, the binding sites to which a particular therapeutic agent binds are often unknown. In order to clarify where different classes of NSAIDs bind on the HSA molecule, competition studies were carried out on a HSA-based HPLC column using site I and site II markers as displacing agents. Results show that all the NSAIDs included in the study were affected by site I and site II markers and that a number of drugs had (an) extra binding site(s) not affected by any of the competitors used in the study. Competition data also suggest that binding of NSAIDs at the benzodiazepine site could in fact occur at two separate subsites, as previously observed for benzodiazepines.

The binding of 5-fluorouracil to native and modified human serum albumin: UV, CD, and 1H and 19F NMR investigation

5-Fluorouracil (FU) is an important and widely used antineoplastic drug that is carried in the serum by plasma proteins. Protein binding studies of this drug to human serum albumin (HSA) have been carried out by several spectroscopic techniques. Difference circular dichroism and UV studies provided information on the class of binding sites involved in the interaction. In particular, displacement experiments showed that FU has at least one secondary binding site in the coumarin binding area, but does not interact with the benzodiazepine binding area. Binding was also investigated by difference 1H NMR and by measuring the increase in the 19F NMR signal of FU when bound to HSA. Finally, evidence was obtained that chemical acetylation of Lys199 results in a decreased apparent binding affinity constant (nK) for FU. Such a modification is induced under physiological conditions by aspirin.

Glyburide protein binding and the effect of albumin glycation in children, young adults, and older adults with diabetes

Because glyburide is a weak acid that is more than 98% bound to albumin, the authors evaluated the binding in vitro and the influence of albumin glycation in children, young adults, and older adults with diabetes mellitus. Glyburide binding to non-glycated albumin was greater than 98%, and remained constant over a total concentration range of 50 to 1000 ng/mL. Increasing the albumin concentration from 0.5 to 5.0 g/dL was logarithmically related to the free fraction of glyburide. After the in vitro glycation of albumin (range, 5.7-15.6%), mean (+/- SD) glyburide binding was 99.05 +/- 0.082%, a value in agreement with that obtained from control serum. Serum samples from 57 subjects with type I and 16 patients with type II diabetes were incubated with 300 ng/mL of unlabeled glyburide and 200 ng/mL of 14C-glyburide. The extent of albumin glycation varied from 5 to 22% for the type I subjects and 5 to 14% for the type II subjects. The free fraction from these groups ranged from 1.07 to 1.75% and 0.66 to 0.88% for the type I and type II subjects, respectively. Although these values did not differ significantly from those of the control samples, the glyburide free fraction in patients with type I diabetes (1.39 +/- 0.85%) was significantly greater than that found for the 16 elderly patients with type II diabetes (0.78 +/- 0.05%). A significant linear relationship was not found between glyburide free fraction and the degree of albumin glycation for either group. Glyburide protein binding did not appear to be influenced by the extent of albumin glycation.

Stereospecific and competitive binding of drugs to human serum albumin: a difference circular dichroism approach

Simultaneous binding of two drugs to human serum albumin (HSA) was investigated by difference circular dichroism (delta CD) spectroscopy. Phenylbutazone and diazepam were chosen as specific markers for binding areas I (cumarines) and II (indoles), respectively, and their stereospecific interactions with protein were selectively characterized. Displacers were drugs known to specifically bind to areas I (salicylate) and II (racemic ibuprofen). The results indicate two different interaction mechanisms: a direct competition one (diazepam-ibuprofen and phenylbutazone-salicylate) and an indirect competition one (diazepam-salicylate and phenylbutazone-ibuprofen). The two major binding areas on HSA are distinct, but not independent, entities. Finally, the dissociation constants of marker ligands and competitors complexed to HSA were determined by quantitative analysis of CD data.

Determination of the total concentration of highly protein-bound drugs in plasma by on-line dialysis and column liquid chromatography: application to non-steroidal anti-inflammatory drugs

The potential of on-line dialysis as a sample preparation procedure for compounds highly bound to plasma proteins is evaluated, using non-steroidal anti-inflammatory drugs as model compounds and column liquid chromatography as the separation technique. Different strategies to reduce the degree of drug-protein binding and so increase the analyte recovery are systematically explored and discussed: alteration of the conformation of the binding protein by changing the pH of the sample or by adding an organic solvent, addition of several displacing compounds and combinations of such approaches. A fully automated method is presented for the determination of ketoprofen, ibuprofen, flurbiprofen, fenoprofen and naproxen in human plasma, in which the absolute analyte recoveries are increased from 0-1% (untreated samples) to 40-65%. Relevant analytical data are given to demonstrate the reliability of the proposed procedure.