Liquid Chromatographic Determination of Glyoxal and Methylglyoxal from Serum of Diabetic Patients using Meso‐Stilbenediamine as Derivatizing Reagent

Theanine Capture of Reactive Carbonyl Species in Humans after Consuming Theanine Capsules or Green Tea

Acrolein (ACR), methylglyoxal (MGO), and glyoxal (GO) are a class of reactive carbonyl species (RCS), which play a crucial role in the pathogenesis of chronic and age-related diseases. Here, we explored a new RCS inhibitor (theanine, THE) and investigated its capture capacity on RCS in vivo by human experiments. After proving that theanine could efficiently capture ACR instead of MGO/GO by forming adducts under simulated physiological conditions, we further detected the ACR/MGO/GO adducts of theanine in the human urine samples after consumption of theanine capsules (200 and 400 mg) or green tea (4 cups, containing 200 mg of theanine) by using ultraperformance liquid chromatography-time-of-flight-high-resolution mass spectrometry. Quantitative assays revealed that THE-ACR, THE-2ACR-1, THE-MGO, and THE-GO were formed in a dose-dependent manner in the theanine capsule groups; the maximum value of the adducts of theanine was also tested. Furthermore, besides the RCS adducts of theanine, the RCS adducts of catechins could also be detected in the drinking tea group. Whereas, metabolite profile analysis showed that theanine could better capture RCS produced in the renal metabolic pathway than catechins. Our findings indicated that theanine could reduce RCS in the body in two ways: as a pure component or contained in tea leaves.

Scavenging Glyoxal and Methylglyoxal by Synephrine and Neohesperidin from Flowers of Citrus aurantium L. var. amara Engl. in Mice and Humans

There is considerable research evidence that α-dicarbonyl compounds, including glyoxal (GO) and methylglyoxal (MGO), are closely related to many chronic diseases. In this work, after comparison of the capture capacity, reaction pathway, and reaction rate of synephrine (SYN) and neohesperidin (NEO) on GO/MGO in vitro, experimental mice were administrated with SYN and NEO alone and in combination. Quantitative data from UHPLC-QQQ-MS/MS revealed that SYN/NEO/HES (hesperetin, the metabolite of NEO) could form the GO/MGO-adducts in mice (except SYN-MGO), and the levels of GO/MGO-adducts in mouse urine and fecal samples were dose-dependent. Moreover, SYN and NEO had a synergistic scavenging effect on GO in vivo by promoting each other to form more GO adducts, while SYN could promote NEO to form more MGO-adducts, although it could not form MGO-adducts. Additionally, human experiments showed that the GO/MGO-adducts of SYN/NEO/HES found in mice were also detected in human urine and fecal samples after drinking flowers of Citrus aurantium L. var. amara Engl. (FCAVA) tea using UHPLC-QTOF-MS/MS. These findings provide a novel strategy to reduce endogenous GO/MGO via the consumption of dietary FCAVA rich in SYN and NEO.

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.

Mitochondria-targeted fluorescent probe for visualization of exogenous and endogenous methylglyoxal in living cells

An activatable mitochondria-targeted fluorescent probe Hcy-OPD was synthesized for the detection of methylglyoxal (MGO). For the introduction of a preorganized isopropylamino group on the aromatic o-diamine framework to regulate the hindrance effect, Hcy-OPD showed high selectivity and sensitivity (0.22 μM) for monitoring MGO. The probe can be applied successfully in the imaging of exogenous and endogenous MGO in living cells.

Activity-based fluorescent probes for sensing and imaging of Reactive Carbonyl species (RCSs)

This review explains various strategies for developing fluorescent probes to detect reactive carbonyl species (RCS). There are sevaral number of mono and diacarbonyls among 30 varieties of reactive carbonyl species (RCSs) so far discovered, which play pivotal roles in pathological processes such as cancer, neurodegenerative diseases, cardiovascular disease, renal failure, and diabetes mellitus. These RCSs play essential roles in maintaining ion channels regulation, cellular signaling pathways, and metabolisms. Among RCSs, Carbon moxide (CO) is also utilized for its cardioprotective, anti-inflammatory, and anti-apoptotic effects. Fluorescence-based non-invasive optical tools have come out as one of the promising methods for analyzing the concentrations and co-localizations of these small metabolites. There has been a tremendous eruption in developing fluorescent probes for selective detection of specific RCSs within cellular and aqueous environments due to its high sensitivity, high spatial and temporal resolution of fluorescence imaging. Fluorescence-based sensing mechanisms such as intramolecular charge transfer (ICT), photoinduced electron transfer (PeT), excited-state intramolecular proton transfer (ESIPT), and fluorescence resonance energy transfer (FRET) are described. In particular, probes for dicarbonyls such as methylglyoxal (MGO), malondialdehyde (MDA), along with monocarbonyls that include formaldehyde (FA), carbon monoxide (CO) and phosgene are discussed. One of the most exciting advances in this review is the summary of fluorescent probes of dicarbonyl compounds.

