Inhibitory effect of quercetin in the formation of advance glycation end products of human serum albumin: An in vitro and molecular interaction study

Glyoxal in Foods: Formation, Metabolism, Health Hazards, and Its Control Strategies

Glyoxal is a highly reactive aldehyde widely present in common diet and environment and inevitably generated through various metabolic pathways in vivo. Glyoxal is easily produced in diets high in carbohydrates and fats via the Maillard reaction, carbohydrate autoxidation, and lipid peroxidation, etc. This leads to dietary intake being a major source of exogenous exposure. Exposure to glyoxal has been positively associated with a number of metabolic diseases, such as diabetes mellitus, atherosclerosis, and Alzheimer's disease. It has been demonstrated that polyphenols, probiotics, hydrocolloids, and amino acids can reduce the content of glyoxal in foods via different mechanisms, thus reducing the risk of exogenous exposure to glyoxal and alleviating carbonyl stresses in the human body. This review discussed the formation and metabolism of glyoxal, its health hazards, and the strategies to reduce such health hazards. Future investigation of glyoxal from different perspectives is also discussed.

Experimental and hypothetical appraisal on inhibition of glucose-induced glycation of bovine serum albumin by quercetin

BACKGROUND

The specificity of protein functions depends on its folding ability into a functional structure. Protein folding is an essential systemic phenomenon that prevents incorrect folding which could result in harmful aggregation. This harmful aggregation of proteins causes neurodegenerative diseases and systemic amyloidosis. Experimental and theoretical approaches were used in this study to explicate the probable mechanisms of action of quercetin in inhibition of glucose-induced glycation through estimations of percentage glycated protein, inhibited induced protein aggregation, and unoxidized bovine serum albumin thiol groups and assessments of molecular interactions of quercetin with the structures of bovine serum albumin, amyloid beta-peptide (1-42) and 3D amyloid-beta (1-42) fibrils retrieved from the protein databank ( http://www.rcsb.org ).

RESULTS

The results showed quercetin inhibited the formation of glycated protein, protein aggregation, and thiol oxidation in a concentration-dependent manner where 200 μg/ml showed the highest inhibition while 50 μg/ml depicted the least inhibition in all the studied assessments. From the docking analysis, it was observed that quercetin had a significantly higher binding affinities - 8.67 ± 0.09 kcal/mol, - 5.37 ± 0.05 kcal/mol and - 5.93 ± 0.13 kcal/mol for the bovine serum albumin, amyloid beta-peptide (1-42) and 3D amyloid-beta (1-42) fibrils respectively compared to the glucose, the inducer. Quercetin and glucose interacted with amino acid residues at the BSA subdomain IIA thus providing a clue that quercetin may impose its inhibition through the binding domain. Also, it is important to mention that the phytochemicals shared a similar interaction profile as that of glucose with the amyloid-beta.

CONCLUSIONS

These findings established the beneficial effects of quercetin as a potential agent that could alleviate hyperglycaemic-initiated disorders associated with elevated serum glucose levels.

Antiglycation and antioxidant potential of coumaric acid isomers: a comparative in-vitro study

Advanced glycation end products (AGEs) are the product of non-enzymatic glycation of serum proteins. AGEs increase reactive oxygen species (ROS) formation, which leads to diabetic complications. Phytochemicals exhibit lesser side effects as compared to conventional therapy. In this study, three isomers of coumaric acid (ortho, meta, para) were used to deduce the better one in terms of reducing diabetic complications. For this purpose, human serum albumin (HSA) was incubated with glucose in the absence and presence of isomers for 28 days. To avoid any growth, NaN3 was added and temperature was kept constant throughout the incubation period. Studies like fluorescence, circular dichroism spectroscopy, fructosamine analysis, free lysine estimation, free thiol group estimation were done. To investigate the ROS production, fluorescence microscopy of isolated lymphocytes using DAPI and dichloro-dihydro-fluorescein diacetate were performed. Molecular docking and molecular dynamic simulations (root-mean-square deviation, root-mean-square fluctuations, radius of gyration and solvent-accessible surface area) of HSA and peroxisome proliferator activated receptor (PPAR) alpha and gamma were also done. It was observed that in glycated protein samples, the level of absorbance, fluorescence, fructosamine and carbonyl group increased along with the loss of secondary structure, free lysine and thiol group. These parameters were found gradually recovered in treated samples. ROS production and apoptosis were found to be reduced in lymphocytes treated with p-Coumaric acid (pCA)-treated protein samples as compared to lymphocyte treated with glycated protein. Computational modelling suggested a stable complex formation of HSA and PPARs with pCA. Results with pCA at 200 µM were consistently better than other two isomers. Our next step is to evaluate this study in rats.Communicated by Ramaswamy H. Sarma.

