Study of high-affinity interactions by quantitative affinity chromatography. Analytical expressions in terms of total ligand concentration

Foreword to 'Quantitative and analytical relations in biochemistry'-a special issue in honour of Donald J. Winzor's 80th birthday

The purpose of this special issue is to honour Professor Donald J. Winzor's long career as a researcher and scientific mentor, and to celebrate the milestone of his 80th birthday. Throughout his career, Don has been renowned for his development of clever approximations to difficult quantitative relations governing a range of biophysical measurements. The theme of this special issue, 'Quantitative and analytical relations in biochemistry', was chosen to reflect this aspect of Don's scientific approach.

Sulfated galactan is a catalyst of antithrombin-mediated inactivation of alpha-thrombin

Novel compounds presenting anticoagulant activity, such as sulfated polysaccharides, open new perspectives in medicine. Elucidation of the molecular mechanism behind this activity is desirable by itself, as well as because it allows for the design of novel compounds. In the present study, we investigated the action of an algal sulfated galactan, which potentiates alpha-thrombin inactivation by antithrombin. Our results indicate the following: 1) both the sulfated galactan and heparin potentiate protease inactivation by antithrombin at similar molar concentrations, however they differ markedly in the molecular size required for their activities; 2) this galactan interacts predominantly with exosite II on alpha-thrombin and, similar to heparin, catalyzes the formation of a covalent complex between antithrombin and the protease; 3) the sulfated galactan has a higher affinity for alpha-thrombin than for antithrombin. We propose that the preferred pathway of sulfated galactan-induced inactivation of alpha-thrombin by antithrombin starts with the polysaccharide binding to the protease through a high-affinity interaction. Antithrombin is then added to the complex and the protease is inactivated by covalent interactions. Finally, the antithrombin-alpha-thrombin covalent complex dissociates from the polysaccharide chain. This mechanism resembles the action of heparin with low affinity for antithrombin, as opposed to heparin with high affinity for serpin.

Quantitative affinity chromatography

The objective of this review is to summarize developments in the use of quantitative affinity chromatography to determine equilibrium constants for solute interactions of biological interest. Affinity chromatography is an extremely versatile method for characterizing interactions between dissimilar reactants because the biospecificity incorporated into the design of the affinity matrix ensures applicability of the method regardless of the relative sizes of the two reacting solutes. Adoption of different experimental strategies, such as column chromatography, simple partition equilibrium experiments, solid-phase immunoassay, and biosensor technology, has led to a situation whereby affinity chromatography affords a means of characterizing interactions governed by an extremely broad range of binding affinities--relatively weak interactions (binding constants below 10(3) M(-1)) through to interactions with binding constants in excess of 10(9) M(-1). In addition to its important role in solute separation and purification, affinity chromatography thus also possesses considerable potential for investigating the functional roles of the reactants thereby purified.

Advantages of quantitative affinity chromatography for the analysis of solute interaction with membrane proteins

The use of membrane proteins as chromatographic stationary phases for the quantitation of biospecific interaction between the proteins and solutes is reviewed. This method is one among the few where a membrane protein is immobilized for repeated analyses of solute binding. To our knowledge, five transmembrane proteins have been immobilized in chromatographic matrices: the glucose and nucleoside transporters from human red blood cells, the photosynthetic reaction center from Rhodobacter sphaeroides, the nicotinic acetylcholine receptor from rat brain and a recombinant P-glycoprotein. Proteoliposomes and membrane vesicles have thereby been entrapped in size-exclusion beads, such as Superdex 200, and membrane proteins have been adsorbed on 'immobilized artificial membrane' monolayers of lipid analogs grafted to silica beads. Encouragingly, immobilized glucose transporter and P-glycoprotein showed constant interactant affinities for months. Analysis is done in the frontal mode at equilibrium because there is no separation between bound and free ligand. Both the affinity constant, which generally coincides with the corresponding constant determined by use of nonchromatographic methods, and the amount of active binding sites are obtained. The method has been successfully applied to functional analysis of membrane proteins in cells or reconstituted in lipid mono- or bilayers, screening of low-molecular interactants, investigation of protein-protein interaction and studies of effects of physico-chemical parameters on solute-protein interaction. The analyses require sensitive detection of the analyte and matching between amount of binding sites and affinity.

From gel filtration to biosensor technology: the development of chromatography for the characterization of protein interactions

The objective of this review is to summarize the development of chromatographic techniques for the determination of reaction stoichiometries and equilibrium constants for solute interactions of biological importance. Gel chromatography is shown to offer a convenient means of characterizing solute self-association as well as solute-ligand interactions. Affinity chromatography is an even more versatile method of characterizing interactions between dissimilar reactants because the biospecificity incorporated into the design of the affinity matrix ensures applicability of the method regardless of the relative sizes of the two reactants. Adoption of different experimental strategies such as column chromatography, simple partition equilibrium experiments and biosensor technology has created a situation wherein affinity chromatography affords a means of characterizing the whole range of reaction affinities-from relatively weak interactions (binding constants less that 10(3)M (-1)) to tight interactions with binding constants greater than 10(9)M (-1). In addition to its established prowess as a means of solute separation and purification, chromatography thus also possesses considerable potential for investigation of the functional roles of the purified reactants-an endeavour that requires characterization as well as identification of the interactions responsible for a physiological phenomenon.