Simultaneous Determination of Multiple Reactive Carbonyl Species in High Fat Diet-Induced Metabolic Disordered Mice and the Inhibitory Effects of Rosemary on Carbonyl Stress

As potential endogenous biomarkers, reactive carbonyl species (RCS) have gained abundant attention for monitoring oxidative and carbonyl stress. However, there is no accurate method to evaluate multiple RCS in biological samples. In this study, a 2,4-dinitrophenylhydrazine (DNPH) derivatization-based LC-MS method was developed and validated to quantitate eight RCS: malondialdehyde (MDA), acrolein (ACR), 4-hydroxy-2-nonenal (4-HNE), 4-oxo-2-nonenal (4-ONE), methylglyoxal (MGO), glyoxal (GO), 3-deoxyglucosone (3-DG), and 2-keto-d-glucose (2-Keto). Subsequently, the method was applied to assess the RCS in low fat (LF), high fat (HF), and HF plus rosemary extract (RE) diet-fed mouse samples. The quantitative results on RCS levels indicated that the HF diet significantly increased the total RCS levels in mouse urine, plasma, and kidney with an average rate of 280.69%, 153.87%, and 61.30%, respectively. The RE administration significantly inhibited the elevated RCS levels induced by the HF diet, especially for MDA, 4-ONE, 4-HNE, and 2-Keto in mouse plasma, and ACR and 2-Keto in mouse kidney. This is the first study to simultaneously measure eight RCS in biological samples and demonstrate that RE was able to eliminate the accumulation of the HF diet-induced RCS.

Diagnosis of methylglyoxal in blood by using far-infrared spectroscopy and o-phenylenediamine derivation

Methyglyoxal (MGO) is an important pathological factor for diabetic cardiovascular complications. Conventional methods for MGO detection in biological samples, such as high performance liquid chromatography (HPLC)-UV spectrometry, LC-fluorescence spectrometry, and HPLC-mass spectrometry, are time-consuming, high-cost, and complicated. Here, we present a method for MGO quantitative detection based on far-IR spectral analyses. Our method uses o-phenylenediamine (OPD) to produce a chemical reaction with MGO, which results in multiple fingerprint feature changes associated with the molar ratio of MGO and OPD. We use the linear relationship between MGO concentration and peak intensity of the reaction product to quantitatively determine MGO concentration. The corresponding linear detectable range is 5∼2500 nmol/mL nmol per mL with a correlation coefficient of 0.999. This quantitative method is also tested by blood samples with adjusted MGO concentrations, and shows 95% accuracy with only 30s testing time. Our method provides a fast, simple and economical approach to determining MGO concentration in blood.

Measurement of α-dicarbonyl compounds in human saliva by pre-column derivatization HPLC

Background α-Dicarbonyl compounds (α-DCs) have been detected in body fluids including plasma and urine and elevation of this sort of compounds in vivo has been associated with the development of many kinds of chronic diseases. However whether α-DCs are present in human saliva, and if their presence/absence can be related with various chronic diseases is yet to be determined. Methods In this study, a pre-column derivatization HPLC-UV method was developed to measure 3-deoxyglucosone (3-DG), glyoxal (GO), methylglyoxal (MGO), diacetyl (DA), and pentane-2,3-dione (PD) in human saliva employing 4-(2,3-dimethyl-6-quinoxalinyl)-1,2-benzenediamine (DQB) as a derivatizing reagent. The derivatization of the α-DCs is fast and the conditions are facile. The method was evaluated and the results show that it is suitable for the quantification of α-DCs in human saliva. Results In the measurements of these α-DCs in the saliva of 15 healthy subjects and 23 type 2 diabetes mellitus (T2DM) patients, we found that the concentrations of GO and MGO in the saliva of the diabetic patients were significantly higher than those in healthy subjects. As far as we know, this is the first time that salivary α-DC concentrations have been determined and associated with T2DM. Conclusions The developed method would be useful for the measurement of the salivary α-DC levels and the data acquired could be informative in the early screening for diabetes.

Dietary Genistein Inhibits Methylglyoxal-Induced Advanced Glycation End Product Formation in Mice Fed a High-Fat Diet

BACKGROUND

Methylglyoxal (MGO), an important precursor of advanced glycation end products (AGEs), circulates at high concentrations in diabetic patients' blood and plays an important role in the pathogenesis of diabetes and other chronic diseases.

