Protein-boronic acid conjugates and their binding to low-molecular-mass cis-diols and glycated hemoglobin

A comprehensive review on fructosyl peptide oxidase as an important enzyme for present hemoglobin A1c assays

Glycated proteins are generated by binding of glucose to the proteins in blood stream through a nonenzymatic reaction. Hemoglobin A1c (HbA1c) is a glycated protein with glucose at the N-terminal of β-chain. HbA1c is extensively used as an indicator for assessing the blood glucose concentration in diabetes patients. There are different conventional clinical methods for the detection of HbA1c. However, enzymatic detection method has newly obtained great attention for its high precision and cost-effectiveness. Today, fructosyl peptide oxidase (FPOX) plays a key role in the enzymatic measurement of HbA1c, and different companies have marketed HbA1c assay systems based on FPOX. Recent investigations show that FPOX could be used in assaying HbA1 without requiring HbA1c primary digestion. It could also be applied as a biosensor for HbA1c detection. In this review, we have discussed the recent improvements of FPOX properties, different methods of FPOX purification, solubility, and immobilization, and also the use of FPOX in HbA1c biosensors.

Molecular Imprinted Polymers on Microneedle Arrays for Point of Care Transdermal Sampling and Sensing of Inflammatory Biomarkers

The skin interstitial fluid (ISF) contains biomarkers that complement other biofluids such as blood, sweat, saliva, and urine. It can be sampled in a minimally invasive manner and used either for point of care testing or real time, continuous monitoring of analytes, the latter using microneedle arrays. The analytes present in the skin ISF are indicative of both systemic and local (i.e., skin) physiology. In this paper, we describe combining microneedle technology with molecularly imprinted polymers to demonstrate the potential of transdermal electrochemical sensing. The molecularly imprinted polymer employed here is easy to produce; it can be thought of as plastic antibody. Its synthesis is scalable, and the resulting sensor has a short measurement time (6 min), with high accuracy and a low limit of detection. It provides the requisite specificity to detect the proinflammatory cytokine IL-6. IL-6 is present in the skin ISF with other cytokines and is implicated in many clinical states including neurodegenerative diseases and fatal pneumonia from SARSCoV 2. The ability to mass produce microneedle arrays and plastic antibodies will allow for low-cost transdermal sensing devices. The transdermal sensors were able to detect IL-6 at concentrations as low as 1 pg/mL in artificial skin ISF, indicating its utility for routine point of care, bloodless measurements in simpler settings, worldwide.

Self-healing boronic acid-based hydrogels for 3D co-cultures

Synthetic hydrogels have been widely adopted as well-defined matrices for three-dimensional (3D) cell culture, with increasing interest in systems that enable the co-culture of multiple cell types for probing both cell-matrix and cell-cell interactions in studies of tissue regeneration and disease. We hypothesized that the unique dynamic covalent chemistry of self-healing hydrogels could be harnessed for not only the encapsulation and culture of human cells but also the subsequent construction of layered hydrogels for 3D co-cultures. To test this, we formed hydrogels using boronic acid-functionalized polymers and demonstrated their self-healing in the presence of physiologically-relevant cell culture media. Two model human cell lines, MDA-MB-231 breast cancer cells and CCL151 pulmonary fibroblasts, were encapsulated within these dynamic materials, and good viability was observed over time. Finally, self-healing of cut hydrogel 'blocks' laden with these different cell types was used to create layered hydrogels for the generation of a dynamic co-culture system. This work demonstrates the utility of self-healing materials for multi-dimensional cultures and establishes approaches broadly useful for a variety of biological applications.

