Studies of protein binding to nonpolar solutes by using zonal elution and high-performance affinity chromatography: interactions of cis- and trans-clomiphene with human serum albumin in the presence of beta-cyclodextrin

Generation of affinity maps for thiazolidinediones with human serum albumin using affinity microcolumns. I. Studies of effects by glycation on multisite drug binding

Human serum albumin (HSA) is known to undergo modifications by glucose during diabetes. This process produces glycated HSA that can have altered binding to some drugs. In this study, high-performance affinity microcolumns and competition studies were used to see how glycation affects the binding by two thiazolidinedione-class drugs (i.e., pioglitazone and rosiglitazone) at specific regions of HSA. These regions included Sudlow sites I and II, the tamoxifen and digitoxin sites, and a drug-binding site located in subdomain IB. At Sudlow site II, the association equilibrium constants (or binding constants) for pioglitazone and rosiglitazone with normal HSA were 1.7 × 105 M-1 and 2.0 × 105 M-1 at pH 7.4 and 37 °C, with values that changed by up to 5.7-fold for glycated HSA. Sudlow site I of normal HSA had binding constants for pioglitazone and rosiglitazone of 3.4 × 105 M-1 and 4.6 × 105 M-1, with these values changing by up to 1.5-fold for glycated HSA. Rosiglitazone was found to also bind a second region that had a positive allosteric effect on Sudlow site I for all the tested preparations of HSA (binding affinity, 1.1-3.2 × 105 M-1; coupling constant for Sudlow site I, 1.20-1.34). Both drugs had a strong positive allosteric effect on the tamoxifen site of HSA (coupling constants, 13.7-19.9 for pioglitazone and 3.7-11.5 for rosiglitazone). Rosiglitazone also had weak interactions at a site in subdomain IB, with a binding constant of 1.4 × 103 M-1 for normal HSA and a value that was altered by up to 6.8-fold with glycated HSA. Neither of the tested drugs had any significant binding at the digitoxin site. The results were used to produce affinity maps that described binding by these thiazolidinediones with HSA and the effects of glycation on these interactions during diabetes.

A chromatographic method for determining the interaction between a drug and two target proteins by fabricating a dual-heterogeneous surface

Characterization of the drug-target interactions is pivotal throughout the whole procedure of drug development. Most of the current assays, particularly, chromatographic methods lack the capacity to reveal drug adsorption on the muti-target surface. To this end, we derived a reliable and workable mathematical equation for revealing drug bindings to dual targets on the heterogeneous surface starting from the mass balance equation. The derivatization relied on the correlation of drug injection amounts with their retention factors. Experimental validation was performed by determining the binding parameters of three canonical drugs on a heterogeneous surface, which was fabricated by fusing angiotensin receptor type I and type II receptors (AT1R and AT2R) at the terminuses of circularly permuted HaloTag (cpHaloTag) and immobilizing the whole fusion protein onto 6-bromohexanoic acid modified silica gel. We proved that immobilized AT1R-cpHalo-AT2R maintained the original ligand- and antibody-binding activities of the two receptors in three weeks. The association constants of valsartan, candesartan, and telmisartan to AT1R were (6.26±0.14) × 105, (9.66±0.71) × 105, and (3.17±0.03) × 105 L/mol. In the same column, their association constants to AT2R were (1.25±0.04) × 104, (2.30±0.08) × 104, and (8.51±0.06) × 103 L/mol. The patterns of the association constants to AT1R/AT2R (candesartan>valsartan>telmisartan) were in good line with the data by performing nonlinear chromatography on control columns containing immobilized AT1R or AT2R alone. This provided proof of the fact that the derivatization allowed the determination of drug bindings on the heterogeneous surface with the utilization of a single series of injections and linear regression. We reasoned that is simple enough to model the bindings of drug adsorption on commercially available adsorbents in fundamental or industrial fields, thus having the potential to become a universal method for analyzing the bindings of a drug to the heterogeneous surface containing multiple targets.