OBJECTIVES

The aim of this study was to determine whether dietary genistein can prevent indicators of metabolic syndrome (MetS) induced by a very-high-fat (VHF) diet or a high-fat (HF) diet plus exogenous MGO, and the accumulation of MGO and AGEs in mice.

METHODS

Male, 6-wk-old C57BL/6J mice (n = 15) were fed a low-fat (LF) diet (10% fat energy) or a VHF diet (60% fat energy) alone or including 0.25% genistein (VHF-G) for 16 wk in study 1. In study 2, 75 similar mice were fed the LF diet (LF) or the HF diet alone (HF) or in combination with up to 0.2% MGO in water (HFM) and 0.067% (HFM-GL) or 0.2% (HFM-GH) dietary genistein for 18 wk. Anthropometric and metabolic data were obtained in both studies to determine the effects of MGO and genistein on variables indicative of MetS.

RESULTS

Body weight gain, fat deposits, dyslipidemia, hyperglycemia, and fatty liver were ameliorated by dietary genistein in both studies. The plasma MGO concentration in VHF-G mice was 52% lower than that in VHF mice. Moreover, the AGE concentrations in plasma, liver, and kidney of VHF-G mice were 73%, 52%, and 49%, respectively, lower than in the VHF group (study 1). Similarly, the concentrations of plasma MGO and AGE in plasma, liver, and kidney of HFM-GH mice were 33.5%, 49%, 69%, and 54% lower than in HFM mice (study 2). Genistein inhibited AGE formation by trapping MGO to form adducts and upregulating the expressions of glyoxalase I and II and aldose reductase in liver and kidney to detoxify MGO in both studies.

CONCLUSIONS

Our data demonstrate for the first time that genistein significantly lowers MGO and AGE concentrations in 2 mouse MetS models via multiple pathways.

Determination of glyoxal and methylglyoxal in serum by UHPLC coupled with fluorescence detection

Glyoxal (GO) and methylglyoxal (MGO) are two important biomarkers in diabetes. Analytical methods for determination of GO and MGO in serum samples are either HPLC with UV-Vis (low sensitivity) or MS/MS (expensive) detection. These disadvantages have hampered the introduction of these biomarkers as a routine analyte for diabetes diagnostics into the clinical laboratory. In this study, we introduce a UHPLC method with fluorescence detection for the measurement of GO and MGO in serum samples by pre-column derivatization at neutral pH with 5, 6-diamino-2,4-dihydroxypyrimidine sulfate (DDP) to form lumazines. The method was validated as per FDA guidelines. Using this method, we have determined GO and MGO in a variety of animal serum samples, and for example, determined the GO and MGO concentration in adult bovine serum to be 852 ± 27 and 192 ± 10 nmol/L, respectively. In human serum, GO and MGO levels in non-diabetic subjects (n = 14) were determined to be 154 ± 88 and 98 ± 27 nmol/L, and in serum samples from subjects with diabetes (n = 14) 244 ± 137 and 190 ± 68 nmol/L, respectively. In addition, interference studies showed that physiological serum components did not lead to an artificial increase in the levels of GO and MGO.

Elevated levels of α-dicarbonyl compounds in the plasma of type II diabetics and their relevance with diabetic nephropathy

The presence of α‑dicarbonyl compounds (α-DCs) in vivo has been associated with the development of complications of diabetes mellitus (DM) and also with other chronic diseases. Therefore, quantitative analysis of α-DCs in body fluids is crucial to understand their roles in the formation of these chronic diseases. We established in this study a practical HPLC-UV method to measure 3‑deoxyglucosone (3-DG), glyoxal (GO), methylglyoxal (MGO), diacetyl (DA), and pentane‑2,3‑dione (PD) in blood plasma using 4‑(2,3‑dimethyl‑6‑quinoxalinyl)‑1,2‑benzenediamine (DQB) as a derivatizing reagent. The derivatizing reaction could be carried out quickly under mild conditions and the HPLC determination is simple, sensitive, and easy to operate. The recoveries of the α-DCs are between 85.26% and 110.20% (intra-day) and 87.25% and 103.18% (inter-day); the RSDs are between 1.28% and 5.69% (intra-day) and 2.26% and 6.34% (inter-day). We found the plasma levels of 3-DG, GO, and MGO in the diabetic patients are all significantly higher than those in healthy subjects. The results also show that the contents of GO and MGO in diabetic nephropathy (DN) patients are both significantly higher than those in simple T2DM patients. Moreover, it is found for the first time that the plasma level of GO might be a potential predictor of DN. The developed method would be useful for the measurements of the plasma α-DCs and the data acquired could be informative in the diagnosis of DM complications.

