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

Generation of affinity maps for thiazolidinediones with human serum albumin using affinity microcolumns. II. Effects of advanced glycation end products on multisite drug binding

Multisite protein interactions by the thiazolidinedione-class drugs pioglitazone and rosiglitazone were examined by using high-performance affinity microcolumns that contained normal human serum albumin (HSA) vs HSA that had been modified to form advanced glycation end products by glyoxal (Go) or methylglyoxal (MGo). The results were used to generate an affinity map for these drugs at several key regions on HSA. Strong binding (∼105 M-1) by these drugs was seen at both Sudlow sites I and II. About a 50 % decrease in the affinities at Sudlow site II was observed for pioglitazone for Go-modified HSA, while either a 47 % decrease or 1.6-fold increase in affinity was seen for MGo-modified HSA, depending on the extent of modification. The binding affinity for rosiglitazone at Sudlow site II had a 40-83 % decrease for Go-modified HSA and either a non-significant change or 1.4-fold increase for MGo-modified HSA. At Sudlow site I, pioglitazone gave a 41 % decrease in affinity for either Go or MGo-modified HSA, and for rosiglitazone up to a 55 % decrease or 1.3-fold increase in affinity was noted. Positive allosteric effects were seen by these drugs with the tamoxifen site of HSA, and neither drug had any notable binding at the digitoxin site for the normal or modified forms of HSA. Rosiglitazone also had weak interactions at a site in subdomain IB, which increased in affinity by up to 5.0-fold with the Go- or MGo-modified HSA. This study illustrated how affinity microcolumns can be used to provide a detailed analysis of solute-protein systems that involve complex interactions. The data obtained should also be valuable in providing a better understanding of how drug interactions with HSA and other proteins can be altered by modifications of these binding agents in diseases such as diabetes.

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.

Characterization of binding by sulfonylureas with normal or modified human serum albumin using affinity microcolumns prepared by entrapment

Modification of proteins can occur during diabetes due to the formation of advanced glycation end-products (AGEs) with reactive dicarbonyls such as glyoxal (Go) and methylglyoxal (MGo). Human serum albumin (HSA) is a serum protein that binds to many drugs in blood and that is known to be modified by Go and MGo. This study examined the binding of various sulfonylurea drugs with these modified forms of HSA by using high-performance affinity microcolumns prepared by non-covalent protein entrapment. Zonal elution experiments were employed to compare the retention and overall binding constants for the drugs with Go- or MGo-modified HSA vs normal HSA. The results were compared to values from the literature, such as measured or estimated using affinity columns containing covalently immobilized HSA or biospecifically-adsorbed HSA. The entrapment-based approach provided estimates of global affinity constants within 3-5 min for most of the tested drugs and with typical precisions of ±10-23%. Each entrapped protein microcolumn was stable for over at least 60-70 injections and one month of use. The results obtained with normal HSA agreed at the 95% confidence level with global affinity constants that have been reported for the given drugs in the literature. It was found for HSA that had been modified with clinically-relevant levels of either Go or MGo that an increase in the global affinity constant of up to 2.1-fold occurred for some of the tested drugs. The information acquired in this study can be used in the future to adapt this entrapment-based approach to study and evaluate interactions between other types of drugs and normal or modified binding agents for clinical testing and biomedical research.

Evaluation of Quantitative Structure Property Relationship Algorithms for Predicting Plasma Protein Binding in Humans

