Characterization of the binding and chiral separation of D- and L-tryptophan on a high-performance immobilized human serum albumin column

Characterization of binding by repaglinide and nateglinide with glycated human serum albumin using high-performance affinity microcolumns

High-performance affinity microcolumns were used to characterize binding by the anti-diabetic drugs repaglinide and nateglinide with normal and glycated forms of human serum albumin. The microcolumns contained only nmol amounts of protein and provided a detailed analysis of these drug interactions with good precision and in a matter of minutes per experiment. The overall binding by repaglinide to normal and glycated albumin fits a model with two types of binding sites: a set of one or two moderate-to-high affinity regions and a larger set of weaker regions with association equilibrium constants of ∼10⁵ and 10³  M^-1 , respectively, at pH 7.4 and 37°C. Competition studies gave site-specific association constants for repaglinide and nateglinide at Sudlow site I of 4.2 × 10⁴ and 5.0 × 10⁴  M^-1 for normal albumin, with a decrease of 26%-30% being seen for nateglinide with glycated albumin and no significant change being noted for repaglinide. At Sudlow site II, repaglinide and nateglinide had association constants for normal albumin of 6.1 × 10⁴ and 7.1 × 10⁵  M^-1 , with glycated albumin giving an increase in the association constant at this site for repaglinide of 1.6- to 1.8-fold and a decrease for nateglinide of 51%-58%.

Enantioresolution and Binding Affinity Studies on Human Serum Albumin: Recent Applications and Trends

The interaction between proteins and drugs or other bioactive compounds has been widely explored over the past years. Several methods for analysis of this phenomenon have been developed and improved. Nowadays, increasing attention is paid to innovative methods, such as high performance affinity liquid chromatography (HPALC) and affinity capillary electrophoresis (ACE), taking into account various advantages. Moreover, the development of separation methods for the analysis and resolution of chiral drugs has been an area of ongoing interest in analytical and medicinal chemistry research. In addition to bioaffinity binding studies, both HPALC and ACE al-low one to perform other type of analyses, namely, displacement studies and enantioseparation of racemic or enantiomeric mixtures. Actually, proteins used as chiral selectors in chromatographic and electrophoretic methods have unique enantioselective properties demonstrating suitability for the enantioseparation of a large variety of chiral drugs or other bioactive compounds. This review is mainly focused in chromatographic and electrophoretic methods using human serum albumin (HSA), the most abundant plasma protein, as chiral selector for binding affinity analysis and enantioresolution of drugs. For both analytical purposes, updated examples are presented to highlight recent applications and current trends.

Serotonin Promotes Serum Albumin Interaction with the Monomeric Amyloid β Peptide

Prevention of amyloid β peptide (Aβ) deposition via facilitation of Aβ binding to its natural depot, human serum albumin (HSA), is a promising approach to preclude Alzheimer's disease (AD) onset and progression. Previously, we demonstrated the ability of natural HSA ligands, fatty acids, to improve the affinity of this protein to monomeric Aβ by a factor of 3 (BBRC, 510(2), 248-253). Using plasmon resonance spectroscopy, we show here that another HSA ligand related to AD pathogenesis, serotonin (SRO), increases the affinity of the Aβ monomer to HSA by a factor of 7/17 for Aβ₄₀/Aβ₄₂, respectively. Meanwhile, the structurally homologous SRO precursor, tryptophan (TRP), does not affect HSA's affinity to monomeric Aβ, despite slowdown of the association and dissociation processes. Crosslinking with glutaraldehyde and dynamic light scattering experiments reveal that, compared with the TRP-induced effects, SRO binding causes more marked changes in the quaternary structure of HSA. Furthermore, molecular docking reveals distinct structural differences between SRO/TRP complexes with HSA. The disintegration of the serotonergic system during AD pathogenesis may contribute to Aβ release from HSA in the central nervous system due to impairment of the SRO-mediated Aβ trapping by HSA.

Determination of l-norvaline and l-tryptophan in dietary supplements by nano-LC using an O-[2-(methacryloyloxy)-ethylcarbamoyl]-10,11-dihydroquinidine-silica hybrid monolithic column

An analytical methodology based on an O-[2-(methacryloyloxy)-ethylcarbamoyl]-10,11-dihydroquinidine (MQD)-silica hybrid monolithic column was developed for the enantioseparation of 9-fluorenylmethoxycarbonyl (FMOC) derivatized amino acids by nano-liquid chromatography. The mobile phase was optimized including the apparent pH, content of ACN, and concentration of the buffer to obtain a satisfactory enantioresolution performance. 27 FMOC derivatized amino acids including 19 protein and 8 non-protein amino acids were tested, and 19 out of them were enantiomerically discriminated obtaining baseline separation for 11 of them. Analytical characteristics of the method were evaluated for norvaline and tryptophan in terms of linearity, precision, accuracy, limits of detection (LOD) and quantitation (LOQ) showing good performance to be applied to the enantiomeric determination of these amino acids in dietary supplements. LOD and LOQ values were 9.3 and 31 μM for norvaline enantiomers and 7.5 and 25 μM for tryptophan enantiomers, respectively. The contents of d-norvaline and d-tryptophan were below their respective LODs in all the analyzed samples. Quantitation of l-tryptophan and l-norvaline showed good agreement with the labeled contents except for one sample which did not show presence of l-norvaline, contrary to the label indication.