Vanillin Affects Amyloid Aggregation and Non-Enzymatic Glycation in Human Insulin

Curcumin is known for its anti-inflammatory, antioxidant and anticancer activity, as well as for its ability to interfere with amyloid aggregation and non-enzymatic glycation reaction, that makes it an attractive potential drug. However, curcumin therapeutic use is limited because of its low systemic bioavailability and chemical stability as it undergoes rapid hydrolysis in physiological conditions. Recently, much attention has been paid to the biological properties of curcumin degradation products as potential bioactive molecules. Between them, vanillin, a natural vanilla extract, is a stable degradation product of curcumin that could be responsible for mediating its beneficial effects. We have analyzed the effect of vanillin, in comparison with curcumin, in the amyloid aggregation process of insulin as well as its ability to prevent the formation of the advanced glycation end products (AGEs). Employing biophysical, biochemical and cell based assays, we show that vanillin and curcumin similarly affect insulin amyloid aggregation promoting the formation of harmless fibrils. Moreover, vanillin restrains AGE formation and protects from AGE-induced cytotoxicity. Our novel findings not only suggest that the main health benefits observed for curcumin can be ascribed to its degradation product vanillin, but also open new avenues for developing therapeutic applications of curcumin degradation products.

Specific bioactive compounds in ginger and apple alleviate hyperglycemia in mice with high fat diet-induced obesity via Nrf2 mediated pathway

Prolonged hyperglycemia activates the formation of advanced glycation end-products (AGEs). Major dicarbonyl compounds such as methylglyoxal or glyoxal are found to be the main precursors of AGEs and N(ε)-(carboxymethyl)lysine (CML) found to be predominantly higher in the diabetic population. We hypothesized that phloretin from apple and [6]-gingerol from ginger inhibit formation of AGEs and suppress the receptor for advanced glycation end products (RAGE) via nuclear factor erythroid-2-related-factor-2 (Nrf2)-dependent pathway. Phloretin and [6]-gingerol were supplemented at two different doses to C57BL/6 mice on high fat diet or standard diet for a period of 17weeks. Phloretin or [6]-gingerol supplementation significantly reduced plasma glucose, alanine aminotransferase, aspartate aminotransferase, AGEs and insulin levels. Phloretin and [6]-gingerol also decreased the levels of AGEs and CML levels, via Nrf2 pathway, enhancing GSH/GSSG ratio, heme oxygenase-1 and glyoxalase 1 in liver tissue. These results suggest that phloretin and [6]-gingerol are potential dietary compounds that can alleviate diabetes-induced complications.

Trapping Methylglyoxal by Genistein and Its Metabolites in Mice

Increasing evidence supports dicarbonyl stress such as methylglyoxal (MGO) as one of the major pathogenic links between hyperglycemia and diabetic complications. In vitro studies have shown that dietary flavonoids can inhibit the formation of advanced glycation end products (AGEs) by trapping MGO. However, whether flavonoids can trap MGO in vivo and whether biotransformation limits the trapping capacity of flavonoids remain virtually unknown. In this study, we investigated whether genistein (GEN), the major soy isoflavone, could trap MGO in mice by promoting the formation of MGO adducts of GEN and its metabolites. Two different mouse studies were conducted. In the acute study, a single dose of MGO and GEN were administered to mice via oral gavage. In the chronic study, MGO was given to mice in drinking water for 1 month and then GEN was given to mice for 4 consecutive days via oral gavage. Two mono-MGO adducts of GEN and six mono-MGO adducts of GEN phase I and microbial metabolites were identified in mouse urine samples from these studies using liquid chromatography/electrospray ionization tandem mass spectrometry. The structures of these MGO adducts were confirmed by analyzing their MS(n) (n = 1-4) spectra as well as by comparing them with the tandem mass spectra of authentic standards. All of the MGO adducts presented in their phase II conjugated forms in mouse urine samples in the acute and chronic studies. To our knowledge, this is the first in vivo evidence to demonstrate the trapping efficacy of GEN in mice and to show that the metabolites of GEN remain bioactive.

