Influence of Hydroxyl Substitution on the Suppression of Flavonol in Harmful Glycation Product Formation and the Inhibition Mechanism Revealed by Spectroscopy and Mass Spectrometry

Quercetin (Que), kaempferol (Kaem), isorhamnetin (Irh), and myricetin (Myri) are typical flavonols that are abundant in plant resources. This research investigated their ability in attenuating harmful glycation product formation and the effect of hydroxyl substitution. The inhibition mechanisms were elucidated by fluorescence spectroscopy and nano-liquid chromatography Orbitrap tandem mass spectrometry. The results indicated that the 3'-OH on the B-ring is critical in alleviating harmful glycation product formation, methylation reduced its inhibition, and the 5'-OH showed much less contribution than the 3'-OH. Que showed the strongest suppression on initial product, 5-hydroxymethylfurfural, and advanced glycation end product formation, with the corresponding percentage inhibitions at 36.58 μM of 81.1, 56.9, and 95.4%. Que and Myri also clearly inhibited fructosamine and acrylaminde production, while no suppression was observed by Irh and Kaem. The number of glycated sites was reduced from ten to seven, five, six, and nine, respectively, when 36.58 μM Que, Myri, Kaem, and Irh was added. Suppressing the conformational changes of ovalbumin induced by glycation, trapping dicarbonyl compounds, altering the microenvironment around tryptophan, and reducing the glycation activity of potential sites were the major inhibition mechanisms. These results suggest that Que and Myri may be promising natural agents for inhibiting harmful glycation and provide theoretical support for the effective screening of natural antiglycation reagents.

Mitigation of isoquercitrin on β-lactoglobulin glycation: Insight into the mechanisms by mass spectrometry and interaction analysis

Formation of advanced glycation end products (AGEs) on foods imposes threats to human health after intaking. This research firstly evaluated the inhibition of isoquercitrin on β-lactoglobulin (β-Lg) glycation, the mechanisms were elucidated by fluorescence spectroscopy, Orbitrap MSⁿ and molecular docking. Fluorescence spectra indicated that isoquercitrin effectively alleviated the formation of AGEs, it could stabilize the conformation structure of glycated β-Lg (G-β-Lg), change the micro-environment in the vicinity of chromophores. SDS-PAGE analysis revealed the suppressed cross-linking of G-β-Lg induced by isoquercitrin. The number of glycation site detected on G-β-Lg was reduced from ten to eight after the addition of isoquercitrin, and the relative glycation degree of substitution of per site (RGDSP) of most glycation sites were also greatly decreased. As indicated by intermolecular interaction, isoquercitrin quenched the fluorescence of β-Lg via a static mechanism, and their combination is an endothermic processing mainly derived by hydrophobic interaction, hydrogen bonds, and van der Waals forces. Isoquercitrin interacted with β-Lg to form an equimolar complex, and one hydrogen bond was formed between isoquercitrin and Lys69 (4.96 Å). Above results proved that isoquercitrin can be a promising anti-glycation agent used in food system to prevent the formation of harmful glycation products.

Observation of the structural changes of α-lactalbumin induced by ultrasonic prior to glycated modification

Bovine α-lactalbumin (BLA) was treated by ultrasonic at 150 W/cm² for different times and subsequently glycated with mannose by dry-heating. Molecular weight, intrinsic fluorescence spectra, glycation sites and degree of modified BLA were observed. The proteinaceous high molecular weight components were formed after ultrasonic prior to glycated modification, while the conformational changes were obvious. Prior to ultrasonic pretreatment, K62, K114, and K122 of BLA were identified. After treated by ultrasound at 150 W/cm² for 5, 10, 15, and 20 min, the sites were increased to four, four, five, and five, respectively. All glycated sites of modified BLA exhibited a higher degree of substitution per peptide (DSP) values compared to native BLA. Ultrasonic at 150 W/cm² for 20 min revealed the most significant change in the BLA structure. Therefore, conformational changes, the intensified glycation site, and DSP value were responsible for the structural changes of BLA. Practical applications BLA is suitable as an ingredient for infant nutrition in food, and has immune-modulating, antioxidant, antibacterial, and antitumor activity etc. This study revealed that the structural changes of BLA induced by ultrasonic prior to glycated modification. It will be beneficial to understand the mechanism of the functional changes of modified BLA. Ultrasonic prior to glycated modification will be more likely to develop a practical technology to modify protein in the food industry, and improve the functional characteristics of food, such as produce hypo-allergenic cow's milk in future.