The location of drug binding sites in human serum albumin

Evidence for isoxicam binding to site I as a primary site and to site II as a secondary site of human serum albumin

Isoxicam binding to HSA was studied using equilibrium dialysis and fluorescence methods. It was shown that this drug binds to or near site I (warfarin or azapropazone site) and to site II (the diazepam site) as a secondary site, although it is generally considered that their respective drug structural requirements are often exclusive. The binding parameters were calculated with different mathematical models; a site oriented model with or without fixing the number of binding sites as integer values and a stoichiometric model. The relevant results are in good agreement under the selected experimental conditions. The stoichiometric method indicates that no positive cooperativity occurred during the binding process but other interactions between the two sites cannot be excluded.

Binding of pirprofen to human serum albumin studied by dialysis and spectroscopy techniques

The interaction of pirprofen with human serum albumin (HSA) was investigated by equilibrium dialysis and spectroscopic (UV absorption, fluorescence, CD, NMR) techniques. It was found that HSA binds pirprofen nonstereospecifically. The binding of pirprofen depends upon the N-B conformational change of albumin. Chloride ions appear to displace the drug from its binding site. The thermodynamic parameters suggest that the interaction may be explained by electrostatic as well as hydrophobic forces. The absorption spectral changes which accompanied the binding of pirprofen to HSA implied that the aromatic portion of drugs was inserted into the hydrophobic crevice in the protein, while the carboxyl group of the drug interacted with a cationic site on the albumin surface. The NMR data indicated that the pyrroline ring and propionic acid parts may be the major binding site for HSA. A specific binding site for pirprofen on the HSA was found to be site II, benzodiazepine site, using fluorescence probes and drug markers. In addition, from the binding data with modified HSA, it seems that Tyr-411 is specifically involved in pirprofen binding.

Location and characterization of the warfarin binding site of human serum albumin. A comparative study of two large fragments

The warfarin binding behaviour of a large tryptic fragment (residues 198-585 which comprise domains two and three) and of a large peptic fragment (residues 1-387 which comprise domains one and two) of human serum albumin has been studied by circular dichroism and equilibrium dialysis in order to locate and characterize the primary warfarin binding site. The induced ellipticity of the warfarin-peptic fragment complex turned out to be pH dependent. This pH dependence occurs in the region of the neutral-to-base transition of the albumin molecule. The induced ellipticity of the warfarin-tryptic fragment complex is pH independent. Difference CD-spectra showed that the peptic fragment and albumin have similar warfarin binding properties whereas the tryptic fragment has deviant warfarin binding properties. The equilibrium dialysis experiments showed that the affinity of warfarin to the peptic fragment and to albumin is practically the same whereas the affinity of warfarin to the tryptic fragment is a factor 2-8 lower than the affinity of warfarin to albumin. Our results indicate that the main part of the primary warfarin binding site is located in domain two of the albumin structure and that domain one plays an important role in the N-B transition of albumin.

Effects of a perfluorochemical blood substitute on diazepam binding by human albumin

The binding of 0.4 microgram mL-1 diazepam and 0.75 mol diazepam mol-1 of albumin by a perfluorochemical (PFC) emulsion, Fluosol-DA, 20%, by human serum albumin (HSA), and by their mixtures, has been examined at ambient temperature. The concentration of free diazepam was determined by standard centrifugation followed by supernatant ultrafiltration. Non-specific loss of diazepam occurred to the ultrafiltration device. This loss was independent of drug concentration and a correction factor was employed to calculate the true free diazepam concentration. Diazepam was extensively bound by the PFC emulsion. The percent free diazepam increased as the emulsion concentration decreased, while the binding of diazepam appeared to be independent of drug concentration. Diazepam did not partition into the pure PFC liquids, indicating that emulsion-bound diazepam is only associated with the emulsifiers of the droplets. Diazepam was extensively bound by HSA and the percent free diazepam increased as drug concentration increased or as HSA concentration decreased. The PFC emulsion significantly displaced HSA bound diazepam in all mixtures examined. Studies with the individual and combined components of the emulsion indicated that this displacement is largely attributed to the oleic acid component and, to a much smaller degree, the Pluronic F-68 component of the emulsion.

The binding of thioureylene compounds to human serum albumin

The binding interactions of some thioureylene compounds to human serum albumin were studied in vitro by ultraviolet spectroscopy and equilibrium dialysis. Binding of 6-n-propyl-2-thiouracil, 6-n-benzyl-2-thiouracil and 2-thiouracil to human serum albumin results in a red shift of the ultraviolet absorption maximum, suggesting that the binding site is a hydrophobic area of the protein. Bindings of 6-n-propyl-2-thiouracil and 6-n-benzyl-2-thiouracil to human serum albumin are characterized by two classes of sites while 6-n-propyl-uracil and 2-thiouracil bind to one low-affinity binding site. In addition, an identification of those sites was performed by measuring the displacement of these drugs. The data show that the moderate-affinity site is common with the warfarin site while the low-affinity site is likely to be shared by benzodiazepines. It is concluded that the binding is enhanced by the hydrophobicity of the substituent in the thioureylene compounds, and it is further shown that thiol-group substitutions in the thioureylene ring will weaken the binding.

Cefazolin serum protein binding and its inhibition by bilirubin, fatty acids and other drugs

This paper describes the protein binding of cefazolin to human serum and to human serum albumin (HSA) using equilibrium dialysis. The drug is exclusively bound to HSA with a moderate affinity, Ka = 16,600 +/- 1600 M-1, and one saturable binding site, n = 0.73 +/- 0.02. Moreover cefazolin shows a dose-dependent binding leading a possible increase of the free fraction (when its total concentration increases). This antibiotic is displaced by free fatty acids (FFA) and bilirubin. Cefazolin binding to human serum and human serum albumin (HSA) was studied in presence of acidic drugs. At low concentrations clofibric acid and phenylbutazone both exhibiting high affinity for HSA displace strongly cefazolin. Valproic and salicylic acids, sulfamethoxazole, cefoperazone which have approximately the same affinity as cefazolin, must be used at higher concentrations to displace this antibiotic. A particular phenomenon was observed with cefazolin on HSA when associated with furosemide. A low concentration (5-25 microM) of this drug induces a positive cooperativity of binding between cefazolin and HSA. But at a molar ratio of furosemide to albumin greater than one, such cooperative interaction disappears and a competitive inhibition of cefazolin binding occurs. For all drugs studied, a competitive inhibition was found except for tryptophan. Finally, it is concluded that cefazolin shares the warfarin binding site on HSA.

