Progress in affinophoresis

Lectin affinity electrophoresis

An interaction or a binding event typically changes the electrophoretic properties of a molecule. Affinity electrophoresis methods detect changes in the electrophoretic pattern of molecules (mainly macromolecules) that occur as a result of biospecific interactions or complex formation. Lectin affinity electrophoresis is a very effective method for the detection and analysis of trace amounts of glycobiological substances. It is particularly useful for isolating and separating the glycoisomers of target molecules. Here, we describe a sensitive technique for the detection of glycoproteins separated by agarose gel-lectin affinity electrophoresis that uses antibody-affinity blotting. The technique is tested using α-fetoprotein with lectin (Lens culinaris agglutinin and Phaseolus vulgaris agglutinin)-agarose gels.

The Cutting Edge of Affinity Electrophoresis Technology

Affinity electrophoresis is an important technique that is widely used to separate and analyze biomolecules in the fields of biology and medicine. Both quantitative and qualitative information can be gained through affinity electrophoresis. Affinity electrophoresis can be applied through a variety of strategies, such as mobility shift electrophoresis, charge shift electrophoresis or capillary affinity electrophoresis. These strategies are based on changes in the electrophoretic patterns of biological macromolecules that result from interactions or complex-formation processes that induce changes in the size or total charge of the molecules. Nucleic acid fragments can be characterized through their affinity to other molecules, for example transcriptional factor proteins. Hydrophobic membrane proteins can be identified by means of a shift in the mobility induced by a charged detergent. The various strategies have also been used in the estimation of association/disassociation constants. Some of these strategies have similarities to affinity chromatography, in that they use a probe or ligand immobilized on a supported matrix for electrophoresis. Such methods have recently contributed to profiling of major posttranslational modifications of proteins, such as glycosylation or phosphorylation. Here, we describe advances in analytical techniques involving affinity electrophoresis that have appeared during the last five years.

Quantitative evaluation of lectin-reactive glycoforms of α(1)-acid glycoprotein using lectin affinity capillary electrophoresis with fluorescence detection

α(1)-Acid glycoprotein (AGP) was previously shown to be a marker candidate of disease progression and prognosis of patients with malignancies by analysis of its glycoforms via lectins. Herein, affinity capillary electrophoresis of fluorescein-labeled AGP using lectins with the aid of laser-induced fluorescence detection was developed for quantitative evaluation of the fractional ratios of concanavalin A-reactive or Aleuria aurantia lectin-reactive AGP. Labeled AGP was applied at the anodic end of a fused-silica capillary (50 μm id, 360 μm od, 27 cm long) coated with linear polyacryloyl-β-alanyl-β-alanine, and electrophoresis was carried out for about 10 min in 60 mM 3-morpholinopropane-1-sulfonic acid-NaOH buffer (pH 7.35). Addition of the lectins to the anode buffer resulted in the separation of lectin-reactive glycoform peaks from lectin-non-reactive glycoform peaks. Quantification of the peak area of each group revealed that the percent of lectin-reactive AGP is independent of a labeling ratio ranging from 0.4 to 1.5 mol fluorescein/mol AGP, i.e. the standard deviation of 0.5% for an average of 59.9% (n=3). In combination with a facile procedure for micro-purification of AGP from serum, the present procedure, marking the reactivity of AGP with lectins, should be useful in determining the prognosis for a large number of patients with malignancies.

A transglucosidase necessary for starch degradation and maltose metabolism in leaves at night acts on cytosolic heteroglycans (SHG)

The recently characterized cytosolic transglucosidase DPE2 (EC 2.4.1.25) is essential for the cytosolic metabolism of maltose, an intermediate on the pathway by which starch is converted to sucrose at night. In in vitro assays, the enzyme utilizes glycogen as a glucosyl acceptor but the in vivo acceptor molecules remained unknown. In this communication we present evidence that DPE2 acts on the recently identified cytosolic water-soluble heteroglycans (SHG) as does the cytosolic phosphorylase (EC 2.4.1.1) isoform. By using in vitro two-step (14)C labeling assays we demonstrate that the two transferases can utilize the same acceptor sites of the SHG. Cytosolic heteroglycans from a DPE2-deficient Arabidopsis mutant were characterized. Compared with the wild type the glucose content of the heteroglycans was increased. Most of the additional glucosyl residues were found in the outer chains of SHG that are released by an endo-alpha-arabinanase (EC 3.2.1.99). Additional starch-related mutants were characterized for further analysis of the increased glucosyl content. Based on these data, the cytosolic metabolism of starch-derived carbohydrates is discussed.

