The role of the thiol group in protein modification with methylglyoxal

Methylglyoxal-induces multiple stable changes in human serum albumin before forming nephrotoxic advanced glycation end-products: Injury demonstration in human embryonic kidney cells

The minor fraction of methylglyoxal that is not metabolized in healthy humans reacts with macromolecules to form AGEs. In diabetics, the formation of MG is accelerated; its level may be enhanced multifold. The glyoxalase enzymes responsible for the regular and effective clearance of excess methylglyoxal may become defective in diabetes mellitus leading to its retention in cells and plasma. The methylglyoxal-modified-HSA was prepared, characterised by multiple biophysical techniques and biochemical (s) and its damaging effect was examined on embryonic kidney cell line HEK 293. The UV results showed hyperchromicity in MG-modified-HSA while nitroblue tetrazolium and fluorescence data suggested AGEs formation in comparison to control HSA. Upward shift of negative peaks in CD suggested reduction in α-helicity. Accelerated mobility and diffused broad bands observed in native and SDS polyacrylamide gel, respectively suggest neutralization of some of the positive charges on MG-modified-HSA as well as generation of cross-links. As observed by trypan blue assay, MTT, LDH activity assay, acridine orange, propidium iodide, ethidium bromide, 4',6-diamidino-2-phenylindole (DAPI) staining and ROS measurements, the MG-HSA AGEs caused damage to human embryonic kidney cells. The data suggest that MG-HSA AGEs may trigger powerful inflammatory responses at cellular level which might set the stage for nephrotoxicity in diabetics.

Bioflavonoid (Hesperidin) Restrains Protein Oxidation and Advanced Glycation End Product Formation by Targeting AGEs and Glycolytic Enzymes

Alpha-amylase (α-amylase) not long ago has acquire recognition as a possible drug target for the management of diabetes. Here, we have investigated the binding and enzyme activity of α-amylase by hesperidin; a naturally occurring flavanone having wide therapeutic potential. Hesperidin exerted an inhibitory influence on α-amylase activity with an IC50 value of 16.6 µM. Hesperidin shows a significant binding toward α-amylase with a binding constant (Ka) of the order of 104 M-1. The evaluation of thermodynamic parameters (∆H and ∆S) suggested that van der Waals force and hydrogen bonding drive seemingly specific hesperidin-α-amylase complex formation. Glycation and oxidation studies were performed using human serum albumin (HSA) as ideal protein. Hesperidin inhibited fructosamine content ≈40% at 50 µM and inhibited advanced glycation end products (AGEs) formation by 71.2% at the same concentration. Moreover, significant recovery was evident in free -SH groups and carbonyl content of HSA. Additionally, molecular docking also entrenched in vitro observations and provided an insight into the important residues (Trp58, Gln63, His101, Glu233, Asp300, and His305) at the heart of hesperidin-α-amylase interaction. This study delineates mechanistic insight of hesperidin-α-amylase interaction and provides a platform for use of hesperidin to treat AGEs directed diseases.

Genotoxicity of advanced glycation end products in vitro is influenced by their preparation temperature, purification, and cell exposure time

Advanced glycation end products (AGEs) are formed via non-enzymatic reactions between amino groups of proteins and the carbonyl groups of reducing sugars. Previous studies have shown that highly glycated albumin prepared using a glucose-bovine serum albumin (Glu-BSA) model system incubated at 60°C for 6 weeks induces genotoxicity in WIL2-NS cells at 9 days of exposure measured by the cytokinesis-block micronucleus cytome (CBMNcyt) assay. However, this AGE model system is not physiologically relevant as normal body temperature is 37°C and the degree of glycation may exceed the extent of albumin modification in vivo. We hypothesised that the incubation temperature and purification method used in these studies may cause changes to the chemical profile of the glycated albumin and may influence the extent of genotoxicity observed at 3, 6 and 9 days of exposure. We prepared AGEs generated using Glu-BSA model systems incubated at 60°C or 37°C purified using trichloroacetic acid (TCA) precipitation or ultrafiltration (UF) and compared their chemical profile (glycation, oxidation, and aggregation) and genotoxicity in WIL2-NS cells using the CBMNcyt assay after 3, 6, and 9 days of exposure. The number of micronuclei (MNi) was significantly higher for cells treated with Glu-BSA incubated at 60°C and purified via TCA (12 ± 1 MNi/1000 binucleated cells) compared to Glu-BSA incubated at 37°C and purified using UF (6 ± 1 MNi/1000 binucleated cells) after 9 days (p < 0.0001). The increase in genotoxicity observed could be explained by a higher level of protein glycation, oxidation, and aggregation of the Glu-BSA model system incubated at 60°C relative to 37°C. This study highlighted that the incubation temperature, purification method and cell exposure time are important variables to consider when generating AGEs in vitro and will enable future studies to better reflect in vivo situations of albumin glycation.

