Insight into the Mechanism of Reduced IgG/IgE Binding Capacity in Ovalbumin as Induced by Glycation with Monose Epimers through Liquid Chromatography and High-Resolution Mass Spectrometry

Insight into the Mechanism Underlying the Reduction of Digestibility and IgG/IgE Binding Ability in Ovalbumin during Different High-Temperature Conduction Modes-Induced Glycation

Effects of different high-temperature conduction modes [high-temperature air conduction (HAC), high-temperature contact conduction (HCC), high-temperature steam conduction (HSC)]-induced glycation on the digestibility and IgG/IgE-binding ability of ovalbumin (OVA) were studied and the mechanisms were investigated. The conformation in OVA-HSC showed minimal structural changes based on circular dichroism, fluorescence, and ultraviolet spectroscopy. The degree of hydrolysis analysis indicated that glycated OVA was more resistant to digestive enzymes. Liquid chromatography-Orbitrap mass spectrometry identified 11, 14, and 15 glycation sites in OVA-HAC, OVA-HCC, and OVA-HSC, respectively. The IgG/IgE-binding ability of OVA was reduced during glycation and digestion, and the interactions among glycation, allergenicity, and digestibility were further investigated. Glycation sites masked the IgG/IgE epitopes resulting in a reduction in allergenicity. Digestion enzymes destroyed the IgG/IgE epitopes thus reducing allergenicity. Meanwhile, the glycation site in proximity to the digestion site of pepsin was observed to cause a reduction in digestibility.

The decrease of Ara h 2 allergenicity by glycation is determined by reducing sugar chain length and isomers

The mechanism of the effect of reducing sugar chain length and isomers on the allergenicity of Ara h 2 after glycation was investigated. Ara h 2 was more prone to glycation with ribose which had a short chain length. The glycation sites of Ara h 2 after glycation with galactose were higher than the glycation sites in galactose's isomers-Ara h 2 conjugates, which might be affected by the configuration differences at position C-4 and the small steric effects in terminal groups -CHO of galactose. Ara h 2-ribose conjugate had the lowest allergenicity, and glycation with galactose was more capable of reducing Ara h 2 allergenicity than its isomers. The results indicated that glycation with ribose caused conformational epitope destruction and linear epitope masking of Ara h 2 greatly. Furthermore, since the small steric effects of -CHO, galactose was more capable of reducing Ara h 2 allergenicity than fructose. This study will provide a theoretical basis for selecting appropriate reducing sugars and preparing hypoallergenic products containing peanuts.

Effects of unfolding treatment assisted glycation on the IgE/IgG binding capacity and antioxidant activity of ovomucoid

Ovomucoid is the immune-dominant allergen in the egg white of hens. Due to its structure based on nine disulfide bonds as well as its resistance to heat and enzymatic hydrolysis, the allergenicity of this food protein is difficult to decrease by technological processes. We sought to reduce its allergenicity through the Maillard reaction. The unfolding of ovomucoid with L-cysteine-mediated reduction was used to increase accessibility to conformational and linear epitopes by modifying the secondary and tertiary structures of the allergen. Glycation with different saccharides revealed the beneficial effect of maltose glycation on the IgG-binding capacity reduction. By determining the better glycation conditions of unfolded ovomucoid, we produced ovomucoid with reduced IgE binding capacity due to the glycation sites (K17, K112, K129, and K164) on epitopes. Moreover, after simulated infant and adult gastrointestinal digestion, the unfolded plus glycated ovomucoid showed higher ABTS˙+ scavenging activity, O2˙- scavenging activity, ˙OH scavenging activity, Fe2+ chelating activity, and a FRAP value; in particular, for ˙OH scavenging activity, there was a sharp increase of more than 100%.

Tolerance to heated egg in egg allergy: Explanations and implications for prevention and treatment

Hen's egg allergy is the second most frequent food allergy found in children. Allergic symptoms can be caused by raw or heated egg, but a majority of egg-allergic children can tolerate hard-boiled or baked egg. Understanding the reasons for the tolerance towards heated egg provides clues about the molecular mechanisms involved in egg allergy, and the differential allergenicity of heated and baked egg might be exploited to prevent or treat egg allergy. In this review, we therefore discuss (i) why some patients are able to tolerate heated egg; by highlighting the structural changes of egg white (EW) proteins upon heating and their impact on immunoreactivity, as well as patient characteristics, and (ii) to what extent heated or baked EW might be useful for primary prevention strategies or oral immunotherapy. We describe that the level of immunoreactivity towards EW helps to discriminate patients tolerant or reactive to heated or baked egg. Furthermore, the use of heated or baked egg seems effective in primary prevention strategies and might limit adverse reactions. Oral immunotherapy is a promising treatment strategy, but it can sometimes cause significant adverse events. The use of heated or baked egg might limit these, but current literature is insufficient to conclude about its efficacy.