The extent of plasma protein binding is an important compound-specific property that influences a compound's pharmacokinetic behavior and is a critical input parameter for predicting exposure in physiologically based pharmacokinetic (PBPK) modeling. When experimentally determined fraction unbound in plasma (fup) data are not available, quantitative structure-property relationship (QSPR) models can be used for prediction. Because available QSPR models were developed based on training sets containing pharmaceutical-like compounds, we compared their prediction accuracy for environmentally relevant and pharmaceutical compounds. Fup values were calculated using Ingle et al., Watanabe et al. and ADMET Predictor (Simulation Plus). The test set included 818 pharmaceutical and environmentally relevant compounds with fup values ranging from 0.01 to 1. Overall, the three QSPR models resulted in over-prediction of fup for highly binding compounds and under-prediction for low or moderately binding compounds. For highly binding compounds (0.01≤ fup ≤ 0.25), Watanabe et al. performed better with a lower mean absolute error (MAE) of 6.7% and a lower mean absolute relative prediction error (RPE) of 171.7 % than other methods. For low to moderately binding compounds, both Ingle et al. and ADMET Predictor performed better than Watanabe et al. with superior MAE and RPE values. The positive polar surface area, the number of basic functional groups and lipophilicity were the most important chemical descriptors for predicting fup. This study demonstrated that the prediction of fup was the most uncertain for highly binding compounds. This suggested that QSPR-predicted fup values should be used with caution in PBPK modeling.

Detection Methods and Research Progress of Human Serum Albumin

Human serum albumin (HSA) is a biological macromolecule with important physiological functions; abnormal HSA levels are associated with coronary heart disease, multiple myeloma, diabetes, nephropathy, neurometabolic disorders, liver cirrhosis and other diseases. Therefore, accurate and quantitative detection of HAS have extremely important research and application value in biological science, molecular biology, clinical medicine and other fields. As for the detection method of HSA, dye-binding method and immune method are the first to be used, and have been applied in clinical detection. In recent years, many new detection technologies have emerged, such as fluorescent probe detection method, nano-materials for HSA detection, biosensor and so on. Although there are many methods developed recently to detect HSA, comprehensive reviews for HSA detection methods are still rare. Thus, writing this review to fill in the blank is in need. In order to highlight the recent progress in the field of HSA detection, in this review, the methods used to detect HSA are summarized and sorted, the advantages and disadvantages of these detection methods are also listed, then the research progress of small molecular fluorescence probe method is emphatically introduced in this paper. Then, we briefly discussed the challenges and future development directions in this field.

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.

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.

Analysis of biomolecular interactions using affinity microcolumns: a review

Affinity chromatography has become an important tool for characterizing biomolecular interactions. The use of affinity microcolumns, which contain immobilized binding agents and have volumes in the mid-to-low microliter range, has received particular attention in recent years. Potential advantages of affinity microcolumns include the many analysis and detection formats that can be used with these columns, as well as the need for only small amounts of supports and immobilized binding agents. This review examines how affinity microcolumns have been used to examine biomolecular interactions. Both capillary-based microcolumns and short microcolumns are considered. The use of affinity microcolumns with zonal elution and frontal analysis methods are discussed. The techniques of peak decay analysis, ultrafast affinity extraction, split-peak analysis, and band-broadening studies are also explored. The principles of these methods are examined and various applications are provided to illustrate the use of these methods with affinity microcolumns. It is shown how these techniques can be utilized to provide information on the binding strength and kinetics of an interaction, as well as on the number and types of binding sites. It is further demonstrated how information on competition or displacement effects can be obtained by these methods.

Characterization of Alizarin Red S binding sites and structural changes on human serum albumin: a biophysical study

Alizarin Red S (ARS), is a water-soluble, widely used anthraquinone dye synthesized by sulfonation of alizarin. In this report, the binding of ARS to human serum albumin (HSA) was characterized by employing fluorescence, UV/vis absorption, circular dichroism (CD), and molecular modeling methods. The data of fluorescence spectra displayed that the binding of ARS to HSA is the formation of HSA-ARS complex at 1:1 stoichiometric proportion. Hydrophobic probe 8-anilino-1-naphthalenesulfonic acid (ANS) was employed and elucidated that the dye was located in subdomain IIIA. This phenomenon corroborates the result of site-specific probe displacement experiments, which demonstrate the dye is at indole-benzodiazepine site (Sudlow's site II); and it is also consistent with guanidine hydrochloride (GuHCl) induced HSA unfolding studies and molecular modeling simulations. The features of the dye, which led to structural perturbations of HSA, have also been studied in detail by methods of UV/vis, CD and three-dimensional fluorescence spectroscopy.