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A method was developed for the enantiomeric separation of amino acids by nano-LC.

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A novel quinidine-silica hybrid monolith was employed as chiral column.

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19 protein and non-protein FMOC-amino acids were enantiomerically discriminated.

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Analytical characteristics of the developed method were evaluated.

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Norvaline and tryptophan were enantiomerically determined in dietary supplements.

Unravelling the interaction of glipizide with human serum albumin using various spectroscopic techniques and molecular dynamics studies

Glipizide is known to stimulate insulin secretion by β-cells of the pancreas. It is a second-generation sulfonylurea drug used in the management of type 2 diabetes. The shorter biological half-life makes it a suitable candidate to be designed as a controlled release formulation. Human serum albumin (HSA), a major plasma protein plays a crucial role in the transportation of drugs, hormones, fatty acids, and many other molecules and determines their physiological fate and biodistribution. In this study, the interaction of glipizide with HSA was investigated under physiological conditions using multi-spectroscopic techniques corroborated with molecular docking and dynamics approach. It was found that glipizide integrates to HSA with a binding constant in the order of 10⁵ M^-1. The mode of fluorescence quenching by glipizide is static in nature with one binding site. Glipizide preferentially interacts with sub-domain IIA of HSA and their complexion is thermodynamically favorable. This interaction results in the loss of α-helical content of HSA. The energy transfer efficiency from HSA to glipizide was found to be 26.72%. In silico molecular docking and simulation studies ratified in vitro findings and revealed that hydrogen bonds and hydrophobic interactions are accountable for glipizide-HSA complex formation.Communicated by Ramaswamy H. Sarma.

Development of an on-line immunoextraction/entrapment system for protein capture and use in drug binding studies by high-performance affinity chromatography

An on-line purification and entrapment system was developed that could extract a protein from a sample such as serum and entrap this protein within a small column for use in high-performance affinity chromatography. Human serum albumin (HSA) was employed as a model protein for this work. Immunoextraction columns containing polyclonal anti-HSA antibodies were developed to capture and isolate HSA from applied samples. This was followed by the use of a strong cation-exchange column to recapture and focus HSA as it eluted from the immunoextraction columns. The recaptured HSA was entrapped within 1.0 cm × 2.1 mm I.D. columns containing hydrazide-activated silica and in the presence of oxidized glycogen as a capping agent. The binding and elution properties of HSA on the various components of this system were examined and optimized. The entrapped columns produced by this system were then evaluated for their use in binding studies with several sulfonylurea drugs. The HSA columns created by this approach typically contained 0.3-0.6 nmol HSA and were stable over several weeks and more than 50-60 sample injections. Drug binding constants could be determined with these columns in 8 min or less by zonal elution and gave good agreement with literature values. The same system could be used for the capture and entrapment of other proteins by utilizing antibodies against the given target for immunoextraction.

Chromatographic studies of chlorpropamide interactions with normal and glycated human serum albumin based on affinity microcolumns

Sulfonylurea drugs have significant binding to proteins in blood, with most of this binding believed to occur with human serum albumin (HSA). High performance affinity chromatography and affinity microcolumns containing immobilized HSA were used to investigate binding by the sulfonylurea drug chlorpropamide to normal HSA and glycated HSA, which is a modified form of HSA that has an increased serum concentration in diabetes. Experiments employing frontal analysis indicated that the binding by chlorpropamide gave a good fit to a two-site model for both normal HSA and glycated HSA samples that were representative of controlled or advanced diabetes. These interactions involved a set of moderate-to-high affinity sites and a set of lower affinity sites, with binding constants in the range of 6.2-9.9 × 10⁴ M^-1 and 0.18-0.57 × 10⁴ M^-1, respectively, at pH 7.4 and 37 °C. Competition studies utilizing a zonal elution format demonstrated that chlorpropamide could interact at both Sudlow sites I and II of HSA, with affinities in the range expected for the moderate-to-high affinity sites of this drug. The affinity of chlorpropamide at Sudlow site I had a small increase of up to 1.2-fold when comparing the normal HSA and glycated HSA samples. Chlorpropamide gave a larger 1.4- to over 1.5-fold increase at Sudlow site II when the affinity of this drug was compared between normal HSA and the same samples of glycated HSA. These results were compared to those obtained previously with other sulfonylurea drugs to help determine how glycation can change the overall and site-selective binding strength of these drugs with HSA at levels of protein modification that are seen in patients with diabetes.

Analysis of solute-protein interactions and solute-solute competition by zonal elution affinity chromatography

Many biological processes involve solute-protein interactions and solute-solute competition for protein binding. One method that has been developed to examine these interactions is zonal elution affinity chromatography. This review discusses the theory and principles of zonal elution affinity chromatography, along with its general applications. Examples of applications that are examined include the use of this method to estimate the relative extent of solute-protein binding, to examine solute-solute competition and displacement from proteins, and to measure the strength of these interactions. It is also shown how zonal elution affinity chromatography can be used in solvent and temperature studies and to characterize the binding sites for solutes on proteins. In addition, several alternative applications of zonal elution affinity chromatography are discussed, which include the analysis of binding by a solute with a soluble binding agent and studies of allosteric effects. Other recent applications that are considered are the combined use of immunoextraction and zonal elution for drug-protein binding studies, and binding studies that are based on immobilized receptors or small targets.

