Synthesis and antiglycation activity of 3-phenacyl substituted thiazolium salts, new analogs of Alagebrium

After preliminary ab initio calculations, 3-phenacyl substituted thiazolium salts, analogs of Alagebrium, were synthesized and investigated in vitro as glycation reaction inhibitors. The most part of investigations focused on the potential of the title compounds to attenuate the formation of fluorescent AGEs as well on their ability to disrupt the cross-linking formation among glycated proteins. Additionally, the capability of thiazolium salts to deglycate in the reaction of early glycation products with nitroblue tetrazolium was determined. Cytotoxicological properties of the title compounds were evaluated using LDH and MTT assays. The leader compound (3-[2-(biphenyl-4-yl)-2-oxoethyl]-1,3-thiazol-3-ium bromide) in a 50 mg/kg dose (p.o. 14 days) was further tested within an in vivo carbonyl stress model (rats, methylglyoxal 86.25 mg/kg/d, i.p., 14 days). As a result, the leader-molecule revealed a high effectiveness against all three examined mechanisms of glycation reaction inhibition in in vitro tests and was able to suppress capacity of methylglyoxal to form AGEs in vivo.

Antiglycation, antifibrillation and antioxidative effects of para coumaric acid and vitamin D; an in-vitro and in-silico comparative-cum-synergistic approach

Diabetes Mellitus is a metabolic disorder that results in impaired utilization of carbohydrates, lipids, and proteins. Severe hyperglycemia is its principal clinical symptom. Human serum albumin (HSA) is used as a model protein since it is viewed as a sign of glycaemic management because it is more likely to get glycated in diabetic people than other proteins. Para-coumaric acid (pCA), a phenolic acid, and Vitamin D (vit-D) are used as protective agents. In the present work, we deduce a synergistic-cum-comparative effect of pCA and vit-D, expecting some improvement in the efficacy of pCA when combined with vit-D. Methods employed are DPPH radical scavenging activity, In-vitro glycation of HSA, UV-vis spectroscopy, fluorescence analysis, and circular dichroism measurement. After treatment, increasein the absorbance and fluorescence intensity were reduced along with normalization of CD value. . The glycation-mediated fibrillation assessed by Congo-Red and Thioflavin T (ThT) were found to be diminishedwhen HSA was treated with equimolar concentration of p-CA and vit-D- treatment. Fructosamine adduct formation and lysine modificationwas also decreased, while inhibition to hemolysis and lipid peroxidation was found to increase upon treatment. The reactive oxygen species generation detection was also performed in lymphocytes treated with various protein samples. Docking results further confirmed theblocking some glycation-prone amino acids by both compounds. The study shows that the combination in the ratio of 1:1 has provided higher overall protection comparable to aminoguanidine (AG), the molecule which is utilized as a positive control.

Non-inhibitory effects of the potent antioxidant C-phycocyanin from Plectonema sp. on the in vitro glycation reaction

When glucose and Amadori products are auto-oxidized, glycation occurs, resulting in the formation of early (Amadori) and late advanced glycation end products (AGEs), as well as free radicals. Glycation and an increase in free radical activity induce diabetic complications. Antioxidant and antiglycation compounds may aid in the prevention of oxidation and glycation. The goal of this study was to assess the antiglycation and antioxidant capacity of C-phycocyanin (C-PC) derived from Plectonema sp. The DPPH (1, 1-diphenyl-2-picrylhydrazyl), nitric oxide, hydroxyl radical scavenging activities and ferric ions reducing antioxidant power (FRAP) assays were used to assess antioxidant activity, while an in vitro bovine serum albumin-methyl glyoxal glycation (BSA-MG) model was used to assess glycation inhibitory potential. Glycation inhibition was measured using a variety of spectroscopic and biochemical parameters, including UV-visible & fluorescence spectroscopy, ketoamine, carbonyl and hydroxymethyl furfural content, as well as free lysine & free arginine estimations. In vitro, C-PC exhibited dose-dependent potent antioxidant activity, but lacked significant antiglycation potential. As a result, it is recommended that further studies be conducted to evaluate the antiglycation potential of C-PC.