Selected pharmacokinetic issues of the use of antiepileptic drugs and parenteral nutrition in critically ill patients

OBJECTIVES

To conduct a systematic review for the evidence supporting or disproving the reality of parenteral nutrition- antiepileptic drugs interaction, especially with respect to the plasma protein-binding of the drug.

METHODS

The articles related to the topic were identified through Medline and PubMed search (1968-Feburary 2010) for English language on the interaction between parenteral nutrition and antiepileptic drugs; the search terms used were anti-epileptic drugs, parenteral nutrition, and/or interaction, and/or in vitro. The search looked for prospective randomized and nonrandomized controlled studies; prospective nonrandomized uncontrolled studies; retrospective studies; case reports; and in vitro studies. Full text of the articles were then traced from the Universiti Sains Malaysia (USM) library subscribed databases, including Wiley-Blackwell Library, Cochrane Library, EBSCOHost, OVID, ScienceDirect, SAGE Premier, Scopus, SpringerLINK, and Wiley InterScience. The articles from journals not listed by USM library were traced through inter library loan.

RESULTS

There were interactions between parenteral nutrition and drugs, including antiepileptics. Several guidelines were designed for the management of illnesses such as traumatic brain injuries or cancer patients, involving the use of parenteral nutrition and antiepileptics. Moreover, many studies demonstrated the in vitro and in vivo parenteral nutrition -drugs interactions, especially with antiepileptics.

CONCLUSIONS

There was no evidence supporting the existence of parenteral nutrition-antiepileptic drugs interaction. The issue has not been studied in formal researches, but several case reports and anecdotes demonstrate this drug-nutrition interaction. However, alteration in the drug-free fraction result from parenteral nutrition-drug (i.e. antiepileptics) interactions may necessitate scrupulous reassessment of drug dosages in patients receiving these therapies. This reassessment may be particularly imperative in certain clinical situations characterized by hypoalbuminemia (e.g., burn patients).

Evaluation of binding and thermodynamic characteristics of interactions between a citrus flavonoid hesperitin with protein and effects of metal ions on binding

The mechanism of interaction of a non-glycosidic citrus flavonoid, hesperitin (HES) with bovine serum albumin (BSA) was studied by UV-vis absorption, fluorescence, FT-IR, circular dichroism, fluorescence anisotropy and synchronous fluorescence spectroscopy in phosphate buffer of pH 7.4. Fluorescence data revealed that the fluorescence quenching of BSA by HES was the result of the formed complex of HES-BSA. The binding constants and thermodynamic parameters at four different temperatures, the location of binding, and the nature of binding force were determined. The hydrogen bonds interactions were found to be the predominant intermolecular forces to stabilize the complex. The conformation of BSA was discussed by synchronous fluorescence and CD methods. The alterations of protein secondary structure upon complexation with HES were evident from the gradual decrease in α-helicity. The distance between the donor (BSA) and acceptor (flavonoid) was calculated from the fluorescence resonance energy transfer and found to be 1.978 nm. Common ions viz., Zn(2+), K(+), Cu(2+), Ni(2+), Mn(2+) and Co(2+) were found to influence the binding of flavonoid to protein.

[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.

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.

Analysis of lidocaine interactions with serum proteins using high-performance affinity chromatography

High-performance affinity chromatography was used to study binding by the drug lidocaine to human serum albumin (HSA) and alpha(1)-acid glycoprotein (AGP). AGP had strong binding to lidocaine, with an association equilibrium constant (K(a)) of 1.1-1.7 x 10(5) M(-1) at 37 degrees C and pH 7.4. Lidocaine had weak to moderate binding to HSA, with a K(a) in the range of 10(3) to 10(4) M(-1). Competitive experiments with site selective probes showed that lidocaine was interacting with Sudlow site II of HSA and the propranolol site of AGP. These results agree with previous observations in the literature and provide a better quantitative understanding of how lidocaine binds to these serum proteins and is transported in the circulation. This study also demonstrates how HPAC can be used to examine the binding of a drug with multiple serum proteins and provide detailed information on the interaction sites and equilibrium constants that are involved in such processes.