The fatty-acid-induced conformational states of human serum albumin investigated by means of multiple co-binding of protons and oleic acid

The binding of oleic acid to human serum albumin causes progressive changes in (a) the pK of some amino acid residues, as detected by pH-stat titration and (b) the induced molar ellipticities of albumin-bound drugs (diazepam and oxyphenbutazone), as measured by c.d. It is concluded that albumin undergoes several conformational transitions as the amount of oleic acid bound increases from 0 to about 9 molecules/molecule of protein. At least three different conformations of the protein seem to be involved. These conformations can be linked with the three classes of oleic acid-binding sites on albumin.

Structural requirements for binding of nonsteroidal anti-inflammatory drugs to human serum albumin

The binding of representative chemical classes of nonsteroidal anti-inflammatory drugs (NSAIDs) to human serum albumin (HSA) was investigated by equilibrium dialysis. Warfarin enantiomers were used as specific markers in displacement studies. Data were analyzed by a computerized nonlinear least squares approach designed for binding of small ligands to macromolecules at equilibrium. The binding data indicated comparable affinities to the primary site by the warfarin enantiomers, phenylbutazone, and meclofenamate sodium. Naproxen, sulindac, and zomepirac showed lower affinity by one order of magnitude. The displacement data revealed stereoselectivity. The R(+) isomer was displaced to a significantly greater extent than the S(-) isomer by meclofenamate sodium, while the reverse was observed for phenylbutazone. Naproxen displaced both isomers to the same extent. No significant displacement of either isomer was seen with sulindac or zomepirac. Examination of the chemical structures of the high affinity compounds indicated the common feature of a hydrophobic area bearing a widely delocalized negative charge. Hydrophobic binding of these compounds to HSA at the warfarin site is possibly stabilized by the attraction of the delocalized negative charge to the basic lysine and arginine residues adjoining the lone tryptophan.

Interactions with the protein binding of 7-hydroxy-methotrexate in human serum in vitro

7-Hydroxy-methotrexate (7-OH-MTX), the major extracellular methotrexate (MTX) metabolite, is 90-95% bound in human serum, with albumin (HSA) as the major binding protein. Reports of an interaction with concomitantly administered non-steroidal antiinflammatory drugs (NSAIDs) during MTX therapy led us to investigate whether these compounds could reduce the binding of 7-OH-MTX in vitro. Equilibrium dialysis experiments demonstrated that naproxen and indomethacin concentration dependently reduced the binding of 1 microM 7-OH-MTX. After ingestion of 1000 mg naproxen, per cent unbound 7-OH-MTX in sera from volunteers increased 2-3-fold in vitro, positively correlated to naproxen concentrations (P less than 0.00015). In addition, etacrynic acid, bilirubin, sulphamethizole and acetylsalicylic acid displaced 7-OH-MTX from its binding protein(s) in a competitive manner. The data suggest that 7-OH-MTX interacts with several exogenous and endogenous substances associated with HSA in human serum. Displacement of 7-OH-MTX from HSA may contribute to the interaction between NSAIDs and MTX.

Expression of domains of mouse alpha-fetoprotein in Escherichia coli

A mouse alpha-fetoprotein complementary DNA containing the coding region for amino acids 256 to 548 was fused to the lac transcriptional and translational control elements contained on the expression vector pOP203-28. The expression of a 35 kD hybrid lac Z'- alpha-fetoprotein polypeptide in Escherichia coli was demonstrated by the chloramphenicol release assay and by immunoprecipitation using rabbit anti-mouse alpha-fetoprotein antibodies as probe.

Cicletanine binding to human plasma proteins and erythrocytes, a particular HSA-drug interaction

The binding of cicletanine to human serum, isolated proteins and red blood cells was studied in vitro by equilibrium dialysis. Our results show this drug is highly bound to serum (97.3%) at therapeutic levels. No saturation to the binding sites was seen. Human serum albumin was shown to mainly responsible for this binding (93.5%) with a saturable process characterized by one binding site with a moderate affinity (K = 75800 M-1) and a non saturable process with a low total affinity (nK = 6400 M-1). Like many basic lipophilic drugs, cicletanine showed a saturable binding to alpha-1-acid glycoprotein with one site and a moderate affinity (K = 38,800 M-1). Its binding to lipoproteins and red blood cells was weak and non saturable. Over the range of therapeutic concentrations, the unbound fraction in blood remains constant (3.6%). Moreover, interactions were studied using bilirubin and non esterified fatty acids at pathological concentrations and these endogenous compounds did not alter cicletanine binding human serum or to human serum albumin likewise cicletanine shared the diazepam-site on HSA but no inhibition could take place between cicletanine and the drugs sharing the same binding site in serum at therapeutic levels.

The pharmacokinetics of bendazac-lysine and 5-hydroxybendazac, its main metabolite, in patients with hepatic cirrhosis

We have studied the pharmacokinetics of bendazac and its major metabolite, 5-hydroxybendazac, in 11 patients with hepatic cirrhosis after the oral administration of a single 500 mg tablet of bendazac-lysine, and compared them with those obtained from 10 healthy adults. The rate of absorption of bendazac, as assessed by tmax and Cmax, is similar in patients and in healthy subjects. The drug is eliminated mostly by metabolism in healthy adults, more than 60% of the dose being excreted in the urine as 5-hydroxybendazac and its glucuronide. Hepatic insufficiency impairs this metabolism, a two-fold decrease in apparent plasma clearance (CL/f) being observed in the patients. Although the plasma unbound fraction of bendazac is increased in patients (the drug is highly bound to plasma albumin), the apparent volume of distribution (V/f) is unchanged. In consequence, the half-life of bendazac is increased two-fold in the patients. Impairment of metabolism decreases the formation of 5-hydroxybendazac, but metabolism remains the main route of its elimination. Renal excretion of bendazac accounts for about 10% of the dose in both patients with cirrhosis and healthy subjects. We conclude that in patients with severe hepatic insufficiency the daily dose of bendazac-lysine should be halved.

