Characterization of advanced glycation end products for biochemical studies: side chain modifications and fluorescence characteristics

UV Fluorescence-Based Determination of Urinary Advanced Glycation End Products in Patients with Chronic Kidney Disease

Advanced glycation end products (AGEs) are a class of proteins or lipids that are non-enzymatically glycated and oxidized after contact with aldose sugars. The accumulation of AGEs results in carbonyl stress, which is characteristic for diabetes mellitus, uremia, atherosclerosis and vascular dysfunction. In recent decades, several innovative methods have been developed to measure the concentration of AGEs in blood or urine. In the present study, we evaluated the use of UV fluorescence as an alternative tool to detect urinary AGEs in four groups of well characterized chronic kidney disease (CKD) patients over a wide range of kidney insufficiency and in a group of healthy subjects. Using an excitation wavelength of 365 nm, the fluorescence spectra of urinary AGEs were recorded in the 400–620 nm emission range. When considering the emission peaks at 440 nm and 490 nm, a significantly higher AGE-specific fluorescence intensity was detected in CKD patients compared to healthy subjects (p < 0.0001 and p = 0.0001, respectively). The urinary creatinine adjusted fluorescence emission spectra in the group of CKD patients with diabetes mellitus were comparable with those of CKD patients without diabetes mellitus. Creatinine-adjusted fluorescence emission spectra were highest in CKD patients with proteinuria, moderate in CKD patients without proteinuria and lowest in healthy controls (p < 0.0001 at both emission wavelengths). In a multiple regression analysis, age, CRP and insulin treatment were predictors of fluorescence intensity at the emission wavelength of 440 nm. Age and insulin treatment were predictors at 490 nm. The presented method is a simple, cheap, alternative method to monitor the AGE-load in the CKD population.

Influence of Human Serum Albumin Glycation on the Binding Affinities for Natural Flavonoids

Increasing the degree of glycation in diabetes could affect the ability of plasma proteins in binding to small molecules and active compounds. In this study, the influence of glycation of Human serum albumin (HSA) on the binding affinities for six dietary flavonoids was investigated by fluorescence spectra. Glycated HSA was prepared through incubation with glucose and characterized by several methods to confirm the glycation. It was found that the level of glycation increased with the increasing incubation time. The glycation of HSA increased the binding affinities for flavonoids by 1.40 to 48.42 times, which indicates that modifications caused by the glycation may have different influences on the interactions of flavonoids with HSA at separate binding sites on this protein. These results are valuable for understanding the influence of diabetes on the metabolism of flavonoids and other bioactive small molecules in human body.

Intensification of serum albumin amyloidogenesis by a glycation-peroxidation loop (GPL)

The interaction of reducing sugars with proteins leads to the formation of advanced glycation end products (AGE) and reactive oxidative species (ROS). ROS peroxidise free or membrane included unsaturated fatty acids, leading to generate reactive aldehydes as advanced lipid peroxidation end products (ALE). Aldehydes from lipid peroxidation (LPO) react with proteins to cause alteration of protein structure to exacerbate complication of diseases. Here we studied serum albumin glycation in the presence and absence of liposomes as a bio-membrane model to investigate protein structural changes using various techniques including intrinsic and extrinsic fluorescence spectroscopies and electron microscopy analysis. Accordingly, serum albumin glycation and fibrillation were accelerated and intensified in the presence of liposomes through a hypothesized glycation-peroxidation loop (GPL). Together, our results shed light on the necessity of reconsidering diabetic protein glycation to make it close to physiological conditions mimicry, more importantly, proteins structural change due to diabetic glycation is intensified in the proximity of cell membranes which probably potentiates programmed cell death distinct from apoptosis.

Propolis nanoparticles prevent structural changes in human hemoglobin during glycation and fructation

Nowadays diabetes, as a metabolic disorder, is increasing at an alarming rate. Glycation and production of advanced glycation end products (AGEs) is the most important factor involved in diabetic complications. Due to the side effects of synthetic drugs, the demand for natural anti-diabetic herbal medicines has increased. Propolis is a natural and resinous material, which iscollected by honeybees. Due to the impact of nanotechnology in medicine and the advantageous role of nanoparticles in treatment, nano-propolis particles (PNP) were prepared. The anti-glycation effect of PNP at various concentrations was investigated on human hemoglobin (Hb) glycation and fructation and compared with aspirin as a common anti-glycation agent using glycation specific AGE fluorescence, AGE-specific absorbance and circular dichroism (CD) methods. Fluorescence spectroscopy results showed that PNP inhibited the formation of AGEs in Hb glycation and fructation by glucose and fructose, respectively. CD results revealed that PNP caused an increase in Hb beta-sheet content while decreasing the alpha helical content. Additionally, the results of UV-Vis spectroscopy and fluorescence emission of heme degradation products revealed the protective effect of PNP on heme during glycation and fructation of human Hb. It is notable that the synergistic effects of combined propolis nanoparticles and aspirin is more than either of them alone. However, having said that, PNP as a natural product has a potential to be an effective drug in the treatment of diabetes.