High performance affinity chromatography and related separation methods for the analysis of biological and pharmaceutical agents

The last few decades have witnessed the development of many high-performance separation methods that use biologically related binding agents. The combination of HPLC with these binding agents results in a technique known as high performance affinity chromatography (HPAC). This review will discuss the general principles of HPAC and related techniques, with an emphasis on their use for the analysis of biological compounds and pharmaceutical agents. Various types of binding agents for these methods will be considered, including antibodies, immunoglobulin-binding proteins, aptamers, enzymes, lectins, transport proteins, lipids, and carbohydrates. Formats that will be discussed for these methods range from the direct detection of an analyte to indirect detection based on chromatographic immunoassays, as well as schemes based on analyte extraction or depletion, post-column detection, and multi-column systems. The use of biological agents in HPLC for chiral separations will also be considered, along with the use of HPAC as a tool to screen or study biological interactions. Various examples will be presented to illustrate these approaches and their applications in fields such as biochemistry, clinical chemistry, and pharmaceutical research.

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.

Chromatographic Studies of Protein-Based Chiral Separations

The development of separation methods for the analysis and resolution of chiral drugs and solutes has been an area of ongoing interest in pharmaceutical research. The use of proteins as chiral binding agents in high-performance liquid chromatography (HPLC) has been an approach that has received particular attention in such work. This report provides an overview of proteins that have been used as binding agents to create chiral stationary phases (CSPs) and in the use of chromatographic methods to study these materials and protein-based chiral separations. The supports and methods that have been employed to prepare protein-based CSPs will also be discussed and compared. Specific types of CSPs that are considered include those that employ serum transport proteins (e.g., human serum albumin, bovine serum albumin, and alpha1-acid glycoprotein), enzymes (e.g., penicillin G acylase, cellobiohydrolases, and α-chymotrypsin) or other types of proteins (e.g., ovomucoid, antibodies, and avidin or streptavidin). The properties and applications for each type of protein and CSP will also be discussed in terms of their use in chromatography and chiral separations.

Analysis of drug-protein binding using on-line immunoextraction and high-performance affinity microcolumns: Studies with normal and glycated human serum albumin

A method combining on-line immunoextraction microcolumns with high-performance affinity chromatography (HPAC) was developed and tested for use in examining drug-protein interactions with normal or modified proteins. Normal human serum albumin (HSA) and glycated HSA were used as model proteins for this work. High-performance immunoextraction microcolumns with sizes of 1.0-2.0 cm × 2.1mm i.d. and containing anti-HSA polyclonal antibodies were developed and tested for their ability to bind normal HSA or glycated HSA. These microcolumns were able to extract up to 82-93% for either type of protein at 0.05-0.10 mL/min and had a binding capacity of 0.34-0.42 nmol HSA for a 1.0 cm × 2.1mm i.d. microcolumn. The immunoextraction microcolumns and their adsorbed proteins were tested for use in various approaches for drug binding studies. Frontal analysis was used with the adsorbed HSA/glycated HSA to measure the overall affinities of these proteins for the drugs warfarin and gliclazide, giving comparable values to those obtained previously using similar protein preparations that had been covalently immobilized within HPAC columns. Zonal elution competition studies with gliclazide were next performed to examine the specific interactions of this drug at Sudlow sites I and II of the adsorbed proteins. These results were also comparable to those noted in prior work with covalently immobilized samples of normal HSA or glycated HSA. These experiments indicated that drug-protein binding studies can be carried out by using on-line immunoextraction microcolumns with HPAC. The same method could be used in the future with clinical samples and other drugs or proteins of interest in pharmaceutical studies or biomedical research.

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.

Studies of metabolite-protein interactions: a review

The study of metabolomics can provide valuable information about biochemical pathways and processes at the molecular level. There have been many reports that have examined the structure, identity and concentrations of metabolites in biological systems. However, the binding of metabolites with proteins is also of growing interest. This review examines past reports that have looked at the binding of various types of metabolites with proteins. An overview of the techniques that have been used to characterize and study metabolite-protein binding is first provided. This is followed by examples of studies that have investigated the binding of hormones, fatty acids, drugs or other xenobiotics, and their metabolites with transport proteins and receptors. These examples include reports that have considered the structure of the resulting solute-protein complexes, the nature of the binding sites, the strength of these interactions, the variations in these interactions with solute structure, and the kinetics of these reactions. The possible effects of metabolic diseases on these processes, including the impact of alterations in the structure and function of proteins, are also considered.

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.