Metformin Monotherapy Downregulates Diabetes-Associated Inflammatory Status and Impacts on Mortality

Aging is the main risk factor for developing diabetes and other age-related diseases. One of the most common features of age-related comorbidities is the presence of low-grade chronic inflammation. This is also the case of metabolic syndrome and diabetes. At the subclinical level, a pro-inflammatory phenotype was shown to be associated with Type-2 diabetes mellitus (T2DM). This low to mid-grade inflammation is also present in elderly individuals and has been termed inflammaging. Whether inflammation is a component of aging or exclusively associated with age-related diseases in not entirely known. We used clinical data and biological readouts in a group of individuals stratified by age, diabetes status and comorbidities to investigate this aspect. While aging is the main predisposing factor for several diseases there is a concomitant increased level of pro-inflammatory cytokines. DM patients show an increased level of sTNFRll, sICAM-1, and TIMP-1 when compared to Healthy, Non-DM and Pre-DM individuals. These inflammatory molecules are also associated with insulin resistance and metabolic syndrome in Non-DM and pre-DM individuals. We also show that metformin monotherapy was associated with significantly lower levels of inflammatory molecules, like TNFα, sTNFRI, and sTNFRII, when compared to other monotherapies. Longitudinal follow up indicates a higher proportion of death occurs in individuals taking other monotherapies compared to metformin monotherapy. Together our finding shows that chronic inflammation is present in healthy elderly individuals and exacerbated with diabetes patients. Likewise, metformin could help target age-related chronic inflammation in general, and reduce the predisposition to comorbidities and mortality.

Methylglyoxal produces more changes in biochemical and biophysical properties of human IgG under high glucose compared to normal glucose level

Hyperglycaemia triggers increased production of methylglyoxal which can cause gross modification in proteins’ structure vis-a-vis function though advanced glycation end products (AGEs). The AGEs may initiate vascular and nonvascular pathologies. In this study, we have examined the biochemical and biophysical changes in human IgG under normal and high glucose after introducing methylglyoxal into the assay mixture. This non-enzymatic reaction mainly engaged lysine residues as indicated by TNBS results. The UV results showed hyperchromicity in modified-IgG samples while fluorescence data supported AGEs formation during the course of reaction. Shift in amide I and amide II band position indicated perturbations in secondary structure. Increase carbonyl content and decrease in sulfhydryl suggests that the modification is accompanied by oxidative stress. All modified-IgG samples showed more thermostability than native IgG; the highest Tm was shown by IgG-high glucose-MGO variant. Results of ANS, Congo red and Thioflavin T dyes clearly suggest increase in hydrophobic patches and aggregation, respectively. SEM and TEM images support aggregates generation in modified-IgG samples.

Glycation in human fingernail clippings using ATR-FTIR spectrometry, a new marker for the diagnosis and monitoring of diabetes mellitus

OBJECTIVES

Although HbA1c is a good diagnostic tool for diabetes, the precarity of the health system and the costs limit the use of this biomarker in developing countries. Fingernail clippings contain ±85% of keratins, which are prone to glycation. Nail keratin glycation may reflect the average glycemia over the last months. We explored if attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) can be used as a non-invasive tool for assessing glycation in diabetes.

DESIGN AND METHODS

Using ATR-FTIR spectroscopy, glycation and deglycation experiments with fructosamine 3-kinase allowed to identify the spectrum that corresponds with keratin glycation in fingernail clippings. Clippings of 105 healthy subjects and 127 diabetics were subjected to the standardized ATR-FTIR spectroscopy method.

RESULTS

In vitro glycation resulted in an increased absorption at 1047cm^-1. Following enzymatic deglycation, this peak diminished significantly, proving that the AUC between 970 and 1140cm^-1 corresponded with glycated proteins. Within-run CV of the assay was 3%. Storage of nail clippings at 37°C for 2weeks did not significantly change results. In diabetics, glycated nail protein concentrations (median: 1.51μmol/g protein, IQR: 1.37-1.85μmol/g protein) were significantly higher than in the controls (median: 1.19μmol/g protein, IQR: 1.09-1.26μmol/g protein) (p<0.0001). ROC analysis yielded an AUC of 0.92 at a cut-off point of 1.28μmol/g nail (specificity: 82%; sensitivity: 90%). No correlation was observed between the glycated nail protein concentrations and HbA1c.

CONCLUSIONS

Protein glycation analysis in fingernails with ATR-FTIR spectroscopy could be an alternative affordable technique for diagnosing and monitoring diabetes. As the test does not consume reagents, and the preanalytical phase is extremely robust, the test could be particularly useful in developing countries.