Gold Nanoparticle-Based Detection of Low Molecular Weight AGEs from In Vitro Glycated Haemoglobin A0 Samples

Protein glycation is a major biochemical event that takes place in the plasma of diabetic patients due to increased sugar levels. Extensive glycation leads to the formation of advanced glycation end products (AGEs) that is well known for having detrimental effects on diabetic patients. In the current work, we have glycated the physiologically important protein Haemoglobin A0 in vitro to study AGE formation and activity by using them as a template for gold nanoparticle (GNPs) synthesis. It was found that the surface plasmon resonance of synthesised GNPs showed high correlation with the extent of glycation. On fractionation, the glycated Haemoglobin A0 segregated into two distinct population of products, one consisting of proteinaceous, cross-linked larger fragments of Haemoglobin A0 and a second population of non-proteinaceous low molecular weight AGEs. Only low molecular weight AGEs contributed to synthesis of GNPs upon using the fractions as a template, substantiating the principle of proposed GNP-based assay. Owing to its physiological importance, AGEs can be used as a diagnostic means for diabetes and its associated complications. In this study, we have employed the high reactivity of AGEs for the development of a GNP-based novel colorimetric sensor to enable their detection. Our proposed GNP-based sensing could have high clinical significance in detecting diabetes and its associated complexities.

Identification of differences in the formation of plasma glycated proteins between dogs and humans under diabetes-like glucose concentration conditions

Dogs have been proposed as a translational model and used for studying aging, diabetes, and diabetes-related complications in humans. However, no studies have ever compared the glycation of plasma proteins between dogs and humans under similar experimental conditions. Thus, the aim of this study was to fill this gap by comparing the plasma protein glycation patterns of dogs and humans in an ex-vivo system. Canine and human plasma samples were incubated with glucose at concentrations comparable to those observed in diabetic patients. The final glucose plasma concentration resulted in similar glucose:albumin ratios in both species. Glycated proteins were evaluated by measuring the content of fructosamine, protein carbonyls, and the formation of advanced glycation end-products (AGEs). The concentrations of fructosamine and protein carbonyls in canine and human plasma increased in a glucose concentration-dependent manner (P < 0.0001). Of note, the relative increment of fructosamine and protein carbonyl content and AGE formation was always higher in human than in dog plasma. Our results reveal that the plasma glycation processes in dogs and humans are not similar. These novel findings could contribute to improve our understating about canine and human diabetes as well as other condition associated in the glycation of proteins.

Repurposing blue laser autofluorescence to measure ocular sun exposure

IMPORTANCE

Excessive ocular sun exposure is linked to various eye pathologies. Conjunctival ultraviolet autofluorescence (CUVAF) is a method of detecting sun-related conjunctival damage; however, the custom-built camera system required is not readily available.

BACKGROUND

We investigated whether blue laser autofluorescence (BAF) on a commonly used confocal scanning laser ophthalmoscope (cSLO) can be utilized to measure CUVAF area.

DESIGN

Cross-sectional evaluation of a diagnostic technology at a medical research institute.

PARTICIPANTS

Sixty-four participants recruited from three on-going observational eye studies in Western Australia.

METHODS

All participants had four images, two of each eye, captured using the CUVAF camera and BAF on the same day. Participants with pterygium or poor quality images were excluded from the analysis. Two graders measured CUVAF area in each image twice. CUVAF area measured by BAF was then compared to measurements determined with the conventional camera system.

MAIN OUTCOME MEASURES

CUVAF area.

RESULTS

After exclusions, 50 participants' images were analysed. Intra- and inter-observer repeatability were similar between the two systems. When comparing CUVAF area measured by BAF to the camera measurement, grader 1 had a mean difference of +1.00 mm² , with 95% limits of agreement -5.75 to 7.77 mm² . Grader 2 had a mean difference of +0.21mm² , with 95% limits of agreement -7.22 to 7.64 mm² .

CONCLUSIONS AND RELEVANCE

BAF on a commercially available cSLO is a valid method for measuring CUVAF area. This finding provides broader opportunity for identifying, monitoring and educating patients with sun-exposure-related ocular conditions and for researching the ocular impacts of sun exposure.