Interaction of disopyramide enantiomers for sites on plasma protein

The binding of disopyramide enantiomers to donor plasma to which had been added human alpha-1 acid glycoprotein (AAG) was characterized alone and in the presence of the opposite enantiomer. At pre-dialysis concentrations of 10(-5) M, S(+)-disopyramide increased the percent of R(-)-disopyramide free (unbound) 2.6-fold from 11 to 30%. At similar pre-dialysis concentrations, R(-)-disopyramide increased the percent of S(+)-disopyramide free 2-fold from 4.1 to 9.0%. Differences in the binding of one enantiomer due to the presence of the other were due to apparent changes in association constant; no changes in capacity to bind the enantiomers were observed. It is concluded that the enantiomers of disopyramide compete with each other for one site on AAG.

Binding in vitro of pipequaline (45319 RP) onto plasma proteins and blood cells in man

Serum binding of pipequaline, a new anxiolytic drug, was studied in vitro by equilibrium dialysis. The percent binding in serum is high, 96.3%, and remains constant within the range of therapeutic concentrations. Investigations performed on isolated proteins with a wide range of concentrations showed one site with a high affinity constant (Ka = 450,000 M-1) for alpha 1-acid glycoprotein and two sites with a lower affinity constant (Ka = 58,000 M-1) for human serum albumin. Binding to lipoproteins was saturable, with an affinity constant of 22,000 less than or equal to Ka less than or equal to 35,000 M-1. Over the range of therapeutic concentrations, the ratio of pipequaline concentrations in serum and red blood cells remained constant (14.4%) and was shown to be dependent on the free fraction of pipequaline in serum.

Influence of time and chloride ions on the interaction of cisplatin with human albumin in-vitro

The interaction of cis-dichlorodiammineplatinum (II) (cisplatin) with human serum albumin (HSA), dissolved in phosphate buffer with or without sodium chloride (0.1 M) has been examined at pH 7.4 and mu = 0.154. Equal volumes of cisplatin and HSA solutions were incubated at 37 degrees C for various times and filterable platinum concentrations versus time measured by flameless atomic absorption spectrophotometry. Binding kinetics differed depending on the buffer solutions used and on the time elapsing between cisplatin dissolution and outset of incubation with HSA. Experimental data were fitted to a theoretical equation used to calculate the number of nucleophilic sites per HSA molecule. Titrations of the HSA sulphydryl group content before and after incubation with a cisplatin solution were made, from which it was shown that the lone SH-group of the HSA macromolecule is involved in cisplatin binding. We also studied HSA's sensitivity towards denaturing agents when it was complexed with cisplatin. This sensitivity was decreased upon cisplatin binding. Also, the binding capacities of HSA and the HSA-Pt(II) complex to both tryptophan and warfarin were compared to determine the possible influence of cisplatin upon the binding to HSA of other drugs; this influence was negligible.

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.

Amino acid substitutions in genetic variants of human serum albumin and in sequences inferred from molecular cloning

The structural changes in four genetic variants of human serum albumin were analyzed by tandem high-pressure liquid chromatography (HPLC) of the tryptic peptides, HPLC mapping and isoelectric focusing of the CNBr fragments, and amino acid sequence analysis of the purified peptides. Lysine-372 of normal (common) albumin A was changed to glutamic acid both in albumin Naskapi, a widespread polymorphic variant of North American Indians, and in albumin Mersin found in Eti Turks. The two variants also exhibited anomalous migration in NaDodSO4/PAGE, which is attributed to a conformational change. The identity of albumins Naskapi and Mersin may have originated through descent from a common mid-Asiatic founder of the two migrating ethnic groups, or it may represent identical but independent mutations of the albumin gene. In albumin Adana, from Eti Turks, the substitution site was not identified but was localized to the region from positions 447 through 548. The substitution of aspartic acid-550 by glycine was found in albumin Mexico-2 from four individuals of the Pima tribe. Although only single-point substitutions have been found in these and in certain other genetic variants of human albumin, five differences exist in the amino acid sequences inferred from cDNA sequences by workers in three other laboratories. However, our results on albumin A and on 14 different genetic variants accord with the amino acid sequence of albumin deduced from the genomic sequence. The apparent amino acid substitutions inferred from comparison of individual cDNA sequences probably reflect artifacts in cloning or in cDNA sequence analysis rather than polymorphism of the coding sections of the albumin gene.

Participation of the lone tryptophan residue of rat alpha-foetoprotein in its drug-binding sites. Comparison with rat serum albumin

The participation in drug binding of the lone tryptophan residue of rat alpha-foetoprotein (alpha-FP) and serum albumin, the two main transport proteins of foetal serum, has been studied by two different techniques. Firstly, the effect on phenylbutazone and warfarin binding of the chemical derivatization of the lone tryptophan residue of both proteins by 2-nitrophenylsulphonyl chloride (NPS) was studied. Secondly, the effect of phenylbutazone binding on the intrinsic fluorescence of the tryptophan residue of rat alpha-FP and albumin was investigated. The specific modification of the proteins by NPS did not affect the binding of warfarin by rat alpha-FP and albumin, but greatly decreased the affinity of the high-affinity sites of rat alpha-FP for phenylbutazone, though the numbers of these sites were not significantly changed. However, for albumin a similar decrease in the affinity constant appeared to be due to the reaction conditions. The spectrofluorimetric studies showed that the lone tryptophan residue of alpha-FP and albumin was quenched by phenylbutazone binding, and the quenching paralleled the fractional saturation of the high-affinity site only in the case of albumin. The effect of phenylbutazone binding on the intrinsic fluorescence of rat alpha-FP indicated that the lone tryptophan residue of this foetal protein is not in the same molecular environment as that of albumin, not participating directly in the high-affinity site for phenylbutazone, and the effect may be via some induced conformational change in rat alpha-FP. These results also confirm our previous suggestion that the high-affinity sites for phenylbutazone and warfarin are different on the rat alpha-FP molecule. The results seem to indicate that this is also the case for albumin, but confirmation is necessary.