Effects of different high-temperature conduction modes on the ovalbumin-glucose model: AGEs production and regulation of glycated ovalbumin on gut microbiota

Food high-temperature processing frequently induces the production of advanced glycation end products (AGEs) in the food industry. In this study, the effects of three high-temperature conduction modes on the AGEs production derived from ovalbumin (OVA)-glucose model and the regulation of glycated OVA on gut microbiota were investigated. The peak time of OVA shifted maximally from 13.72 to 13.57 due to the rise in molecular weight, confirming successful coupling between OVA and glucose. The inhibition of superheated steam (SS) on AGEs was observed, with the sample treated by SS showing the lowest content among glycated OVA groups. The analysis revealed an increase in AGEs during digestion and a decrease in fermentation, suggesting the release during digestion and the availability by intestinal flora. Furthermore, an expansion of Bifidobacterium and Lactobacillus, and the inhibition of Desulfovibrio and Escherichia-Shigella were observed, indicating the prebiotic activity of glycated OVA and its potential to improve intestinal health. These results provide valuable information for controlling high-temperature processing to inhibit AGEs formation and highlight the positive effects of glycated proteins on intestinal health.

Comparison of ovalbumin glycation induced by high-temperature steaming and high-temperature baking: A study combining conventional spectroscopy with high-resolution mass spectrometry

High-temperature steaming (HTS) and high-temperature baking (HTB)-induced ovalbumin (OVA)-glucose glycation (140 °C, 1-3 min) were compared, and the different mechanisms were evaluated by changes in protein conformation, glycation sites and average degree of substitution per peptide molecule (DSP) values as well as the antioxidant activity of glycated OVA. Conventional spectroscopic results suggested that in comparison with HTB, HTS promoted protein expansion, increased β-sheet content and made OVA structure more orderly. Liquid chromatography-high resolution mass spectrometry (LC-HRMS) analysis showed that 10 glycation sites were found under HTB, while 4 new glycation sites R111, R200, R219 and K323 appeared under HTS, and 2 of them (R219 and K323) were located in internal β-sheet chains. The antioxidant activities of glycated OVA increased with increasing treatment time, and HTS showed stronger enhancement effect than HTB. Furthermore, the DSP values were generally higher under HTS than HTB. Compared with HTB, HTS with high penetrability could enhance the change of OVA primary structure and spatial conformation, making the protein structure more unfolded and stable, leading to more protein-sugar collisions occurred in inner OVA molecular and significantly promoted glycation. In conclusion, HTS is a promising method for high-temperature short-time glycation reaction, with drastically increasing the protein antioxidant activities.

A comprehensive review on glycation and its potential application to reduce food allergenicity

Food allergens are a major concern for individuals who are susceptible to food allergies and may experience various health issues due to allergens in their food. Most allergenic foods are subjected to heat treatment before being consumed. However, thermal processing and prolonged storage can cause glycation reactions to occur in food. The glycation reaction is a common processing method requiring no special chemicals or equipment. It may affect the allergenicity of proteins by altering the structure of the epitope, revealing hidden epitopes, concealing linear epitopes, or creating new ones. Changes in food allergenicity following glycation processing depend on several factors, including the allergen's characteristics, processing parameters, and matrix, and are therefore hard to predict. This review examines how glycation reactions affect the allergenicity of different allergen groups in allergenic foods.