Advanced Glycation End Products (AGE) Potently Induce Autophagy through Activation of RAF Protein Kinase and Nuclear Factor κB (NF-κB)

Advanced glycation end products (AGE) accumulate in diabetic patients and aging people because of high amounts of three- or four-carbon sugars derived from glucose, thereby causing multiple consequences, including inflammation, apoptosis, obesity, and age-related disorders. It is important to understand the mechanism of AGE-mediated signaling leading to the activation of autophagy (self-eating) that might result in obesity. We detected AGE as one of the potent inducers of autophagy compared with doxorubicin and TNF. AGE-mediated autophagy is inhibited by suppression of PI3K and potentiated by the autophagosome maturation blocker bafilomycin. It increases autophagy in different cell types, and that correlates with the expression of its receptor, receptor for AGE. LC3B, the marker for autophagosomes, is shown to increase upon AGE stimulation. AGE-mediated autophagy is partially suppressed by inhibitor of NF-κB, PKC, or ERK alone and significantly in combination. AGE increases sterol regulatory element binding protein activity, which leads to an increase in lipogenesis. Although AGE-mediated lipogenesis is affected by autophagy inhibitors, AGE-mediated autophagy is not influenced by lipogenesis inhibitors, suggesting that the turnover of lipid droplets overcomes the autophagic clearance. For the first time, we provide data showing that AGE induces several cell signaling cascades, like NF-κB, PKC, ERK, and MAPK, that are involved in autophagy and simultaneously help with the accumulation of lipid droplets that are not cleared effectively by autophagy, therefore causing obesity.

Bioactive ginger constituents alleviate protein glycation by trapping methylglyoxal

Considerable evidence suggests that long-term pathological diabetes is a result of the accumulation of tissue macromolecules that have been progressively modified by nonenzymatic glycation of protein. Methylglyoxal (MGO) is a highly reactive endogenous dicarbonyl metabolite derived from multiple sources such as glucose and lipids and is thought to contribute greatly to protein glycation and the formation of advanced glycation end products (AGEs). In this study, we demonstrated for the first time that both [6]-shogaol (6S) and [6]-gingerol (6G), the major active components in ginger, markedly trapped MGO in vitro and consequently formed mono-MGO adducts, 6S-MGO and 6G-MGO, which were purified from the respective chemical reaction and characterized as novel compounds by NMR experiments and LC-MS/MS approaches. We revealed that the α-carbon of the carbonyl group in the side chain of 6S or 6G is the major active site for trapping MGO. We also demonstrated that 6S and 6G could effectively inhibit the formation of MGO-induced AGEs via trapping MGO in a time-dependent manner in the human serum albumin (HSA)-MGO system. Mono-MGO adducts, 6S-MGO and 6G-MGO, were determined to be the major conjugates in 6S- and 6G-treated HSA-MGO assays, respectively, using LC-ESI-MS techniques. These findings showed the potential effects of 6S and 6G on the prevention of protein glycation, suggesting regular consumption of ginger root extract may attenuate the progression of MGO-associated diabetic complications in patients.

Resveratrol protects against methylglyoxal-induced hyperglycemia and pancreatic damage in vivo

Methylglyoxal (MG) has been found to cause inflammation and insulin resistance in vitro and in vivo in recent studies. Resveratrol has been proposed as an effective treatment that helps lower the risk of developing complications of diabetes. To study the significance of glycosylation-related stress on the pathology of diabetes, the effects of resveratrol were examined in a mouse model of diabetes induced by MG. Resveratrol was given via oral gavage in MG-treated mice, and diabetes-related tests and markers were assessed using biochemical and immunohistochemical analyses. Treatment with resveratrol markedly improved blood glucose level from the oral glucose tolerance test and promoted nuclear factor erythroid 2-related factor-2 (Nrf2) phosphorylation (p < 0.05) in the pancreas of MG-treated mice. However, these effects were abolished by retinoic acid, Nrf2 inhibitor, in resveratrol and retinoic acid-treated and MG-induced mice. These findings support that resveratrol may be useful in the treatment of type-2 diabetes by protecting against pancreatic cell dysfunction.

Actinidia callosa peel (kiwi fruit) ethanol extracts protected neural cells apoptosis induced by methylglyoxal through Nrf2 activation

CONTEXT

Methylglyoxal (MG) is a reactive dicarbonyl compound generated as an intermediate of glycolysis during the physical glycation in the diabetic condition. MG itself has been commonly implicated in the development of diabetic neuropathy. Several active compounds in Actinidia callosa have been found to inhibit glycation and MG-protein reaction.

OBJECTIVE

This study investigated the protective effects of A. callosa (kiwi fruits) peel ethanol extracts (ACE) on MG-induced Neuro-2A cell apoptosis.

MATERIALS AND METHODS

The Neuro-2A cells pre-treated by ACE (50-200 μg/mL) or allyl-isothiocyanate (AITC) (50 μM) for 6 h, in turn, the cells were treated with MG (250 μM) for 24 h.