The glycan substrate of the cytosolic (Pho 2) phosphorylase isozyme from Pisum sativum L.: identification, linkage analysis and subcellular localization

The subcellular distribution of starch-related enzymes and the phenotype of Arabidopsis mutants defective in starch degradation suggest that the plastidial starch turnover is linked to a cytosolic glycan metabolism. In this communication, a soluble heteroglycan (SHG) from leaves of Pisum sativum L. has been studied. Major constituents of the SHG are galactose, arabinose and glucose. For subcellular location, the SHG was prepared from isolated protoplasts and chloroplasts. On a chlorophyll basis, protoplasts and chloroplasts yielded approximately 70% and less than 5%, respectively, of the amount of the leaf-derived SHG preparation. Thus, most of SHG resides inside the cell but outside the chloroplast. SHG is soluble and not membrane-associated. Using membrane filtration, the SHG was separated into a <10 kDa and a >10 kDa fraction. The latter was resolved into two subfractions (I and II) by field-flow fractionation. In the protoplast-derived >10 kDa SHG preparation the subfraction I was by far the most dominant compound. beta-Glucosyl Yariv reagent was reactive with subfraction II, but not with subfraction I. In in vitro assays the latter acted as glucosyl acceptor for the cytosolic (Pho 2) phosphorylase but not for rabbit muscle phosphorylase. Glycosidic linkage analyses of subfractions I and II and of the Yariv reagent reactive glycans revealed that all three glycans contain a high percentage of arabinogalactan-like linkages. However, SHG possesses a higher content of minor compounds, namely glucosyl, mannosyl, rhamnosyl and fucosyl residues. Based on glycosyl residues and glycosidic linkages, subfraction I possesses a more complex structure than subfraction II.

Determination of the affinity constants of recombinant human galectin-1 and -3 for simple saccharides by capillary affinophoresis

The affinity constants of recombinant human galectin-1 and galectin-3 for sugars were determined by capillary affinophoresis. The monoliganded affinophore contains p-aminophenyl-beta-lactoside as an affinity ligand in the matrix of succinylglutathione and has three negative charges. An analysis of the mobility change of the lectins caused by the affinophore and its inhibition by neutral sugars allowed, for the first time, a determination of the affinity constants between the binding sites of the lectins and sugars. The relative magnitude of the affinity constants for each of the sugars in terms of dissociation constants found to be consistent with previously reported data on the concentrations of sugars that caused a 50% inhibition (I50) in the binding assay of the lectin to oligosaccharide-immobilized agarose beads but the absolute values of the dissociation constants were considerably smaller than the I50 values. Capillary affinophoresis indicated microheterogeneity of the lectin preparations and enabled the separate analysis of the affinity of each component simultaneously showing the advantage in using a separation method for analysis of bioaffinity.

Prevention of evaporation of small-volume sample solutions for capillary electrophoresis using a mineral-oil overlay

The suppression of evaporation of water from small volumes of sample solutions or reagents for capillary electrophoresis by the use of a mineral-oil overlay was investigated in affinophoresis applications, in which the affinity constant of a mutant protein of recombinant human galectin-1 to a lactose affinophore, a triply negative charged ion having a lactoside as an affinity ligand, was determined. When an injection was carried out from a minimum of 20 microL of an aqueous solution beneath the oil overlay, no oil contamination inside the capillary was observed, provided the capillary was cleanly cut so that the end was flat, and the polyimide coating had been removed for a distance of about 2 mm from the end. Affinophoresis was carried out using 20 microL of an affinophore solution covered with an oil overlay. The abnormalities in the electropherograms as the result of the evaporation of the water from the solution during storage prior to use in an automatic operation of a capillary electrophoresis instrument were suppressed, with respect to the formation of a base line gap, an increase in the detection time of a marker ion and an increase in the initial current. A solution in a vial could be used repeatedly for a longer period of time when overlaid with mineral oil than in the absence of an overlay. The use of a mineral-oil overlay is a simple but very efficient technique for solving the problem of the evaporation of water from small volumes of aqueous solutions for use in capillary electrophoresis.