Potential of biosensor technology for the characterization of interactions by quantitative affinity chromatography

This review places the characterization of interactions by biosensor technology in the broader context of their study by quantitative affinity chromatography. The general reluctance to consider biosensor-based characterization as a form of quantitative affinity chromatography on the grounds of a difference in aims of the two techniques reflects a mistaken belief that BIAcore and IAsys studies characterize the kinetics of the chemical reaction responsible for biospecific adsorption of a soluble reactant to an immobilized form of its affinity partner. It now transpires that the association and dissociation rate constants thereby determined refer to thermodynamic characterization of biospecific adsorption in terms of a single-phase model in which affinity sites are distributed uniformly throughout the liquid-phase volume accessible to the partitioning reactant--the model used for characterization of biospecific adsorption by quantitative affinity chromatography. In that light the most important attribute of biosensor technology is its potential for thermodynamic characterization of biospecific adsorption by virtue of its ability to monitor complex formation directly; and hence its potential for the characterization of interactions with affinities that are too strong for study by forms of quantitative affinity chromatography that monitor complex formation on the basis of reactant depletion from the liquid phase. Kinetic as well as thermodynamic analyses of biosensor data are described for attainment of that potential.

Chromatography on cells and biomolecular assemblies

Red cells, biomembrane vesicles, proteoliposomes and liposomes non-covalently immobilized in gel particles or beads have been used as stationary phases for biomembrane affinity analyses and ion-exchange chromatographic separation. Lipid monolayers coupled to silica beads have been utilized for membrane protein purification in detergent solution and plant cell walls for group separation of macromolecules according to size and charge. Further methodological studies are essential to implement general practical application.

Glucose affinity for the glucose transporter Glut1 in native or reconstituted lipid bilayers. Temperature-dependence study by biomembrane affinity chromatography

The affinity of D-glucose and the transport inhibitor cytochalasin B (CB) for the glucose transporter Glut1 was studied at 5-42 degrees C by quantitative frontal affinity chromatography on sterically immobilized human red cell membrane vesicles, and on proteoliposomes containing reconstituted red cell membrane proteins. Glut1 in the vesicles showed the highest glucose affinity; the dissociation constant Kd(glc) was nearly constant (16 +/- 3 mM) from 15 degrees C to 37 degrees C. For Glut1 in proteoliposomes Kd(glc) decreased from 56 mM at 5 degrees C to 26 mM at 42 degrees C. The CB-Glut1 affinity was strongest around 20 degrees C and was mostly higher with the vesicles, Kd (CB) being 49 nM at 19 degrees C. The entropy and entropy and enthalpy changes for the interactions were calculated.

Characterisation of silica-based heparin affinity sorbents from equilibrium binding studies on plasma fractions containing thrombin

The binding properties of rigid heparin sorbents, synthesised by end-point-attachment of heparin onto aminopropylderivatised silica through reductive amination, were characterised through batch-adsorption studies employing human plasma fractions containing thrombin. Thrombin was quantified using a chromogenic assay that had been specially modified for these studies. These investigations yielded information regarding the maximum adsorption capacities/stoichiometries and binding affinities for thrombin present in complex protein mixtures. Of the two types of heparin-silica evaluated, heparin-Fractosil 1000, with a pore size of 1000 A, displayed a capacity of 2.4 mol of thrombin/mol of heparin (mol T-mol H). This stoichiometry was significantly higher than the value of 1.8 mol T-mol H obtained for the commercial soft gel heparin-Sepharose CL-6B. Furthermore, the heparin-Fractosil 1000 sorbents were superior in capacity and binding site accessibility to heparin-LiChroprep Si60 sorbents, where the smaller pore size of 60 A largely restricts the ligand-protein interactions to the outer surface of the sorbent particles. Nevertheless, heparin-LiChroprep Si60 sorbents were useful, in that they simulated a non-porous particle system, in which intra-pore diffusion effects are eliminated. The batch adsorption results with these sorbents indicated that the adsorption involved both high and low binding affinity characteristics. This bimodal binding mechanism was also evident with the commercial heparin-Sepharose sorbent. Binding stoichiometries and affinities in the high concentration range were similar to values reported for a largely non-specific electrostatic thrombin-heparin interaction. Dissociation constants in the nanomolar range were observed in the low concentrations range. This stronger binding affinity is more similar to highly specific bio-affinity interactions. Thus, the results indicated that heparin-thrombin interactions with these systems involve both a weak electrostatic and a strong biospecific interaction component.

Recent developments in quantitative affinity chromatography

This review surveys developments during the past decade in the use of quantitative affinity chromatography as a means of evaluating equilibrium constants for solute-ligand and solute-matrix interactions. Topics include allowance for multivalency of the partitioning solute, removal of the myth that highly substituted affinity matrices are unsuitable for zonal quantitative affinity chromatography, adaptation of the technique to allow characterization of high-affinity interactions and the application of quantitative affinity chromatography theory to the characterization of biospecific adsorption phenomena in cellular systems.