Determination of free sulfhydryl contents for proteins including monoclonal antibodies by use of SoloVPE

Free sulfhydryls are important properties of protein products including monoclonal antibodies (mAbs). Here, a new technology, variable pathlength extension (SoloVPE), is employed to quantify the amount of free sulfhydryl in monoclonal antibodies (mAbs) using the well-known Ellman reagent. Briefly, the unbound thiols (free sulfhydryls) of proteins including mAbs react with Ellman reagent to produce a 2-nitro-5-thiobenzoate (TNB^2-) which is detected at visible wavelength of 412 nm and quantified. The method does not require dilution of antibody samples, is simple, reproducible and takes less than one hour to complete. Values obtained by the new method are compared to literature values from traditional UV or fluorescence methods with agreements. Qualification and trending data over two years of method utilization in our labs support that assay variability is minimal with an intermediate precision of relative standard deviation (RSD) ≤ 10 % and a limit of quantification (LOQ) of 0.1 mol/mol, which is sufficient to measure free sulfhydryl content in proteins including mAbs.

Ginsenoside derivatives inhibit advanced glycation end-product formation and glucose-fructose mediated protein glycation in vitro via a specific structure-activity relationship

Ginseng (Panax ginseng and red ginseng) extract has been reported to inhibit the formation of advanced glycation end-products (AGEs); however, the potential inhibitory activity of its major constituents (ginsenosides) against AGE formation is still unknown. In the present study, we investigated the inhibitory effect of ginsenoside derivatives on AGE formation. Herein, we assessed the activity of 22 ginsenosides, most of which significantly inhibited fluorescent AGE formation. Notably, ginsenoside Rh2, ginsenoside Rh1, and compound K exhibited the most potent AGE inhibitory potential with IC₅₀ values of 3.38, 8.42, and 10.85 µM, respectively. The structure- activity relationship revealed that the presence of sugar moieties, hydroxyl groups, and their linkages, and the stereostructure of the ginsenoside skeleton played an important role in the inhibition of AGE formation. Furthermore, the inhibitory activity of the most active ginsenoside Rh2 on fructose-glucose-mediated protein glycation and oxidation of bovine serum albumin (BSA) was explored. Rh2 (0.1-12.5 µM) inhibited the formation of fluorescent AGE and non-fluorescent AGE, as well as the level of fructosamine and prevented protein oxidation by decreasing protein carbonyl formation and protein thiol group modification. Rh2 also suppressed the formation of the β-cross amyloid structure of BSA. Ginsenosides might be promising new anti-glycation agents for the prevention of diabetic complications via inhibition of AGE formation and oxidation-dependent protein damage.

Inhibitory Effect of Antidesma bunius Fruit Extract on Carbohydrate Digestive Enzymes Activity and Protein Glycation In Vitro

Antidesma bunius (L.) spreng (Mamao) is widely distributed in Northeastern Thailand. Antidesma bunius has been reported to contain anthocyanins, which possess antioxidant and antihypertensive actions. However, the antidiabetic and antiglycation activity of Antidesma bunius fruit extract has not yet been reported. In this study, we investigated the inhibitory activity of anthocyanin-enriched fraction of Antidesma bunius fruit extract (ABE) against pancreatic α-amylase, intestinal α-glucosidase (maltase and sucrase), protein glycation, as well as antioxidant activity. A liquid chromatography-tandem mass spectrometry (LC-MS/MS) chromatogram revealed that ABE contained phytochemical compounds such as cyanidin-3-glucoside, delphinidin-3-glucoside, ellagic acid, and myricetin-3-galactoside. ABE inhibited intestinal maltase and sucrase activity with the IC₅₀ values of 0.76 ± 0.02 mg/mL and 1.33 ± 0.03 mg/mL, respectively. Furthermore, ABE (0.25 mg/mL) reduced the formation of fluorescent AGEs and the level of N^ε-carboxymethyllysine (N^ε-CML) in fructose and glucose-induced protein glycation during four weeks of incubation. During the glycation process, the protein carbonyl and β-amyloid cross structure were decreased by ABE (0.25 mg/mL). In addition, ABE exhibited antioxidant activity through DPPH radical scavenging activity and Trolox equivalent antioxidant capacity (TEAC) with the IC₅₀ values 15.84 ± 0.06 µg/mL and 166.1 ± 2.40 µg/mL, respectively. Meanwhile, ferric reducing antioxidant power (FRAP) showed an EC₅₀ value of 182.22 ± 0.64 µg/mL. The findings suggest that ABE may be a promising agent for inhibiting carbohydrate digestive enzyme activity, reducing monosaccharide-induced protein glycation, and antioxidant activity.