Silybin, a flavonolignan from milk thistle seeds, restrains the early and advanced glycation end product modification of albumin

Silybin exhibited a protective effect towards the non-enzymatic glycation mediated structure functional changes in albumin.

N-(3-Aminoalkyl)proline derivatives with potent antigycation activity

The synthesis and anti-glycation properties of non-naturalN-aminoalkylproline derivatives are reported. They also exhibit good anti-oxidant properties and may be useful in the treatment of complications that arise as a result of increased glycation.

Protein-phenolic interactions and inhibition of glycation - combining a systematic review and experimental models for enhanced physiological relevance

BACKGROUND

while antiglycative capacity has been attributed to (poly)phenols, the exact mechanism of action remains unclear. Studies so far are often relying on supra-physiological concentrations and use of non-bioavailable compounds.

METHODS

to inform the design of a physiologically relevant in vitro study, we carried out a systematic literature review of dietary interventions reporting plasma concentrations of polyphenol metabolites. Bovine Serum Albumin (BSA) was pre-treated prior to in vitro glycation: either no treatment (native), pre-oxidised (incubated with 10 nM H2O2, for 8 hours) or incubated with a mixture of phenolic acids at physiologically relevant concentrations, for 8 hours). In vitro glycation was carried out in the presence of (i) glucose only (0, 5 or 10 mM), (ii) glucose (0, 5 or 10 mM) plus H2O2 (10 nM), or (iii) glucose (0, 5 or 10 mM) plus phenolic acids (10-160 nM). Fructosamine was measured using the nitro blue tetrazolium method.

RESULTS

following (high) dietary polyphenol intake, 3-hydroxyphenylacetic acid is the most abundant phenolic acid in peripheral blood (up to 338 μM) with concentrations of other phenolic acids ranging from 13 nM to 200 μM. The presence of six phenolic acids with BSA during in vitro glycation did not lower fructosamine formation. However, when BSA was pre-incubated with phenolic acids, significantly lower concentration of fructosamine was detected under glycoxidative conditions (glucose 5 or 10 mM plus H2O2 10 nM) (p < 0.001 vs. native BSA).

CONCLUSION

protein pre-treatment, either with oxidants or phenolic acids, is an important regulator of subsequent glycation in a physiologically relevant system. High quality in vitro studies under conditions closer to physiology are feasible and should be employed more frequently.

Role of oxidative stress in physiological albumin glycation: a neglected interaction

Protein glycation is a key mechanism involved in chronic disease development in both diabetic and nondiabetic individuals. About 12-18% of circulating proteins are glycated in vivo in normoglycemic blood, but in vitro studies have hitherto failed to demonstrate glucose-driven glycation below a concentration of 30mM. Bovine serum albumin (BSA), reduced BSA (mercaptalbumin) (both 40g/L), and human plasma were incubated with glucose concentrations of 0-30mM for 4 weeks at 37°C. All were tested preoxidized for 8h before glycation with 10nM H2O2 or continuously exposed to 10nM H2O2 throughout the incubation period. Fructosamine was measured (nitroblue tetrazolium method) at 2 and 4 weeks. Oxidized BSA (both preoxidized and continuously exposed to H2O2) was more readily glycated than native BSA at all glucose concentrations (p = 0.03). Moreover, only oxidized BSA was glycated at physiological glucose concentration (5mM) compared to glucose-free control (glycation increased by 35% compared to native albumin, p < 0.05). Both 5 and 10mM glucose led to higher glycation when mercaptalbumin was oxidized than when unoxidized (p < 0.05). Fructosamine concentration in human plasma was also significantly higher when oxidized and exposed to 5mM glucose, compared to unoxidized plasma (p = 0.03). The interaction between glucose concentration and oxidation was significant in all protein models (p < 0.05). This study has for the first time demonstrated albumin glycation in vitro, using physiological concentrations of albumin, glucose, and hydrogen peroxide, identifying low-grade oxidative stress as a key element early in the glycation process.