Glycation induced conformational transitions in cystatin proceed to form biotoxic aggregates: A multidimensional analysis

Hyperglycaemic conditions facilitate the glycation of serum proteins which may have predisposition to aggregation and thus lead to complications. The current study investigates the glycation induced structural and functional modifications of chickpea cystatin (CPC) as well as biological toxicity of the modified protein forms, using CPC-glucose as a model system. Several structural intermediates were formed during the incubation of CPC with glucose (day 4, 8, 12, & 16) as revealed by circular dichroism (CD), altered intrinsic fluorescence, and high ANS binding. Further incubation of CPC with glucose (day 21) formed abundant β structures as revealed by Fourier transform infrared spectroscopy and CD analysis which may be due to the aggregation of protein. High thioflavin T fluorescence intensity and increased Congo red absorbance together with enhanced turbidity and Rayleigh scattering by this modified form confirmed the aggregation. Electron microscopy finally provided the valid physical authentication about the presence of aggregate structures. Functional inactivation of glucose incubated CPC was also observed with time. Single cell electrophoresis of lymphocytes and plasmid nicking assays in the presence of modified CPC showed the DNA damage which confirmed its biological toxicity. Hence, our study suggests that glycation of CPC not only leads to structural and functional alterations in proteins but also to biotoxic AGEs and aggregates.

Variation in Polyphenolic Profiles, Antioxidant and Antimicrobial Activity of Different Achillea Species as Natural Sources of Antiglycative Compounds

A comparative study was carried out on the methanolic extracts from six Achillea species and the examined polyphenols from these plants on the formation of advanced glycation end-products (AGE) in vitro. A. pachycephala which was richer in flavonoids (15 mg quercetin/g W) and phenolics (111.10 mg tannic acid/g DW) with substantial antioxidant activity (IC₅₀  = 365.5 μg/ml) presented strong anti-AGE properties. Chlorogenic acid, luteolin, quercetin and caffeic acid were identified as the major polyphenols in the extracts by HPLC. In general, polyphenolic content follows the order of A. pachycephalla > A. nobilis > A. filipendulina > A. santolina > A. aucheri > A. millefolium. Most extracts exhibited marked anti-AGE ability in the bovine serum albumin (BSA)/methylglyoxal (MG) system, though A. pachycephala showed the highest potential. The formation of AGEs was assessed by monitoring the production of fluorescent products and circular dichroism (CD) spectroscopy. Diminution in free radical production (assessed by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays) is discussed as potential mechanism for delay or reduced AGE. The results demonstrate the antiglycative, antioxidant and antimicrobial potential of Achillea species which can be attributed to polyphenols content and the effectiveness on generation of AGEs, thus Achillea species can be considered as natural sources for slowing down glycation related diseases.

Methylglyoxal induced glycation and aggregation of human serum albumin: Biochemical and biophysical approach

Serum protein glycation and formation of advanced glycation end products (AGEs) correlates with many diseases viz. diabetes signifying the importance of studying the glycation pattern of serum proteins. In our present study, methylglyoxal was investigated for its effect on the structure of human serum albumin (HSA); exploring the formation of AGEs and aggregates of HSA. The analytical tools employed includes intrinsic and extrinsic fluorescence, UV spectroscopy, far UV circular dichroism, Thioflavin T fluorescence, congo red binding, polyacrylamide gel electrophoresis (PAGE). UV and fluorescence spectroscopy revealed the structural transition of native HSA evident by new peaks and increased absorbance in UV spectra and quenched fluorescence in the presence of MG. Far UV CD spectroscopy revealed MG induced secondary structural alteration evident by reduced α-helical content. AGEs formation was confirmed by AGEs specific fluorescence. Increased ThT fluorescence and CR absorbance of 10mM MG incubated HSA suggests that glycated HSA results in the formation of aggregates of HSA. SEM and TEM were reported to have an insight of these aggregates. Molecular docking was also utilized to see site specific interaction of MG-HSA. This study is clinically significant as HSA is a clinically relevant protein which plays a crucial role in many diseases.

Advanced glycation end products induce neural tube defects through elevating oxidative stress in mice

Our previous study showed an association between advanced glycation end products (AGEs) and neural tube defects (NTDs). To understand the molecular mechanisms underlying the effect of AGEs on neural tube development, C57BL/6 female mice were fed for 4 weeks with commercial food containing 3% advanced glycation end product bovine serum albumin (AGE-BSA) or 3% bovine serum albumin (BSA) as a control. After mating mice, oxidative stress markers including malondialdehyde and H₂O₂ were measured at embryonic day 7.5 (E7.5) of gestation, and the level of intracellular reactive oxygen species (ROS) in embryonic cells was determined at E8.5. In addition to evaluating NTDs, an enzyme-linked immunosorbent assay was used to determine the effect of embryonic protein administration on the N-(carboxymethyl) lysine reactivity of acid and carboxyethyl lysine antibodies at E10.5. The results showed a remarkable increase in the incidence of NTDs at E10.5 in embryos of mice fed with AGE-BSA (no hyperglycemia) compared with control mice. Moreover, embryonic protein administration resulted in a noticeable increase in the reactivity of N-(carboxymethyl) lysine and N(ε)-(carboxyethyl) lysine antibodies. Malondialdehyde and H₂O₂ levels in embryonic cells were increased at E7.5, followed by increased intracellular ROS levels at E8.5. Vitamin E supplementation could partially recover these phenomena. Collectively, these results suggest that AGE-BSA could induce NTDs in the absence of hyperglycemia by an underlying mechanism that is at least partially associated with its capacity to increase embryonic oxidative stress levels.