Physical Separation of Amphiprotic-Polar AproticS for Simultaneous Extraction and Clean-up of Clomiphene from Plasma before Liquid Chromatographic Analysis

An efficient and quantitative two phase freezing (TPF) method coupled with high performance liquid chromatography and UV-Vis detector was developed for the extraction, clean up, and determination of clomiphene citrate (CLC) in plasma samples. The separation of two miscible solvents by TPF method permits that the CLC was efficiently removed from proteins and transferred into the relative aprotic dipolar organic phase and in consequence, gave a higher recovery. The TPF method was compared to conventional liquid-liquid extraction and it gave more clean solution with better reproducibility. Linear range, limit of detection, and limit of quantification for CLC in plasma were obtained in the range of 0.06-18, 0.02, and 0.06 µg mL^-1, respectively. The intraday and interday reproducibility for concentration of 1.0 µg mL^-1 (%RSD) were 3.2% and 4.6%, respectively. In addition, the trueness, ruggedness, and reality of TPF were assessment. Finally, several real plasma samples were successfully analyzed using the developed method.

Analysis of stereoselective drug interactions with serum proteins by high-performance affinity chromatography: A historical perspective

The interactions of drugs with serum proteins are often stereoselective and can affect the distribution, activity, toxicity and rate of excretion of these drugs in the body. A number of approaches based on affinity chromatography, and particularly high-performance affinity chromatography (HPAC), have been used as tools to study these interactions. This review describes the general principles of affinity chromatography and HPAC as related to their use in drug binding studies. The types of serum agents that have been examined with these methods are also discussed, including human serum albumin, α1-acid glycoprotein, and lipoproteins. This is followed by a description of the various formats based on affinity chromatography and HPAC that have been used to investigate drug interactions with serum proteins and the historical development for each of these formats. Specific techniques that are discussed include zonal elution, frontal analysis, and kinetic methods such as those that make use of band-broadening measurements, peak decay analysis, or ultrafast affinity extraction.

High-Performance Affinity Chromatography: Applications in Drug-Protein Binding Studies and Personalized Medicine

The binding of drugs with proteins and other agents in serum is of interest in personalized medicine because this process can affect the dosage and action of drugs. The extent of this binding may also vary with a given disease state. These interactions may involve serum proteins, such as human serum albumin or α1-acid glycoprotein, or other agents, such as lipoproteins. High-performance affinity chromatography (HPAC) is a tool that has received increasing interest as a means for studying these interactions. This review discusses the general principles of HPAC and the various approaches that have been used in this technique to examine drug-protein binding and in work related to personalized medicine. These approaches include frontal analysis and zonal elution, as well as peak decay analysis, ultrafast affinity extraction, and chromatographic immunoassays. The operation of each method is described and examples of applications for these techniques are provided. The type of information that can be obtained by these methods is also discussed, as related to the analysis of drug-protein binding and the study of clinical or pharmaceutical samples.

Analysis of multi-site drug-protein interactions by high-performance affinity chromatography: Binding by glimepiride to normal or glycated human serum albumin

High-performance affinity chromatography (HPAC) was used in a variety of formats to examine multi-site interactions between glimepiride, a third-generation sulfonylurea drug, and normal or in vitro glycated forms of the transport protein human serum albumin (HSA). Frontal analysis revealed that glimepiride interacts with normal HSA and glycated HSA at a group of high affinity sites (association equilibrium constant, or Ka, 9.2-11.8×10(5)M(-1) at pH 7.4 and 37°C) and a group of lower affinity regions (Ka, 5.9-16×10(3)M(-1)). Zonal elution competition studies were designed and carried out in both normal- and reversed-role formats to investigate the binding by this drug at specific sites. These experiments indicated that glimepiride was interacting at both Sudlow sites I and II. Allosteric effects were also noted with R-warfarin at Sudlow site I and with tamoxifen at the tamoxifen site on HSA. The binding at Sudlow site I had a 2.1- to 2.3-fold increase in affinity in going from normal HSA to the glycated samples of HSA. There was no significant change in the affinity for glimepiride at Sudlow site II in going from normal HSA to a moderately glycated sample of HSA, but a slight decrease in affinity was seen in going to a more highly glycated HSA sample. These results demonstrated how various HPAC-based methods can be used to profile and characterize multi-site binding by a drug such as glimepiride to a protein and its modified forms. The information obtained from this study should be useful in providing a better understanding of how drug-protein binding may be affected by glycation and of how separation and analysis methods based on HPAC can be employed to study systems with complex interactions or that involve modified proteins.