RESULTS

ACE or AITC treatment markedly inhibited the generation of reactive oxygen species (ROS) and the elevation of caspase-3 and capase-9 levels induced by MG in Neuro-2A cells. ACE and AITC elevated Bcl2 and inhibited Bax expressions in MG-induced Neuro-2A cells. ACE elevated Nrf2 transcriptional activity and nuclear translocation in MG-induced Neuro-2A cells. Nrf2 down-stream molecules including HO-1 and GCL were elevated by ACE or AITC treatment in MG-induced Neuro-2A cells. The protective effects of ACE on MG-induced Neuro-2A apoptosis were attenuated while Nrf2 knockdown.

DISCUSSION AND CONCLUSION

We established the first evidence that ACE might contribute to the prevention of the development of diabetic neuropathy by blocking the MG-mediated intracellular glycation system.

Dietary phenolics as reactive carbonyl scavengers: potential impact on human health and mechanism of action

Previous studies have demonstrated that accumulation of reactive carbonyl compounds in human tissue will accelerate the vascular damage in both diabetes and uremia. Moreover, advanced glycation progressively and irreversibly modify the proteins over time and yield advanced glycation end products (AGEs). AGEs are thought to contribute to the development of diabetes mellitus and its complications. Therefore, we propose a novel approach to decrease the levels of dicarbonyl compounds by direct trapping of dietary polyphenolic compounds, and consequently, inhibit the formation of AGEs and prevent the development of diabetic complications and age-related diseases.

Monascin and AITC attenuate methylglyoxal-induced PPARγ phosphorylation and degradation through inhibition of the oxidative stress/PKC pathway depending on Nrf2 activation

Abnormal cellular accumulation of the dicarbonyl metabolite methylglyoxal (MG) results in cell damage, inflammation, and oxidative stress. It is also associated with increased protein linkage to form advanced glycation end products (AGEs) or induce DNA strand breaks. The association between peroxisome proliferator-activated receptor-γ (PPARγ) and nuclear factor-erythroid 2-related factor 2 (Nrf2) is unclear. This study investigated Nrf2 activator protection against PPARγ phosphorylation and degradation to maintain pancreatic function. MG was used at a noncytotoxic concentration (200 μM) to induce protein kinase C (PKC) and PPARγ phosphorylation in pancreatic RINm5F cells. For in vivo studies, MG (60 mg/kg bw) was intraperitoneally (IP) injected into Balb/C mice for 28 d to induce pancreas damage, at which point we investigated the effect of monascin protection (PPARγ and Nrf2 activator), rosiglitazone (PPARγ activator), allyl isothiocyanate (AITC; Nrf2 activator), or N-acetylcysteine (NAC) on pancreatic function. The in vitro and in vivo results indicated that MG leads to marked PPARγ phosphorylation (serine 82); this effect led to reduction in pancreatic and duodenal homeobox-1 (PDX-1), glucokinase (GCK), and insulin expression. However, monascin and rosiglitazone may protect PPARγ degradation by elevating PDX-1, GCK, and as a result, insulin expression. Monascin and AITC can attenuate PKC activation to suppress PPARγ phosphorylation caused by oxidative stress through the Nrf2 pathway. Similarly, the N-acetylcysteine (NAC) antioxidant also improved oxidative stress and pancreatic function. This study examined whether MG caused impairment of PDX-1, GCK, and insulin through PPARγ phosphorylation and degradation. MG and AGE accumulation improved on Nrf2 activation, thereby protecting against pancreas damage. Taken together, PPARγ activation maintained pancreatic PDX-1, GCK, and insulin expression levels to regulate blood glucose levels.

Effects of monascin on anti-inflammation mediated by Nrf2 activation in advanced glycation end product-treated THP-1 monocytes and methylglyoxal-treated wistar rats

Hyperglycemia is associated with advanced glycation end products (AGEs). This study was designed to evaluate the inhibitory effects of monascin on receptor for advanced glycation end product (RAGE) signal and THP-1 monocyte inflammation after treatment with S100b, a specific ligand of RAGE. Monascin inhibited cytokine production by S100b-treated THP-1 monocytes via up-regulation of nuclear factor-erythroid 2-related factor-2 (Nrf2) and alleviated p47phox translocation to the membrane. Methylglyoxal (MG, 600 mg/kg bw) was used to induce diabetes in Wistar rats. Inhibitions of RAGE and p47phox by monascin were confirmed by peripheral blood mononuclear cells (PBMCs) of MG-induced rats. Silymarin (SM) was used as a positive control group. It was found that monascin promoted heme oxygenase-1 (HO-1) expression mediated by Nrf2. Suppressions of AGEs, tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-β) in serum of MG-induced rats were attenuated in the monascin administration group treated with retinoic acid (RA). RA treatment resulted in Nrf2 inactivation by increasing RA receptor-α (RARα) activity, suggesting that RA acts as an inhibitor of Nrf2. The results showed that monascin exerted anti-inflammatory and antioxidative effects mediated by Nrf2 to prevent the development of diseases such as type 2 diabetes caused by inflammation.