Antibacterial Activity of Polyphenols: Structure-Activity Relationship and Influence of Hyperglycemic Condition

Polyphenols are plant-derived natural products with well-documented health benefits to human beings, such as antibacterial activities. However, the antibacterial activities of polyphenols under hyperglycemic conditions have been rarely studied, which could be relevant to their antibacterial efficacy in disease conditions, such as in diabetic patients. Herein, the antibacterial activities of 38 polyphenols under mimicked hyperglycemic conditions were evaluated. The structure-antibacterial activity relationships of polyphenols were also tested and analyzed. The presence of glucose apparently promoted the growth of the bacterial strains tested in this study. The OD600 values of tested bacteria strains increased from 1.09-fold to 1.49-fold by adding 800 mg/dL glucose. The polyphenols showed structurally dependent antibacterial activities, which were significantly impaired under the hyperglycemic conditions. The results from this study indicated that high blood glucose might promote bacterial infection, and the hyperglycemic conditions resulting from diabetes were likely to suppress the antibacterial benefits of polyphenols.

Imaging Lenticular Autofluorescence in Older Subjects

Purpose

To evaluate whether a practical method of imaging lenticular autofluorescence (AF) can provide an individualized measure correlated with age-related lens yellowing in older subjects undergoing tests involving shorter wavelength lights.

Methods

Lenticular AF was imaged with 488-nm excitation using a confocal scanning laser ophthalmoscope (cSLO) routinely used for retinal AF imaging. There were 75 older subjects (ages 47–87) at two sites; a small cohort of younger subjects served as controls. At one site, the cSLO was equipped with an internal reference to allow quantitative AF measurements; at the other site, reduced-illuminance AF imaging (RAFI) was used. In a subset of subjects, lens density index was independently estimated from dark-adapted spectral sensitivities performed psychophysically.

Results

Lenticular AF intensity was significantly higher in the older eyes than the younger cohort when measured with the internal reference (59.2 ± 15.4 vs. 134.4 ± 31.7 gray levels; P < 0.05) as well as when recorded with RAFI without the internal reference (10.9 ± 1.5 vs. 26.1 ± 5.7 gray levels; P < 0.05). Lenticular AF was positively correlated with age; however, there could also be large differences between individuals of similar age. Lenticular AF intensity correlated well with lens density indices estimated from psychophysical measures.

Conclusions

Lenticular AF measured with a retinal cSLO can provide a practical and individualized measure of lens yellowing, and may be a good candidate to distinguish between preretinal and retinal deficits involving short-wavelength lights in older eyes.

Control of Maillard Reactions in Foods: Strategies and Chemical Mechanisms

Maillard reactions lead to changes in food color, organoleptic properties, protein functionality, and protein digestibility. Numerous different strategies for controlling Maillard reactions in foods have been attempted during the past decades. In this paper, recent advances in strategies for controlling the Maillard reaction and subsequent downstream reaction products in food systems are critically reviewed. The underlying mechanisms at play are presented, strengths and weaknesses of each strategy are discussed, and reasonable reaction mechanisms are proposed to reinforce the evaluations. The review includes strategies involving addition of functional ingredients, such as plant polyphenols and vitamins, as well as enzymes. The resulting trapping or modification of Maillard targets, reactive intermediates, and advanced glycation endproducts (AGEs) are presented with their potential unwanted side effects. Finally, recent advances in processing for control of Maillard reactions are discussed.

Anti-glycation and anti-oxidative effects of a phenolic-enriched maple syrup extract and its protective effects on normal human colon cells

Oxidative stress and free radical generation accelerate the formation of advanced glycation endproducts (AGEs) which are linked to several chronic diseases. Published data suggest that phenolic-rich plant foods, show promise as natural anti-AGEs agents due to their anti-oxidation capacities. A phenolic-enriched maple syrup extract (MSX) has previously been reported to show anti-inflammatory and neuroprotective effects but its anti-AGE effects remain unknown. Therefore, herein, we investigated the anti-glycation and anti-oxidation effects of MSX using biochemical and biophysical methods. MSX (500 μg mL^-1) reduced the formation of AGEs by 40% in the bovine serum albumin (BSA)-fructose assay and by 30% in the BSA-methylglyoxal (MGO) assay. MSX also inhibited the formation of crosslinks typically seen in the late stage of glycation. Circular dichroism and differential scanning calorimeter analyses demonstrated that MSX maintained the structure of BSA during glycation. In the anti-oxidant assays, MSX (61.7 μg mL^-1) scavenged 50% of free radicals (DPPH assay) and reduced free radical generation by 20% during the glycation process (electron paramagnetic resonance time scan). In addition, the intracellular levels of hydrogen peroxide induced reactive oxygen species were reduced by 27-58% with MSX (50-200 μg mL^-1) in normal/non-tumorigenic human colon CCD-18Co cells. Moreover, in AGEs and MGO challenged CCD-18Co cells, higher cellular viabilities and rapid extracellular signal-regulated kinase (ERK) phosphorylation were observed in MSX treated cells, indicating its protective effects against AGEs-induced cytotoxicity. Overall, this study supports the biological effects of MSX, and warrants further investigation of its potential as a dietary agent against diseases mediated by oxidative stress and inflammation.