Analysis of glipizide binding to normal and glycated human serum albumin by high-performance affinity chromatography

In diabetes, the elevated levels of glucose in the bloodstream can result in the nonenzymatic glycation of proteins such as human serum albumin (HSA). This type of modification has been shown to affect the interactions of some drugs with HSA, including several sulfonylurea drugs that are used to treat type II diabetes. This study used high-performance affinity chromatography (HPAC) to examine the interactions of glipizide (i.e., a second-generation sulfonylurea drug) with normal HSA or HSA that contained various levels of in vitro glycation. Frontal analysis indicated that glipizide was interacting with both normal and glycated HSA through two general groups of sites: a set of relatively strong interactions and a set of weaker interactions with average association equilibrium constants at pH 7.4 and 37 °C in the range of 2.4-6.0 × 10(5) and 1.7-3.7 × 10(4) M(-1), respectively. Zonal elution competition studies revealed that glipizide was interacting at both Sudlow sites I and II, which were estimated to have affinities of 3.2-3.9 × 10(5) and 1.1-1.4 × 10(4) M(-1). Allosteric effects were also noted to occur for this drug between the tamoxifen site and the binding of R-warfarin at Sudlow site I. Up to an 18% decrease in the affinity for glipizide was observed at Sudlow site I ongoing from normal HSA to glycated HSA, while up to a 27% increase was noted at Sudlow site II. This information should be useful in indicating how HPAC can be used to investigate other drugs that have complex interactions with proteins. These results should also be valuable in providing a better understanding of how glycation may affect drug-protein interactions and the serum transport of drugs such as glipizide during diabetes.

Review: Glycation of human serum albumin

Glycation involves the non-enzymatic addition of reducing sugars and/or their reactive degradation products to amine groups on proteins. This process is promoted by the presence of elevated blood glucose concentrations in diabetes and occurs with various proteins that include human serum albumin (HSA). This review examines work that has been conducted in the study and analysis of glycated HSA. The general structure and properties of HSA are discussed, along with the reactions that can lead to modification of this protein during glycation. The use of glycated HSA as a short-to-intermediate term marker for glycemic control in diabetes is examined, and approaches that have been utilized for measuring glycated HSA are summarized. Structural studies of glycated HSA are reviewed, as acquired for both in vivo and in vitro glycated HSA, along with data that have been obtained on the rate and thermodynamics of HSA glycation. In addition, this review considers various studies that have investigated the effects of glycation on the binding of HSA with drugs, fatty acids and other solutes and the potential clinical significance of these effects.

Research Spotlight: Research in bioanalysis and separations at the University of Nebraska – Lincoln

The Chemistry Department at the University of Nebraska – Lincoln (UNL) is located in Hamilton Hall on the main campus of UNL in Lincoln, NE, USA. This department houses the primary graduate and research program in chemistry in the state of Nebraska. This program includes the traditional fields of analytical chemistry, biochemistry, inorganic chemistry, organic chemistry and physical chemistry. However, this program also contains a great deal of multidisciplinary research in fields that range from bioanalytical and biophysical chemistry to nanomaterials, energy research, catalysis and computational chemistry. Current research in bioanalytical and biophysical chemistry at UNL includes work with separation methods such as HPLC and CE, as well as with techniques such as MS and LC–MS, NMR spectroscopy, electrochemical biosensors, scanning probe microscopy and laser spectroscopy. This article will discuss several of these areas, with an emphasis being placed on research in bioanalytical separations, binding assays and related fields.