Poncirin, an orally active flavonoid exerts antidiabetic complications and improves glucose uptake activating PI3K/Akt signaling pathway in insulin resistant C2C12 cells with anti-glycation capacities

Poncirin, a natural flavanone glycoside present abundantly in many citrus fruits, contains an extensive range of biological activities. However, the antidiabetic mechanism of poncirin is unexplored yet. In this study, we examined the anti-diabetic prospective of poncirin by evaluating its ability to inhibit protein tyrosine phosphatase 1B (PTP1B), α-glucosidase, human recombinant AR (HRAR), rat lens aldose reductase (RLAR), and advanced glycation end-product (AGE) formation (IC₅₀ = 7.76 ± 0.21, 21.31 ± 1.26, 3.56 ± 0.33, 11.91 ± 0.21, and 3.23 ± 0.09 µM, respectively). Kinetics data and docking studies showed the lowest binding energy and highestaffinityforthemixed and competitivetypeof inhibitorsof poncirin. Moreover, the molecular mechanisms underlying the antidiabetic outcomes of poncirin in insulin resistant C2C12 skeletal muscle cells were explored, which significantly increased glucose uptake and decreased the expression of PTP1B in C2C12 cells. Consequently, poncirin increased GLUT-4 expression level by activating the IRS-1/PI3K/Akt/GSK-3 signaling pathway. Moreover, poncirin (0.5-50 µM) remarkably inhibited the formation of fluorescent AGE, nonfluorescent CML, fructosamine, and β-cross amyloid structures in glucose-fructose-induced BSA glycation during 4 weeks of study. Poncirin also notably prevented protein oxidation demonstrated with decreasing the protein carbonyl and the consumption of protein thiol in the dose-dependent manner. The results clearly expressed the promising activity of poncirin for the therapy of diabetes and its related complications.

The interplay between copper(II), human serum albumin, fatty acids, and carbonylating agent interferes with Cys 34 thiol reactivity and copper binding

Cys34 thiol group of human serum albumin (HSA) represents major plasma antioxidant. Its reactivity is influenced by multiple factors. The influence of fatty acids (FA; saturated, mono, and poly unsaturated acids from fish oil) binding to HSA, on copper(II) binding affinity and Cys34 thiol group accessibility/reactivity, in the presence of carbonylation agent (methylglyoxal, MG) was examined. HSA-copper(II) content, thiol group reactivity, and HSA carbonylation level were monitored spectrophotometrically. Changes in HSA were followed by fluorescence spectroscopy and native PAG electrophoresis. FA/HSA molar ratio was screened by GC. Together, binding of copper(II) ions and FA to HSA increase the reactivity of Cys34 thiol group (depending on the type of FA), with constant contribution of copper(II) ions of one-third. Carbonylation of FA-HSA-Cu(II) complexes caused a decrease in the Cys34 thiol group content, accompanied by a decrease in the content of HSA-bound copper. The carbonylation level of guanidine groups was not affected by FAs and copper(II) binding. Fluorescent emission spectra of FA-HSA-Cu(II)-MG complexes showed conformational changes in HSA molecule. Although binding of fatty acids and copper ions caused a significant increase in the thiol group reactivity, Cys34 thiol from FA-HSA-Cu(II) complexes reacted with MG in smaller extent than expected, probably as a consequence of conformational changes introduced by carbonylation. Increase in the percentage of reacted-free thiol groups with MG (due to FA and copper binding) may not seem to be very significant, but it is very important in complex biological systems, where catalytic metal is present.