Fluorescence energy transfer study of the relationship between the lone tryptophan residue and drug binding sites in human serum albumin

The relationship between the lone tryptophan residue at position 214 and drug binding sites (Sites I and II) in human serum albumin (HSA) was studied by fluorescence energy transfer. The distance between the lone tryptophan residue and ligands bound to HSA was estimated by Förster's equation, taking into consideration the degree of ligand binding at these sites, as determined from binding parameters (binding constant, k, and the number of binding sites, n). For all ligands investigated, the distance in each case appeared to asymptotically decrease when the occupation ratio of the binding sites increased with ligand concentration. When the primary binding site of each ligand in HSA was almost saturated, the distance attained a constant value, making possible a somewhat more exact determination of the distance. The distance ranged from approximately 22 to 23 A for ligands typical of Site I (warfarin, dansylamide, dansylglutamine), and approximately 16.1 to 17.5 A for ligands typical of Site II (dansylsarcosine, dansylproline, dansylglycine, diazepam, flufenamic acid).

The extracellular control of intracellular aspirin hydrolysis

We have previously presented evidence that an intra-erythrocyte arylesterase is primarily responsible for the deacetylation of aspirin to salicylate. We now report studies on extracellular control factors. The fractional acetylsalicylic acid (ASA) hydrolysis at the completion of 20-minute incubations in suspensions of washed red blood cells (RBC) with human serum albumin (HSA) decreased sharply, from 0.6 at 0 gm/dl HSA to 0.2 at 5.0 gm/dl HSA. In RBC suspensions with oleic, linoleic, and lauric acid, ASA hydrolysis increased as fatty acid:HSA ratios increased. The addition of salicylate (SA) also resulted in an increase in the fraction hydrolyzed. In a separate, more direct, ultracentrifugation assay, unbound ASA levels were similarly affected by HSA and the other ligands. When SA and oleic acid were incubated with ASA in RBC, additive competition for binding sites was apparent at relatively low levels of the fatty acid. In the presence of increasing concentrations of sodium SA, 5-amino SA, and 4-amino SA, ASA hydrolysis in RBC suspensions was increased by an average of 137%, 75%, and 54%, respectively. The most reasonable explanation for these findings is that the intact ASA molecule binds to albumin and that the binding of competing ligands can modulate the levels of free ASA in the circulation. This, in turn, affects the amount of drug available as substrate for penetration into the RBC and reaction with the intracellular hydrolase which catalyzes ASA deacetylation. Such factors controlling drug availability, if validated in vivo, could be important with regard to the antiinflammatory and anti-platelet properties of aspirin.

Pharmacokinetics of bendazac-lysine and 5-hydroxybendazac in patients with renal insufficiency

The pharmacokinetics of bendazac and its major metabolite, 5-hydroxybendazac, have been investigated in 15 patients with moderate to severe renal insufficiency and renal failure following a single oral dose of 500 mg bendazac-lysine. The pharmacokinetic parameters were compared to those obtained in 10 healthy adult volunteers. The rate and the extent of absorption of bendazac was not modified in the patients with moderate and severe renal insufficiency, nor was there any change in plasma tmax, Cmax, apparent elimination t1/2 and AUC. There was a significant increase in the unbound fraction of bendazac in renal failure patients undergoing haemodialysis, with a consequent increase in the apparent volume of distribution (V/F) and apparent plasma clearance (CL/F), and a decrease in plasma Cmax and AUC. Simultaneous changes of V/F and CL/F lead to an unchanged plasma t1/2 in these patients. Renal clearance (CLR) was decreased, but CL/F was not affected, since renal excretion is a minor route of elimination of bendazac. Bendazac is mostly eliminated by metabolism to 5-hydroxybendazac, in healthy subjects greater than 60% of a dose being excreted in urine as 5-hydroxybendazac and its glucuronide. In patients with renal insufficiency urinary excretion of 5-hydroxybendazac was decreased and the systemic availability of the metabolite (AUC), was increased about three-fold, irrespective of the degree of renal failure. Plasma 5-hydroxybendazac glucuronide accumulated according to the degree of renal insufficiency. Overall it can be assumed that the pharmacological effect of the drug will not be enhanced in renal failure and that the dosage regimen of bendazac-lysine in such patients need not be modified.

Protein binding of tolfenamic acid in the plasma from patients with renal and hepatic disease

The protein binding of tolfenamic acid in plasma from patients with renal and hepatic disorders was studied by equilibrium dialysis. Drug binding to the cellular components of whole blood and blood cell suspensions was also measured. Salicylic acid was used as the reference drug in all experiments. Renal and hepatic diseases increased the unbound fraction of tolfenamic acid. Free drug fractions were significantly correlated with changes in creatinine, urea, and total bilirubin, but not with those in albumin or total protein in plasma. Comparison of the theoretical binding parameters in control plasma and similar changes in protein binding in all the plasma samples studied revealed that tolfenamic acid and salicylic acid probably share a common primary binding site. The significance of the correlation permits use of salicylic acid as a model drug for predicting changes in the protein binding of tolfenamic acid. The measurements of binding properties in whole blood and blood cell--buffer suspension showed that tolfenamic acid interacts with the lipid membrane structures of blood cells, while salicylic acid is distributed into the aqueous cell space.

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.

The effect of intra-articular steroids on plasma salicylate concentrations

Plasma salicylate concentrations and salicylate clearances were evaluated in 10 patients before and then 3 days after the intra-articular administration of glucocorticoid steroids. All patients were on chronic salicylate treatment, the dosage varying from 1.3-4.6 g day-1 of enteric-coated aspirin. The mean steady state plasma concentration (+/- s.e. mean) of salicylate decreased significantly after intra-articular steroid (90.9 +/- 14.8 mg l-1-64.7 +/- 8.9, P less than 0.05), whilst the mean clearance increased significantly (87.3 +/- 20.6 ml min-1-120.6 +/- 28.3, P less than 0.05). In these paired studies, the intra-articular administration of steroids lead to a significant decrease in plasma salicylate levels in patients taking enteric-coated aspirin.