Analysis of drug interactions with modified proteins by high-performance affinity chromatography: binding of glibenclamide to normal and glycated human serum albumin

High-performance affinity chromatography (HPAC) was used to examine the changes in binding that occur for the sulfonylurea drug glibenclamide with human serum albumin (HSA) at various stages of glycation for HSA. Frontal analysis on columns containing normal HSA or glycated HSA indicated glibenclamide was interacting through both high affinity sites (association equilibrium constant, K(a), 1.4-1.9 × 10(6)M(-1) at pH 7.4 and 37 °C) and lower affinity sites (K(a), 4.4-7.2 × 10(4)M(-1)). Competition studies were used to examine the effect of glycation at specific binding sites of HSA. An increase in affinity of 1.7- to 1.9-fold was seen at Sudlow site I with moderate to high levels of glycation. An even larger increase of 4.3- to 6.0-fold in affinity was noted at Sudlow site II for all of the tested samples of glycated HSA. A slight decrease in affinity may have occurred at the digitoxin site, but this change was not significant for any individual glycated HSA sample. These results illustrate how HPAC can be used as tool for examining the interactions of relatively non-polar drugs like glibenclamide with modified proteins and should lead to a more complete understanding of how glycation can alter the binding of drugs in blood.

Unraveling complex small-molecule binding mechanisms by using simple NMR spectroscopy

Heteronuclear NMR spectroscopy provides a unique way to obtain site-specific information about protein-ligand interactions. Usually, such studies rely on the availability of isotopically labeled proteins, thereby allowing both editing of the spectra and ligand signals to be filtered out. Herein, we report that the use of the methyl SOFAST correlation experiment enables the determination of site-specific equilibrium binding constants by using unlabeled proteins. By using the binding of L- and D-tryptophan to serum albumin as a test case, we determined very accurate dissociation constants for both the high- and low-affinity sites present at the protein surface. The values of site-specific dissociation constants were closer to those obtained by isothermal titration calorimetry than those obtained from ligand-observed methods, such as saturation transfer difference. The possibility of measuring ligand binding to serum albumin at physiological concentrations with unlabeled proteins may open up new perspectives in the field of drug discovery.

Pharmaceutical and biomedical applications of affinity chromatography: recent trends and developments

Affinity chromatography is a separation technique that has become increasingly important in work with biological samples and pharmaceutical agents. This method is based on the use of a biologically related agent as a stationary phase to selectively retain analytes or to study biological interactions. This review discusses the basic principles behind affinity chromatography and examines recent developments that have occurred in the use of this method for biomedical and pharmaceutical analysis. Techniques based on traditional affinity supports are discussed, but an emphasis is placed on methods in which affinity columns are used as part of HPLC systems or in combination with other analytical methods. General formats for affinity chromatography that are considered include step elution schemes, weak affinity chromatography, affinity extraction and affinity depletion. Specific separation techniques that are examined include lectin affinity chromatography, boronate affinity chromatography, immunoaffinity chromatography, and immobilized metal ion affinity chromatography. Approaches for the study of biological interactions by affinity chromatography are also presented, such as the measurement of equilibrium constants, rate constants, or competition and displacement effects. In addition, related developments in the use of immobilized enzyme reactors, molecularly imprinted polymers, dye ligands and aptamers are briefly considered.

High-performance affinity chromatography and the analysis of drug interactions with modified proteins: binding of gliclazide with glycated human serum albumin

This study used high-performance affinity chromatography (HPAC) to examine the binding of gliclazide (i.e., a sulfonylurea drug used to treat diabetes) with the protein human serum albumin (HSA) at various stages of modification due to glycation. Frontal analysis conducted with small HPAC columns was first used to estimate the number of binding sites and association equilibrium constants (K(a)) for gliclazide with normal HSA and glycated HSA. Both normal and glycated HSA interacted with gliclazide according to a two-site model, with a class of high-affinity sites (average K(a), 7.1-10 × 10(4) M(-1)) and a group of lower-affinity sites (average K(a), 5.7-8.9 × 10(3) M(-1)) at pH 7.4 and 37 °C. Competition experiments indicated that Sudlow sites I and II of HSA were both involved in these interactions, with the K(a) values for gliclazide at these sites being 1.9 × 10(4) and 6.0 × 10(4) M(-1), respectively, for normal HSA. Two samples of glycated HSA had similar affinities to normal HSA for gliclazide at Sudlow site I, but one sample had a 1.9-fold increase in affinity at this site. All three glycated HSA samples differed from normal HSA in their affinity for gliclazide at Sudlow site II. This work illustrated how HPAC can be used to examine both the overall binding of a drug with normal or modified proteins and the site-specific changes that can occur in these interactions as a result of protein modification.

CHROMATOGRAPHIC ANALYSIS OF DRUG INTERACTIONS IN THE SERUM PROTEOME

The binding of drugs with serum proteins and binding agents such as human serum albumin, α1-acid glycoprotein, and lipoproteins is an important process in determining the activity and fate of many pharmaceuticals in the body. A variety of techniques have been used to study drug interactions with serum proteins, but there is still a need for faster or better methods for such work. High-performance liquid chromatography (HPLC) is one tool that has been utilized in many formats for these types of measurements. Advantages of using HPLC for this application include its speed and precision, its ability to be automated, its good limits of detection, and its compatibility with a wide range of assay formats and detectors. This review will discuss various approaches in which HPLC can be employed for the study of drug-protein interactions. These techniques include the use of soluble proteins in zonal elution and frontal analysis methods or vacancy techniques such as the Hummel-Dreyer method. Zonal elution and frontal analysis methods that make use of immobilized proteins and high-performance affinity chromatography will also be presented. A variety of applications will be examined, ranging from the determination of free drug fractions to the measurement of the strength or rate of a drug-protein interaction. Newer developments that will be discussed include recent work in the creation of novel mathematical approaches for HPLC studies of drug-protein binding, the use of HPLC methods for the high-throughput screening of drug-protein binding, and the creation and use of affinity monoliths or affinity microcolumns for examining drug-protein systems.