Fenugreek seed ethanolic extract inhibited formation of advanced glycation end products via scavenging reactive carbonyl intermediates

Senescence is a natural phenomenon of growing old. It accelerates under certain conditions like diabetes mellitus resulting in early decline of bodily functions, which can be avoided by many claimed functional foods. The present study aims to investigate the anti-aging ability of Fenugreek seeds (Trigonellafoenum-graecum); a common ingredient of Indo-Pak cuisines. Briefly, the Fenugreek seeds extract (FgSE) in concentrationsof0.1, 0.5 and 1 mg/ml inhibited the formation of Advanced Glycation End products (AGEs) and fructosamine adducts in Bovine serum albumin (BSA)/fructose model in vitro. The BSA conformational analysis via Circular Dichorism and Congo red assays showed that it preserves secondary structure of BSA in aforementioned model. Although mechanistic studies revealed insignificant lysine blocking ability of Fenugreek by OPA assay, however carbonyl entrapping was found to be 24%, 34% and 42% at 0.1, 0.5 and 1 mg/ml, respectively. In vivo model of High Fructose diet (HFD) induced glycation, FgSE treatment in doses of 10, 25 & 50 mg/kg markedly improved Escape latency (p < 0.01) and preserved cognition in Morris Water Maze. Our data further exhibits significant decrease of CML (Nε-carboxymethyl lysine) levels in serum and hippocampus byFgSE treatment in comparison with HFD group. Therefore, we deduced that FgSE prevents glycation-induced memory decline via entrapping the reactive carbonyl intermediates, formed during production of AGEs. Hence, as a promising functional food it slows down the harmful process of glycation and aging associated morbidities.

The mechanism of in vitro non-enzymatic glycosylation inhibition by Tartary buckwheat's rutin and quercetin

Tartary buckwheat is rich in rutin, quercetin, and other flavonoids, which exert prominent effects by inhibiting non-enzymatic glycosylation. In this study, an in vitro non-enzymatic glycosylation model was established, and the inhibitory effects of rutin and quercetin on the early, middle, and late products of non-enzymatic glycosylation were determined. Furthermore, their effects on the formation of advanced glycation end products (AGEs) and on protein functional groups and secondary structure were analyzed. These findings provided a theoretical basis for further investigation of the mechanism via which Tartary buckwheat's rutin and quercetin inhibited non-enzymatic glycosylation. The results showed that rutin and quercetin inhibited the formation of fructosamine, dicarbonyl compounds, and fluorescent AGE in a concentration-dependent manner. Rutin and quercetin exhibited antioxidant activity and could reduce the formation of protein oxidation products. The highest clearance rates for DPPH and ABTS⁺ were 62.74 % and 71.14 %, respectively.

Synthesis of thiazole-based-thiourea analogs: as anticancer, antiglycation and antioxidant agents, structure activity relationship analysis and docking study

This work reports the convenient approach for the synthesis of thiazole based thiourea derivatives (1-21) from 2-bromo-1-(4-fluorophenyl)thiazole-1-one and phenyl isothiocyanates. The scope and diversity were achieved from readily available phenyl isothiocyanates. This protocol involves an oxidative C-S bond formation. Moreover, hybrid thiazole based thiourea scaffolds (1-21) according to literature known protocol were screened in vitro for anticancer Potential against breast cancer, antiglycation and antioxidant inhibitory profile. All newly developed scaffolds were showed moderate to good inhibitory potentials ranging from 0.10 ± 0.01 µM to 11.40 ± 0.20 µM, 64.20 ± 0.40 µM to 385.10 ± 1.70 µM and 8.90 ± 0.20 µM to 39.20 ± 0.50 µM against anticancer, antiglycation and antioxidant respectively. Among the series, compounds 12 (IC50 = 0.10 ± 0.01 µM), 10 (IC50 = 64.20 ± 0.40 µM) and 12 (IC50 = 8.90 ± 0.20 µM) with flouro substitution at phenyl ring of thiourea were identified to be the most potent among the series having excellent anticancer, antiglycation and antioxidant potential. The structure of all the newly synthetics scaffolds were confirmed by using different types of spectroscopic techniques such as HREI-MS, 1H- and 13C-NMR spectroscopy. To find structure-activity relationship, molecular docking studies were carried out to understand the binding mode of active inhibitors with active site of enzymes and results supported the experimental data.Communicated by Ramaswamy H. Sarma.

Phytochemicals as Potential Inhibitors of Advanced Glycation End Products: Health Aspects and Patent Survey

BACKGROUND

The glycation of proteins and lipids synthesizes the advanced glycation end products (AGEs), i.e., substances that irreversibly damage macromolecules present in tissues and organs, which contribute to the impairment of biological functions. For instance, the accumulation of AGEs induces oxidative stress, the inflammatory responses, and consequently the on set/worsening of diseases, including obesity, asthma, cognitive impairment, and cancer. There is a current demand on natural and low-cost sources of anti-AGE agents. As a result, food phytochemicals presented promising results to inhibit glycation and consequently, the formation of AGEs.

OBJECTIVE

Here we describe how the AGEs are present in food via Maillard reaction and in organs via natural aging, as well as the effects of AGEs on the worsening of diseases. Also we described the methods used to detect AGEs in samples, and the current findings on the use of phytochemicals (phenolic compounds, phytosterols, carotenoids, terpenes and vitamins) as natural therapeuticals to inhibit health damages via inhibition of AGEs in vitro and in vivo.

METHODS

This manuscript reviewed publications available in the PubMed and Science Direct databases dated from the last 20 years on the uses of phytochemicals for the inhibition of AGEs. Recent patents on the use of anti-AGEs drugs were reviewed with the use of Google Advanced Patents database.

RESULTS AND DISCUSSION

There is no consensus about which concentration of AGEs in blood serum should not be hazardous to the health of individuals. Food phytochemicals derived from agroindustry wastes, including peanut skins, and the bagasses derived from citrus and grapes are promising anti-AGEs agents via scavenging of free radicals, metal ions, the suppression of metabolic pathways that induces inflammation, the activation of pathways that promote antioxidant defense, and the blocking of AGE connection with the receptor for advanced glycation endproducts (RAGE).