Certain high molecular weight heparin chains have high affinity for vitronectin

Vitronectin is a 70-kDa protein that is found in both the extracellular matrix as well as serum. Vitronectin is one of the few proteins that regulates both the complement and the coagulation systems. Heparin is known to bind to vitronectin. Review of the literature reveals apparently conflicting outcomes of the interaction of heparin, vitronectin, and the complement system. Previous studies demonstrated that heparin diminishes vitronectin inhibition of complement activity. Numerous studies have also demonstrated that heparin exerts a net inhibitory effect on complement. We used two dimensional affinity resolution electrophoresis (2DARE) to examine this apparent paradox. 2DARE allowed simultaneous determination of binding affinity of heparin for vitronectin as well as the M(r) of the heparin species. In the 2DARE experiment, the interaction of heparin with vitronectin caused retardation of the movement of the heparin through the tube gel in the first dimension. The degree of the retardation of movement was used to calculate the approximate K(d) of that interaction. The heparin from the tube gel was then subjected to a second dimension electrophoresis to determine the M(r) of the heparin. 2DARE analysis of the interaction of heparin with vitronectin clearly demonstrated that a sub-population of heparin chains with M(r) > 8000 bound vitronectin with high affinity whereas most high M(r) chains and all lower M(r) chains showed little to no affinity for vitronectin. Our findings are consistent with the hypothesis that a unique binding domain exists in certain heparin chains for vitronectin.

Capillary zone electrophoretic separation of neutral species of chloro-s-triazines in the presence of cationic surfactant monomers

Chloro-s-triazines are difficult to separate by capillary zone electrophoresis (CZE), due to their low pKa values. However, these analytes can be effectively separated by CZE in the presence of cationic surfactant monomers, such as tetradecylammonium bromide (TTAB) and dodecyltrimethylammonium bromide (DTAB). The separation mechanism based on a 1:1 binding of analytes to cationic surfactant monomers is proposed. The binding constants of chloro-s-triazines to cationic surfactant monomers are estimated. The results show that the strength of the interactions of these analytes with TTAB monomers is considerably strong, whereas that of the corresponding analyte with DTAB monomers is about 12- to 14-fold weaker. A linear correlation of binding constants with log P(ow) (the logarithm of the partition coefficient of analytes between 1-octanol and aqueous phases) indicates that the migration order of these chloro-s-triazines depends primarily on their hydrophobicity. Moreover, the skewed peaks of chloro-s-triazines observed may reveal the occurrence of adsolubilization of these analytes in the adsorbed cationic surfactant layer on the capillary surface.

Electrophoretic methods for studying protein-protein interactions

Protein-protein interactions are involved in many biological processes ranging from DNA replication, to signal transduction, to metabolism control, to viral assembly. The understanding of those interactions would allow the effective design of new drugs and further manipulation of those interactions. Several useful analytical methods are available for the study of protein-protein binding, and among them, electrophoresis is commonly used. We describe two types of electrophoresis: gel electrophoresis and capillary electrophoresis. Gel electrophoresis is a well-established method used to study protein-protein interactions and includes overlay gel electrophoresis, charge shift method, band shift assay, countermigration electrophoresis, affinophoresis, affinity electrophoresis, rocket immunoelectrophoresis, and crossed immunoelectrophoresis. These techniques are briefly described along with their advantages and limitations. Capillary electrophoresis, on the other hand, is a relatively new method and affinity capillary electrophoresis has demonstrated its value in the measurement of binding constants, the estimation of kinetic rate constants, and the determination of stoichiometry of biomolecular interactions. It offers short analysis time, requires minute amounts of protein samples, usually involves no radiolabeled compounds, and, most importantly, is carried out in solution. We summarize the principles of affinity capillary electrophoresis for studying protein-protein interactions along with current limitations and describe in depth its application to the determination of stoichiometries of tight and weak binding protein-protein interactions. The protocol presented in the experimental section details the use of affinity capillary electrophoresis for the determination of stoichiometry of protein complexes.

Capillary affinophoresis of pea lectin with polyliganded affinophores: a model study of divalent-polyvalent interactions

Affinophoresis is a type of affinity electrophoresis using an affinophore, a soluble ionic carrier bearing affinity ligand(s). It was reported previously that an affinophore, prepared by coupling multiple p-aminophenyl alpha-D-mannoside ligands to a part of the carboxyl groups of succinylated polylysine, specifically changed the mobility of pea lectin in agarose gel. The affinophoresis of this divalent lectin with the polyliganded affinophore was investigated by using capillary electrophoresis. Analysis of the mobility change of the lectin in the presence of differently modified affinophores showed that the affinity was larger for affinophores having higher ligand density. Analysis of the inhibition of the mobility change by a neutral ligand, with a known affinity constant for the lectin, allowed estimation of the contributions of monovalent and divalent interactions to the binding in the lectin-affinophore complex. The proportion of divalent complexes was greater for affinophores having higher ligand density. This approach to estimate the contribution of divalency in complex formation should be generally applicable to the analysis of divalent interactions with different techniques other than electrophoresis.