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.

Evaluation of an olive leaf extract as a natural source of antiglycative compounds

Advanced Glycation End-products (AGEs) have been associated to diabetes, neurodegenerative and cardiovascular diseases. Mitigating the formation of AGEs is a strategy to avoid detrimental physiopathological effects of age-related chronic diseases. An olive leaf extract (OLE), obtained under acidic conditions, and two fractions, obtained by solid-phase extraction, were characterized by LC-MS/MS. Antiglycative capacity of OLE and fractions were investigated in different in vitro models. The OLE significantly inhibited the formation of Amadori products at the early stage as well as the formation of fluorescent AGEs at the advanced stage of the glycation. Carboxymethyllysine was significantly inhibited by the OLE but it showed weaker activity against argpyrimidine and carboxyethyllysine. The antiglycative activity of each OLE fraction independently did not explain the activity reached in the whole extract, being necessary the compounds present in both fractions. OLE and its fractions were highly effective for trapping reactive dicarbonyl compounds (glyoxal, methylglyoxal, 3-deoxyglucosone and 3-deoxygalactosone). Different adducts resulting from the conjugation of methylglyoxal and hydroxytyrosol in OLE were identified. Results pointed out that OLE exert a broad-spectrum in vitro antiglycative activity.

Ameliorating Effects of Green Synthesized Silver Nanoparticles on Glycated End Product Induced Reactive Oxygen Species Production and Cellular Toxicity in Osteogenic Saos-2 Cells

Advanced glycation end-products (AGEs) that result from nonenzymatic glycation are one of the major factors involved in diabetes and its secondary complications and diseases. This necessitates our urge to discover new compounds that may be used as potential AGEs inhibitors without affecting the normal structure and function of biomolecules. In the present study, we investigated the inhibitory effects of AgNP (silver nanoparticles) on AGEs formation as well as their inhibitory effects on glycation mediated cell toxicity via reactive oxygen species (ROS) production and DNA damage. The excitation-emission fluorescence spectroscopy was employed to investigate the interaction of AgNP during glycation. The values of conditional stability constant (log K_a = 4.44) derived from the Stern-Volmer equation indicate that AgNP have strong binding capacity for glycated protein. UV-vis, fluorescence, and Fourier transform infrared spectral data reveal complexation of AgNP with glycated bovine serum albumin, which significantly inhibits AGEs formation in a concentration-dependent manner. Cytotoxic evaluations suggest that simultaneous administration of AgNP and glycated product reduces cell death (42.82% ± 3.54) as compared to the glycated product alone. Similarly, ROS production in AgNP treated cells is significantly less compared to only glycated product treated cells. Although DNA damage studies show DNA damage in both GP and GP-AgNP treated cells, fluorescence activated cell sorting analysis demonstrates that glycated products induce cell death by necrosis, while AgNP cause cell death via apoptotic pathways. AgNP have a positive effect on restoring native protein structure deduced from spectral studies, and hence, inferences can be drawn that AgNP have ameliorating effects on glycated induced cytotoxicity observed in osteogenic Saos-2 cells.

The role of acetoacetate in Amadori product formation of human serum albumin

Amadori product is an important and stable intermediate, which is produced during glycation process. It is a marker of hyperglycemia in diabetes mellitus, and its accumulation in the body contributes to microvascular complication of diabetes including diabetic nephropathy and retinopathy. In this study, the effect of acetoacetate on the formation of Amadori products and biophysical properties of human serum albumin (HSA), after incubation with glucose, was investigated using various methods. These included circular dichroism (CD), Fourier transform infrared (FTIR) spectroscopy, and UV-visible and fluorescence spectroscopy. Our results indicated that the production of Amadori products in HSA incubated with glucose (GHSA) was increased in the presence of acetoacetate. We also detected alterations in the secondary and tertiary structure of GHSA, which was increased in the presence of acetoacetate. These changes were attributed to the formation of covalent bonds between the carbonyl group of acetoacetate and the nucleophilic groups (lysine residues) of HSA. Thus, acetoacetate can enhance the production of Amadori products through formation of covalent bonds with biomaterials.

Vitamin D prevents glycation of proteins: an in vitro study

Human serum albumin (HSA) is an important protein involved in the transport of hormones, fatty acids, drugs, and other macromolecules. Under hyperglycemic conditions, this molecule undergoes irreversible modification that affects its structure and function. In this study, we explored the effect of two forms of vitamin D, a nutraceutical, on glycation modification in HSA. The protein was incubated with a physiologically high concentration of glucose in the presence of vitamin D metabolites. After 21 days, samples were tested for secondary structural changes, side chain modification, and the presence of advanced glycation end products. Vitamin D metabolites could reduce glycation modification, albeit only to a small extent. Interaction studies reveal that Vitamin D interaction with HSA can prevent protein glycation.