Evaluation of alternatives to warfarin as probes for Sudlow site I of human serum albumin: characterization by high-performance affinity chromatography

Warfarin is often used as a site-specific probe for examining the binding of drugs and other solutes to Sudlow site I of human serum albumin (HSA). However, warfarin has strong binding to HSA and the two chiral forms of warfarin have slightly different binding affinities for this protein. Warfarin also undergoes a slow change in structure when present in common buffers used for binding studies. This report examined the use of four related, achiral compounds (i.e., coumarin, 7-hydroxycoumarin, 7-hydroxy-4-methylcoumarin, and 4-hydroxycoumarin) as possible alternative probes for Sudlow site I in drug binding studies. High-performance affinity chromatography and immobilized HSA columns were used to compare and evaluate the binding properties of these probe candidates. Binding for each of the tested probe candidates to HSA was found to give a good fit to a two-site model. The first group of sites had moderate-to-high affinities for the probe candidates with association equilibrium constants that ranged from 6.4 x 10(3)M(-1) (coumarin) to 5.5 x 10(4)M(-1) (4-hydroxycoumarin) at pH 7.4 and 37 degrees C. The second group of weaker, and probably non-specific, binding regions, had association equilibrium constants that ranged from 3.8 x 10(1)M(-1) (7-hydroxy-4-methylcoumarin) to 7.3 x 10(2)M(-1) (coumarin). Competition experiments based on zonal elution indicated that all of these probe candidates competed with warfarin at their high affinity regions. Warfarin also showed competition with coumarin, 7-hydroxycoumarin and 7-hydroxy-4-methycoumarin for their weak affinity sites but appeared to not bind and/or compete for all of the weak sites of 4-hydroxycoumarin. It was found from this group that 4-hydroxycoumarin was the best alternative to warfarin for examining the interactions of drugs at Sudlow site I on HSA. These results also provided information on how the major structural components of warfarin contribute to the binding of this drug at Sudlow site I.

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.

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

The binding of verapamil to the protein human serum albumin (HSA) was examined by using high-performance affinity chromatography. Many previous reports have investigated the binding of verapamil with HSA, but the exact strength and nature of this interaction (e.g. the number and location of binding sites) is still unclear. In this study, frontal analysis indicated that at least one major binding site was present for R- and S-verapamil on HSA, with estimated association equilibrium constants on the order of 10(4)M(-1) and a 1.4-fold difference in these values for the verapamil enantiomers at pH 7.4 and 37 degrees C. The presence of a second, weaker group of binding sites on HSA was also suggested by these results. Competitive binding studies using zonal elution were carried out between verapamil and various probe compounds that have known interactions with several major and minor sites on HSA. R/S-Verapamil was found to have direct competition with S-warfarin, indicating that verapamil was binding to Sudlow site I (i.e. the warfarin-azapropazone site of HSA). The average association equilibrium constant for R- and S-verapamil at this site was 1.4 (+/-0.1)x10(4)M(-1). Verapamil did not have any notable binding to Sudlow site II of HSA but did appear to have some weak allosteric interactions with l-tryptophan, a probe for this site. An allosteric interaction between verapamil and tamoxifen (a probe for the tamoxifen site) was also noted, which was consistent with the binding of verapamil at Sudlow site I. No interaction was seen between verapamil and digitoxin, a probe for the digitoxin site of HSA. These results gave good agreement with previous observations made in the literature and help provide a more detailed description of how verapamil is transported in blood and of how it may interact with other drugs in the body.