Effect of glycation derived from α-dicarbonyl compounds on the in vitro digestibility of ovalbumin: Tracing of advanced glycation end-products and immuno-active peptides

Currently, the biological consequences of advanced glycation end-products (AGEs) and their link to the antigenicity of food allergens are largely unknown due to the uncertainty in their digestive fates within the body. In this study, the influence of glycation derived from α-dicarbonyl compounds (α-DCs), precursors of AGEs, on digestive behaviors of ovalbumin (OVA) was investigated in a two-step simulated gastrointestinal (GI) model. Methylglyoxal (MGO), glyoxal (GO), and 3-deoxyglucosone were selected as typical α-DCs to obtain glycated OVA with different AGE-modifications (AGE-Ms). It was unveiled that α-DC-glycation reduced the digestibility of OVA via blocking tryptic cleavage sites and inducing steric hindrance, especially seen in the GO- and MGO-OVA groups. The formed AGE-Ms, depending on the precursor type, showed masking effects on the epitopes of OVA, which counteracted the negative effects of reduced digestibility on its antigenicity. Substantial changes in the peptide release patterns were also noted in glycated OVA, including alterations in the sequences and structures of several known protease-resistant epitopes of OVA. This study provides new insights into the nutritional and healthy effects of MRPs in heat-processed foods, as well as their potential connection to the modulation of egg allergy.

Post-synthesis of covalent organic frameworks as a hydrophilic platform for specific detection of egg ovalbumin under physiological pH

Ovalbumin (OVA) is an important protein source in our daily life. Unfortunately, the food safety problem has become more and more serious, such as protein allergy and contaminated protein. Therefore, it is necessary to detect vital proteins efficiently and rapidly. Mass spectrometry (MS) is a powerful tool for the detection of proteins. Herein, dual amino acids functionalized covalent organic frameworks containing disulfide covalent bonds (COF@SS@GC, where G is glutathione and C is cysteine) were facilely prepared for OVA enrichment through hydrophilic interaction liquid chromatography (HILIC) under physiological pH. The results showed that COF@SS@GC had displayed sensitive detection (0.1 fmol), good selectivity (OVA: BSA = 1:100), adsorption capacity (311 mg/g), stability, reproducibility, linearity, LOQ level (42 μg/mL) and recovery ratio (64.83 %) for OVA. COF@SS@GC also demonstrated satisfactory purification ability in the enrichment of egg white, indicating that COF@SS@GC had great potential in the enrichment of protein from complex samples.

Molecular imprinted electrochemical sensor for ovalbumin detection based on boronate affinity and signal amplification approach

Ovalbumin (OVA), a class of glycoproteins, is the main allergen in hen egg white that often causes allergies in humans, especially in babies. Therefore, it is pivotal to be able to detect and separate OVA. This work presents an ingenious sandwich-structured magnetic molecular imprinted electrochemical sensor for OVA detection by utilizing boronate affinity and signal amplification strategy. With anti-OVA antibody-modified gold nanoparticles (AuNPs) as amplifiers, the imprinted cavities in the probe could capture protein to form a sandwich structure. Due to its specific recognition of antibody and molecular imprinted polymers and the signal amplification of AuNPs, the sensor had good selectivity and sensitivity toward OVA and a low detection limit of 3.0 fg/mL. The sensor also had excellent stability and could satisfactorily detect OVA in real red wine samples.

Allergenicity, assembly and applications of ovalbumin in egg white: a review

Ovalbumin (OVA), the most abundant protein in egg whites, has been widely used in various industries. Currently, the structure of OVA has been clearly established, and the extraction of high-purified OVA has become feasible. However, the allergenicity of OVA is still a serious problem because it can cause severe allergic reactions and may even be life-threatening. The structure and allergenicity of the OVA can be altered by many processing methods. In this article, a detailed description on the structure and a comprehensive overview on the extraction protocols and the allergenicity of OVA was documented. Additionally, the information on assembly and potential applications of OVA was summarized and discussed in detail. Physical treatment, chemical modification, and microbial processing can be applied to alter the IgE-binding capacity of OVA by changing its structure and linear/sequential epitopes. Furthermore, research indicated that OVA could assemble with itself or other biomolecules into various forms (particles, fibers, gels, and nanosheets), which expanded its application in the food field. OVA also shows excellent application prospects, including food preservation, functional food ingredients and nutrient delivery. Therefore, OVA demonstrates significant investigation value as a food grade ingredient.