CONCLUSION

Phytochemicals derived from agroindustry are promising anti-AGEs, which can be included to replace synthetic drugs to inhibit AGE formation, and consequently to act as therapeutical strategy to prevent and treat diseases caused by AGEs, including diabetes, ovarian cancer, osteoporosis, and Alzheimer's disease.

Research Advances on the Damage Mechanism of Skin Glycation and Related Inhibitors

Our skin is an organ with the largest contact area between the human body and the external environment. Skin aging is affected directly by both endogenous factors and exogenous factors (e.g., UV exposure). Skin saccharification, a non-enzymatic reaction between proteins, e.g., dermal collagen and naturally occurring reducing sugars, is one of the basic root causes of endogenous skin aging. During the reaction, a series of complicated glycation products produced at different reaction stages and pathways are usually collectively referred to as advanced glycation end products (AGEs). AGEs cause cellular dysfunction through the modification of intracellular molecules and accumulate in tissues with aging. AGEs are also associated with a variety of age-related diseases, such as diabetes, cardiovascular disease, renal failure (uremia), and Alzheimer's disease. AGEs accumulate in the skin with age and are amplified through exogenous factors, e.g., ultraviolet radiation, resulting in wrinkles, loss of elasticity, dull yellowing, and other skin problems. This article focuses on the damage mechanism of glucose and its glycation products on the skin by summarizing the biochemical characteristics, compositions, as well as processes of the production and elimination of AGEs. One of the important parts of this article would be to summarize the current AGEs inhibitors to gain insight into the anti-glycation mechanism of the skin and the development of promising natural products with anti-glycation effects.

4-Chloro-1,2-phenylenediamine induced structural perturbation and genotoxic aggregation in human serum albumin

4-Chloro-1,2-phenylenediamine (4-Cl-OPD) is a halogenated aromatic diamine used as a precursor in permanent hair color production. Despite its well-documented mutagenic and carcinogenic effects in various in vitro and in vivo models, its role in fibrillar aggregate formation and their genotoxic effect in therapeutic proteins has received less attention. The significance of human serum albumin (HSA) arises from its involvement in bio-regulatory and transport processes. HSA misfolding and aggregation are responsible for some of the most frequent neurodegenerative disorders. We used various complementary approaches to track the formation of amyloid fibrils and their genotoxic effect. Molecular dynamics study demonstrated the complex stability. The impact of 4-Cl-OPD on the structural dynamics of HSA was confirmed by Raman spectroscopy, X-ray diffraction, HPLC and SDS-PAGE. Fibrilllar aggregates were investigated using Congo red assay, DLS, and SEM. The genotoxic nature of 4-Cl-OPD was confirmed using plasmid nicking assay and DAPI staining, which revealed DNA damage and cell apoptosis. 4-Cl-OPD provides a model system for studying fibrillar aggregation and their genotoxic potential in the current investigation. Future studies should investigate the inhibition of the aggregation/fibrillation process, which may yield valuable clinical insights.

Urolithin A alleviates advanced glycation end-product formation by altering protein structures, trapping methylglyoxal and forming complexes

Urolithin A (UroA) is a first-in-class natural compound derived from the gut microbiota-derived metabolites of ellagitannins. This research for the first time evaluates the mechanisms of UroA inhibiting advanced glycation end-product (AGE) formation by fluorescence spectroscopy, molecular docking, liquid chromatography (LC) and LC-Oribitrap tandem mass spectrometry. The results indicated that UroA exhibited a good suppression effect on the formation of AGEs in human serum albumin (HSA)-fructose and HSA-methylglyoxal (MGO) systems. Further mechanism analysis revealed that UroA alleviated AGE formation by changing the conformational structure of HSA, trapping reactive MGO to form mono-MGO-UroA complexes, promoting the exposure of chromophores to a more hydrophobic micro-environment, and forming stable UroA-HSA complexes. UroA bound with HSA in an equimolar manner, the binding was an exothermic spontaneous process, subdomain IIIA was the preferred binding pocket, and hydrogen bonding, hydrophobic interactions and van der Waals forces were the major interaction forces. The number of glycation sites detected in glycated HSA was reduced by 1 and 2, respectively, when 181.82 and 363.64 μM UroA was added. These could provide an insight into the mechanism of UroA inhibiting HSA glycation, and highlight its value as a promising glycation inhibitor in the prevention of diabetic complications.

Dietary Quercetin Reduces Plasma and Tissue Methylglyoxal and Advanced Glycation End Products in Healthy Mice Treated with Methylglyoxal

BACKGROUND

Methylglyoxal (MGO), a precursor of advanced glycation end products (AGEs), has been linked to AGEs-associated diseases.

OBJECTIVES

This study investigated the efficacy and mechanisms of dietary quercetin in decreasing plasma and tissue concentrations of MGO and AGEs in MGO-administered mice.