D-Ribosylated Tau forms globular aggregates with high cytotoxicity

Although the glycation of Tau that is involved in paired helical filament formation in Alzheimer's disease has been widely studied, little attention has been paid to the role of D-ribose in the glycation of Tau. Here, we show that Tau is rapidly glycated in the presence of D-ribose, resulting in oligomerization and polymerization. Glycated derivatives appeared after 24 h incubation. Western blotting indicated the formation of advanced glycation end-products (AGEs) during initial stages of glycation. Thioflavin T-positive (ThT-positive) aggregations that appeared from day 4 indicated the globular-like features. Atomic force microscopy revealed that the surface morphology of ribosylated Tau40 was globular-like. Kinetic studies suggested that D-ribosylated Tau is slowly oligomerized and rapidly polymerized with ThT-positive features. Moreover, D-ribosylated Tau aggregates were highly toxic to SHSY5Y cells and resulted in both apoptosis and necrosis. This work has demonstrated that D-ribose reacted with Tau protein rapidly, producing ThT-positive aggregations which had high cytotoxicity.

Are free radical reactions increased in the diabetic eye?

Reactive oxygen species (ROS) are thought to play a significant role in the development of diabetic retinopathy; however, no direct evidence supports ROS generation in vivo. This study used in vivo electron spin resonance (ESR) spectroscopy with a surface resonator to detect local free radical reactions. The ESR signal decay of carbamoyl-PROXYL was enhanced in the eyes of streptozotocin (STZ)-induced diabetic mice. This enhanced signal decay was suppressed by the administration of SOD or the pretreatment with aminoguanidine. We demonstrate, for the first time, specific free radical reactions in the eyes of mice with STZ-induced diabetes.

Novel fluorescent sensing system for alpha-fructosyl amino acids based on engineered fructosyl amino acid binding protein

A novel fluorescent sensing system for alpha-glycated amino acids was created based on fructosyl amino acid binding protein (FABP) from Agrobacterium tumefaciens. The protein was found to bind specifically to the alpha-glycated amino acids fructosyl glutamine (Fru-Gln) and fructosyl valine (Fru-Val) while not binding to epsilon-fructosyl lysine. An Ile166Cys mutant of FABP was created by genetic engineering and modified with the environmentally sensitive fluorophore acrylodan. The acrylodan-conjugated mutant FABP showed eight-fold greater sensitivity to Fru-Val than the unconjugated protein and could detect concentrations as low as 17 nM, making it over 100-fold more sensitive than enzyme-based detection systems. Its high sensitivity and specificity for alpha-substituted fructosyl amino acids makes the new sensing system ideally suited for the measurement of hemoglobin A1c (HbA1c), a major marker of diabetes.

Site-specific synthesis of Amadori-modified peptides on solid phase

Glycation of peptides and proteins is a slow chemical reaction of reducing sugars modifying the amino groups. The first intermediates of this nonenzymatic glycosylation are the Amadori products that can undergo further chemical reactions, finally leading to advanced glycation end products (AGEs). The formation of AGEs was not only linked to aging of tissues and organs in general but also to several diseases such as diabetes mellitus and Alzheimer's disease. Because of the importance of these modifications and their potential use as diagnostic markers, a global postsynthetic approach on solid phase was developed. The peptides were synthesized by Fmoc/(t)Bu-chemistry, with the lysine residue to be modified being protected with the very acid-labile methyltrityl group. Incubation of the peptides with D-glucose in DMF at elevated temperatures resulted in product yields of 35%. Neighboring residues with bulky protecting groups reduced the yields only slightly. The major by-products were the unmodified peptide and an oxidation product. Whereas the unmodified peptide eluted before the glycated peptide, all other by-products eluted later in RP-HPLC, allowing simple purification.

SocA is a novel periplasmic binding protein for fructosyl amino acid

Bacterial periplasmic proteins (bPBPs) undergo drastic conformational changes upon binding substrate, making them appealing as novel molecular recognition tools for biosensing. A putative bPBP-encoding gene, socA, belongs to the soc operon responsible for santhopine (fructosyl glutamine, FQ) catabolism of Agrobacterium tumefaciens. The socA gene was isolated and expressed in Escherichia coli as a soluble 28.8kDa periplasmic protein to investigate its properties as a potential bPBP for fructosyl amino acid (FA). The autofluorescence of SocA was used to monitor the protein's conformational change resulting from substrate binding. The fluorescence intensity changed upon binding FQ in a concentration dependent manner with a calculated K(d) of 2.1muM, but was unaffected by the presence of sugars or amino acid. Our results demonstrate that SocA is a novel FA bPBP that can be utilized as a novel molecular recognition element for the monitoring of FA.