Estimating protein-ligand binding affinity using high-throughput screening by NMR

Many of today's drug discovery programs use high-throughput screening methods that rely on quick evaluations of protein activity to rank potential chemical leads. By monitoring biologically relevant protein-ligand interactions, NMR can provide a means to validate these discovery leads and to optimize the drug discovery process. NMR-based screens typically use a change in chemical shift or line width to detect a protein-ligand interaction. However, the relatively low throughput of current NMR screens and their high demand on sample requirements generally makes it impractical to collect complete binding curves to measure the affinity for each compound in a large and diverse chemical library. As a result, NMR ligand screens are typically limited to identifying candidates that bind to a protein and do not give any estimate of the binding affinity. To address this issue, a methodology has been developed to rank binding affinities for ligands based on NMR screens that use 1D (1)H NMR line-broadening experiments. This method was demonstrated by using it to estimate the dissociation equilibrium constants for twelve ligands with the protein human serum albumin (HSA). The results were found to give good agreement with previous affinities that have been reported for these same ligands with HSA.

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.

A new drug binding subsite on human serum albumin and drug-drug interaction studied by X-ray crystallography

3'-Azido-3'-deoxythymidine (AZT) is the first clinically effective drug for the treatment of human immunodeficiency virus infection. The drug interaction with human serum albumin (HSA) has been an important component in understanding its mechanism of action, especially in drug distribution and in drug-drug interaction on HSA in the case of multi-drug therapy. We present here crystal structures of a ternary HSA-Myr-AZT complex and a quaternary HSA-Myr-AZT-SAL complex (Myr, myristate; SAL, salicylic acid). From this study, a new drug binding subsite on HSA Sudlow site 1 was identified. The presence of fatty acid is needed for the creation of this subsite due to fatty acid induced conformational changes of HSA. Thus, the Sudlow site 1 of HSA can be divided into three non-overlapped subsites: a SAL subsite, an indomethacin subsite and an AZT subsite. Binding of a drug to HSA often influences simultaneous binding of other drugs. From the HSA-Myr-AZT-SAL complex structure, we observed the coexistence of two drugs (AZT and SAL) in Sudlow site 1 and the competition between these two drugs in subdomain IB. These results provide new structural information on HSA-drug interaction and drug-drug interaction on HSA.

Applications of silica supports in affinity chromatography

The combined use of silica-based chromatographic supports with immobilized affinity ligands can be used in many preparative and analytical applications. One example is the use of silica-based affinity columns in HPLC, giving rise to a method known as high-performance affinity chromatography (HPAC). This review discusses the role that silica has played in the development of affinity chromatography and HPAC and the applications of silica in these methods. This includes a discussion of the types of ligands that have been employed with silica and the methods by which these ligands have been immobilized. Various formats have also been presented for the use of silica in affinity chromatographic methods, including assays involving direct or indirect analyte detection, on-line or off-line affinity extraction, and chiral separations. The use of silica-based affinity columns in studies of biological systems based on zonal elution and frontal analysis methods will also be considered.

Kinetics parameter estimation for the binding process of salicylic acid to human serum albumin (HSA) with capacitive sensing technique

A novel method for real-time investigating the binding interaction between human serum albumin (HSA) and salicylic acid with capacitive sensing technique was successfully proposed. HSA was immobilized on the surface of a gold electrode modified with an insulating poly (o-phenylenediamine) (o-PD) film and colloid Au nanoparticles layers. The bioactivity of HSA was remained and major binding sites were available because of the excellent biocompatibility of gold nanoparticles. The capacitance and interfacial electron resistance of the sensor were altered, owing to the binding of HSA to salicylic acid. The time courses of the capacitance change were acquired with capacitive sensing technique during the binding process. Based on the capacitance response curves with time, the response model for the binding was derived in theory and the corresponding regression parameters were determined by fitting the real-time experimental data to the model. The binding and the dissociation rate constants (k(1) and k(-1)) were estimated to be 54.8 (mol l(-1))(-1) s(-1) and 2.9 x 10(-3) s(-1), respectively. And the binding equilibrium constant (K(a)) was calculated to be 1.89 x 10(4) (mol l(-1))(-1).