Antioxidant and pro-oxidant actions of resveratrol on human serum albumin in the presence of toxic diabetes metabolites: Glyoxal and methyl-glyoxal

Methylglyoxal (MGO) and glyoxal (GO) are attracting considerable attention because of their role in the onset of diabetes symptoms. Therefore, to comprehend the molecular fundamentals of their pathological actions is of the utmost importance. In this study, the molecular interactions between resveratrol (RES) and human serum albumin (HSA) and the ability of the stilbene to counteract the oxidative damage caused by pathological concentrations of MGO and GO to the human plasma protein, was assessed. The oxidation of Cys34 in HSA as well as the formation of specific protein semialdehydes AAS (α-aminoadipic), GGS (γ-glutamic) and the accumulation of Advanced Glycation End-products (AGEs) was investigated. Resveratrol was found to neutralize both α-dicarbonyls by forming adducts detected by HESI-Orbitrap-MS. This antioxidant action was manifested in a significant reduction of AGEs. However, RES-α-dicarbonyl conjugates oxidized Cys34 and lysine, arginine and/or proline by a nucleophilic attack on SH and ε-NH groups in HSA. The formation of specific semialdehydes in HSA after incubation with GO and MGO at pathological concentrations was reported for the first time in this study, and may be used as early and specific biomarkers of the oxidative stress undergone by diabetic patients. The pro-oxidative role of the RES-α-dicarbonyl conjugates should be further investigated to clarify whether this action leads to positive or harmful clinical consequences. The biological relevance of human protein carbonylation as a redox signaling mechanism and/or as a reflection of oxidative damage and disease should also be studied in future works.

Simultaneous anti-diabetic and anti-vascular calcification activity of Nocardia sp. UTMC 751

Alpha-amylase can act as a significant player in causing hyperglycaemia, leading to protein glycation, which is the main complication in this condition, besides causing vascular calcification (VC), an important vascular failure caused due to this. In order to find a natural source of the biocompounds with inhibitory effects on α-amylase, 15 fermentation broth extracts of actinobacteria (FBEA) (200 μg ml^-1 ) have been screened. Finally, the effects of the most efficient FBE have been investigated on osteopontin (OPN, a VC marker) mRNA level in the vascular smooth muscle cells under the calcification conditions, and the chemical constituents of the most efficient FBE were analysed using gas chromatography and mass spectrometry (GC-MS) analysis. The tested FBEA showed anti-amylase (7·2-21%) and anti-denaturation (7·5-37%) activities. Among the tested FBEA, Nocardia sp. UTMC 751 FBE showed the highest anti-amylase activity (21%). This treatment group also displayed the minimum fructosamine and the maximum thiol groups content. In addition, this FBE reduced the mRNA level of the OPN (fourfold). The GC-MS analysis demonstrated the existence of three volatile and known antioxidants including pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-3-(2-methylpropyl)-, pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-3-(phenylmethyl)- and methyl ester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionic acid in the FBE of Nocardia sp. UTMC 751. The results indicated that Nocardia sp. UTMC 751 is a considerable source of bioactive compounds that are effective against the direct and indirect pathological targets involved in diabetes. This study highlights the significant potential of rare Actinomycetes in producing pharmaceutically important biocompounds.

SIGNIFICANCE AND IMPACT OF THE STUDY

Actinobacteria are one of the best natural libraries for discovering drugs. Various commercial drugs have been developed against infectious and metabolic disorders from actinobacteria; however, there is no report on their simultaneous inhibitory effect against diabetes, a life-threatening disease, and its related pathological processes, like inflammation and vascular calcification (VC). In this research, after several screening, Nocardia sp. UTMC 751 was introduced as the first microbial source exhibiting a simultaneous inhibitory activity on the targets, including hyperglycaemia and protein glycation, and other involved pathological processes like inflammation and VC.