Molecular structure, IgE binding capacity and gut microbiota of ovalbumin conjugated to fructose and galactose:A comparative study

Ovalbumin (OVA) was modified by fructose (Fru) and galactose (Gal) to study the structure, IgG/IgE binding capacity and effects on human intestinal microbiota of the conjugated products. Compared with OVA-Fru, OVA-Gal has a lower IgG/IgE binding capacity. The reduction of OVA is not only associated with the glycation of R84, K92, K206, K263, K322 and R381 in the linear epitopes, but also with conformational epitope changes, manifested as secondary and tertiary structural changes caused by Gal glycation. In addition, OVA-Gal could alter the structure and abundance of gut microbiota at phylum, family, and genus levels and restore the abundance of bacteria associated with allergenicity, such as Barnesiella, Christensenellaceae_R-7_group, and Collinsela, thereby reducing allergic reactions. These results indicate that OVA-Gal glycation can reduce the IgE binding capacity of OVA and change the structure of human intestinal microbiota. Therefore, Gal glycation may be a potential method to reduce protein allergenicity.

Effects of Allergen-Specific and Non-Specific AGEs on the Allergenicity of Ovalbumin in a Mouse Model of Food Allergy

SCOPE

Epidemiologic studies suggest a link between the incidence of food allergy and the consumption of dietary advanced glycation end-products (AGEs). However, the pathogenic role of dietary AGEs in food allergy is largely unknown. This study aims to investigate the effect of allergen-specific and non-specific AGEs on the allergenic manifestation of ovalbumin (OVA), a typical food allergen in vivo.

METHODS AND RESULTS

OVA is glycated by methylglyoxal to prepare allergen-specific AGEs (i.e., OVA-AGE), and a standard AIN-93G diet is heated to obtain allergen-non-specific AGEs. A BALB/c mouse model orally sensitizes to OVA with different forms of AGEs is established and the outcomes are measured as clinical signs, specific antibodies, type-2/type-2 cytokines, immune cell subpopulations, intestinal barrier function, and gut microbiota (GM) composition. The OVA-AGE which has a lower immunoglobulin E (IgE)-binding level in vitro does not reduce the allergenicity of OVA but promotes a stronger T helper 2 cells (Th2)-response than native OVA in vivo. Both forms of AGEs up-regulate the expression of splenic RAGE and aggravate the destruction of gut barrier and GM dysbiosis, especially when exposes to non-relevant AGEs.

CONCLUSION

This study highlights the role of dietary AGEs in food allergy and helps to understand the biological consequences of immune-toxic compounds in modern diet.

Evaluation of the structure-activity relationship between allergenicity and spatial conformation of ovalbumin treated by pulsed electric field

The aim of the present study was to investigate the effect of pulsed electric field (PEF) treatment on ovalbumin (OVA) induced allergens and reveal potential allergy regulatory mechanisms. At 10 kV/cm, OVA-induced allergic symptoms were significantly reduced, and the capacity of OVA to bind with specific IgG1 and IgE was reduced by 10.32% and 3.61%, respectively. Furthermore, the degranulation of RBL-2H3 cells and allergen activity were also reduced by 4.63% and 22.15%, respectively. Interestingly, the α-helix content was reduced by 5.81% and the fluorescence intensity was increased by 6.90% with PEF treatment. At 10 kV/cm, water contact angle and surface hydrophobicity increased by 8.40% and 0.18%, respectively, indicating that PEF treatment increased the exposure of hydrophobic amino acid residues. PEF treatment alters the hydrogen bonding and hydrophobic interactions in the protein, which masks the binding sites of sensitized epitopes, and consequently reduces allergies.

Investigation of differences in allergenicity of protein from different soybean cultivars through LC/MS-MS

Soybean allergy is a health-threatening issue and identifying raw soybeans with low allergenicity is important for producing hypoallergenic soybean products. Soybean allergy is mainly triggered by soybean proteins. In this study, the protein profiles, allergen compositions, and epitopes in protein from different soybean cultivars (R1, R2 and R3) were evaluated by SDS-PAGE and LC/MS-MS, and their allergenicity was assessed by indirect ELISA and Western blot analysis using the serum IgE of patients allergic to soybeans. The lowest allergenicity was observed in R3, probably resulting from the low concentration of Gly m 4-Gly m 6. The allergenicity of soybeans is affected by multiple allergens rather than a single allergen. Venn diagram, PCA, heatmap, and peptide map analyses have shown the differences in protein and peptide profiles among soybean proteins from different soybean cultivars. Epitope analysis further demonstrated that low contents of dominant epitopes in Gly m 4 and Gly m 5 contributed to low allergenicity in R3, although R3 contained high contents of no-dominant epitopes.