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.

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.

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 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.

High-Performance Affinity Monolith Chromatography: Development and Evaluation of Human Serum Albumin Columns

Several immobilization methods were explored for the preparation of high-performance affinity monolithic columns containing human serum albumin (HSA). These monoliths were based on a copolymer of glycidyl methacrylate and ethylene dimethacrylate. In one method, the epoxy groups of this copolymer were used directly for the immobilization of HSA through its amine residues (i.e., the epoxy method); in other approaches, these epoxy groups were converted to diols for later use in the carbonyldiimidazole, disuccinimidyl carbonate, and Schiff base methods. Each HSA monolith was evaluated in terms of its total protein content and its retention of several model compounds, including (R/S)-warfarin and D/L-tryptophan. The greatest amount of immobilized HSA was obtained by the Schiff base method, whereas the epoxy method gave the lowest protein content. The Schiff base method also gave the best resolution in chiral separations of (R/S)-warfarin and D/L-tryptophan. All of the immobilization methods gave similar relative activities for HSA in its binding to (R)- and (S)-warfarin, but some differences were noted in the activity of the immobilized HSA for D- and L-tryptophan. The efficiency of these monoliths was found to be greater than that of silica-based HSA columns for (R/S)-warfarin (i.e., analytes with high retention), but little or no difference was seen for D- and L-tryptophan (analytes with weak retention).

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.

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.

Piezoelectric quartz crystal impedance and electrochemical impedance study of HSA-diazepam interaction by nanogold-structured sensor

Human serum albumin (HSA) was immobilized on the surface of colloidal Au and exposed to diazepam. Colloidal Au were at first self-assembled on the gold electrode through the thiol groups of a 1,6-hexanedithiol monolayer. The real-time course of the resonant frequency and equivalent circuit parameters of the sensor during the protein-diazepam binding was determined for the first time by piezoelectric quartz crystal impedance (PQCI). On the basis of the multidimensional information provided by the PQCI analysis, it was concluded that the decrement of the observed frequency was mainly ascribable to the mass loading on the sensor surface. Compared with a bare gold electrode, the gold electrode self-assembled from nanogold colloids exhibits maintained biocompatibility, increased capacity, and more bioactivity. Cyclic voltammetry and electrochemical impedance techniques were used to investigate the immobilization of HSA and the interaction between HSA and diazepam. Results testified that gold colloid could play the role of an efficient electron-conducting tunnel and have a very high ratio of surface to volume. Additionally, the kinetics of the binding process was investigated. The estimated binding constant (K) and the number of binding site (n) on one HSA molecule were 1.66 x 10(6) mol l(-1) and 1.28, respectively.

In silico prediction of drug-binding strengths to human serum albumin

Drug binding to Human Serum Albumin (HSA) is an area of intense research. The pharmacokinetics and pharmacodynamics of drugs are strongly affected by their binding to this protein. In this article, the field is reviewed, as well as our models to predict drug-binding affinities to HSA from drug structure. The physiological role of HSA is described, as well as its influence in drug action. The crystal structures of this protein are discussed, emphasizing the two drug-binding sites and the fatty acids binding sites observed therein. The advantages of using high-performance affinity chromatography to rapidly screen drugs for HSA binding are explained. The different QSAR models for HSA binding of restricted families of drugs (both from other groups and our group) are enumerated. Finally, a detailed description of our general models to predict drug-binding strengths to HSA from structure is given. It is expected for these models to be useful in drug design and pharmaceutical research.