The structural damages of lens crystallins induced by peroxynitrite and methylglyoxal, two causative players in diabetic complications and preventive role of lens antioxidant components

Peroxynitrite (PON) and methylglyoxal (MGO), two diabetes-associated compounds, are believed to be important causative players in development of diabetic cataracts. In the current study, different spectroscopic methods, gel electrophoresis, lens culture and microscopic assessments were applied to examine the impact of individual, subsequent or simultaneous modification of lens crystallins with MGO and PON on their structure, oligomerization and aggregation. The protein modifications were confirmed with detection of the significantly increased quantity of carbonyl groups and decreased levels of sulfhydryl, tyrosine and tryptophan. Also, lens proteins modification with these chemical agents was accompanied with important structural alteration, oligomerization, disulfide/chromophore mediated protein crosslinking and important proteolytic instability. All these structural damages were more pronounced when the lens proteins were modified in the presence of both mentioned chemical agents, either in sequential or simultaneous manner. Ascorbic acid and glutathione, as the main components of lens antioxidant defense mechanism, were also capable to markedly prevent the damaging effects of PON and MGO on lens crystallins, as indicated by gel electrophoresis. The results of this study may highlight the importance of lens antioxidant defense system in protection of crystallins against the structural insults induced by PON and MGO during chronic hyperglycemia in the diabetic patients.

Isoferulic Acid Action against Glycation-Induced Changes in Structural and Functional Attributes of Human High-Density Lipoprotein

Glycation-induced high-density lipoprotein (HDL) modification by aldehydes can result in loss of its antiinflammatory/antioxidative properties, contributing to diabetes-associated cardiovascular diseases. Isoferulic acid, a major active ingredient of Cimicifuga heracleifolia, shows antiinflammatory, antiviral, antioxidant, and antidiabetic properties. Thus, this study investigated the antiglycation effect of isoferulic acid against compositional modifications of HDL and loss of biological activity of HDL-paraoxonase induced on incubation with different aldehydes. Protective effect of isoferulic acid was assessed by subjecting purified HDL from human plasma to glycation with methylglyoxal, glyoxal, or glycolaldehyde and varying concentrations of isoferulic acid. The effect of isoferulic acid was analyzed by determining amino group number, tryptophan and advanced glycation end-product fluorescence, thermal denaturation studies, carboxymethyl lysine content, and activity of HDL-paraoxonase. Concentration-dependent inhibitory action of isoferulic acid was observed against extensive structural perturbations, decrease in amino group number, increase in carboxymethyl lysine content, and decrease in the activity of HDL-paraoxonase caused by aldehyde-associated glycation in the HDL molecule. Isoferulic acid, when taken in concentration equal to that of aldehydes, was most protective, as 82-88% of paraoxonase activity was retained for all studied aldehydes. Isoferulic acid shows antiglycation action against aldehyde-associated glycation in HDL, which indicates its therapeutic potential for diabetic patients, especially those with micro-/macrovascular complications.

Phenolics from Garcinia mangostana Inhibit Advanced Glycation Endproducts Formation: Effect on Amadori Products, Cross-Linked Structures and Protein Thiols

Accumulation of Advanced Glycation Endproducts (AGEs) in body tissues plays a major role in the development of diabetic complications. Here, the inhibitory effect of bioactive metabolites isolated from fruit hulls of Garcinia mangostana on AGE formation was investigated through bio-guided approach using aminoguanidine (AG) as a positive control. Including G. mangostana total methanol extract (GMT) in the reaction mixture of bovine serum albumin (BSA) and glucose or ribose inhibited the fluorescent and non-fluorescent AGEs formation in a dose dependent manner. The bioassay guided fractionation of GMT revealed isolation of four bioactive constituents from the bioactive fraction; which were identified as: garcimangosone D (1), aromadendrin-8-C-glucopyranoside (2), epicatechin (3), and 2,3',4,5',6-pentahydroxybenzophenone (4). All the tested compounds significantly inhibited fluorescent and non-fluorescent AGEs formation in a dose dependent manner whereas compound 3 (epicatechin) was found to be the most potent. In search for the level of action, addition of GMT, and compounds 2-4 inhibited fructosamine (Amadori product) and protein aggregation formation in both glucose and ribose. To explore the mechanism of action, it was found that addition of GMT and only compound (3) to reaction mixture increased protein thiol in both glucose and ribose while compounds 1, 2 and 4 only increased thiol in case of ribose. In conclusion, phenolic compounds 1-4 inhibited AGEs formation at the levels of Amadori product and protein aggregation formation through saving protein thiol.