Beta-D-glucoside protects against advanced glycation end products (AGEs)-mediated diabetic responses by suppressing ERK and inducing PPAR gamma DNA binding

Accumulation of advanced glycation end products (AGEs), due to excessive amounts of 3- or 4-carbon sugars derived from glucose; cause multiple consequences in diabetic patients and older persons. The transcription factor, peroxisome proliferator-activated receptor gamma (PPARγ), is down regulated in the diabetic condition. Drugs targeting PPARγ were developed for diabetes therapy. We found that AGE inhibited PPARγ activity in different cell types induced by PPARγ activators, like troglitazone, rosiglitazone, oleamide, and anandamide. AGE induced translocation of PPARγ from nucleus to cytoplasm, increased on activation of ERK in cells. Antioxidants that inhibit AGE-induced NF-κB activation by preventing ROI generation were unable to protect AGE-mediated decrease in PPARγ activity. Only mangiferin, a β-D-glucoside, prevented AGE-mediated decrease in PPARγ activity and inhibited phosphorylation of ERK and cytoplasmic translocation of PPARγ. Mangiferin interacts with PPARγ and enhanced its DNA binding activity as predicted by in silico and shown by in vitro DNA-binding activity. Overall, the data suggest that (i) mangiferin inhibited AGE-induced ERK activation thereby inhibited PPARγ phosphorylation and cytoplasmic translocation; (ii) mangiferin interacts with PPARγ and enhances its DNA-binding ability. With these dual effects, mangiferin can be a likely candidate for developing therapeutic drug against diabetes.

Trapping of methylglyoxal by curcumin in cell-free systems and in human umbilical vein endothelial cells

Curcumin, the most active compound of curcuminoids, has been shown to inhibit formation of advanced glycation end products (AGEs) in streptozotocin-induced diabetic rats. However, little is known on whether curcumin may trap methylglyoxal (MGO), a major reactive dicarbonyl compound, to inhibit AGE formation. We found that one molecule of curcumin effectively trapped one molecule of MGO at a 1:3 ratio at 24 h of incubation under physiological conditions (pH 7.4, 37 °C). Curcumin decreased N(ε)-(carboxymethyl)lysine (CML) expression in human umbilical vein endothelial cells. We further used two curcumin analogues, dimethoxycurcumin (DIMC) and ferulic acid, to investigate the possible MGO-trapping mechanism of curcumin. Results reveal that DIMC, but not ferulic acid, exhibited MGO-trapping capacity, indicating curcumin traps MGO at the electron-dense carbon atom (C10) between the two keto carbon groups. Thus, curcumin may prevent MGO-induced endothelial dysfunction by directly trapping MGO.

Advanced glycation end products (AGEs) induce apoptosis via a novel pathway: involvement of Ca2+ mediated by interleukin-8 protein

Advanced glycation end products (AGEs) accumulate in diabetic patients due to high blood glucose levels and cause multiple deleterious effects. In this study, we provide evidence that the AGE increased cell death, one such deleterious effect. Methyl glyoxal-coupled human serum albumin (AGE-HSA) induced transcription factors such as NF-κB, NF-AT, and AP-1. AGE acts through its cell surface receptor, RAGE, and degranulates vesicular contents including interleukin-8 (IL-8). The number of RAGEs, as well as the amount of NF-κB activation, is low, but the cell death is higher in neuronal cells upon AGE treatment. Degranulated IL-8 acts through its receptors, IL-8Rs, and induces sequential events in cells: increase in intracellular Ca(2+), activation of calcineurin, dephosphorylation of cytoplasmic NF-AT, nuclear translocation of NF-AT, and expression of FasL. Expressed FasL increases activity of caspases and induces cell death. Although AGE increases the amount of reactive oxygen intermediate, accompanying cell death is not dependent upon reactive oxygen intermediate. AGE induces autophagy, which partially protects cells from cell death. A novel mechanism of AGE-mediated cell death in different cell types, especially in neuronal cells where it is an early event, is provided here. Thus, this study may be important in several age-related neuronal diseases where AGE-induced apoptosis is observed because of high amounts of AGE.

Methylglyoxal: its presence in beverages and potential scavengers

Nonenzymic glycation, also known as the Maillard reaction, is a complex series of reactions between reducing sugars and amino compounds. Previous studies have demonstrated that reactive dicarbonyl compounds (e.g., methylglyoxal [MG] and glyoxal [GO]), formed as intermediate products of the Maillard reaction, irreversibly and progressively modify proteins over time and yield advanced glycation end products (AGEs), which are thought to contribute to the development of diabetes mellitus and its complications. Several studies have shown that higher levels of MG are present in diabetic patients' plasma than in healthy people's plasma. Thus, decreasing the levels of MG and GO will be an effective approach to reduce the formation of AGEs and the development of diabetic complications. Here, we briefly describe our effort in searching for non- or less-toxic trapping agents of reactive dicarbonyl species from dietary sources. In addition, we have discovered that commercial beverages contain extremely high levels of MG. The potential hazardous effects of dietary MG on humans remain to be explored.