Capillary affinophoresis as a versatile tool for the study of biomolecular interactions: a mini-review

Combination of capillary electrophoresis and bioaffinity interaction gave rise to powerful research tools for analyzing molecular recognition. They take advantages of the electrophoretic behavior of the complex formed between a target biomolecule and a specially designed mobile ligand molecule (affinophore or affinity probe), and enable detection of complex formation, determination of the equilibrium constants and stoichiometry, etc.

New approaches in clinical chemistry: on-line analyte concentration and microreaction capillary electrophoresis for the determination of drugs, metabolic intermediates, and biopolymers in biological fluids

The use of capillary electrophoresis (CE) for clinically relevant assays is attractive since it often presents many advantages over contemporary methods. The small-diameter tubing that holds the separation medium has led to the development of multicapillary instruments, and simultaneous sample analysis. Furthermore, CE is compatible with a wide range of detectors, including UV-Vis, fluorescence, laser-induced fluorescence, electrochemistry, mass spectrometry, radiometric, and more recently nuclear magnetic resonance, and laser-induced circular dichroism systems. Selection of an appropriate detector can yield highly specific analyte detection with good mass sensitivity. Another attractive feature of CE is the low consumption of sample and reagents. However, it is paradoxical that this advantage also leads to severe limitation, namely poor concentration sensitivity. Often high analyte concentrations are required in order to have injection of sufficient material for detection. In this regard, a series of devices that are broadly termed 'analyte concentrators' have been developed for analyte preconcentration on-line with the CE capillary. These devices have been used primarily for non-specific analyte preconcentration using packing material of the C18 type. Alternatively, the use of very specific antibody-containing cartridges and enzyme-immobilized microreactors have been demonstrated. In the current report, we review the likely impact of the technology of capillary electrophoresis and the role of the CE analyte concentrator-microreactor on the analysis of biomolecules, present on complex matrices, in a clinical laboratory. Specific examples of the direct analysis of physiologically-derived fluids and microdialysates are presented, and a personal view of the future of CE in the clinical environment is given.

Lectin affinity electrophoresis

Lectin affinity electrophoresis is a powerful technique to investigate the interaction between a lectin and its ligand. Affinity electrophoresis results from the reduced mobility of a charged species owing to its interaction with an immobile species. In this protocol, a two-dimensional lectin affinity electrophoresis experiment is described that affords separation of oligosaccharides. The first-dimension is composed of a weak, polyacrylamide, capillary tube gel containing a lectin. The example described involves a mixture of fluorescently labeled disaccharides. The mobility of only the lectin-binding disaccharide is reduced affording a separation in the first-dimension. The tube gel is then extruded and placed onto the second-dimension gradient polyacrylamide gel and subjected to electrophoresis. Mobility in the second-dimension is dependent on molecular size and visualization si by fluorescence under transillumination. This method is also applicable, with appropriate modifications, for the separation and analysis of glycopeptides and glycoproteins.

Affinity-based reverse micellar extraction and separation (ARMES): a facile technique for the purification of peroxidase from soybean hulls

A new technique for the purification of proteins has been developed which combines the high selectivity of affinity interaction with the scalability and ease of operation of liquid-liquid extraction. The approach is called affinity-based reverse micellar extraction and separation (ARMES). The salient features of ARMES include the following: (1) intraphasic interaction between the ligand and ligate which provides for high ligand utilization; (2) no chemical modification of the ligand is needed; and (3) ease of operation and inherent scalability due to the use of liquid-liquid extraction. This technique has been used to purify the peroxidase from soybean hulls using the lectin concanavalin A (con A) as a sugar-binding affinity ligand. A purification factor of 30 is achieved to provide a nearly pure peroxidase solution (as determined by HPLC and SDS-PAGE) with nearly complete regeneration of the con A ligand. We propose that ARMES will be useful in the facile purification of complex biomolecules such as glycoform protein variants using lectins as affinity ligands and proteins of therapeutic importance using antibodies as affinity ligands.