[Determination of binding domains of paeonol on immobilized human serum albumin by high-performance affinity chromatography]

High-performance affinity chromatography was used to investigate the binding of paeonol (PAE) to immobilized human serum albumin (HSA) under the condition of pH 7.4 and temperature of 37 degrees C. Self-competition zonal elution indicated that there was only one major binding site on HSA for PAE. The association constant of PAE with HSA was 4.84 x 10(3) L/mol. Competition studies based on zonal elution were carried out between PAE and various probes which have been known to interact with several major and minor sites on HSA. PAE was found to have direct competition with L-tryptophan. The results indicated that PAE interacted with Sudlow site II on HSA. The thermodynamic analysis indicated that the main force between the paeonol and HSA was hydrogen bond or van der Waals force.

Characterization of drug-protein interactions in blood using high-performance affinity chromatography

The binding of drugs with proteins in blood, serum, or plasma is an important process in determining the activity, distribution, rate of excretion, and toxicity of drugs in the body. High-performance affinity chromatography (HPAC) has received a great deal of interest as a means for studying these interactions. This review examines the various techniques that have been used in HPAC to examine drug-protein binding and discusses the types of information that can be obtained through this approach. A comparison of these techniques with traditional methods for binding studies (e.g., equilibrium dialysis and ultrafiltration) will also be presented. The use of HPAC with specific serum proteins and binding agents will then be discussed, including HSA and alpha(1)-acid glycoprotein (AGP). Several examples from the literature are provided to illustrate the applications of such research. Recent developments in this field are also described, such as the use of improved immobilization techniques, new data analysis methods, techniques for working directly with complex biological samples, and work with immobilized lipoproteins. The relative advantages and limitations of the methods that are described will be considered and the possible use of these techniques in the high-throughput screening or characterization of drug-protein binding will be discussed.

Quantitative studies of allosteric effects by biointeraction chromatography: analysis of protein binding for low-solubility drugs

A new chromatographic method was developed for characterizing allosteric interactions between an immobilized binding agent and low-solubility compounds. This approach was illustrated by using it to characterize the interactions between tamoxifen and warfarin during their binding to the protein human serum albumin (HSA), with beta-cyclodextrin being employed as a solubilizing agent for these drugs. It was confirmed in this work through several experiments that warfarin had a single binding site on HSA with an association equilibrium constant of (2-5) x 10(5) M(-1) (average, 3.9 x 10(5) M(-1)) at 37 degrees C, in agreement with previous reports. It was also found that tamoxifen had a single major binding site on HSA, with an association equilibrium constant of (3-4) x 10(7) M(-1) (average, 3.5 x 10(7) M(-1)) at 37 degrees C. When warfarin was used as a mobile-phase additive in competition studies with tamoxifen, this had a positive allosteric effect on tamoxifen/HSA binding, giving a coupling constant of 2.3 (+/-0.3). Competitive studies using tamoxifen as a mobile-phase additive indicated that tamoxifen had a negative allosteric effect on warfarin/HSA binding, providing a coupling constant of 0.79 (+/-0.03). A unique feature of the technique described in this report was its ability to independently examine both directions of the warfarin/tamoxifen allosteric interaction. This approach is not limited to warfarin, tamoxifen, and HSA but can also be used to study other solutes and binding agents.

Studies by biointeraction chromatography of binding by phenytoin metabolites to human serum albumin

Biointeraction studies based on high performance affinity chromatography were used to investigate the binding of human serum albumin (HSA) to two major phenytoin metabolites: 5-(3-hydroxyphenyl)-5-phenylhydantoin (m-HPPH) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH). This was initially examined by conducting self-competition zonal elution experiments in which m-HPPH or p-HPPH were placed in both the mobile phase and injected sample. It was found that each metabolite had a single major binding site on HSA. Competitive zonal elution experiments using l-tryptophan, warfarin, digitoxin, and cis-clomiphene as site-selective probes indicated that m-HPPH and p-HPPH were interacting with the indole-benzodiazepine site of HSA. The estimated association equilibrium constants for m-HPPH and p-HPPH at this site were 3.2 (+/-1.2)x10(3) and 5.7 (+/-0.7)x10(3)M(-1), respectively, at pH 7.4 and 37 degrees C. Use of these metabolites as competing agents for injections of phenytoin demonstrated that m-HPPH and p-HPPH had direct competition with this drug at the indole-benzodiazepine site. However, the use of phenytoin as a competing agent indicated that this drug had additional negative allosteric interactions on the binding of these metabolites to HSA. These results agreed with previous studies on the binding of phenytoin to HSA and its effects on the interactions of HSA with site-selective probes for the indole-benzodiazepine site.