Binding of tolbutamide to glycated human serum albumin

The presence of elevated levels of glucose in blood during diabetes can lead to the non-enzymatic glycation of serum proteins such as human serum albumin (HSA). This study examined the changes that occur in binding of the sulfonylurea drug tolbutamide to HSA as the level of glycation for this protein was increased. High-performance affinity chromatography was used in this work along with columns containing various preparations of in vitro glycated HSA. It was found in frontal analysis experiments that the binding of tolbutamide with all of the tested preparations of glycated HSA could be described by a two-site model involving both strong and weak affinity interactions. The association equilibrium constants (K(a)) for tolbutamide at its high affinity sites on glycated HSA were in the range of 0.8-1.2 x 10⁵ M⁻¹ and increased by 1.4-fold in going from normal HSA to mildly glycated HSA. It was found through competition studies that tolbutamide was binding at both Sudlow sites I and II on the glycated HSA, in agreement with previous studies. The K(a) for tolbutamide at Sudlow site II increased by 1.1- to 1.4-fold in going from normal HSA to glycated HSA. At Sudlow site I, the K(a) for tolbutamide increased by 1.2- to 1.3-fold in going from normal HSA to the glycated HSA samples. This information demonstrates the effects that glycation can have on drug interactions on HSA and should provide a better quantitative understanding of how the protein binding of tolbutamide in serum may be affected for individuals with diabetes.

Chromatographic analysis of acetohexamide binding to glycated human serum albumin

Acetohexamide is a drug used to treat type II diabetes and is tightly bound to the protein human serum albumin (HSA) in the circulation. It has been proposed that the binding of some drugs with HSA can be affected by the non-enzymatic glycation of this protein. This study used high-performance affinity chromatography to examine the changes in acetohexamide-HSA binding that take place as the glycation of HSA is increased. It was found in frontal analysis experiments that the binding of acetohexamide to glycated HSA could be described by a two-site model involving both strong and weak affinity interactions. The average association equilibrium constant (K(a)) for the high affinity interactions was in the range of 1.2-2.0×10(5)M(-1) and increased in moving from normal HSA to HSA with glycation levels that might be found in advanced diabetes. It was found through competition studies that acetohexamide was binding at both Sudlow sites I and II on the glycated HSA. The K(a) for acetohexamide at Sudlow site I increased by 40% in going from normal HSA to minimally glycated HSA but then decreased back to near-normal values in going to more highly glycated HSA. At Sudlow site II, the K(a) for acetohexamide first decreased by about 40% and then increased in going from normal HSA to minimally glycated HSA and more highly glycated HSA. This information demonstrates the importance of conducting both frontal analysis and site-specific binding studies in examining the effects of glycation on the interactions of a drug with HSA.

The effects of glycation on the binding of human serum albumin to warfarin and L-tryptophan

Diabetes leads to elevated levels of glucose in blood which, in turn, can lead to the non-enzymatic glycation of serum proteins such as human serum albumin (HSA). It has been suggested that this increase in glycation can alter the ability of HSA to bind to drugs and other small solutes. This study used high-performance affinity chromatography (HPAC) to see if there is any significant change related to glycation in the binding of HSA to warfarin and l-tryptophan, which are often used as probe compounds for Sudlow sites I and II of HSA in drug binding studies with this protein. It was found through frontal analysis studies that both of these compounds gave a good fit to a single-site binding model with glycated HSA under the conditions used in this study. There was no significant change in the association equilibrium constants or specific activities for warfarin with HSA at pH 7.4 and 37 degrees C under glycation conditions that were representative of those expected in pre-diabetes or diabetes, but a 4.7- to 5.8-fold increase in binding affinity for l-tryptophan with glycated HSA was observed. These results indicate that warfarin and l-tryptophan can be successively used as site-selective probes for glycated HSA; however, changes in the affinity of l-tryptophan may need to be considered in such an application. These results should be valuable in future competition studies using these compounds as probes to examine the interactions of other drugs and solutes with Sudlow sites I and II and to determine how changes in HSA glycation can affect the serum protein binding of various pharmaceutical agents during diabetes.

Evaluation of affinity microcolumns containing human serum albumin for rapid analysis of drug-protein binding

This study examined the use of affinity microcolumns as tools for the rapid analysis and high-throughput screening of drug-protein binding. The protein used was immobilized human serum albumin (HSA) and the model analytes were warfarin and L-tryptophan, two solutes often used as site-specific probes for drug binding to Sudlow sites I and II of HSA, respectively. The use of HSA microcolumns in binding studies was examined by using both zonal elution and frontal analysis formats. The zonal elution studies were conducted by injecting the probe compounds onto HSA microcolumns of varying lengths while measuring the resulting retention factors, plate heights and peak asymmetries. A decrease in the retention factor was noted when moving from longer to shorter column lengths while using a constant amount of injected solute. However, this change could be corrected, in part, by determining the relative retention factor of a solute versus a reference compound injected onto the same microcolumn. The plate height values were relatively consistent for all column lengths and gave an expected increase at higher linear velocities. The peak asymmetries were similar for all columns up to 1 mL/min but shifted to larger values at higher flow rates and when using short microcolumns (e.g., 1 mm length). The association equilibrium constants and number of binding sites estimated by frontal analysis for warfarin with HSA were consistent at the various column sizes that were tested and gave good agreement with previous literature values. These results confirmed affinity microcolumns provide comparable results to those obtained with longer columns and can be used in the rapid analysis of drug-protein binding and in the high-throughput screening of such interactions.