Acetoacetate promotes the formation of fluorescent advanced glycation end products (AGEs)

Acetoacetate (AA) is an important ketone body, which produces reactive oxygen species (ROS). Advanced glycation end products (AGEs) are defined as final products of glycation process whose production is influenced by the levels of ROS. The accumulation of AGEs in the body contributes to pathogenesis of many diseases including complications of diabetes, and Alzheimer's and Parkinson's disease. Here, we evaluated the impact of AA on production of AGEs upon incubation of human serum albumin (HSA) with glucose. The effect of AA on the AGEs formation of HSA was studied under physiological conditions after incubation with glucose for 35 days. The physical techniques including circular dichroism (CD) and fluorescence spectroscopy were used to assess the impact of AA on formation and structural changes of glycated HSA (GHSA). Our results indicated that the secondary and tertiary structural changes of GHSA were increased in the presence of AA. The fluorescence intensity measurements of AGEs also showed an increase in AGEs formation. Acetoacetate has an activator effect in formation of AGEs through ROS production. The presence of AA may result in enhanced glycation in the presence of glucose and severity of complications associated with accumulation of AGEs.

Alteration of human serum albumin tertiary structure induced by glycation. Spectroscopic study

The modification of human serum albumin (HSA) structure by non-enzymatic glycation is one of the underlying factors that contribute to the development of complications of diabetes and neurodegenerative diseases. The aim of the present work was to estimate how glycation of HSA altered its tertiary structure. Changes of albumin conformation were investigated by comparison of glycated (gHSA) and non-glycated human serum albumin (HSA) absorption spectra, red edge excitation shift (REES) and synchronous spectra. Effect of glycation on human serum albumin tertiary structure was also investigated by (1)H NMR spectroscopy. Formation of gHSA Advanced Glycation End-products (AGEs) caused absorption of UV-VIS light between 310 nm and 400 nm while for non-glycated HSA in this region no absorbance has been registered. Analysis of red edge excitation shift effect allowed for observation of structural changes of gHSA in the hydrophobic pocket containing the tryptophanyl residue. Moreover changes in the microenvironment of tryptophanyl and tyrosyl residues brought about AGEs on the basis of synchronous fluorescence spectroscopy have been confirmed. The influence of glycation process on serum albumin binding to 5-dimethylaminonaphthalene-1-sulfonamide (DNSA), 2-(p-toluidino) naphthalene-6-sulfonic acid (TNS), has been studied. Fluorescence analysis showed that environment of both binding site I and II is modified by galactose glycation.

The hydrolyzable gallotannin, penta-O-galloyl-β-D-glucopyranoside, inhibits the formation of advanced glycation endproducts by protecting protein structure

Glycation is a spontaneous process initiated by a condensation reaction between reducing sugars and proteins that leads to the formation of advanced glycation endproducts (AGEs). The in vivo accumulation of AGEs is associated with several chronic human diseases and, thus, the search for AGE inhibitors is of great research interest. Hydrolyzable tannins (gallotannins and ellagitannins) are bioactive plant polyphenols which show promise as natural inhibitors of glycation and AGE formation. Notably, the gallotannin, 1,2,3,4,6-penta-O-galloyl-β-D-glucose (PGG), is a key intermediate involved in the biosynthesis of hydrolyzable tannins in plants. Herein, we investigated the effects of PGG on the individual stages of protein glycation and on protein structure (using bovine serum albumin; BSA). MALDI-TOF data demonstrated that PGG inhibited early glycation by 75% while the synthetic AGE inhibitor, aminoguanidine (AG), was not active (both at 50 μM). In addition, PGG reduced the formation of middle and late stage AGEs by 90.1 and 60.5%, respectively, which was superior to the positive control, AG. While glycation induced conformational changes in BSA from α-helix to β-sheets (from circular dichroism and congo red binding studies), PGG (at 50 μM) reduced this transition by 50%. Moreover, BSA treated with PGG was more stable in its structure and retained its biophysical properties (based on zeta potential and electrophoretic mobility measurements). The interaction between PGG and BSA was further supported by molecular docking studies. Overall, the current study adds to the growing body of data supporting the anti-AGE effects of hydrolyzable tannins, a ubiquitous class of bioactive plant polyphenols.

Carbonyl scavenging and chemical chaperon like function of essential amino acids attenuates non-enzymatic glycation of albumin

Essential amino acids showed potent antiglycation activity by preventing formation of both early and advanced glycation end products (AGEs).