A Boronate Affinity-Assisted SERS Tag Equipped with a Sandwich System for Detection of Glycated Hemoglobin in the Hemolysate of Human Erythrocytes

Phenylboronic acid-functionalized, Ag shell-coated, magnetic, monodisperse polymethacrylate microspheres equipped with a glycoprotein-sensitive sandwich system were proposed as a surface-enhanced Raman scattering (SERS) substrate for quantitative determination of glycated hemoglobin (HbA1c). The magnetization of the SERS tag and the formation of the Ag shell on the magnetic support were achieved using the bifunctional reactivity of newly synthesized polymethacrylate microspheres. The hemolysate of human red blood cells containing both HbA1c and nonglycated hemoglobin was used for determination of HbA1c. The working principle of the proposed SERS tag is based on the immobilization of HbA1c by cyclic boronate ester formation between glycosyl residues of HbA1c and boronic acid groups of magnetic polymethacrylate microspheres and the binding of p-aminothiophenol (PATP)-functionalized Ag nanoparticles (Ag NPs) carrying another boronic acid ligand via cyclic boronate ester formation via unused glycosyl groups of bound HbA1c. Then, in situ formation of a Raman reporter, 4,4'-dimercaptoazobenzene from PATP under 785 nm laser irradiation allowed for the quantification of HbA1c bound onto the magnetic SERS tag, which was proportional to the HbA1c concentration in the hemolysate of human erythrocytes. The sandwich system provided a significant enhancement in the SERS signal intensity due to the plasmon coupling between Ag NPs and Ag shell-coated magnetic microspheres, and low HbA1c concentrations down to 50 ng/mL could be detected. The calibration curve obtained with a high correlation coefficient between the SERS signal intensity and HbA1c level showed the usability of the SERS protocol for the determination of the HbA1c level in any person.

Selective label-free electrochemical impedance measurement of glycated haemoglobin on 3-aminophenylboronic acid-modified eggshell membranes

We propose a novel alternative approach to long-term glycaemic monitoring using eggshell membranes (ESMs) as a new immobilising platform for the selective label-free electrochemical sensing of glycated haemoglobin (HbA1c), a vital clinical index of the glycaemic status in diabetic individuals. Due to the unique features of a novel 3-aminophenylboronic acid-modified ESM, selective binding was obtained via cis-diol interactions. This newly developed device provides clinical applicability as an affinity membrane-based biosensor for the identification of HbA1c over a clinically relevant range (2.3 - 14 %) with a detection limit of 0.19%. The proposed membrane-based biosensor also exhibited good reproducibility. When analysing normal and abnormal HbA1c levels, the within-run coefficients of variation were 1.68 and 1.83%, respectively. The run-to-run coefficients of variation were 1.97 and 2.02%, respectively. These results demonstrated that this method achieved the precise and selective measurement of HbA1c. Compared with a commercial HbA1c kit, the results demonstrated excellent agreement between the techniques (n = 15), demonstrating the clinical applicability of this sensor for monitoring glycaemic control. Thus, this low-cost sensing platform using the proposed membrane-based biosensor is ideal for point-of-care diagnostics.

BSA-boronic acid conjugate as lectin mimetics

We report bovine serum albumin (BSA)-boronic acid (BA) conjugates as lectin mimetics and their glyco-capturing capacity. The BSA-BA conjugates were synthesized by amidation of carboxylic acid groups in BSA with aminophenyl boronic acid in the presence of EDC, and were characterized by Alizarin Red S (ARS) assay and SDS-PAGE gel. The BSA-BA conjugates were immobilized onto maleimide-functionalized silica beads and their sugar capturing capacity and specificity were confirmed by ARS displacement assay. Further, surface plasmon resonance (SPR) analysis of the glyco-capturing activity of the BSA-BA conjugates was conducted by immobilizing BSA-BA onto SPR gold chip. Overall, we demonstrated a BSA-BA-based lectin mimetics for glyco-capturing applications. These lectin mimetics are expected to provide an important tool for glycomics and biosensor research and applications.

Use of boronic acid nanoparticles in glycoprotein enrichment

Glyco-specific enrichment methods for mass spectrometry pretreatment are invaluable for the detection of low abundant glycoproteins or glycopeptides. For example, boronic acid can specifically interact with glycans in nonaqueous or basic aqueous solutions. Here, we describe a glyco-specific enrichment method which uses a boronic acid-functionalized "core-satellite" composite nanoparticle to isolate glycoproteins or glycopeptides from complex biological samples. Furthermore, we also demonstrate detection limit improvements and show how to evaluate the percent recovery from the glycoprotein or glycopeptide enrichment process via SDS-PAGE and (16)O/(18)O labeling strategies.