Measurement of drug-protein dissociation rates by high-performance affinity chromatography and peak profiling

The rate at which a drug or other small solute interacts with a protein is important in understanding the biological and pharmacokinetic behavior of these agents. One approach that has been developed for examining these rates involves the use of high-performance affinity chromatography (HPAC) and estimates of band-broadening through peak profiling. Previous work with this method has been based on a comparison of the statistical moments for a retained analyte versus nonretained species at a single, high flow rate to obtain information on stationary phase mass transfer. In this study an alternative approach was created that allows a broad range of flow rates to be used for examining solute-protein dissociation rates. Chromatographic theory was employed to derive equations that could be used with this approach on a single column, as well as with multiple columns to evaluate and correct for the impact of stagnant mobile phase mass transfer. The interaction of L-tryptophan with human serum albumin was used as a model system to test this method. A dissociation rate constant of 2.7 (+/-0.2) s(-1) was obtained by this approach at pH 7.4 and 37 degrees C, which was in good agreement with previous values determined by other methods. The techniques described in this report can be applied to other biomolecular systems and should be valuable for the determination of drug-protein dissociation rates.

Evaluation of indole-based probes for high-throughput screening of drug binding to human serum albumin: Analysis by high-performance affinity chromatography

There has been growing interest in the use of rapid and selective separation methods such as high-performance affinity chromatography (HPAC) or affinity capillary electrophoresis (ACE) for the characterization of drug-protein interactions. L-Tryptophan is commonly used in these and other methods as a site-selective probe for examining the binding of small solutes and drugs at Sudlow site II on the protein HSA. However, solutions of L-tryptophan can be unstable and are generally prepared fresh daily for these studies. In this report, HPAC was used to examine other indole compounds as possible replacements for L-tryptophan as a site-selective probe for use in the high-throughput screening of drug binding to HSA; the implications of these results in the use of such compounds in ACE were also considered. The probe candidates that were tested included indole-3-acetic acid, indole-3-carboxylic acid, indole-3-butyric acid, indole-3-propionic acid, indole-3-methanol, 3-acetylindole, and 3-methylindole. All of these compounds were found by (1)H NMR and UV-Vis spectroscopy to be stable for up to 3 wk at room temperature when kept in a pH 7.4, 0.067 M phosphate buffer. The binding of these compounds was examined by using columns that contained immobilized HSA. 3-Acetylindole was found to be the best candidate in this group for use as an alternative probe to L-tryptophan for Sudlow site II. This probe had the same binding site and a similar affinity to L-tryptophan but was more stable in aqueous solution, making it suitable for high-throughput screening of drug-HSA binding in both HPAC and ACE.

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

Binding by the drug imipramine to the protein human serum albumin (HSA) was studied by using high-performance affinity chromatography. The association equilibrium constants and number of binding sites for imipramine with HSA were first estimated by utilizing frontal analysis. Imipramine was found to have one major binding site on HSA with an association equilibrium constant of 1.6 x 10(5) M(-1) at pH 7.4 and 37 degrees C, as well as a second group of weaker and non-specific binding regions (8-9 mol/mol HSA) with an average association equilibrium constant of 1.5 x 10(3) M(-1). Competition studies based on zonal elution were performed to identify the location of the major binding site for imipramine on HSA. Imipramine was found to have direct competition with L-tryptophan, which indicated that imipramine was interacting with Sudlow site II, or the indole-benzodiazepine site of HSA. No competition or allosteric effects were noted between imipramine and warfarin, a probe for Sudlow site I or the warfarin-azapropazone site of HSA. The association equilibrium constant found for imipramine at its site of competition with L-tryptophan also agreed with the value that was obtained for the major binding site of imipramine in the frontal analysis studies. These results confirmed that Sudlow site II was the location of the major binding site for imipramine on HSA. These results gave good agreement with previous observations made in the literature and should provide a more detailed description of how imipramine is transported in blood and of how it may interact with other drugs in the body.

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.

Rapid analysis of the interactions between drugs and human serum albumin (HSA) using high-performance affinity chromatography (HPAC)

This study used a combination of zonal elution and frontal affinity chromatography on immobilized human serum albumin (HSA) high-performance affinity chromatography (HPAC) column to examine the association constants of various compounds that have been studied by equilibrium dialysis or ultra filtration. A standard plot was generated from retention factors of reference compounds using zonal elution chromatography against association constants of reference compounds using frontal affinity chromatography. The linear relationship was established (r2=0.9993) between retention factors and association constants of reference compounds. This standard plot was later used for rapid determination of association constants of various drugs which show low to medium binding affinity to HSA. Association constants of those drugs from this study were compared to that of more generally used methods (i.e., equilibrium dialysis or ultra filtration) from literature and resulted in a relatively high correlation (r2=0.945) value. This combination of zonal elution and frontal affinity chromatography method for determining association constants showed several advantages against traditional methods. Depending on drugs of interest, an association constant of drug to HSA can be measured as fast as 1.5 min. Other notable advantages include an ease of automation and its ability to distinguish association constants of chiral compounds at the same time. The same approach could be used for studying interaction of other drugs and proteins and should further improve overall drug screening process.