METHODS

Male, 6-wk-old CD-1 mice were administered AIN-93G diet and water (Con) or 0.12% MGO in water (MGO) or MGO plus 0.2% (0.2Q) dietary quercetin for 1 wk (n = 5) (experiment 1), and water (Con), 0.12% MGO (MGO), or MGO plus 0.1% (0.1Q), 0.2% (0.2Q), or 0.4% (0.4Q) dietary quercetin for 6 wk (n = 10) (experiment 2). The plasma, kidney, and liver concentrations of MGO, quercetin, and isorhamnetin and their trapping adducts with MGO were determined by LC-MS, and AGE concentrations were measured by the fluorescent method. Furthermore, the expressions of glyoxalase I/II (GLO I/II) and aldose reductase (AR), MGO detoxification enzymes, were determined by Western blot. One-factor ANOVA and post hoc Dunnett's or Tukey's test were used to analyze the data.

RESULTS

After 1 wk of treatment, the MGO concentrations in plasma (20.2%) and kidney (29.9%) in 0.2Q mice were significantly lower than those in MGO mice. After 6 wk of treatment, the concentrations of MGO in the plasma (14.7-18.6%), kidney (20-20.8%), liver (15.4-18.6%), and tissue AGEs (28-36.8%) in 0.1Q, 0.2Q, and 0.4Q mice were significantly lower than those in MGO mice. The plasma concentrations of quercetin, isorhamnetin, and their MGO adducts were dose-dependently increased after quercetin administration. In addition, after 6 wk of quercetin administration, the expressions of GLO I/II and AR in the liver and kidney were significantly upregulated to promote MGO detoxification compared with MGO-treated mice.

CONCLUSIONS

Quercetin reduced plasma and tissue MGO concentrations and inhibited AGE formation by trapping MGO and regulating the MGO detoxification systems in MGO-administered healthy mice.

Investigation into the mechanisms of quercetin-3-O-glucuronide inhibiting α-glucosidase activity and non-enzymatic glycation by spectroscopy and molecular docking

The inhibition of α-glucosidase and glycation is closely related to the treatment of type 2 diabetes mellitus (DM) and its complications. In this study, quercetin-3-O-glucuronide (Q3GA) showed reversible and mixed-mode inhibition of α-glucosidase activity, with an IC₅₀ value of 108.11 ± 4.61 μM. This was mainly due to the spontaneous formation of Q3GA-α-glucosidase driven by hydrogen bonding and van der Waals forces, which could change the microenvironments and conformation of α-glucosidase. In addition, Q3GA showed strong suppression of the formation of glycation products, including fructosamine, advanced glycation end products (AGEs), and 5-hydroxymethylfurfural (5-HMF). Molecular docking analysis demonstrated that Q3GA entered the hydrophobic pocket of ovalbumin to form six hydrogen bonds with amino acid residues, which affected the glycation process. These findings indicate that Q3GA is an excellent inhibitor of α-glucosidase and glycation, and promote its development as a drug or dietary supplement for DM.

Hydroxyl radical induced structural perturbations make insulin highly immunogenic and generate an auto-immune response in type 2 diabetes mellitus

Reactive oxygen species (ROS) cause oxidative damage to proteins and generate deleterious by-products which induce a breakdown of immune tolerance and produce antibodies against host macromolecules with implication in human diseases. This study characterizes the hydroxyl radical (OH) modifications of insulin, evaluates its cytotoxicity and immunogenicity, and probes its role in type 2 diabetes (T2DM) autoimmunity. The results demonstrate susceptibility of insulin to modifications induced by OH, causing exposure of its chromophoric aromatic amino acid residues, quenching of tyrosine fluorescence intensity, loss of α-helix and gain in β content. Modification causes re-arrangement of native interactions of the aromatic residues in insulin. It enhanced the carbonyl content in insulin, exposed its hydrophobic patches and generated non-fibrillar, amorphous type of aggregates that are cytotoxic in nature. Native insulin induced low titre antibodies in immunized rabbits, whereas OH modified insulin generated a strong immune response. Competitive ELISA studies showed high specificity of antibodies generated against OH modified insulin towards the modified protein. Cross reaction studies showed the presence of common antigenic determinants on various oxidised proteins. Since T2DM patients show increased ROS production, oxidation of insulin is expected to occur, which might amplify autoimmune reactions against insulin. True to the assumption, direct binding ELISA showed the presence of anti-OH insulin circulating antibodies in T2DM patients which are specific for the oxidized insulin. In conclusion, insulin loses structural integrity to OH, forms cytotoxic amorphous aggregates, turns highly immunogenic and elicits humoral response in T2DM patients.

Adrenomedullin 2 improves bone regeneration in type 1 diabetic rats by restoring imbalanced macrophage polarization and impaired osteogenesis

Background

Both advanced glycation end products (AGEs) and AGE-mediated M1 macrophage polarization contribute to bone marrow mesenchymal stem cell (BMSC) dysfunction, leading to impaired bone regeneration in type 1 diabetes mellitus (T1DM). Adrenomedullin 2 (ADM2), an endogenous bioactive peptide belonging to the calcitonin gene-related peptide family, exhibits various biological activities associated with the inhibition of inflammation and reduction of insulin resistance. However, the effects and underlying mechanisms of ADM2 in AGE-induced macrophage M1 polarization, BMSC dysfunction, and impaired bone regeneration remain poorly understood.