Stereoselective binding of 3-acetoxy-, and 3-hydroxy-1,4-benzodiazepine-2-ones to human serum albumin. Selective allosteric interaction with warfarin enantiomers

Stereoselective binding of oxazepam, lorazepam, temazepam and methyl lorazepam as well as of their acetates to human serum albumin was investigated by different techniques. The 2'-chlorine and the N(1)-methyl substitution exert opposite effects on the antipodes. Enantiomers of oxazepam acetate (OAc) and lorazepam acetate (LAc) displace diazepam. Allosteric interactions with warfarin were manifested by either mutually increased or decreased binding depending on the structure of benzodiazepine and on the configuration of both benzodiazepine and warfarin. The most remarkable effect could be observed in the simultaneous binding of (S)-lorazepam acetate and (S)-warfarin.

Drug-binding properties of human alpha-foetoprotein

The drug-binding properties of human alpha-foetoprotein (alpha FP) were investigated by a fluorescence-spectral method. Human alpha FP was shown to bind to albumin's site I marker (warfarin, phenylbutazone), site II marker (L-tryptophan), but not site III marker (cholic acid, digoxin). The binding of human alpha FP towards lower alcohols was examined, and this binding seems to depend partly on the hydrophobicity of the ligands. The binding of human alpha FP is discussed in comparison with human serum albumin or rat alpha FP.

Evidence for the fatty acid-induced heterogeneity of the N and B conformations of human serum albumin

The influence of oleic acid on the interaction between albumin and warfarin, oxyphenbutazone or diazepam has been studied by circular dichroism and equilibrium dialysis. The pH dependences of the molar ellipticity of the drug-albumin complexes and of the free fraction of drug are completely changed by the presence of oleic acid. This phenomenon is attributed to an oleic acid-induced conformational change in both the neutral (N) and the basic (B) conformation of albumin, a change to which the warfarin-oxyphenbutazone binding area and the diazepam binding site is sensitive. The oleic acid-induced conformational states of albumin, the so-called N* and B* conformations, show binding properties that are different from the binding properties of the N and B conformations.

Aromatic amino acid metabolites as potential protein binding inhibitors in human uremic plasma

Decreased binding of aromatic acidic drugs and endogenous metabolites to plasma proteins of patients with severe renal failure appears to be due to accumulation of unknown solutes. Both the warfarin and indole binding sites of albumin, the principal binding protein for these ligands, are affected. We used a large number of endogenous aromatic acids and synthetic congeners as displacers (a) better to characterize the chemical requirements for binding to each site and (b) to derive clues to the chemical structure of the undefined binding inhibitors in uremic plasma. 14C-tryptophan, 14C-warfarin and 14C-salicylate were used as bound ligands. Numerous indoles, quinolines and phenyl derivatives were moderate to strong displacers with several structural correlates. Increasing apolar side chain length enhanced displacing potency. A hydroxyl group at the 5 position of indoles and at the para position of phenyl derivatives severely reduced activity. The two ends of amphophilic molecules showed opposite requirements for displacement of tryptophan: the greater the polarity at the hydrophilic end, the greater the tryptophan displacing potency. Conversely, the greater the total hydrophobic mass of the remainder of the molecule, the more potent the inhibition of binding. The dipeptides l-tryptophyl-l-tryptophan and l-tryptophyl-l-phenylalanine were potent displacers. Computer-assisted analysis of warfarin binding in the presence of xanthurenic acid revealed inhibition by a mechanism other than simple competition, probably via a third albumin binding locus. We conclude that decreased binding in uremic plasma is most likely the summation effect of a number of retained aromatic acids, peptides, or both types of ligands.

Protein binding of oxazepam and its glucuronide conjugates to human albumin

The binding of oxazepam and its glucuronide conjugates to human serum albumin (HSA), as well as the binding interactions of the drug and its metabolites, were examined by equilibrium dialysis and kinetic probe studies. Oxazepam and its S(+) glucuronide are bound to the HSA molecule with affinity constants of 3.5 X 10(5) M-1 and 5.5 X 10(4) M-1, respectively, which were independent of protein concentration over a range of 0.1 to 5.0 g/dl. The R(-) glucuronide bound weakly to albumin, with the binding parameter, N X K, increasing at lower albumin concentrations. Pre-acetylation of fatty acid free-HSA resulted in decreased binding of all three compounds, probably by altering the conformation of the binding sites. Kinetic probe studies with p-nitrophenyl acetate indicate that oxazepam and its S(+) glucuronide shared a common binding site on HSA, but that the R(-) glucuronide bound at another site. Oxazepam binding was unaffected by the presence of its glucuronide conjugates but was inhibited by fatty acids. The percentage of oxazepam bound to plasma proteins in patients with renal impairment (94%) was lower than in normal volunteers (97%). This lower binding can neither be attributed to lower albumin concentrations because of the large binding capacity of the protein and linearity of N X K nor to displacement by elevated concentrations of glucuronide conjugates, but it may be ascribed partly to increased plasma fatty acids.

Diclofenac binding to albumin and lipoproteins in human serum

The binding of diclofenac to human serum albumin (HSA) and to lipoproteins was studied in vitro by equilibrium dialysis. Binding to HSA is characterized by two classes of sites with one site each (K1 = 5 X 10(5) M-1 and K2 = 0.6 X 10(5) M-1). The binding to lipoproteins was shown to be saturable with a larger number of binding sites and low association constants. The evidence of two specific binding sites on HSA was confirmed by circular dichroism data. In addition, an identification of those sites was performed by displacement of fluorescent probes. The data show that the high affinity site (K1 = 5 X 10(5) M-1) is likely to be shared by benzodiazepines while the second one (K2 = 0.6 X 10(5) M-1) is common with the warfarin site.