Development of an affinity silica monolith containing human serum albumin for chiral separations

An affinity monolith based on silica and containing immobilized human serum albumin (HSA) was developed and evaluated in terms of its binding, efficiency and selectivity in chiral separations. The results were compared with data obtained for the same protein when used as a chiral stationary phase with HPLC-grade silica particles or a monolith based on a copolymer of glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA). The surface coverage of HSA in the silica monolith was similar to values obtained with silica particles and a GMA/EDMA monolith. However, the higher surface area of the silica monolith gave a material that contained 1.3-2.2-times more immobilized HSA per unit volume when compared to silica particles or a GMA/EDMA monolith. The retention, efficiency and resolving power of the HSA silica monolith were evaluated using two chiral analytes: d/l-tryptophan and R/S-warfarin. The separation of R- and S-ibuprofen was also considered. The HSA silica monolith gave higher retention and higher or comparable resolution and efficiency when compared with HSA columns that contained silica particles or a GMA/EDMA monolith. The silica monolith also gave lower back pressures and separation impedances than these other materials. It was concluded that silica monoliths can be valuable alternatives to silica particles or GMA/EDMA monoliths when used with immobilized HSA as a chiral stationary phase.

Drug binding to human serum albumin: abridged review of results obtained with high-performance liquid chromatography and circular dichroism

The drug binding to plasma and tissue proteins are fundamental factors in determining the overall pharmacological activity of a drug. Human serum albumin (HSA), together with alpha1-acid glycoprotein (AGP), are the most important plasma proteins, which act as drug carriers, with drug pharmacokinetic implications, resulting in important clinical impacts for drugs that have a relatively narrow therapeutic index. This review focuses on the combination of biochromatography and circular dichroism as an effective approach for the characterization of albumin binding sites and their enantioselectivity. Furthermore, their applications to the study of changes in the binding properties of the protein arising by the reversible or covalent binding of drugs are discussed, and examples of physiological relevance reported. Perspectives of these studies reside in supporting the development of new drugs, which require miniaturization to facilitate the screening of classes of compounds for their binding to the target protein, and a deeper characterization of the mechanisms involved in the molecular recognition processes.

Binding behavior of amino acid conjugates of indole-3-acetic acid to immobilized human serum albumin

The affinity of indole-3-acetic acid (IAA), indole-3-propionic acid, indole-3-butyric acid and 24 of their amino acid conjugates to immobilized human serum albumin, as expressed by the retention factor k (determined by HPLC), was dependent on (1) lipophilicity, (2) chirality and (3) functional groups in the amino acid moiety; in some cases conformation plays an additional role. Two lipophilicity-related parameters afforded quantitative correlations with k: retention on a C18 reversed-phase column (experimental approach) and the distance between the hydrophilic and hydrophobic poles of the molecules (in silico approach). Most compounds examined are possible metabolic precursors of IAA, an experimental tumor therapeutic.

Development of an affinity silica monolith containing alpha1-acid glycoprotein for chiral separations

An affinity monolith based on silica and containing immobilized alpha(1)-acid glycoprotein (AGP) was developed and evaluated in terms of its binding, efficiency and selectivity in chiral separations. The results were compared with data obtained for the same protein when used as a chiral stationary phase with HPLC-grade silica particles or monoliths based on a copolymer of glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA). The surface coverage of AGP in the silica monolith was 18% higher than that obtained with silica particles and 61% higher than that measured for a GMA/EDMA monolith. The higher surface area of the silica monolith gave materials that contained 1.5- to 3.6-times more immobilized protein per unit volume when compared to silica particles or a GMA/EDMA monolith. The retention, efficiency and resolving power of the AGP silica monolith were evaluated by injecting two chiral analytes onto this column (i.e., R/S-warfarin and R/S-propranolol). In each case, the AGP silica monolith gave higher retention plus better resolution and efficiency than AGP columns containing silica particles or a GMA/EDMA monolith. The AGP silica monolith also gave lower back pressures and separation impedances than these other materials. It was concluded that silica monoliths can be valuable alternatives to silica particles or GMA/EDMA monoliths when used with AGP as a chiral stationary phase.

Development of sulfhydryl-reactive silica for protein immobilization in high-performance affinity chromatography

Two techniques were developed for the immobilization of proteins and other ligands to silica through sulfhydryl groups. These methods made use of maleimide-activated silica (the SMCC method) or iodoacetyl-activated silica (the SIA method). The resulting supports were tested for use in high-performance affinity chromatography by employing human serum albumin (HSA) as a model protein. Studies with normal and iodoacetamide-modified HSA indicated that these methods had a high selectivity for sulfhydryl groups on this protein, which accounted for the coupling of 77-81% of this protein to maleimide- or iodoacetyl-activated silica. These supports were also evaluated in terms of their total protein content, binding capacity, specific activity, nonspecific binding, stability, and chiral selectivity for several test solutes. HSA columns prepared using maleimide-activated silica gave the best overall results for these properties when compared to HSA that had been immobilized to silica through the Schiff base method (i.e., an amine-based coupling technique). A key advantage of the supports developed in this work is that they offer the potential of giving greater site-selective immobilization and ligand activity than amine-based coupling methods. These features make these supports attractive in the development of protein columns for such applications as the study of biological interactions and chiral separations.