Highly specific revelation of rat serum glycopeptidome by boronic acid-functionalized mesoporous silica

Although the specific profiling of endogenous glycopeptides in serum is highly inclined towards the discovery of disease biomarkers, studies on the endogenous glycopeptides (glycopeptidome) have never been conducted because of several factors. These factors include the high dynamic range of serum proteins, the inadequacy of traditional sample preparation techniques in proteomics for low-molecular-weight (LMW) proteins, and the relatively low abundances of glycopeptides. Boronic acid-functionalized mesoporous silica was synthesized in this study to overcome the limitations of the state-of-the-art methods for glycopeptidome research. The boronic acid-functionalized mesoporous silica exhibited excellent selectivity by analyzing glycopeptides in the mixture of glycopeptides/non-glycopeptides at molar ratio of 1:100, extreme sensitivity (the limit of detection was at the fmol level), good binding capacity (40 mg g(-1)), as well as the high post-enrichment recovery of glycopeptides (up to 88.10%). The as-prepared material possessing both glycopeptide-suitable pore size and glycopeptide-specific selectivity has shown special capability for enriching the endogenous glycopeptides. Fifteen unique glycosylation sites mapped to 15 different endogenous glycopeptides were identified in rat serum. The established protocol revealed for the first time the rat serum glycopeptidome.

Bioelectrocatalytic detection of glycated hemoglobin (HbA1c) based on the competitive binding of target and signaling glycoproteins to a boronate-modified surface

We developed an electrochemical glycated hemoglobin (HbA(1c)) biosensor for diagnosing diabetes in whole human blood based on the competitive binding reaction of glycated proteins. Until now, no studies have reported a simple and accurate electrochemical biosensor for the quantification of HbA(1c) in whole blood. This is because it is very difficult to correctly distinguish HbA(1c) from large amounts of hemoglobin and other components in whole blood. To detect glycated hemoglobin, we used electrodes modified with boronic acid, which forms a covalent bond between its diol group and the cis-diol group of the carbohydrate moiety of glycated proteins. For accurate HbA(1c) biosensing, we first removed blood components (except for hemoglobin) such as glycated proteins and blood glucose as they interfere with the boronate-based HbA(1c) competition analysis by reacting with the boronate-modified surface via a cis-diol interaction. After hemoglobin separation, target HbA(1c) and GOx at a predetermined concentration were reacted through a competition onto the boronate-modified electrode, allowing HbA(1c) to be detected linearly within a range of 4.5-15% of the separated hemoglobin sample (HbA(1c)/total hemoglobin). This range covers the required clinical reference range of diabetes mellitus. Hence, the proposed method can be used for measuring %HbA(1c) in whole human blood, and can also be applied to measuring the concentration of various glycated proteins that contain peripheral sugar groups.

Calcium fructoborate--potential anti-inflammatory agent

Calcium fructoborate is a boron-based nutritional supplement. Its chemical structure is similar to one of the natural forms of boron such as bis-manitol, bis-sorbitol, bis-fructose, and bis-sucrose borate complexes found in edible plants. In vitro studies revealed that calcium fructoborate is a superoxide ion scavenger and anti-inflammatory agent. It may influence macrophage production of inflammatory mediators, can be beneficial for the suppression of cytokine production, and inhibits progression of endotoxin-associated diseases, as well as the boric acid and other boron sources. The mechanisms by which calcium fructoborate exerts its beneficial anti-inflammatory effects are not entirely clear, but some of its molecular biological in vitro activities are understood: inhibition of the superoxide within the cell; inhibition of the interleukin-1β, interleukin-6, and nitric oxide release in the culture media; and increase of the tumor necrosis factor-α production. Also, calcium fructoborate has no effects on lipopolysaccharide-induced cyclooxygenase-2 protein express. The studies on animals and humans with a dose range of 1-7 mg calcium fructoborate (0.025-0.175 mg elemental boron)/kg body weight/day exhibited a good anti-inflammatory activity, and it also seemed to have negligible adverse effect on humans.