Methods

The polarization of bone marrow-derived macrophages was verified using flow cytometry analysis. Alkaline phosphatase (ALP) staining, ALP activity detection, and alizarin red staining were performed to assess the osteogenesis of BMSCs. Quantitative real-time polymerase chain reaction, enzyme-linked immunosorbent assay, western blotting, and immunofluorescence staining were used to assess polarization markers, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling, and osteogenic markers. In vivo, a distraction osteogenesis (DO) rat model with T1DM was established, and tibia samples were collected at different time points for radiological, biomechanical, and histological analyses, to verify the effects of ADM2 on bone regeneration and M2 polarization under diabetic conditions.

Results

ADM2 treatment reversed AGE-induced M1 macrophage polarization towards the M2 phenotype, which was partially achieved by the peroxisome proliferator-activated receptor γ (PPARγ)-mediated inhibition of NF-κB signaling. The PPARγ inhibitor GW9662 significantly attenuated the effects of ADM2. Besides, ADM2 treatment improved the AGE-impaired osteogenic potential of BMSCs in vitro. Furthermore, ADM2 accelerated bone regeneration, as revealed by improved radiological and histological manifestations and biomechanical parameters, accompanied by improved M2 macrophage polarization in diabetic DO rats, and these effects were partially blocked by GW9662 administration.

Conclusions

These results indicate that ADM2 enhances diabetic bone regeneration during DO, by attenuating AGE-induced imbalances in macrophage polarization, partly through PPARγ/NF-κB signaling, and improving AGE-impaired osteogenic differentiation of BMSCs simultaneously. These findings reveal that ADM2 may serve as a potential bioactive factor for promoting bone regeneration under diabetic conditions, and imply that management of inflammation and osteogenesis, in parallel, may present a promising therapeutic strategy for diabetic patients during DO treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02368-9.

Influence of Dopamine on Fluorescent Advanced Glycation End Products Formation Using Drosophila melanogaster

Non-enzymatic glycation and covalent modification of proteins leads to Advanced Glycation End products (AGEs). AGEs are biomarkers of aging and neurodegenerative disease, and can be induced by impaired neuronal signaling. The objective of this study was to investigate if manipulation of dopamine (DA) in vitro using the model protein, bovine serum albumin (BSA), and in vivo using the model organism Drosophila melanogaster, influences fluorescent AGEs (fAGEs) formation as an indicator of dopamine-induced oxidation events. DA inhibited fAGEs-BSA synthesis in vitro, suggesting an anti-oxidative effect, which was not observed when flies were fed DA. Feeding flies cocaine and methamphetamine led to increased fAGEs formation. Mutants lacking the dopaminergic transporter or the D1-type showed further elevation of fAGEs accumulation, indicating that the long-term perturbation in DA function leads to higher production of fAGEs. To confirm that DA has oxidative properties in vivo, we fed flies antioxidant quercetin (QUE) together with methamphetamine. QUE significantly decreased methamphetamine-induced fAGEs formation suggesting that the perturbation of DA function in vivo leads to increased oxidation. These findings present arguments for the use of fAGEs as a biomarker of DA-associated neurodegenerative changes and for assessment of antioxidant interventions such as QUE treatment.

Protective actions of bioactive flavonoids chrysin and luteolin on the glyoxal induced formation of advanced glycation end products and aggregation of human serum albumin: In vitro and molecular docking analysis

The post-translational modification of proteins by nonenzymatic glycation (NEG) and the accumulation of AGEs are the two underlying factors associated with the long-term pathogenesis in diabetes. Glyoxal (GO) is a reactive intermediate which has the ability to modify proteins and generate AGEs at a faster rate. Human serum albumin (HSA) being the most abundant serum protein has a higher chance to be modified by NEG. The key objective of the present study is to investigate the potency of chrysin and luteolin as antiglycating and antifibrillating agents in the GO-mediated glycation and fibril formation of HSA. AGEs formation were confirmed from the absorption and fluorescence spectral measurements. Both the flavonoids were able to quench the AGEs fluorescence intensity in vitro indicating the antiglycating nature of the molecules. The formation of fibrils in the GO-modified HSA was confirmed by the Thioflavin T (ThT) fluorescence assay and the flavonoids were found to exihibit the antifibrillation properties in vitro. Docking results suggested that both the flavonoids interact with various amino acid residues of subdomain IIA including glycation prone lysines and arginines via non-covalent forces and further stabilized the structure of HSA, which further explains their mechanisms of action as antiglycating and antifibrillating agents.

In vitro evaluation of anti-methylglyoxal/glyoxal activity of three phytosterols using glycated bovine serum albumin models

Reactive intermediate dicarbonyls, such as methylglyoxal (MGO) and glyoxal (GO), have received extensive attention recently due to their high reactivity and capability to form advanced glycation end products (AGEs) in foods, which have been implicated in the progression of age-related complaints. We aimed to investigate the effects of three structurally different phytosterols (PS), including stigmasterol (SS), β-sitosterol (βS), and γ-oryzanol (γO), on AGEs-formation by measuring their anti-GO/MGO activity. The glycoxidation-based products, SDS-PAGE intensity, free lysine, protein thiols, fluorescence microscopy clicks, scavenging of dicarbonyl activity, and protein aggregation in bovine serum albumin (BSA) models were therefore measured. The results showed that PS could strongly inhibit fluorescent-AGEs, lysine residues, intermediate di-carbonyls, beside their disaggregation effects in a dose and structure dependent manner. Additionally, γ-oryzanol strongly inhibited AGEs more than the other PS, mostly due to its distinctive structure. Our results will provide a new foundation for development of different structure of PS as natural AGEs-inhibitors.