5'-O-Alkylpyridoxamines: Lipophilic Analogues of Pyridoxamine Are Potent Scavengers of 1,2-Dicarbonyls

Pyridoxamine (PM) is a prospective drug for the treatment of diabetic complications. In order to make zwitterionic PM more lipophilic and improve its tissue distribution, PM derivatives containing medium length alkyl groups on the hydroxymethyl side chain were prepared. The synthesis of these alkylpyridoxamines (alkyl-PMs) starting from pyridoxine offers high yields and is amenable to bulk preparations. Interestingly, alkyl-PMs were found to react with methylglyoxal (MGO), a major toxic product of glucose metabolism and autoxidation, several orders of magnitude faster than PM. This suggests the formation of nonionic pyrido-1,3-oxazine as the key step in the reaction of PM with MGO. Since the primary target of MGO in proteins is the guanidine side chain of arginine, alkyl-PMs were shown to be more effective than PM in reducing the modification of N-α-benzoylarginine by MGO. Alkyl-PMs in the presence of MGO also protected the enzymatic activity of lysozyme that contains several arginine residues next to its active site. Alkyl-PMs can be expected to trap MGO and other toxic 1,2-carbonyl compounds more effectively than PM, especially in lipophilic tissue environments, thus protecting macromolecules from functional damage. This suggests potential therapeutic uses for alkyl-PMs in diabetes and other diseases characterized by the elevated levels of toxic dicarbonyl compounds.

Anti-glycation activities of phenolic constituents from Silybum marianum (Milk Thistle) flower in vitro and on human explants

Glycation is an ageing reaction of naturally occurring sugars with dermal proteins, with clinical signs appearing in vivo around age 30, and increasing steadily/regularly with age. The suppleness of the dermis is affected by the formation of bridges between proteins and sugars (Maillard's reaction). The accumulation of advanced glycation end products (AGEs) in skin plays a very important role in skin ageing. Therefore, natural compounds or extracts that possess antiglycation activities may have great anti-ageing potential. In the present study, Silybum marianum flower extract (SMFE) was demonstrated to possess antiglycation activity. We found that SMFE inhibits glycation reaction between BSA and glucose. In addition, antiglycation activity of SMFE was confirmed in a human skin explants model. SMFE reduced Nε-(carboxymethyl) lysine (CML) expression, whereas SMFE stimulated fibrillin-1 expression compared to treatment with methyglyoxal. An active ingredient contributing to the observed activities was identified as silibinin. The antiglycation activity of silibinin was dose-dependent. The beneficial effects of silibinin may be applied to prevention or management of AGE-mediated pathologies, targeting in a pleiotropic and complementary way the biochemical and cellular bases of skin aging.

Protective effects of cyanidin-3-rutinoside against monosaccharides-induced protein glycation and oxidation

Cyanidin-3-rutinoside (C3R), a naturally occurring anthocyanin, is present in various fruits and vegetables as a colorant. C3R has been well characterized and demonstrated a number of biological activities attributed to its antioxidant properties. The present study compared the effectiveness of C3R against monosaccharide-induced protein glycation and oxidation in vitro using bovine serum albumin (BSA).The results demonstrated that C3R (0.125-1.00 mM) inhibited the formation of fluorescent AGEs in ribose-glycated BSA (2-52%), fructose-glycated BSA (81-93%), glucose-glycated BSA (30-74%) and galactose-glycated BSA (6-79%).Correspondingly, C3R (1.00 mM) decreased the level of N(ɛ)-(carboxymethyl) lysine (56-86%) in monosaccharide-induced glycation in BSA. C3R also reduced the level of fructosamine, β-amyloid cross structure, protein carbonyl content as well as the depletion of thiol in BSA/monosaccharide system. In summary, C3R might offer a new promising antiglycation agent for the prevention of diabetic complications by inhibiting AGE formation and oxidation-dependent protein damage.

HSA carbonylation with methylglyoxal and the binding/release of copper(ii) ions

HSA carbonylation in vitro (with MG) and in vivo (diabetes) leads to copper(ii) binding affinity decrease and copper(ii) release from copper–HSA complexes depending mainly on the redox state of the Cys34-SH group.

Mesona Chinensis Benth extract prevents AGE formation and protein oxidation against fructose-induced protein glycation in vitro

BACKGROUND

Mesona chinensis Benth (Chinese Mesona), an economically significant agricultural plant, is the most widely consumed as an herbal beverage in Southeast Asia and China. The objective of this study was to evaluate the inhibitory activity of Mesona chinensis (MC) extract on the formation of advanced glycation end products (AGEs) and protein oxidation in an in vitro model of fructose-mediated protein glycation.