Micellar liquid chromatography with dodecyl dimethyl betaine as an in vitro method for prediction of protein-drug binding

With the accelerating development of new drugs, there is a high need for rapid and simple screening technologies. In this paper, a new in vitro method, dodecyl dimethyl betaine (BS-12) micellar liquid chromatography (MLC) was presented for prediction of protein-drug binding based on the similar property of BS-12 micelles to protein. The predictive possibility of this method was validated by comparing the retention factors of drugs (antidiabetic and antibacterial drugs) on C18 modified by different surfactants with those on the protein column. Through the investigation of the concentration and pH effect on the retention of the drugs in BS-12 MLC, quantitative retention-protein binding relationships were established according to the retention factors in 0.2 M BS-12 (pH 7.4) MLC and those on the protein column. According to the relationships established, the protein binding of seven drugs for psychiatric disorders, six potential drugs for antibiotics and four commercial antibiotics were predicted. The results were consistent with those on the BSA column very well. This indicated, BS-12 MLC was a simple, fast and reproducible method to predict protein-drug binding.

Identification and quantitative studies of protein immobilization sites by stable isotope labeling and mass spectrometry

A method was developed for characterizing immobilization sites on a protein based on stable isotope labeling and MALDI-TOF mass spectrometry. The model for this work was human serum albumin (HSA) immobilized onto silica by the Schiff base method. The immobilized HSA was digested by various proteolytic enzymes in the presence of normal water, while soluble HSA was digested in (18)O-enriched water for use as an internal standard. These two digests were mixed and analyzed, with the (18)O/(16)O ratio for each detected peptide then being measured. Several peptides in the tryptic, Lys-C, and Glu-C digests gave significantly higher (18)O/(16)O ratios than other peptides in the same digests, implying their involvement in immobilization. Analysis of these results led to identification of the N-terminus and several lysines as likely immobilization sites for HSA (e.g., K4, K41, K190, K225, K313, and K317). It was also possible from these results to quantitatively rank these sites in terms of the relative degree to which each might take part in immobilization. This method is not limited to HSA and silica but can be used with other proteins and supports.

Chiral resolution of tryptophan derivatives by CE using canine serum albumin and bovine serum albumin as chiral selectors

This work deals with the application of BSA and canine serum albumin (CSA) for enantioseparation of tryptophan derivatives with CE. The aim of this work was the investigation of the influence of different functional groups of tryptophan derivatives on enantioseparation. CSA as a chiral selector was tested to compare its selector properties with those of BSA. The enantiomers of the tryptophan derivatives were separated by adding BSA or CSA to the BGE. The influence of pH, temperature, BSA and CSA concentration and organic modifiers was investigated. It was found that the stereoselectivity for the different tryptophan derivatives is dependent on the albumin species. It turned out that the different functional groups of the derivatives showed a significant influence on stereoselectivity.

Chromatographic analysis of carbamazepine binding to human serum albumin

Recent studies with carbamazepine on human serum albumin (HSA) columns have noted an appreciable degree of non-specific binding on supports prepared by the Schiff base immobilization method. This work examines an alternative immobilization method for HSA based on N-hydroxysuccinimide (NHS)-activated silica. This support was prepared by reacting HPLC-grade silica directly with disuccinimidyl carbonate. The resulting material was compared to an HSA support prepared by the Schiff base method in terms of its activity for carbamazepine and non-specific interactions with this drug. When examined by frontal analysis, both supports gave comparable association equilibrium constants for carbamazepine interactions with HSA ((0.53-0.55) x 10(4)M(-1) at 37 degrees C). However, columns prepared by the Schiff base method gave greater non-specific binding. These columns, as well as control columns prepared using the carbonyldiimidazole (CDI) immobilization method, were also evaluated for their non-specific binding to a variety of other solutes known to interact with HSA. From these results it was concluded that the NHS method was an attractive alternative to the Schiff base technique in the preparation of immobilized HSA for HPLC-based binding studies for carbamazepine. However, it was also noted that non-specific binding varies from one drug to the next in these immobilization methods, indicating that such properties should be evaluated on a case-by-case basis in the use and development of HSA columns for binding studies.

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.

Combining lectin microcolumns with high-resolution separation techniques for enrichment of glycoproteins and glycopeptides

Silica-based lectin microcolumns are described in this study together with the chemical procedures necessary for their preparation. The analytical merits of Canavalia ensiformis and Sambucus nigra lectins, [immobilized on activated macroporous silica], such as binding capacity, trapping reproducibility, and substrate selectivity, have been evaluated using model glycoproteins. The described microcolumns are applicable to high-pressure analytical schemes utilizing microvalving procedures, washing steps, and quantitative desorption for LC/MS analysis. The described analytical systems are amenable to the applications aiming at fractionation of complex glycopeptide mixtures and determination of the sites of glycosylation.