Antiglycation and Antioxidant Properties of Ficus deltoidea Varieties

The present study aimed to evaluate the potential of standardized methanolic extracts from seven Ficus deltoidea varieties in inhibiting the formation of AGEs, protein oxidation, and their antioxidant effects. The antiglycation activity was analyzed based on the inhibition of AGEs, fructosamine, and thiol groups level followed by the inhibition of protein carbonyl formation. The antioxidant activity (DPPH radical scavenging activity and reducing power assay) and total phenolic contents were evaluated. After 28 days of induction, all varieties of Ficus deltoidea extracts significantly restrained the formation of fluorescence AGEs by 4.55–5.14 fold. The extracts also reduced the fructosamine levels by 47.0–86.5%, increased the thiol group levels by 64.3–83.7%, and inhibited the formation of protein carbonyl by 1.36–1.76 fold. DPPH radical scavenging activity showed an IC₅₀ value of 66.81–288.04 μg/ml and reducing power activity depicted at 0.02–0.24 μg/ml. The extent of phenolic compounds present in the extracts ranged from 70.90 to 299.78 mg·GAE/g. Apart from that, correlation studies between the activities were observed. This study revealed that seven varieties of Ficus deltoidea have the potential to inhibit AGEs formation and possess antioxidant activity that might be attributed to the presence of phenolic compounds.

Mulberry anthocyanins exert anti-AGEs effects by selectively trapping glyoxal and structural-dependently blocking the lysyl residues of β-lactoglobulins

Advanced glycation end-products (AGEs), which instigate many disorders, are mostly mediated by dicarbonyl rearrangements. We studied the corresponding mechanisms of the anti-glycation effects of two anthocyanins purified from mulberry fruits, namely cyanidin 3-glucoside (C3G) and cyanidin 3-rutinoside (C3R), on glycated β-lactoglobulins (β-Lg). Both mulberry anthocyanins (MAs) inhibited the AGEs-formation in a dose-dependent manner, but the effect of C3R was significantly stronger than that of C3G (p < 0.05). MAs inhibited AGEs-formation by selectively trapping dicarbonyls, especially glyoxal. The UPLC-ESI-Q-TOF-MS results characterized that C3R formed mono- and di-glyoxal adducts, where C3G only created di-glyoxal adducts. Additionally, C3R could directly interact with some of the glycation sites of β-Lg. Overall, GO-trapping and β-Lg-MAs covalent/noncovalent binding are disclosed as the key mechanisms of the anti-AGEs activity of MAs on β-Lg, which could be valorised as effectual AGEs inhibitors in proteins-rich matrices.

Mediterranean products as promising source of multi-target agents in the treatment of metabolic syndrome

Alteration of nutritional habits play an essential role on the risk of developing Metabolic Syndrome (MetS). Several epidemiological studies have shown that assuming diets rich of foods included in the Mediterranean diet (MetDiet) pattern like, such as olive oil, nuts, fruit, fiber, vegetables, wine and grain cereals has protective effects on the different risk factors characterizing the MetS. The beneficial effects of the MetDiet in the MetS are mainly due to the antioxidant and anti-inflammatory properties of the most abundant phytochemical components of such foods as polyphenols like resveratrol and oleuropein, allyl sulfides, ellagic acid, mono- and poly-unsaturated fatty acids (MUFA and PUFA), tocopherols and flavonoids like quercetin, which have shown positive results in the prevention of cardiovascular diseases (CVDs), with related risk factors, like hypertension, hypercholesterolemia and obesity. In this review, we highlighted the multi-target activities of the bioactive components contained in some foods typical of the Mediterranean area like olive oil, onion, liquorice, rosemary, oregano, hazelnut, pistachio, "Melannurca" apple, red wine, hot pepper, Citrus sp. fruits, saffron and garlic, with particular focus on their impact on health outcomes in relation to MetS main key factors, such as insulin resistance (IR) and type 2 diabetes mellitus (T2DM), endothelial dysfunctions, inflammatory response, oxidative stress and dyslipidaemic and hypercholesterolemic effects.

Caffeic Acid Inhibits the Formation of Advanced Glycation End Products (AGEs) and Mitigates the AGEs-Induced Oxidative Stress and Inflammation Reaction in Human Umbilical Vein Endothelial Cells (HUVECs)

The advanced glycation end products (AGEs) are the compounds produced by non-enzymatic glycation reaction of proteins and sugars, which can induce the generation of free radicals and the expression of inflammatory factors, thereby playing an important role in vascular dysfunction in diabetes. To investigate the effects of caffeic acid (CA) on glycation formed by glucose and protein, various spectroscopic techniques and molecular docking methods were carried out. Furthermore, the protective effects of CA on human umbilical vein endothelial cells (HUVECs) damaged by AGEs were detected. The results indicated that CA inhibited AGEs formation in vitro, decreased the expression of IL-1β, IL-18, ICAM-1, VCAM-1, NLRP3, Caspase-1 and CRP (C-reactive protein) and reduced the ROS in HUVECs exposed to AGEs. Our findings suggested that the supplementation with dietary CA could prevent and delay the AGEs-induced vascular dysfunction in diabetes.