A field effect transistor (FET)-based immunosensor for detection of HbA1c and Hb

A field effect transistor (FET)-based immunosensor was developed for diabetes monitoring by detecting the concentrations of glycated hemoglobin (HbA1c) and hemoglobin (Hb). This immunosensor consists of a FET-based sensor chip and a disposable extended-gate electrode chip. The sensor chip was fabricated by standard CMOS process and was integrated with signal readout circuit. The disposable electrode chip, fabricated on polyester plastic board by Micro-Electro-Mechanical-Systems (MEMS) technique, was integrated with electrodes array and micro reaction pool. Biomolecules were immobilized on the electrode based on self-assembled monolayer and gold nanoparticles. Experimental results showed that the immunosensor achieved a linear response to HbA1c with the concentration from 4 to 24 μg/ml, and a linear response to Hb with the concentration from 60 to 180 μg/ml.

On-plate-selective enrichment of glycopeptides using boronic acid-modified gold nanoparticles for direct MALDI-QIT-TOF MS analysis

In this study, an on-plate-selective enrichment method is developed for fast and efficient glycopeptide investigation. Gold nanoparticles were first spotted and sintered on a stainless-steel plate, then modified with 4-mercaptophenylboronic acid to provide porous substrate with large specific surface and dual functions. These spots were used to selectively capture glycopeptides from peptide mixtures and the captured target peptides could be analyzed by MALDI-MS simply by deposition of 2,5-dihydroxybenzoic acid matrix. Horseradish peroxidase was employed as a standard glycoprotein to investigate the enrichment efficiency. In this way, the enrichment, washing and detection steps can all be fulfilled on a single MALDI target plate. The relatively small sample amount needed, low detection limit and rapid selective enrichment have made this on-plate strategy promising for online enrichment of glycopeptides, which could be applied in high-throughput proteome research.

Affinity interactions between phenylboronic acid-carrying self-assembled monolayers and flavin adenine dinucleotide or horseradish peroxidase

A method is provided for the recognition of glycated molecules based on their binding affinities to boronate-carrying monolayers. The affinity interaction of flavin adenine dinucleotide (FAD) and horseradish peroxidase (HRP) with phenylboronic acid monolayers on gold was investigated by using voltammetric and microgravimetric methods. Conjugates of 3-aminophenylboronic acid and 3,3'-dithiodipropionic acid di(N-hydroxysuccinimide ester) or 11-mercaptoundecanoic acid were prepared and self-assembled on gold surfaces to generate monolayers. FAD is bound to this modified surface and recognized by a pair of redox peaks with a formal potential of -0.433 V in a 0.1 M phosphate buffer solution, pH 6.5. Upon addition of a sugar to the buffer, the bound FAD could be replaced, indicating that the binding is reversible. Voltammetric, mass measurements, and photometric activity assays show that the HRP can also be bound to the interface. This binding is reversible, and HRP can be replaced by sorbitol or removed in acidic solution. The effects of pH, incubation time, and concentration of H(2)O(2) were studied by comparing the catalytic reduction of H(2)O(2) in the presence of the electron-donor thionine. The catalytic current of the HRP-loaded electrode was proportional to HRP concentrations in the incubation solution in the range between 5 microg mL(-1) and 0.1 mg mL(-1) with a linear slope of 3.34 microA mL mg(-1) and a correlation coefficient of 0.9945.

Analysis of glycated insulin by MALDI-TOF mass spectrometry

Non-enzymatic glycation of protein is mediated via an interaction between the aldehyde group of a reducing sugar and available alpha- or epsilon-amino moieties of the protein. The above event can alter the biological activity of the protein and therefore, it is of particular interest to monitor the glycation of proteins having important functional roles in metabolism. In the present study, matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) has been used to determine the non-enzymatic glycation of bovine insulin. The degree of insulin glycation was increased in both concentration- and time-dependent manner in relation to exposure to glucose, and the event was more pronounced for monoglycation reaction than that noticed for the diglycation of the hormone. Enzymatic digestion of insulin preparations with endoproteinase Glu C has revealed that each of the B 1-13 and B 22-30 peptide fragments of glycated insulin contains a site of binding of a single glucose molecule. Finally, attempt has been made in order to increase the sensitivity of the glycation assay through efficient enrichment of the glycated insulin on magnetic beads containing immobilized 3-aminophenylboronic acid (APBA) on their surface.