Predicting plasma protein binding of drugs: a new approach

In spite of the large amount of plasma protein binding data for drugs, it is not obvious and there is no clear consensus among different disciplines how to deal with this parameter in multidimensional lead optimization strategies. In this work, we have made a comprehensive study on the importance of plasma protein binding and the influencing factors in order to get new insights for this molecular property. Our analysis of the distribution of percentage plasma protein binding among therapeutic drugs showed that no general rules for protein binding can be derived, except for the class of chemotherapeutics, where a clear trend towards lower binding could be observed. For the majority of indication areas, however, empirical rules are missing. We present here an extensive list of multiply determined primary association constants for binding to human serum albumin (HSA) for 138 compounds from the literature. Correlating these binding constants with the percentage fraction of protein bound showed that the percentage data above 90%, corresponding to a binding constant below 6 microM, are of insufficient accuracy. Furthermore, it could be demonstrated that the lipophilicity of drugs, traditionally felt to dominate binding to HSA, is not the only relevant descriptor. Here, we report a generic model for the prediction of drug association constants to HSA, which uses a pharmacophoric similarity concept and partial least square analysis (PLS) to construct a quantitative structure-activity relationship. It is able to single out the submicromolar to nanomolar binders, i.e. to differentiate between 99.0 and 99.99% plasma protein binding. Depending on the system, this can be important in medicinal chemistry programs and may together with other computed physicochemical and ADME properties assist in the prioritization of synthetic strategies.

Monitoring and kinetic parameter estimation for the binding process of berberine hydrochloride to bovine serum albumin with piezoelectric quartz crystal impedance analysis

A new method for monitoring, in real time, the drug-binding process to protein with piezoelectric quartz crystal impedance (PQCI) is proposed. The method was used to monitor the binding process of berberine hydrochloride to bovine serum albumin (BSA). BSA was immobilized on the silver electrode surface of a piezoelectric quartz crystal and the optimized experimental conditions were established. The BSA-coated piezoelectric sensor was in contact with berberine solution. The time courses of the resonant frequency and equivalent circuit parameters of the sensor during the protein-drug binding were simultaneously obtained. On the basis of the analysis of the multidimensional information provided by PQCI, it was concluded that the observed frequency decrease was mainly ascribed to the mass increase of the sensor surface resulting from the binding. According to the frequency decrease with time, the kinetics of the binding process were quantitatively studied. A piezoelectric response model for the binding was theoretically derived. Fitting the experimental data to the model, the kinetic parameters, such as the binding and dissociation rate constants (k(1) and k(-1)) and the binding equilibrium constant (K(a)), were determined. The k(1), k(-1), and K(a) values obtained at 25 degrees C were 67.5 (+/-0.1) (mol liter(-1))(-1) s(-1), 1.7 (+/- 0.1) x 10(-3) s(-1), and 3.97 (+/- 0.06) x10(4) (mol liter(-1))(-1), respectively.

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.

Study of competitive binding of enantiomers to protein by affinity capillary electrochromatography

Affinity capillary electrochromatography (CEC) with zonal elution method was used to probe the competitive interactions of enantiomers with protein. In this approach, a known concentration of a competing agent is continuously applied to a CEC column with bovine serum albumin (BSA) physically adsorbed on SAX packing while injections of a small amount of analyte are made. The binding sites of solutes on the BSA molecule were determined by the changes in the retention factors of the solutes resulted from the addition of competitive agent. By using D- or L-tryptophan as competitive agents and D-, L-tryptophan and benzoin enantiomers as injected analytes showed that BSA molecule has a primary site to strongly bind L-tryptophan, but D-tryptophan dose not bind at this site; D- and L-tryptophan share a weak binding site on the BSA molecule. Benzoin enantiomers do not share any binding sites with either D- or L-tryptophan. Non-chiral compounds of trichloroacetic acid and n-hexanoic acid were applied as the competitive agents to study the binding of warfarin enantiomers to BSA, it was observed that trichloroacetic acid and n-hexanoic acid had a same binding site for warfarin enantiomers binding to BSA molecule.