Salivary markers of oxidative stress in oral diseases

Saliva is an interesting alternative diagnostic body fluid with several specific advantages over blood. These include non-invasive and easy collection and related possibility to do repeated sampling. One of the obstacles that hinders the wider use of saliva for diagnosis and monitoring of systemic diseases is its composition, which is affected by local oral status. However, this issue makes saliva very interesting for clinical biochemistry of oral diseases. Periodontitis, caries, oral precancerosis, and other local oral pathologies are associated with oxidative stress. Several markers of lipid peroxidation, protein oxidation and DNA damage induced by reactive oxygen species can be measured in saliva. Clinical studies have shown an association with oral pathologies at least for some of the established salivary markers of oxidative stress. This association is currently limited to the population level and none of the widely used markers can be applied for individual diagnostics. Oxidative stress seems to be of local oral origin, but it is currently unclear whether it is caused by an overproduction of reactive oxygen species due to inflammation or by the lack of antioxidants. Interventional studies, both, in experimental animals as well as humans indicate that antioxidant treatment could prevent or slow-down the progress of periodontitis. This makes the potential clinical use of salivary markers of oxidative stress even more attractive. This review summarizes basic information on the most commonly used salivary markers of oxidative damage, antioxidant status, and carbonyl stress and the studies analyzing these markers in patients with caries or periodontitis.

Dicarbonyl Induced Structural Perturbations Make Histone H1 Highly Immunogenic and Generate an Auto-Immune Response in Cancer

Increased oxidative stress under hyperglycemic conditions, through the interaction of AGEs with RAGE receptors and via activation of interleukin mediated transcription signalling, has been reported in cancer. Proteins modifications are being explored for their roles in the development and progression of cancer and autoantibody response against them is gaining interest as a probe for early detection of the disease. This study has analysed the changes in histone H1 upon modification by methylglyoxal (MG) and its implications in auto-immunopathogenesis of cancer. Modified histone showed modifications in the aromatic residues, changed tyrosine microenvironment, intermolecular cross linking and generation of AGEs. It showed masking of hydrophobic patches and a hypsochromic shift in the in ANS specific fluorescence. MG aggressively oxidized histone H1 leading to the accumulation of reactive carbonyls. Far UV CD measurements showed di-carbonyl induced enhancement of the alpha structure and the induction of beta sheet conformation; and thermal denaturation (Tm) studies confirmed the thermal stability of the modified histone. FTIR analysis showed amide I band shift, generation of a carboxyethyl group and N-Cα vibrations in the modified histone. LCMS analysis confirmed the formation of Nε-(carboxyethyl)lysine and electron microscopic studies revealed the amorphous aggregate formation. The modified histone showed altered cooperative binding with DNA. Modified H1 induced high titre antibodies in rabbits and the IgG isolated form sera of rabbits immunized with modified H1 exhibited specific binding with its immunogen in Western Blot analysis. IgG isolated from the sera of patients with lung cancer, prostate cancer, breast cancer and cancer of head and neck region showed better recognition for neo-epitopes on the modified histone, reflecting the presence of circulating autoantibodies in cancer. Since reports suggest a link between AGE-RAGE axis and carcinogenesis, glycoxidation of histone H1 and its immunogenicity paves ways for understanding role of glycoxidatively damaged nuclear proteins in cancer.

Protection by beta-Hydroxybutyric acid against insulin glycation, lipid peroxidation and microglial cell apoptosis

BACKGROUND

Diabetes mellitus is characterized jointly by hyperglycemia and hyperinsulinemia that make insulin more prone to be glycated and evolve insulin advanced glycation end products (Insulin- AGE). Here, we report the effect of beta-hydroxy butyrate (BHB) (the predominant ketone body) on the formation of insulin-AGE, insulin glycation derived liposomal lipid peroxidation and insulin-AGE toxicity in microglial cells.

METHODS

The inhibitory effect of BHB was monitored as a result of insulin incubation in the presence of glucose or fructose using AGE-dependent fluorescence, Tyr fluorescence as well as anilinonaphthalenesulfonate (ANS) andthioflavin T (ThT) binding, and circular dichroism (CD) investigations. To study lipid peroxidation induced by insulin glycation, thiobarbituric acid (TBA) assay and thiobarbituric acid reactive substance (TBARS) monitoring were used. The effect of insulin-AGE on microglial viability was investigated by 3-(4, 5 dimethylthiazol-2-yl)-2, 5-diphenyltetrazoliumbromide (MTT) cell assay and Annexin V/propidium iodide (PI) staining.

RESULTS

Here we are reporting the inhibitory effect of BHB on insulin glycation and generation of insulin-AGE as a possible explanation for insulin resistance. Moreover, the protective effect of BHB on consequential glycation derived liposomal lipid peroxidation as a causative event in microglial apoptosis is reported.

CONCLUSION

The reduced insulin fibril formation, structural inertia to glycation involved conformational changes, anti-lipid peroxidation effect, and increasing microglia viability indicated the protective effect of BHB that disclose insight on the possible preventive effect of BHB on Alzheimer's disease.