Protein binding as a component of drug interaction in cellular pharmacokinetic studies. Effects of probenecid on transport and accumulation of methotrexate in Ehrlich ascites tumor cells in vitro

The organic acid probenecid has been shown to interfere with the active extrusion of methotrexate (MTX) from L1210 tumor cells in vitro leading to enhanced MTX accumulation and increased formation of MTX polyglutamate derivatives. In the presence of serum albumin (4 g/100 ml), to which probenecid is bound, the inhibition by probenecid of [3H]MTX efflux from the Ehrlich ascites tumor cell was reduced markedly. While half-maximal inhibition of MTX efflux occurred with 0.12 mM probenecid in the absence of albumin, 1.45 mM probenecid was required in its presence. The presence of albumin also modified the probenecid-induced elevation of steady-state MTX levels in the tumor cell. Maximal elevation of cellular MTX levels occurred with 0.5 mM probenecid in the absence of albumin, and 3 mM probenecid in its presence. Serum albumin further reversed the effects of probenecid on MTX influx. While probenecid inhibited influx of 1 microM [3H]MTX in the absence of albumin (half-maximal inhibition at approximately 1 mM probenecid), probenecid stimulated MTX influx in its presence (half-maximal effect at 0.5 to 1 mM). Equilibrium dialysis studies demonstrated that probenecid displaced MTX from albumin, increasing the effective free concentration of MTX in the incubation medium, and hence the rate of MTX influx. Therefore, probenecid may enhance the accumulation of MTX in the tumor cells by increasing the level of free (as opposed to albumin bound) MTX in the extracellular medium as well as by direct inhibition of MTX efflux. These observations may provide an additional explanation for probenecid enhancement of the therapeutic efficacy of MTX in tumor bearing mice and highlight the importance of assessing drug-protein interactions in an in vitro experimental model.

Auramine O as a fluorescent probe for the binding of basic drugs to human alpha 1-acid glycoprotein (alpha 1-AG). The development of a simple fluorometric method for the determination of alpha 1-AG in human serum

A cationic fluorescent dye, auramine O (AO), exhibited an intense increase in fluorescence after binding to human alpha 1-acid glycoprotein (alpha 1-AG). The interaction between AO and the protein was studied by fluorescence spectroscopy and by equilibrium dialysis. AO binds to the protein via a single site with a dissociation constant of 24 microM. Various basic drugs such as chlorpromazine, imipramine, desipramine, quinidine, propranolol and lidocaine, which are known to bind to the protein, competitively inhibited the AO binding to the protein. The dissociation constants of these basic drugs obtained from such inhibitory experiments were comparable to those obtained with other methods (equilibrium dialysis, quenching of protein intrinsic fluorescence, and the difference spectrophotometric method) and from the literature. It is concluded that AO may be a useful fluorescent probe that binds to a single basic drug binding site on alpha 1-AG. In addition, a simple fluorometric method for the determination of alpha 1-AG in serum was developed using AO, and the validity of this method was confirmed by comparing it with the conventional radial immunodiffusion method.

Human tissue-type plasminogen activator is related to albumin and alpha-fetoprotein

Using a computer program designed to detect evolutionary relationships between proteins, I find that residues 72-110 of the mature sequence of human tissue-type plasminogen activator (t-PA) and 39 residues at the carboxy terminus of human albumin have a comparison score that is 8.8 standard deviation units higher than that obtained with a comparison of randomized sequences of these proteins. The probability (p) of getting this score by chance is approximately 10(-18), indicating that part of t-PA and albumin are derived from a common ancestor. I also find that alpha-fetoprotein, a relative of albumin is related to t-PA. Part of this region on t-PA has been previously shown to be related to epidermal growth factor. t-PA, albumin, alpha-fetoprotein, and epidermal growth factor have diverse biological activities. The finding that these proteins are related suggests some new approaches for studying their functions.

A study on the allosteric interaction between the major binding sites of human serum albumin using microcalorimetry

The binding of warfarin and oxyphenbutazone to albumin has been studied at pH 6.8 and pH 9.2 by measuring the heat of binding of these ligands to their high-affinity binding sites on albumin (delta Ho'1). The -delta Ho'1 values for the binding of warfarin at pH 6.8 and 9.2 and oxyphenbutazone at pH 6.8 and 9.2 were found to be 16.9(+/- 0.6), 28.8(+/- 0.6), 10.5(+/- 0.4) and 17.4(+/- 0.6) kJmol-1, respectively. The Gibbs energies (delta Go'1) corresponding to these delta Ho'1 values cover a much smaller range. The pH dependences of delta Go'1 and delta Ho'1 are explained in terms of pK shifts in the albumin upon binding warfarin or oxyphenbutazone. Diazepam, which binds to a site on albumin which is different from the warfarin-oxyphenbutazone binding site, increases - delta Ho'1 for the binding of warfarin and oxyphenbutazone to albumin at pH 6.8, but it does not influence the -delta Ho'1 at pH 9.2. This phenomenon may be attributed to an allosteric interaction between the diazepam binding site and the warfarin binding site. This allosteric interaction must have its origin in a phenomenon other than the N-B transition.

Relations between high-affinity binding sites of markers for binding regions on human serum albumin

Binding of warfarin, digitoxin, diazepam, salicylate and Phenol Red, individually or in different pair combinations, to defatted human serum albumin at ligand/protein molar ratios less than 1:1 was studied at pH 7.0. The binding was determined by ultrafiltration. Some of the experiments were repeated with the use of equilibrium dialysis in order to strengthen the results. Irrespective of the method used, all ligands bind to one high-affinity binding site with an association constant in the range 10(4)-10(6) M-1. High-affinity binding of the following pair of ligands took place independently: warfarin-Phenol Red, warfarin-diazepam, warfarin-digitoxin and digitoxin-diazepam. Simultaneous binding of warfarin and salicylate led to a mutual decrease in binding of one another, as did simultaneous binding of digitoxin and Phenol Red. Both effects could be accounted for by a coupling constant. The coupling constant is the factor by which the primary association constants are affected; in these examples of anti-co-operativity the factor has a value between 0 and 1. In the first example it was calculated to be 0.8 and in the latter 0.5. Finally, digitoxin and salicylate were found to compete for a common high-affinity binding site. The present findings support the proposal of four separate primary binding sites for warfarin, digitoxin (and salicylate), diazepam and Phenol Red. An attempt to correlate this partial binding model for serum albumin with other models in the literature is made.

Gossypol inhibits estrogen binding to rat alpha-fetoprotein

We find that gossypol, a male anti-fertility compound, is a reversible competitive inhibitor of estrogen binding to rat alpha-fetoprotein (AFP). The Kd of gossypol for rat AFP is 1.75 microM, which is similar to gossypol's affinity for lactate dehydrogenase isozyme X, the putative site where gossypol exerts its anti-fertility effects. Reacting sodium cyanoborohydride with gossypol reduces its affinity for AFP, showing that intact aldehyde groups on gossypol are important for binding to rat AFP and indicating that gossypol is specifically inter-acting with a nucleophilic site on AFP that influences estrogen binding.