Immobilization of aminophenylboronic acid on magnetic beads for the direct determination of glycoproteins by matrix assisted laser desorption ionization mass spectrometry

Aminophenylboronic acid (APBA) has been immobilized on magnetic beads for the direct determination of glycoprotein by matrix assisted laser desorption/ionizaton time of flight mass spectrometry (MALDI-TOF-MS). An APBA layer was formed on the surface of carboxylic acid terminated magnetic beads by coupling with carbodiimide and subsequently reacted with an N-hydroxysuccinimide moiety. The immobilized APBA was identified by MALDI-TOF-MS without a matrix. Glycoproteins, such as HbA1c, fibrinogen, or RNase B were separated and desalted using APBA magnetic beads by simply washing the magnetic beads and then separating them by external magnet. Proteins can be identified by direct determination of proteins on beads on MALDI plate and confirmed again by peptide mass finger printing after digestion of proteins on magnetic beads by trypsin. Fluorescence image with a FITC tagging protein using confocal laser microscopy showed the difference of immobilization efficiency between glycoproteins and nonglycoproteins. The methods developed within this work allow the simple treatment and enrichment of glycoproteins as well as direct determination of proteins on beads by MALDI-TOF-MS.

Immobilization of peroxidase glycoprotein on gold electrodes modified with mixed epoxy-boronic Acid monolayers

The development of bioelectronic enzyme applications requires the immobilization of active proteins onto solid or colloidal substrates such as gold. Coverage of the gold surface with alkanethiol self-assembled monolayers (SAMs) reduces nonspecific adsorption of proteins and also allows the incorporation onto the surface of ligands with affinity for complementary binding sites on native proteins. We present in this work a strategy for the covalent immobilization of glycosylated proteins previously adsorbed through weak, reversible interactions, on tailored SAMs. Boronic acids, which form cyclic esters with saccharides, are incorporated into SAMs to weakly adsorb the glycoprotein onto the electrode surface through their carbohydrate moiety. To prevent protein release from the electrode surface, we combine the affinity motif of boronates with the reactivity of epoxy groups to covalently link the protein to heterofunctional boronate-epoxy SAMs. The principle underlying our strategy is the increased immobilization rate achieved by the weak interaction-induced proximity effect between slow reacting oxyrane groups in the SAM and nucleophilic residues from adsorbed proteins, which allows the formation of very stable covalent bonds. This approach is exemplified by the use of phenylboronates-oxyrane mixed monolayers as a reactive support and redox-enzyme horseradish peroxidase as glycoprotein for the preparation of peroxidase electrodes. Quartz crystal microbalance, atomic force microscopy, and electrochemical measurements are used to characterize these enzymatic electrodes. These epoxy-boronate functional monolayers are versatile, stable interfaces, ready to incorporate glycoproteins by incubation under mild conditions.

Phenylboronic acid-salicylhydroxamic acid bioconjugates. 1. A novel boronic acid complex for protein immobilization

A chemical affinity system exhibiting antibody-like properties is described. The system exploits bioconjugates with appended phenylboronic acid (PBA) moieties and a support-bound phenylboronic acid complexing reagent derived from salicylhydroxamic acid (SHA) for protein immobilization on a chromatographic support. The structure of the PBA.SHA complex was characterized by 11B NMR and mass spectrometry and compared with complexes derived from model compounds. Protein modification reagents were synthesized from 3-aminophenylboronic acid and utilized to prepare bioconjugates from alkaline phosphatase (AP) and horseradish peroxidase (HRP). AP obtained from one source afforded PBA bioconjugates exhibiting significant loss of enzymatic activity, whereas AP obtained from a second source afforded PBA bioconjugates exhibiting only a modest loss of enzymatic activity. Conversely, HRP afforded PBA bioconjugates exhibiting no loss of enzymatic activity. SHA-modified Sepharose was prepared by reaction of methyl 4-[(6-aminohexanoylamino)methyl]salicylate with CNBr-activated Sepharose 4B, followed by treatment with aqueous alkaline hydroxylamine. PBA-AP and PBA-HRP conjugates were efficiently immobilized on SHA-Sepharose at pH 8.3. PBA-AP conjugates were retained after washing with acidic buffers at pH 6.7, 4.2, and 2.5, whereas PBA-HRP conjugates were retained after washing with buffer at pH 6.7, but were eluted to some extent at and below pH 4.2. The results are interpreted in terms of multivalent interactions involving boronic acid complex formation between the enzyme bioconjugates and immobilized complexing reagent.