Pomegranate phenolics inhibit formation of advanced glycation endproducts by scavenging reactive carbonyl species

Advanced Glycation Endproducts (AGEs) are a heterogeneous group of molecules produced from non-enzymatic glycation. Accumulation of AGEs in vivo plays an important role in the pathology of chronic human diseases including type-2 diabetes and Alzheimer's disease. Natural AGEs inhibitors such as the pomegranate (Punica granatum) fruit show great potential for the management of these diseases. Herein, we investigated the in vitro anti-glycation effects of a pomegranate fruit extract (PE), its phenolic constituents [punicalagin (PA), ellagic acid (EA) and gallic acid (GA)], and their in vivo derived colonic metabolites [urolithin A (UA) and urolithin B (UB)]. All of the samples showed anti-glycation activities and PE, PA, and EA were more potent inhibitors than the positive control, aminoguanidine. PE and the purified phenolics also exhibited carbonyl scavenger reactivity. Our study suggests that pomegranate may offer an attractive dietary strategy for the prevention and treatment of AGE-related diseases such as type-2 diabetes and Alzheimer's disease.

Glycation of H1 Histone by 3-Deoxyglucosone: Effects on Protein Structure and Generation of Different Advanced Glycation End Products

Advanced glycation end products (AGEs) culminate from the non-enzymatic reaction between a free carbonyl group of a reducing sugar and free amino group of proteins. 3-deoxyglucosone (3-DG) is one of the dicarbonyl species that rapidly forms several protein-AGE complexes that are believed to be involved in the pathogenesis of several diseases, particularly diabetic complications. In this study, the generation of AGEs (N^ε-carboxymethyl lysine and pentosidine) by 3-DG in H1 histone protein was characterized by evaluating extent of side chain modification (lysine and arginine) and formation of Amadori products as well as carbonyl contents using several physicochemical techniques. Results strongly suggested that 3-DG is a potent glycating agent that forms various intermediates and AGEs during glycation reactions and affects the secondary structure of the H1 protein. Structural changes and AGE formation may influence the function of H1 histone and compromise chromatin structures in cases of secondary diabetic complications.

Dietary CML-enriched protein induces functional arterial aging in a RAGE-dependent manner in mice

SCOPE

Advanced glycation end-products (AGEs) are endogenously produced and are present in food. N(ε)-carboxymethyllysine (CML) is an endothelial activator via the receptor for AGEs (RAGEs) and is a major dietary AGE. This work investigated the effects of a CML-enriched diet and RAGE involvement in aortic aging in mice.

METHODS AND RESULTS

After 9 months of a control diet or CML-enriched diets (50, 100, or 200 μg(CML)/g of food), endothelium-dependent relaxation, RAGE, vascular cell adhesion molecule-1, and sirtuin-1 expression, pulse wave velocity and elastin disruption were measured in aortas of wild-type or RAGE(-/-) male C57BL/6 mice. Compared to the control diet, endothelium-dependent relaxation was reduced in the wild-type mice fed the CML-enriched diet (200 μg(CML)/g) (66.8 ± 12.26 vs. 94.3 ± 2.6%, p < 0.01). RAGE and vascular cell adhesion molecule-1 (p < 0.05) expression were increased in the aortic wall. RAGE(-/-) mice were protected against CML-enriched diet-induced endothelial dysfunction. Compared to control diet, the CML-enriched diet (200 μg(CML)/g) increased the aortic pulse wave velocity (86.6 ± 41.1 vs. 251.4 ± 41.1 cm/s, p < 0.05) in wild-type animals. Elastin disruption was found to a greater extent in the CML-fed mice (p < 0.05). RAGE(-/-) mice fed the CML-enriched diet were protected from aortic stiffening.

CONCLUSION

Chronic CML ingestion induced endothelial dysfunction, arterial stiffness and aging in a RAGE-dependent manner.

Quercetin as a finer substitute to aminoguanidine in the inhibition of glycation products

Non-enzymatic glycation is the addition of a free carbonyl group of a reducing sugar to the free amino groups of proteins, which results in the formation of early and advanced glycation end-products (AGEs). Glycation reaction is profoundly associated with diabetes and its secondary complications, such as nephropathy and neuropathy. Glyoxal is a carbonyl species that reacts rapidly with the free amino groups of proteins to form AGEs. While the formation of AGEs with various glycating agents has previously been demonstrated, no extensive studies have been conducted to assess the role of quercetin in all three stages of glycation (early, intermediate and late). In this study, we report the glycation of HSA (human serum albumin) and its characterization by several spectroscopic techniques. Furthermore, inhibition of products at all stages of glycation was studied by various assays. Spectroscopic analysis suggests structural perturbations in the HSA macromolecule as a result of modification, which might be due to the generation of free radicals and the formation of AGEs. Inhibition in the formation of glycation has established that quercetin is a better and a more potent antiglycating agent than aminoguanidine at all stages of glycation.