Studies of phenytoin binding to human serum albumin by high-performance affinity chromatography

High-performance affinity chromatography was used to study the binding of phenytoin to an immobilized human serum albumin (HSA) column. This was accomplished through frontal analysis and competitive binding zonal elution experiments, the latter of which used four probe compounds for the major and minor binding sites of HSA injected into the presence of mobile phases containing known concentrations of phenytoin. It was found that phenytoin can interact with HSA at the warfarin-azapropazone, indole-benzodiazepine, tamoxifen, and digitoxin sites of this protein. The association constants for phenytoin at the indole-benzodiazepine and digitoxin sites were determined to be 1.04 (+/-0.05) x 10(4)M(-1) and 6.5 (+/-0.6) x 10(3)M(-1), respectively, at pH 7.4 and 37 degrees C. Both allosteric interactions and direct binding for phenytoin appear to take place at the warfarin-azapropazone and tamoxifen sites. This rather complex binding system indicates the importance of identifying the binding regions on HSA for specific drugs as a means for understanding the transport of such substances in blood and in characterizing their potential for drug-drug interactions.

Chromatographic analysis of carbamazepine binding to human serum albumin

In this study, high-performance affinity chromatography was used to characterize the binding of carbamazepine to an immobilized human serum albumin (HSA) column. Frontal analysis was first used to determine the association equilibrium constant and binding capacity for carbamazepine on this column at various temperatures. The non-specific binding of carbamazepine within the column was also considered. The results indicated that carbamazepine had a single binding site on HSA with an association equilibrium constant of 5.3 x 10(3)M(-1) at pH 7.4 and 37 degrees C. This was confirmed through zonal elution self-competition studies. The value of DeltaG for this reaction was -5.35 kcal/mol at 37 degrees C, with an associated change in enthalpy (DeltaH) of -6.45 kcal/mol and a change in entropy (DeltaS) of -3.56 cal/molK. The location of this binding region was examined by competitive zonal elution experiments using probe compounds with known sites on HSA. It was found that carbamazepine had direct competition with l-tryptophan, a probe for the indole-benzodiazepine site of HSA, but allosteric interactions with probes for the warfarin, tamoxifen and digitoxin sites. Changes in the pH, ionic strength, and organic modifier content of the mobile phase were used to identify the predominant forces in the carbamazepine-HSA interaction.

Chromatographic analysis of allosteric effects between ibuprofen and benzodiazepines on human serum albumin

The effects of (R)- and (S)-ibuprofen on the binding of benzodiazepines to human serum albumin (HSA) were examined by biointeraction chromatography. The displacement of benzodiazepines from HSA by (R)- and (S)-ibuprofen was found to involve negative allosteric interactions (or possible direct competition) for most (R)-benzodiazepines. However, (S)-benzodiazepines gave positive or negative allosteric effects and direct competition when displaced by (R)- or (S)-ibuprofen. Association equilibrium constants and coupling constants measured for these effects indicated that they involved two classes of ibuprofen binding regions (i.e., low- and high-affinity sites). Based on these results, a model was proposed to explain the binding of benzodiazepines to HSA and their interactions with ibuprofen. This model gave good agreement with previous reports examining the binding of benzodiazepines to HSA.