Inhibition of salicylate binding to normal plasma by extracts of uremic fluids

We previously reported that an extract of uremic plasma reduces binding of phenytoin and tryptophan by normal plasma and plasma albumin. This effect appears to reproduce the impaired binding of many drugs and several endogenous metabolites by uremic plasma. In the present study we further characterized the properties of extracts from uremic sera and body fluids using binding of salicylate as a model. Salicylate was chosen because it binds to both of the main albumin binding loci for aromatic, acidic drugs. Using a computer-assisted, least-squares, curve-fitting program, LIGAND, we found that the most satisfactory model for salicylate binding to 1:10 diluted normal plasma was a binding number (n) of 2 mol of salicylate per mole of albumin with an association constant (k) of 2.85 X 10(4) L/mol, an additional binding of 0.5 mol to other sites on albumin or to other proteins, and nonspecific binding of 21%. Addition of uremic pleural fluid extract to diluted normal plasma produced a monotonic decline in k to 0.17 X 10(4) L/mol with no change in n except possibly at the highest dose of uremic inhibitor. This pattern of competitive inhibition indicates presence of unknown ligands in the uremic extract that compete at both binding loci. More efficient extraction methods might also yield additional ligand(s) that inhibit through a noncompetitive mechanism.

Drug-binding properties of rat alpha 1-foetoprotein. Binding of warfarin, phenylbutazone, azapropazone, diazepam, digitoxin and cholic acid

As part of an investigation into whether alpha 1-foetoprotein (alpha 1-FP) plays the same transport role in foetal serum as albumin does in the adult, the binding properties of both proteins were compared with respect to the binding of a series of compounds known to be bound by albumin's specific drug-binding sites. The binding of warfarin, phenylbutazone, azapropazone, diazepam, digitoxin and cholic acid by rat alpha 1-FP and serum albumin was studied by equilibrium dialysis at 4 degrees C. Rat alpha 1-FP was shown to have neither albumin's high-affinity site II (diazepam as marker) nor its site III (digitoxin and cholic acid as markers). High-affinity binding by alpha 1-FP was found for the specific markers (warfarin, phenylbutazone, azapropazone) of albumin's drug-binding site I. However, instead of albumin's one high-affinity site/molecule, a mean value of 0.5 site/molecule was obtained with rat alpha 1-FP. Charcoal treatment at neutral pH of rat serum albumin did not affect its measured binding properties, but treatment of the alpha 1-FP led to an increased affinity for warfarin, phenylbutazone and azapropazone without a change in the measured number of sites, indicating competition for binding at this site by (an) endogenous ligand(s). These results are discussed in terms of the structures of the two proteins and with respect to the physiological implications of the differences found.

The binding of aryl carboxylic acid derivatives to human serum albumin--a structure-activity study

The binding interactions of some aryl carboxylic acid derivatives have been examined by circular dichroism and fluorescence spectroscopy. With specific probes, we have shown that the seven ligands under study bind primarily to the benzodiazepine site on HSA. Their association constants are in the range of 10(5)-10(6) M-1 as found by spectropolarimetric titration, and appear to be closely related to some chemical features. It is concluded that the binding is enhanced by the lengthening of the carbon chain substituent with a terminating carboxyl moiety and by halogen substitution in the aromatic rings. It is further shown that hydrophilic substitutions such as hydroxyl or ketone groups in the carbon chain substituent will decrease the binding.

Characterization of microenvironments of 2-(4'-hydroxyphenylazo)benzoic acid bound to bovine serum albumin by studying the solvent effects

The binding and thermodynamic parameters of the two binding forms of 2-(4'-hydroxyphenylazo)benzoic acid to bovine serum albumin were estimated. The number of the binding sites of the azo form was 3.2 and that of the hydrazone form was 0.7. The binding of the hydrazone form was affected only by enthalpy changes, in contrast to that of the azo form which was affected by both enthalpy and entropy changes. To estimate the complex microenvironments of 2-(4'-hydroxyphenylazo)benzoic acid molecules on bovine serum albumin, the solvent effects were studied. The 2-carboxyl group of 2-(4'-hydroxyphenylazo)benzoic acid participates in the azo-hydrazone tautomerism . The 2-carboxylate ion forms an ion pair with triethylamine in ethylene dichloride, chloroform, and benzene, resulting in the appearance of the hydrazone form. The hydrazone formation in the system of 2-(4'-hydroxyphenylazo)benzoic acid-triethylamine (1:1) in chloroform was affected only by enthalpy changes, in the same manner as in the system of 2-(4'-hydroxyphenylazo)benzoic acid-bovine serum albumin. We speculate the presence of the two kinds of ion pairs on the basis of the changes of the azo-hydrazone tautomerism in chloroform, and that the azo form takes the contact ion pair and the hydrazone form takes the solvent-separated ion pair. A new possible model for the interaction of the azo and hydrazone forms and bovine serum albumin is proposed.

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.

Heat inhibition of in vitro lipolysis and 14C ibuprofen protein binding in plasma from heparinized uraemic subjects