Synthetically prepared Aamadori-glycated phosphatidylethanolaminecan trigger lipid peroxidation via free radical reactions

This study for the first time confirmed the peroxidative role of the Amadori product derived from the glycation of phosphatidylethanolamine (PE), namely Amadori-PE. The product was synthesized from the reaction of dioleoyl PE with D-glucose, and then purified by a solid-phase extraction procedure, which was a key step in the next HPLC technique for the isolation of essentially pure Amadori-PE. When the synthetically prepared Amadori-PE was incubated with linoleic acid in the presence of Fe(3+) in micellar system, a remarkable formation of thiobarbituric acid reactive substances was observed together with increases in lipid hydroperoxides. In addition, the lipid peroxidation caused by Amadori-PE was effectively inhibited by superoxide dismutase, mannitol, catalase and metal chelator. These results indicated that Amadori-PE triggers oxidative modification of lipids via the generation of superoxide, and implied the involvement of 'lipid glycation' along with membrane lipid peroxidation in the pathogenesis of diabetes and aging.

Glycohemoglobin filter assay for doctors’ offices based on boronic acid affinity principle

We present a new filter assay for the determination of glycohemoglobin as a unique application of the boronic acid affinity principle. With the use of a water-soluble blue-colored boronic acid derivative and a specific precipitation method for hemoglobin, total hemoglobin including bound boronic acid is precipitated and collected on a filter strip before quantification. Hemoglobin and boronic acid are quantified by a dual-wavelength reflectometric measurement, and the result is reported directly as percent glycohemoglobin. The test is simple, quick, and designed as a doctors’ office test for the monitoring and management of diabetes. The imprecision of the assay is <4% over the range 3–18% Hb A1c, and the method is linear up to at least 20% Hb A1c. Comparisons with four well-established glycohemoglobin methods yielded correlation coefficients ranging from 0.94 to 0.99, with slopes from 0.94 to 1.01.

Chromatographic and electrophoretic methods for modified hemoglobins

The discovery of the clinically important glycohemoglobin adducts and their relation to diabetes mellitus have greatly stimulated the study of other minor post-translational modifications of hemoglobin. Chromatographic and electrophoretic procedures have played an important role in these studies. Today several hemoglobin adducts are known and the formation of adducts with glucose, phosphorylated carbohydrates, urea/cyanate, aspirin, vitamins, acetaldehyde, penicillin and acetyl CoA have been described. Furthermore, new adducts, such as those observed using hemoglobin as a biochemical marker monitoring environmental, occupational and lifestyle exposures to reactive toxic chemicals are constantly being reported. This review deals with chromatographic and electrophoretic separation methods available for the study of non-enzymatic post-translational modifications of hemoglobin. Suitability, perspectives and biomedical applications are discussed.

Soluble highly coloured phenylboronic acids and their use in glycohemoglobin quantification

Water soluble dye-phenylboronic acid conjugates (dye-PBAs) possessing strong absorption of visible light are introduced as new reagents for the determination of glycohemoglobin. Their functionality and prospective use are demonstrated in a semi-homogenous glycohemoglobin assay. The assay is based on cis-diol esterification of dye-PBA to glycohemoglobin followed by selective precipitation of hemoglobin from solution, co-precipitating bound dye-PBA. Quantification of the molar "dye-PBA/Hb"-ratio in redissolved precipitates using either absorption or fluorescence spectroscopy, reflects the glycation level of the blood samples used. Future development of the assay principle is illustrated in a filter based assay, collecting the precipitated hemoglobin on a filter followed by reflectometric readings directly on the precipitate. The significance of this work lies first, in the demonstration of a new principle for the determination of glycohemoglobin, and second, as an illustration of the prospective use of water soluble, signal-forming non-immobilised boronic acids in the determination of cis-diol containing analytes.