Deciphering Changes in the Structure and IgE-Binding Ability of Ovalbumin Glycated by α-Dicarbonyl Compounds under Simulated Heating

As a complex reaction, biological consequences of the Maillard reaction (MR) on dietary proteins need to be deciphered. Despite previous studies on the structural and antigenic properties of ovalbumin (OVA) by MR, associated changes induced by specific MR intermediates and their downstream products are largely unknown. This study focused on the impacts of glycation by α-dicarbonyl compounds (α-DCs), intermediates of MR and precursors of advanced glycation end-products (AGEs), on the structural and IgE-binding properties of ovalbumin (OVA) under simulated heating. Methylglyoxal (MGO), glyoxal (GO), and butanedione (BU) were selected as typical α-DCs to generate glycated OVA with different AGE-modifications (AGE-Ms). The results showed that reactions between OVA and α-DCs generated OVA-AGE with various degrees of modification and conformational unfolding, and the reactivity of α-DCs followed the order GO > MGO > BU. Depending on the precursor type, the levels of 10 specific AGEs were verified, and the amounts of total AGEs increased with heating temperature and α-DC dosage. Compared to native OVA, glycated OVA showed reduced IgE-binding levels but with sRAGE-binding ligands, the extent of which was associated with the contents of total AGEs and Nε-carboxymethyllysine, and changes in certain protein conformational structures. High-resolution mass spectrometry further identified different AGE-Ms on the Lys and Arg residues of OVA, confirming variations in the glycation sites and their associations with the immunoreactive epitopes of OVA under different conditions.

Maillard Reaction Induced Changes in Allergenicity of Food

Food allergy is increasing in prevalence, posing aheavier social and financial burden. At present, there is still no widely accepted treatment for it. Methods to reduce or eliminate the allergenicity of trigger foods are urgently needed. Technological processing contributes to producing some hypoallergenic foods. Among the processing methods, the Maillard reaction (MR) is popular because neither special chemical materials nor sophisticated equipment is needed. MR may affect the allergenicity of proteins by disrupting the conformational epitope, disclosing the hidden epitope, masking the linear epitope, and/or forming a new epitope. Changes in the allergenicity of foods after processing are affected by various factors, such as the characteristics of the allergen, the processing parameters, and the processing matrix, and they are therefore variable and difficult to predict. This paper reviews the effects of MR on the allergenicity of each allergen group from common allergenic foods.

Characterization of the Reduced IgE Binding Capacity in Boiled and Autoclaved Soybeans through Proteomic Approaches

The study investigated the changes in IgE binding capacity, protein profiles and peptide compositions after soybeans were boiled and autoclaved. The results of ELISA showed that the IgE binding capacity of soybean was reduced by 69.3% and 88.9% after boiling and autoclaving, respectively. Above 43 and 10 kDa proteins disappeared in boiled and autoclaved soybeans from SDS-PAGE, respectively. A Venn diagram and heat map showed that there was no change in allergen types and a reduction in allergen contents in the boiled and autoclaved soybeans. The changes in peptide compositions were also observed in the boiled and autoclaved soybeans through Venn diagram, PCA and heat map. LC/MS-MS and peptide mapping analysis demonstrated that boiling and autoclaving masked many epitopes in Gly m 4 and Gly m 5, such as ALVTDADNVIPK, SVENVEGNGGPGTIKK and KITFLEDGETK of Gly m 4 and VEKEECEEGEIPRPRPRPQHPER of Gly m 5, resulting in a reduction of IgE binding capacity in the extracted proteins. By contrast, the exposure of many epitopes in Gly m 6 was observed in boiled and autoclaved soybeans, which might be mainly responsible for the existing IgE binding capacity in the treated soybean proteins. Interestingly, the IgE binding capacity of soybeans showed a positive correlation with the total contents and number of peptides in Gly m 4-Gly m 6.