Capillary gas chromatographic determination of methylglyoxal from serum of diabetic patients by precolumn derivatization with 1,2-diamonopropane

Capillary gas chromatographic (GC) determination of methylglyoxal (MGo) was developed on the basis of precolumn derivatization with 1,2-diaminopropane (DAP) at pH 3. The elution was carried out on an HP-5 (30 m x 0.32 mm i.d.) connected with FID. The linear calibration curve was obtained for MGo within 0.09-1.04 microg/ml with detection limit of 40 ng/ml. Dimethylglyoxal (DMGo) also formed derivative with DAP and eluted and separated from MGo at column temperature 100 degrees C for 1 min with heating rate 30 degrees C/min up to 200 degrees C with run time 4.6 min. The nitrogen flow rate was 1.5 ml/min with split ratio of 10:1, v/v. MGo was determined from serum and urine of diabetics and healthy volunteers. The amounts of MGo from serum and urine of diabetic patients were 0.180-0.260 microg/ml and 0.170-0.250 microg/ml with relative standard deviation (R.S.D.) within 1-4% and 1-3%, respectively. The amounts of MGo from serum of healthy volunteers were 0.032-0.054 microg/ml with an R.S.D. of 1.5-3%. DMGo was not detected from the biological fluids and was used as an internal standard.

Determination of glyoxal and methylglyoxal in the serum of diabetic patients by MEKC using stilbenediamine as derivatizing reagent

An analytical method has been developed for the separation of glyoxal (Go), methylglyoxal (MGo), and dimethylglyoxal (DMGo) by MEKC using stilbenediamine (SD) as derivatizing reagent, separation time 6.5 min, SDS as micellar medium at pH 8, and sodium tetraborate (0.1 M) as buffer. Uncoated fused-silica capillary, effective length 50 cm x 75 microm id; applied voltage 20 kV and photodiode array detection, were used. Calibration was linear within 0.02-150 microg/mL with detection limits 3.5-5.8 ng/mL. Go and MGo, observed for diabetic and healthy volunteers, were within 0.098-0.193 microg/mL Go and 0.106-0.245 microg/mL MGo with RSD 1.6-3.5 and 1.7-3.4%, respectively, in diabetics against 0.016-0.046 microg/mL Go and 0.021-0.066 microg/mL MGo with RSDs 1.5-3.5 and 1.4-3.6%, respectively, in healthy volunteers. Go and MGo in diabetics were also measured by standard addition and DMGo as an internal standard. Additives do not contribute significantly to Go and MGo matrix.

High-performance liquid chromatographic determination of glyoxal and methylglyoxal in urine by prederivatization to lumazinic rings using in serial fast scan fluorimetric and diode array detectors

Glyoxal and methylglyoxal are two important markers of oxidative stress and both are involved in the evaluation of several diseases. A new HPLC method for determining glyoxal and methylglyoxal in urine was developed. The method is based on the reaction of alpha-dialdehydes, glyoxal and methylglyoxal, with 5,6-diamino-2,4-hydroxypyrimidine sulfate in basic medium to form highly fluorescent lumazine derivatives. Creatinine was also included in the method even though it does not react with the reagent. The derivatives and creatinine are separated on a C(18) reversed-phase column with a mobile phase consisting of acetonitrile:citrate buffer, pH 6.0 (3:97 v/v). The flow rate was 1.0mLmin(-1) and the effluent was monitored photometrically at 250 nm for determination of creatinine and fluorimetrically at 500 nm (exciting at 330 nm) for determination of glyoxal and methylglyoxal derivatives. Recording time of the separation is less than 10 min. Determination of the analytes is performed in urine after incubation of the sample, with the reagent in alkaline medium, for 30 min at 60 degrees C. Urinary levels of glyoxal and methylglyoxal, expressed as glyoxal/creatinine and methylglyoxal/creatinine ratios, in healthy young women and men were determined. For women, values of 0.80+/-0.37 and 0.60+/-0.22 microg/mg of creatinine were found for glyoxal and methylglyoxal, respectively. For men, values of 0.63+/-0.15 and 0.49+/-0.05 microg/mg of creatinine were found for glyoxal and methylglyoxal, respectively. These results were also related to the body mass index of each individual.