METHODS

The content of total polyphenolic compounds was measured by using Folin-Ciocalteu assay. Antiglycation activity was determined using the formation of AGE fluorescence intensity, Nϵ-(carboxymethyl)lysine (CML), the level of fructosamine, and the formation of amyloid cross β-structure. The protein oxidation was examined using the level of protein carbonyl content and thiol group.

RESULTS

Our results revealed that the content of total polyphenolic compound in MC extract was 212.4 ± 5.6 mg gallic acid equivalents/g dried extract. MC extract (0.25-1.00 mg/mL) significantly inhibited the formation of fluorescence AGEs in fructose-glycated bovine serum albumin (BSA) during 4 weeks of study. Furthermore, MC extract also decreased the level of Nϵ-CML, fructosamine, and amyloid cross β-structure in fructose-glycated BSA. While the total thiol group was elevated and the protein carbonyl content was decreased in BSA incubated with fructose and MC extract.

CONCLUSIONS

The extract of MC inhibits fructose-mediated protein glycation and protein oxidation. This edible plant could be a natural rich source of antiglycation agent for preventing AGE-mediated diabetic complication.

The efficiency of compounds with α-amino-β-mercapto-ethane group in protection of human serum albumin carbonylation and cross-linking with methylglyoxal

Substances containing an α-amino-β-mercapto-ethane pharmacophore may be used as effective methylglyoxal scavengers and inhibitors of protein carbonylation and cross-linking.

A comparative study of ferulic acid on different monosaccharide-mediated protein glycation and oxidative damage in bovine serum albumin

Three dietary monosaccharides, (glucose, fructose, and ribose), have different rates of protein glycation that accelerates the production of advanced glycation end-products (AGEs). The present work was conducted to investigate the effect of ferulic acid (FA) on the three monosaccharide-mediated protein glycations and oxidation of BSA. Comparing the percentage reduction, FA (1-5 mM) reduced the level of fluorescence AGEs (F-AGEs) and N(ε)-(carboxymethyl) lysine (N(ε)-CML) in glucose-glycated BSA (F-AGEs = 12.61%-36.49%; N(ε)-CML = 33.61%-66.51%), fructose-glycated BSA (F-AGEs = 25.28%-56.42%; N(ε)-CML = 40.21%-62.91%), and ribose-glycated BSA (F-AGEs = 25.63%-51.18%; N(ε)-CML = 26.64%-64.08%). In addition, the percentages of FA reduction of fructosamine (Frc) and amyloid cross β-structure (Amy) were Frc = 20.45%-43.81%; Amy = 17.84%-34.54% in glucose-glycated BSA, Frc = 25.17%-36.92%; Amy = 27.25%-39.51% in fructose-glycated BSA, and Frc = 17.34%-29.71%; Amy = 8.26%-59.92% in ribose-glycated BSA. FA also induced a reduction in protein carbonyl content (PC) and loss of protein thiol groups (TO) in glucose-glycated BSA (PC = 37.78%-56.03%; TO = 6.75%-13.41%), fructose-glycated BSA (PC = 36.72%-52.74%; TO = 6.18%-20.08%), and ribose-glycated BSA (PC = 25.58%-33.46%; TO = 20.50%-39.07%). Interestingly, the decrease in fluorescence AGEs by FA correlated with the level of N(ε)-CML, fructosamine, amyloid cross β-structure, and protein carbonyl content. Therefore, FA could potentially be used to inhibit protein glycation and oxidative damage caused by monosaccharides, suggesting that it might prevent AGEs-mediated pathologies during diabetic complications.

Isoferulic acid, a new anti-glycation agent, inhibits fructose- and glucose-mediated protein glycation in vitro

The inhibitory activity of isoferulic acid (IFA) on fructose- and glucose-mediated protein glycation and oxidation of bovine serum albumin (BSA) was investigated. Our data showed that IFA (1.25–5 mM) inhibited the formation of fluorescent advanced glycation end products (AGEs) and non-fluorescent AGE [N^ε-(carboxymethyl) lysine: CML], as well as the level of fructosamine. IFA also prevented protein oxidation of BSA indicated by decreasing protein carbonyl formation and protein thiol modification. Furthermore, IFA suppressed the formation of β-cross amyloid structures of BSA. Therefore, IFA might be a new promising anti-glycation agent for the prevention of diabetic complications via inhibition of AGEs formation and oxidation-dependent protein damage.