Protein binding determination in post heparin plasma samples is complicated by the continued post heparin lipase activity, in vitro, during the binding analysis. The decomposition of lipoproteins and accumulation of nonesterified fatty acids (NEFA) results in artifically elevated free fractions of many drugs. This artefact is particularly accentuated in haemodialysis patients who are frequently hypertriglyceridaemic and receive large doses of heparin. Rapid heat treatment (60 degrees for 15 min) of plasma from heparinized uraemic subjects is shown to inhibit the in vitro lipolysis occurring during 2 hours of equilibrium dialysis at 37 degrees (ED). Mean NEFA concentrations in heat treated plasma after ED (means = 400 +/- 141 mumol/L) were not different (p greater than 0.05, n = 9) from the baseline values in fresh plasma (means 351 +/- 117 mumol/L) but were considerably less (p less than 0.005) than NEFA levels in untreated plasma after ED (means = 1025 +/- 523 mumol/L). The degree of in vitro lipolysis inhibition (92 +/- 6.6%) was very much greater than using the chemical inhibitors phenyl methyl sulphonyl fluoride, EDTA, Triton X100 or protamine sulphate. Heat treatment at 60 degrees for 15 min increased the percentage of free 14C ibuprofen in 3.5% isolated human serum albumin from 0.34% to 0.62%. Reduced binding as a result of heat treatment was not observed however in whole plasma. The percentage free ibuprofen in heat treated, whole plasma from both heparinized and non heparinized subjects (means = 1.22 +/- 0.19; n = 29) was not different (p greater than 0.05) from the percentage free determined in plasma from a non heparinized group (means = 1.16 +/- 0.23; n = 15). In contrast the % free ibuprofen in untreated plasma from heparinized subjects was markedly higher (means = 1.56 +/- 0.41; n = 24; p less than 0.05). There was a strong correlation between % free ibuprofen and plasma NEFA concentration (r = 0.8; p less than 0.005; n = 68). The heat treatment of plasma for 15 min at 60 degrees is proposed as an effective means of controlling heparin induced lipolysis in vitro and may be valuable in overcoming the post heparin binding artefact.

Esterase-like activity of human serum albumin II: reaction with N-trans-cinnamoylimidazoles

To elucidate the details of the esterase activity of human serum albumin, the reaction of N-trans-cinnamoylimidazoles with albumin was investigated kinetically at various pHs at 25 degrees. The reaction consisted of the acylation of albumin (probably the tyrosine-411 residue) by the substrate and the deacylation of cinnamoyl-albumin. The acylation was approximately 10--100-fold faster than the spontaneous hydrolysis of the substrate over the pH range examined. The pH profile for the deacylation rate constant indicated the participation of a group having a pKa of approximately 9.4. The deacylation was subjected to the effect of deuterium oxide. The electron-withdrawing substituent facilitated the deacylation; the Hammett rho value was 1.63. These results suggest that the deacylation proceeded via general base catalysis by this group.

Kinetics of the rapid modification of human serum albumin with trinitrobenzenesulfonate and localization of its site

The rapid reaction of human serum albumin with trinitrobenzenesulfonate (I) and the location of the reactive site were investigated to characterize the chemical modification of albumin by I. The modification proceeds through trinitrophenylation of a lysine residue of albumin and monoaddition of the byproduct, sulfite ion, to the trinitrophenylalbumin, as reported previously. The individual kinetic parameters for both reactions were determined at various pH values and 25 degrees. The epsilon-amino group of the lysine residue which has a pKa value of approximately 8.9 was the reactive group involved in the trinitrophenylation. The dissociation constant of the sulfite monoadduct was about 10-fold smaller than that of the monoadduct of the model compound trinitrophenyl alpha-acetyllysine. The modification of albumin by I reduced the fluorescence intensity of the tryptophan-214 residue in the albumin amino acid sequence. Acetylation of the lysine-199 residue with aspirin and 5-nitroaspirin decreased the trinitrophenylation rate of albumin with I. These results on the fluorescence spectroscopy and the effect of the acetylation suggest that the reactive group for I is the lysine-199 residue located near the tryptophan-214 residue.

Characterization of a common binding site for basic drugs on human alpha 1-acid glycoprotein (orosomucoid)

The interaction of chlorpromazine, DL-propranolol, and imipramine with isolated alpha 1-acid glycoprotein is characterized by relatively high association constants and only one binding site per protein molecule. The mutual displacement between the three drugs indicates that all three compounds are bound to the same binding site. Several other basic drugs from different pharmacological and chemical classes also displace chlorpromazine, DL-propranolol, and imipramine with potencies, one would predict from their association constants or from the degree of their plasma binding in humans. It is concluded that displacement phenomena like those observed in this study in vitro are likely to occur also in vivo.

Relations between high-affinity binding sites for L-tryptophan, diazepam, salicylate and Phenol Red on human serum albumin

Binding of L-tryptophan, diazepam, salicylate and Phenol Red to defatted human serum albumin was studied by ultrafiltration at pH 7.0. All ligands bind to one high-affinity binding site with association constants of the order of 10(4)-10(5)M-1. The number of secondary binding sites was found to vary from zero to five, with association constants about 10(3)M-1. Competitive binding studies with different pairs of the ligands were performed. Binding of both ligands was determined simultaneously. L-Tryptophan and diazepam were found to compete for a common high-affinity binding site on albumin. The following combinations of ligands do not bind competitively to albumin: L-tryptophan-Phenol Red, L-tryptophan-salicylate and Phenol Red-salicylate. On the other hand, high-affinity bindings of the three ligands do not take place independently but in such a way that binding of one of the ligands results in a decrease in binding of the other ligands. The decreases in binding are reciprocal and can be accounted for by introducing a coupling constant. The magnitude of the constant is dependent on the ligands being bound. In the present study, the mutual decrease in binding was more pronounced with L-tryptophan-salicylate and Phenol Red-salicylate than with L-tryptophan-Phenol Red.

The effects of N-B transition of human serum albumin on the specific drug-binding sites

In the pH range 6-9, human serum albumin undergoes a conformational change termed the neutral-base (N-B) transition. Recently, it has been shown that the N-B transition causes enhanced binding at the warfarin-binding site (site I). The present study used fluorescence and equilibrium dialysis to investigate the effects of the N-B transition, chloride, calcium and fatty acids on the specific binding sites I and II on human serum albumin. The effect of the N-B transition of human serum albumin provides a further distinction between site I and II binding characteristics. The N-B transition of albumin caused a change in conformation at site I which resulted in increased binding of drugs and fluorescent probes at this site, whereas there was no effect on acidic drug binding at site II. These effects on site I and II are qualitatively similar to those induced by fatty acids (increased drug binding at site I and no change at site II). However, the effects of increasing pH and fatty acids were additive, showing that they were caused by two different conformational changes. The effect of Cl- on site I binding was pH-dependent and was abolished by the presence of fatty acid. Ca2+ reduced the fluorescence of site I probes but had no effect on a site II fluorescent probe. Effects of pH were also investigated with drugs not binding to site I or II. Increasing pH caused a decrease in binding to indomethacin, increases in binding of L-tryptophan, tolmetin and quinidine and no change in the binding of salicylic acid, diflunisal and phenytoin.