Insight into the mechanism of d-allose in reducing the allergenicity and digestibility of ultrasound-pretreated α-lactalbumin by high-resolution mass spectrometry

The mechanism of d-allose in reducing the allergenicity and digestibility of ultrasound-pretreated α-lactalbumin (α-LA) was studied. The intensity reduction and peak red shift occurred in fluorescence spectra of glycated samples. Enzyme-linked immunosorbent assay and basophil degranulation analysis showed that d-allose significantly reduced the allergenicity of α-LA, and ultrasound-pretreated α-LA showed the lowest allergenicity after glycation. Compared with α-LA, the degree of hydrolysis decreased in glycated samples. Size-exclusion high-performance liquid chromatography showed that the glycated α-LA was resistant to digestive enzymes. The glycated sites and average degree of substitution per peptide molecule were determined using LC Orbitrap MS/MS. These results suggested that the masking of linear allergenic epitopes by glycation could reduce the allergenicity. Therefore, the combination of ultrasound pretreatment and glycation is a potential method to reduce protein allergenicity in food processing and provides a useful approach for the application of rare sugars in food processing.

Investigation on the Anaphylaxis and Anti-Digestive Stable Peptides Identification of Ultrasound-Treated α-Lactalbumin during In-Vitro Gastroduodenal Digestion

Our previous studies indicated that ultrasound treatment can increase the anaphylaxis of protein. However, investigation on the anaphylaxis changes of ultrasound-treated α-lactalbumin (ALA) during digestion is lacking. The anaphylaxis of ultrasound-treated ALA and its digesta was investigated. The anti-digestive stable peptides were identified by high-resolution mass spectrometry. Ultrasound induced the tertiary structure of ALA to unfold and increased its anaphylaxis. During digestion, the anaphylaxis of both gastric and gastroduodenal digesta was further increased. There are two reasons for this phenomenon. On the one hand, linear epitopes played an important role in affecting anaphylaxis compared with the conformational epitope, and some linear epitopes were still retained on the anti-digestive stable peptides produced after gastroduodenal digestion, resulting in increased anaphylaxis after digestion. On the other hand, the presence of intact ALA molecules after digestion still remained strong anaphylaxis. Compared with the digesta of untreated ALA, the digesta of ultrasound-treated ALA possessed higher anaphylaxis. The results indicated that ultrasound increased the anaphylaxis of ALA during digestion.

Mechanism of the Reduced IgG/IgE Binding Abilities of Glycated β-Lactoglobulin and Its Digests through High-Resolution Mass Spectrometry

Glycation between proteins and reducing sugars is the common chemical modification in food protein, and many studies have focused on the allergenicity of the glycated protein. However, a systemic study on the allergenicity change of its digests is lacking. In this work, we explored the change rule of the digestibility and allergenicity of glycated β-Lg during in vitro gastrointestinal digestion and interpreted the mechanism using high-resolution mass spectrometry. Glycation with arabinose increased the resistance of β-Lg to digestive enzyme, with a low hydrolysis value. Indirect competitive ELISA showed that the IgG/IgE binding rates of β-Lg were reduced after glycation and further reduced after digestion, in comparison with the digests of unglycated β-Lg. There are two reasons for this phenomenon. On the one hand, 11 glycated sites were determined in the lowest allergenicity arabinose-β-Lg conjugation (Ara-β-Lg), which was distributed in the IgG and IgE linear allergic epitopes of β-Lg. On the other hand, glycation masking linear allergenic epitopes had a more significant effect on reducing allergenicity in comparison to digestive enzyme hydrolysis. These results indicated that the allergenicity of Ara-β-Lg in the human body might be lower than that of unglycated β-Lg.

Insight into the mechanism of urea inhibit ovalbumin-glucose glycation by conventional spectrometry and liquid chromatography-high resolution mass spectrometry

The inhibition effect of urea on ovalbumin (OVA) glycation was investigated, and the mechanism was evaluated through the changes in protein structure as well as glycation sites and average degree of substitution per peptide molecule (DSP) by conventional spectrometry and liquid chromatography-high resolution mass spectrometry (LC-HRMS). A urea concentration of 3 M was chosen as the optimum condition. Ultraviolet and fluorescence spectra suggested that both glycation and urea treatment could unfold the OVA, but urea inhibited the glycation-induced protein unfolding. Circular dichroism spectra showed that urea treatment could increase the β-sheet content and reduce the α-helix content of OVA. LC-HRMS indicated that the number of glycation sites was reduced from 15 to 3, and DSP values decreased with urea treatment. In conclusion, urea could significantly inhibit the OVA-glucose glycation, and the sites competition as well as structure unfolding inhibition resulted from urea could be the main factors.