Anti-diabetic study of vitamin B6 on hyperglycaemia induced protein carbonylation, DNA damage and ROS production in alloxan induced diabetic rats

Oxidative stress performs an imperative role in the onset and progression of diabetes. Metabolic enzymes and cellular organelles are detrimental to increased levels of free radicals and the subsequent reduction in anti-oxidant defence. Pyridoxamine (vitamin B6) is an indispensible nutrient for humans and is considered to be an important food additive too. The aim of this research was to examine the effect of vitamin B6 in a diabetic environment. This study reports the effects of pyridoxamine supplementation in alloxan induced diabetic rats. Diabetes was induced by the single intra peritoneal dose of alloxan (120 mg per kg body weight). Diabetic rats were treated with pyridoxamine (10 and 15 mg per kg body weight) and compared with a control set of diabetic rats without supplementation. Pyridoxamine treatment showed dose dependent recovery in all parameters. A notable decline in oxidative stress parameters and ROS production with reductions in fasting blood glucose levels along with normal patterns of the glucose tolerance test has been reported here. Histological studies reveal damage recovery in the liver as well as kidney tissues. A notable amount of recovery was observed in cellular DNA distortion and damage. It is thus advocated that pyridoxamine might help in reducing problems associated with diabetes. A probable mechanism pertaining to the action of pyridoxamine is proposed as well.

Insights into the Mechanism of Quercetin against BSA-Fructose Glycation by Spectroscopy and High-Resolution Mass Spectrometry: Effect on Physicochemical Properties

Quercetin has been reported to suppress protein glycation or the formation of advanced glycation end-products (AGEs), but the inhibition mechanism related to protein structure and glycation sites and the influence on physicochemical properties remain unclear. The aim of the current research was to investigate the mechanism of quercetin against glycation with BSA-fructose as model by spectroscopic and spectrometric techniques. Changes in physicochemical properties were evaluated by antioxidant activity and emulsifying properties. The results indicated that quercetin dose-dependently inhibited the glycation of BSA by attenuating the alteration of conformational structure and microenvironment induced by glycation. It could also suppress the cross-linking or aggregation of glycated BSA, which reflected in the decreased molecular weight determined by SDS-PAGE and MALDI-TOF. Nanoliquid chromatography coupled to Q-Exactive tandem mass spectrometry analysis revealed the mapping of 20, 23, 19, and 19 glycation sites in glycated BSA with 0, 0.5, 1.5, and 3.0 mM quercetin, respectively. Quercetin changed the glycation sites of BSA, but it could not reduce the number greatly. In addition, quercetin reduced the antioxidant ability and increased the emulsifying properties of BSA, while negligible efficiency was observed on the antioxidant activity and emulsifying activity index of glycated BSA.

Modulating impacts of quercetin/sitagliptin combination on streptozotocin-induced diabetes mellitus in rats

BACKGROUND

Since many diabetic patients require combination therapy, the use of herbal remedies with anti-diabetic activity represents a vital option in diabetes mellitus (DM) management. It has been reported that quercetin has hypoglycemic alongside anti-inflammatory and antioxidant activities.

AIM

The present study aimed to investigate the effectiveness of combining quercetin with sitagliptin; a selective dipeptidyl peptidase-IV (DPP-IV) inhibitor, in the management of streptozotocin (STZ)-induced diabetic rats.

METHODS

DM was induced by a single injection of STZ (45 mg/kg, i.p.) in male adult albino Wistar rats. Diabetic rats were orally treated with sitagliptin (70 mg/kg), quercetin (50 mg/kg) or their combination daily for three consecutive weeks. Serum levels of glucose, C-peptide, total cholesterol, triglycerides, malondialdehyde (MDA), superoxide dismutase, (SOD), reduced glutathione (GSH), tumor necrosis factor alpha, (TNF-α), nuclear factor kappa-B, (NF-κB) and adiponectin were estimated. In addition, histopathological, morphometrical and immunohistochemical examinations of pancreatic tissues were conducted.

RESULTS

The combined administration of quercetin and sitagliptin normalized serum C-peptide, MDA, and significantly increased SOD, GSH and decreased NF-κB more than sitagliptin alone. Moreover, this combination normalized Islet number, β-cells' number, area and perimeter alongside restoring the immunostaining intensity of β-cells.

CONCLUSION

Our results suggest the use of quercetin/sitagliptin combination for treating DM based on the observed improvements in glycemic control, metabolic profile, oxidative and inflammatory status, islet structure as well as β-cells function compared with either treatment alone.

Quercetin, but Not Epicatechin, Decreases Plasma Concentrations of Methylglyoxal in Adults in a Randomized, Double-Blind, Placebo-Controlled, Crossover Trial with Pure Flavonoids

Background

Methylglyoxal (MGO) is the most potent precursor of advanced glycation end products (AGEs). MGO and AGEs have been associated with diabetes, its complications, and other age-related diseases. Experimental studies have shown that the flavonoids quercetin and epicatechin are able to scavenge MGO and lower AGE formation.

Objective

Data on the effects of these flavonoids on MGO and AGE concentrations in humans are not yet available. We therefore investigated the effect of quercetin and epicatechin on the concentrations of MGO and AGEs in a post hoc analysis.