Naturally occurring inhibitors against the formation of advanced glycation end-products

Advanced glycation end-products (AGEs) are the final products of the non-enzymatic reaction between reducing sugars and amino groups in proteins, lipids and nucleic acids. Recently, the accumulation of AGEs in vivo has been implicated as a major pathogenic process in diabetic complications, atherosclerosis, Alzheimer's disease and normal aging. The early recognition of AGEs can ascend to the late 1960s when a non-enzymatic glycation process was found in human body which is similar to the Maillard reaction. To some extent, AGEs can be regarded as products of the Maillard reaction. This review firstly introduces the Maillard reaction, the formation process of AGEs and harmful effects of AGEs to human health. As AGEs can cause undesirable diseases or disorders, it is necessary to investigate AGE inhibitors to offer a potential therapeutic approach for the prevention of diabetic or other pathogenic complications induced by AGEs. Typical effective AGE inhibitors with different inhibition mechanisms are also reviewed in this paper. Both synthetic compounds and natural products have been evaluated as inhibitors against the formation of AGEs. However, considering toxic or side effects of synthetic molecules present in clinical trials, natural products are more promising to be developed as potent AGE inhibitors.

The glycation of albumin: structural and functional impacts

Oxidative stress and protein modifications are frequently observed in numerous disease states. Glucose constitutes a vital nutrient necessary to cellular oxygen metabolism. However, hyperglycemia-associated damage is an important factor in diabetes disorders. Albumin, the major circulating protein in blood, can undergo increased glycation in diabetes. From recent studies, it has become evident that protein glycation has important implications for protein activity, unfolding, and degradation, as well as for cell functioning. After giving a brief overview of the key role of albumin in overall antioxidant defense, this review examines its role as a target of glycation reactions. A synthesis of state of the art methods for measuring and characterizing albumin glycation is detailed. In light of recent data, we then report the impact of glycation on the structure of albumin and its various activities, especially its antioxidant and binding capacities. The biological impact of glycated albumin on cell physiology is also discussed, specifically the role of the protein as a biological marker of diabetes.

Influence of the microenvironment of thiol groups in low molecular mass thiols and serum albumin on the reaction with methylglyoxal

Methylglyoxal (MG), a reactive alpha-oxoaldehyde that is produced in higher quantities in diabetes, uremia, oxidative stress, aging and inflammation, reacts with the thiol groups (in addition to the amino and guanidino groups) of proteins. This causes protein modification, formation of advanced glycated end products (AGEs) and cross-linking. Low molecular mass thiols can be used as competitive targets for MG, preventing the reactions mentioned above. Therefore, this paper investigated how the microenvironment of the thiol group in low molecular mass thiols (cysteine, N-acetylcysteine (NAcCys), carboxymethylcysteine (CMC) and glutathione (GSH)) and human serum albumin (HSA) affected the thiol reaction with MG. The SH group reaction course was monitored by (1)H-NMR spectroscopy and spectrophotometric quantification. Changes in the HSA molecules were monitored by SDS-PAGE. The microenvironment of the SH group had a major effect on its reactivity and on the product yield. The reactivity of SH groups decreased in the order Cys>GSH>NAcCys. CMC did not react. The percentages of the reacted SH groups in the equilibrium state were almost equal, regardless of the ratio of thiol compound/MG (1:1, 1:2, 1:5): 38.1 + or - 0.9%; 38.2 + or - 0.7% and 39.0 + or - 0.8% for Cys; 26.5 + or - 0.6%; 26.6 + or - 2.6% and 27.4 + or - 2.5% for GSH; 10.8 + or - 0.9%; and 11.2 + or - 0.7% and 12.2 + or - 0.9% for NAcCys, respectively. Our results explain why substances containing alpha-amino-beta-mercapto-ethane as a pharmacophore are successful scavengers of MG. In equilibrium, HSA SH reacted in high percentages both with an insufficient amount and with an excess of MG (55% and 65%, respectively). An analysis of the hydrophobicity of the microenvironment of the SH group on the HSA surface showed that it could contribute to high levels of SH modification, leading to an increase in the scavenging activity of the albumin thiol.