Characterization of drug interactions with soluble beta-cyclodextrin by high-performance affinity chromatography

This study examined the use of an immobilized human serum albumin (HSA) column to study solution-phase reactions between drugs and beta-cyclodextrin (beta-CD). Chromatographic equations were developed to characterize the binding of chemicals to a soluble ligand (beta-CD) in the presence of an independent immobilized ligand (HSA). Situations considered included the presence of both a homogeneous and heterogeneous immobilized ligand, as well as complex interactions between the chemical of interest and soluble ligand. Three drugs (warfarin, tamoxifen, and phenytoin) were examined by this approach. This method involved injecting a small amount of each drug onto an HSA column in the presence of various concentrations of beta-CD in the mobile phase. By measuring the change in the drug's retention factor as the concentration of beta-CD was varied, it was possible to determine the stability constant between the injected drug and beta-CD. With this approach, warfarin and beta-CD were found to have 1:1 interactions with a stability constant of 5.2 x 10(2) M(-1) at 37 degrees C and pH 7.4, a result in close agreement with previous literature values. Tamoxifen and phenytoin were also found to have 1:1 interactions with beta-CD and had stability constants of 0.9-1.2 x 10(4) and 6-9 x 10(2) M(-1) respectively. With these latter solutes, the effects of secondary binding to the chromatographic support had to be considered. The theory and methods described in this report are not limited to these drugs and beta-CD but can be applied to other analytes and soluble ligands.

Quantitative analysis of allosteric drug-protein binding by biointeraction chromatography

Allosteric interactions are important in many biological processes. They occur when the interactions of one substance with a binding agent changes the interactions of a second substance with the same agent at a separate site. Such interactions are often observed during the binding of drugs to blood proteins such as human serum albumin (HSA). Most previous studies of allosteric interactions have involved only qualitative observations of increased or decreased binding. In this study, we present an approach for quantitatively characterizing such allosteric effects using protein columns. The method is used to examine the interactions of ibuprofen/S-lorazepam acetate, S-oxazepam hemisuccinate/R-oxazepam hemisuccinate, and L-tryptophan/phenytoin during their binding to HSA. This approach can be applied to other receptors or biopolymers and can be used to independently examine the effects of two competing agents during an allosteric interaction.

Survey of binding properties of fatty acid-binding proteins

Fatty acid-binding proteins (FABPs) are members of a super family of lipid-binding proteins, and occur intracellularly in vertebrates and invertebrates. This review briefly addresses the structural and molecular properties of the fatty acid binding proteins, together with their potential physiological role. Special attention is paid to the methods used to study the binding characteristics of FABPs. An overview of the conventional (Lipidex, the ADIFAB and ITC) and innovative separation-based techniques (chromatographic and electrophoretic methods) for the study of ligand-protein interactions is presented along with a discussion of their strengths, weak points and potential applications. The best conventional approaches with natural fatty acids have generally revealed only limited information about the interactions of fatty acid proteins. In contrast, high-performance affinity chromatography (HPAC) studies of several proteins provide full information on the binding characteristics. The review uses, as an example, the application of immobilized liver basic FABP as a probe for the study of ligand-protein binding by high-performance affinity chromatography. The FABP from chicken liver has been immobilized on aminopropyl silica and the developed stationary phase was used to examine the enantioselective properties of this protein and to study the binding of drugs to FABP. In order to clarify the retention mechanism, competitive displacement studies were also carried out by adding short chain fatty acids to the mobile phase as displacing agents and preliminary quantitative structure-retention relationship (QSRRs) correlations were developed to describe the nature of the interactions between the chemical structures of the analytes and the observed chromatographic results. The results of these studies may shed light on the proposed roles of these proteins in biological systems and may find applications in medicine and medicinal chemistry. This knowledge will yield a deeper insight into the mechanism of fatty acid binding in order to indisputably show the central role played by FABPs in cellular FA transport and utilization for a proper lipid metabolism.