Methods

Thirty-seven apparently healthy, nonsmoking adults with a systolic blood pressure between 125 and 160 mm Hg at screening were included in a randomized, double-blind, placebo-controlled crossover trial. Participants ingested (-)-epicatechin (100 mg/d), quercetin 3-glucoside (160 mg/d), or placebo capsules for periods of 4 wk separated by 4-wk washout periods. Fasting blood samples were collected at the start and end of each intervention period. Liquid chromatography-tandem mass spectrometry was used to determine plasma concentrations of the dicarbonyl compounds MGO, glyoxal (GO), and 3-deoxyglucosone (3-DG) and free and protein-bound AGEs. Gene expression of glyoxalase 1 (GLO1), the enzyme involved in the degradation of MGO, was determined by either microarray or quantitative reverse transcriptase-polymerase chain reaction.

Results

The treatment effect (Δtreatment - Δplacebo) of quercetin on MGO was -40.2 nmol/L (95% CI: -73.6, -6.8 nmol/L; P = 0.019), a decrease of 11% from baseline values, whereas GO, 3-DG, and free and protein-bound AGEs did not change significantly. Epicatechin did not affect the concentrations of dicarbonyls and free and protein-bound AGEs. We did not find a significant change in expression of GLO1.

Conclusions

In apparently healthy (pre)hypertensive men and women, quercetin but not epicatechin decreased plasma MGO concentrations. Quercetin may potentially form a new treatment strategy for diseases in which MGO plays a pivotal role. This study was registered at clinicaltrials.gov as NCT01691404.

Insight into the binding interaction of kaempferol-7-O-α-L-rhamnopyranoside with human serum albumin by multiple fluorescence spectroscopy and molecular modeling

Human serum albumin (HSA) is a transporting protein that has multiple functions. The binding interaction between HSA and small molecules affects its function and efficacy of small molecules. The present study reports that kaempferol-7-O-α-L-rhamnopyranoside (KR) interacts with HSA as indicated by multiple fluorescence spectroscopy and molecular modeling. KR can quench the intrinsic fluorescence of HSA through the formation of a KR-HSA complex in a static manner. In addition, the binding site is located in subdomain IIA as confirmed by competitive experiments using site-specific warfarin and ibuprofen, and the driving forces include hydrogen bonds, van der Waals forces and electrostatic interaction derived from a thermodynamic analysis. The formation of KR-HSA is exothermic and spontaneous. Although there is no hydrophobic interaction around Tyr and Trp residues, the secondary structure of HSA changes through the formation of the KR-HSA complex. In addition, docking results visualized and further supported these results. Finally, these results can provide more information to further investigate the use of KR on the prevention of diabetic complications.

Influence of Quercetin and Its Methylglyoxal Adducts on the Formation of α-Dicarbonyl Compounds in a Lysine/Glucose Model System

Increasing evidence has identified α-dicarbonyl compounds, the reactive intermediates generated during Maillard reaction, as the potential factors to cause protein glycation and the development of chronic diseases. Therefore, there is an urgent need to decrease the levels of reactive dicarbonyl compounds in foods. In this study, we investigated the inhibitory effect of quercetin, a major dietary flavonoid, and its major mono- and di-MGO adducts on the formation of dicarbonyl compounds, such as methylglyoxal (MGO) and glyoxal (GO), in a lysine/glucose aqueous system, a model system to reflect the Maillard reaction in food process. Our result indicated that quercetin could efficiently inhibit the formation of MGO and GO in a time-dependent manner. Further mechanistic study was conducted by monitoring the formation of quercetin oxidation and conjugation products using LC-MS/MS. Quercetin MGO adducts, quercetin quinones, and the quinones of quercetin MGO adducts were detected in the system, indicating quercetin plays a dual role in inhibiting the formation of MGO and GO by scavenging free radicals generated in the system and trapping of MGO and GO to form MGO adducts. In addition, we prepared the mono- and di-MGO quercetin adducts and investigated their antioxidant activity and trapping capacity of MGO and GO. Our results indicated that both mono- and di-MGO quercetin adducts could scavenge the DPPH radical in a dose-dependent manner with >40% DPPH scavenged by the MGO adducts at 10 μM, and the di-MGO quercetin adduct could further trap MGO to generate tri-MGO adducts. Therefore, we demonstrate for the first time that quercetin MGO adducts retain the antioxidant activity and trapping capacity of reactive dicarbonyl species.

Characterizing harmful advanced glycation end-products (AGEs) and ribosylated aggregates of yellow mustard seed phytocystatin: Effects of different monosaccharides

Advanced glycation end products (AGEs) are at the core of variety of diseases ranging from diabetes to renal failure and hence gaining wide consideration. This study was aimed at characterizing the AGEs of phytocystatin isolated from mustard seeds (YMP) when incubated with different monosaccharides (glucose, ribose and mannose) using fluorescence, ultraviolet, circular dichroism (CD) spectroscopy and microscopy. Ribose was found to be the most potent glycating agent as evident by AGEs specific fluorescence and absorbance. YMP exists as a molten globule like structure on day 24 as depicted by high ANS fluorescence and altered intrinsic fluorescence. Glycated YMP as AGEs and ribose induced aggregates were observed at day 28 and 32 respectively. In our study we have also examined the anti-aggregative potential of polyphenol, resveratrol. Our results suggested the anti-aggregative behavior of resveratrol as it prevented the in vitro aggregation of YMP, although further studies are required to decode the mechanism by which resveratrol prevents the aggregation.

Synergistic effect of quercetin and 6-gingerol treatment in streptozotocin induced type 2 diabetic rats and poloxamer P-407 induced hyperlipidemia

Ginger and quercetin have been reported to have significant antidiabetic effects.