Drug affinity to immobilized target bio-polymers by high-performance liquid chromatography and capillary electrophoresis

This review addresses the use of high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) as affinity separation methods to characterise drugs or potential drugs-bio-polymer interactions. Targets for the development of new drugs such as enzymes (IMERs), receptors, and membrane proteins were immobilized on solid supports. After the insertion in the HPLC system, these immobilized bio-polymers were used for the determination of binding constants of specific ligands, substrates and inhibitors of pharmaceutical interest, by frontal analyses and zonal elution methods. The most used bio-polymer immobilization techniques and methods for assessing the amount of active immobilized protein are reported. Examples of increased stability of immobilized enzymes with reduced amount of used protein were shown and the advantages in terms of recovery for reuse, reproducibility and on-line high-throughput screening for potential ligands are evidenced. Dealing with the acquisition of relevant pharmacokinetic data, examples concerning human serum albumin binding studies are reviewed. In particular, papers are reported in which the serum carrier has been studied to monitor the enantioselective binding of chiral drugs and the mutual interaction between co-administered drugs by CE and HPLC. Finally CE, as merging techniques with very promising and interesting application of microscale analysis of drugs' binding parameters to immobilized bio-polymers is examined.

High-performance affinity chromatography: a powerful tool for studying serum protein binding

High-performance affinity chromatography (HPAC) is a method in which a biologically-related ligand is used as a stationary phase in an HPLC system. This approach is a powerful means for selectively isolating or quantitating agents in complex samples, but it can also be employed to study the interactions of biological systems. In recent years there have been numerous reports in which HPAC has been used to examine the interactions of drugs, hormones and other substances with serum proteins. This review discusses how HPAC has been used in such work. Particular attention is given to the techniques of zonal elution and frontal analysis. Various applications are provided for these techniques, along with a list of factors that need to be considered in their optimization and use. New approaches based on band-broadening studies and rapid immunoextraction are also discussed.

Chromatographic and electrophoretic studies of protein binding to chiral solutes

Protein interactions are important in determining the transport, metabolism and/or activity of many chiral compounds within the body. This review examines data that have been obtained on these interactions by various chromatographic and electrophoretic methods, especially those based on either high-performance liquid chromatography or capillary electrophoresis. Zonal elution, frontal analysis and vacancy methods are each considered, as are approaches that employ either soluble or immobilized proteins. There are a variety of different items that can be learned about a solute-protein system through these techniques. This includes information on the binding constants and number of binding sites for a solute-protein system, as well as the thermodynamic parameters, rate constants, interaction forces and binding site structure for the protein and solute. Numerous examples are provided throughout this review, as taken from the literature and from work performed within the author's laboratory.

High-performance affinity chromatography and immobilized serum albumin as probes for drug- and hormone-protein binding

The binding of drugs and hormones to proteins within the blood is an important process in determining the transport, excretion, metabolism and activity of such agents. This paper discusses the combined use of immobilized serum albumin and high-performance affinity chromatography (HPAC) as tools for the study of such binding processes. The general approaches that are used in such work and are illustrated by several examples taken from previous work in the author's laboratory. The type of qualitative and quantitative information that can be obtained by such work is described, including the comparison of relative binding affinities, competitive displacement by other agents or the measurement of equilibrium and rate constants based on immobilized albumin columns. A comparison is also provided between the results that are obtained by these methods and those that are provided by solution-phase albumin. Some newer advances that are highlighted include use of HPAC to examine the binding of non-polar compounds to albumin, the effects of binding site heterogeneity on HPAC measurements and the use of chemically-modified albumin as a tool to examined the site-specific interactions of solutes with albumin.

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.

Characterisation of the binding of digitoxin and acetyldigitoxin to human serum albumin by high-performance affinity chromatography

Zonal elution and high-performance affinity chromatography were used to examine interactions of the drugs digitoxin and acetyldigitoxin with the protein human serum albumin (HSA). This was done by injecting small amounts of digitoxin and acetyldigitoxin onto an immobilized HSA column in the presence of mobile phases that contained various concentrations of digitoxin, acetyldigitoxin or other solutes as competing agents. A fixed concentration of beta-cyclodextrin was also present in the mobile phase as a solubilising agent. It was found that digitoxin and acetyldigitoxin each had strong interactions at a single common binding site on HSA, but with slightly different equilibrium constants for this region. Neither compound showed any competition with warfarin or L-tryptophan, which were used as probes for binding at the warfarin-azapropazone and indole-benzodiazepine sites of HSA. These results confirmed the presence of a separate binding region on HSA for digitoxin-related compounds.