Intervention against the Maillard reaction in vivo

The field of Maillard/glycation reactions in vivo has grown enormously during the past 20 years, going from 25 to 500 publications per year. It is now well recognized that many of the "advanced" products form oxidatively or anaerobically and can have deleterious effects on macromolecular and biological function. The feasibility of developing pharmacological agents with beneficial in vivo properties, based on in vitro inhibition of glycation, has been surprisingly successful. This Editorial sets the stage for a series of articles by experts in the field, who have made key contributions to our understanding of the Maillard reaction in vivo.

Paradoxical impact of antioxidants on post-Amadori glycoxidation: Counterintuitive increase in the yields of pentosidine and Nepsilon-carboxymethyllysine using a novel multifunctional pyridoxamine derivative

The inhibition of post-Amadori advanced glycation end product (AGE) formation by three different classes of AGE inhibitors, carbonyl group traps, chelators, and radical-trapping antioxidants, challenge the current paradigms that: 1) AGE inhibitors will not increase the formation of any AGE product, 2) transition metal ions are required for oxidative formation of AGE, and 3) screening AGE inhibitors only in systems containing transition metal ions represents a valid estimate of potential in vivo mechanisms. This work also introduces a novel multifunctional AGE inhibitor, 6-dimethylaminopyridoxamine (dmaPM), designed to function as a combined carbonyl trap, metal ion chelator, and radical-trapping antioxidant. Other AGE inhibitors including pyridoxamine, aminoguanidine, o-phenylenediamine, dipyridoxylamine, and diethylenetriaminepentaacetic acid were also examined. The results during uninterrupted and interrupted ribose glycations show: 1) an unexpected increase in the yield of pentosidine in the presence of radical-trapping phenolic antioxidants such as Trolox and dmaPM, 2) significant formation of Nepsilon-carboxymethyllysine (CML) in the presence of strong chelators and phenolic antioxidants, which implies that there must be nonradical routes to CML, 3) prevention of intermolecular cross-links with radical-trapping inhibitors, and 4) that dmaPM shows excellent inhibition of AGE. Glucose glycations reveal the expected inhibition of pentosidine and CML with all compounds tested, but in a buffer free of trace metal ions the yield of CML in the presence of radical-trapping antioxidants was between the metal ion-free and metal ion-containing controls. Protein molecular weight analyses support the conclusion that Amadori decomposition pathways are constrained in the presence of metal ion chelators and radical traps.

Glucose enhancement of LDL oxidation is strictly metal ion dependent

Recent evidence suggests that lipoprotein oxidation is increased in diabetes, however, the mechanism(s) for such observations are not clear. We examined the effect of glucose on low-density lipoprotein (LDL) oxidation using metal ion-dependent and -independent oxidation systems. Pathophysiological concentrations of glucose (25 mM) enhanced copper-induced LDL oxidation as determined by conjugated diene formation or relative electrophoretic mobility (REM) on agarose gels. Similarly, iron-induced LDL oxidation was stimulated by glucose resulting in 4- to 6-fold greater REM than control incubations without glucose. In contrast, glucose had no effect on metal ion-independent LDL oxidation by aqueous peroxyl radicals. The effect of glucose on metal ion-dependent LDL oxidation was associated with enhanced reduction of metal ions, and in the case of iron-induced LDL oxidation, was completely inhibited by superoxide dismutase. The effect of glucose was mimicked by other reducing sugars, such as fructose and mannose, and the extent to which each sugar enhanced LDL oxidation was closely linked to its metal ion-reducing activity. Thus, promotion of LDL oxidation by glucose is specific for metal ion-dependent oxidation and involves increased metal ion reduction. These results provide one potential mechanism for enhanced LDL oxidation in diabetes.

Spontaneous generation of superoxide anion by human lens proteins and by calf lens proteins ascorbylated in vitro

The proteins isolated from aged human lenses and brunescent cataracts exhibit extensive disulfide bond formation. Diabetic rat lenses similarly contain disulfide-bonded protein aggregates. These observations are consistent with the known link between diabetes, glycation and oxidative damage, and suggest a role for reactive oxygen species (ROS) in this process. To assess whether the glycation-related modifications in human lens proteins spontaneously generate ROS, superoxide anion formation was measured using both cataractous lens proteins and calf lens proteins glycated in vitro with ascorbic acid (ascorbylated). The water-insoluble fraction from aged normal human lenses generated 0.3-0.6 nmol superoxide h(-1)mg protein(-1), whereas the activity increased to 0.5-1.8 nmol h(-1)mg protein(-1)with the WI fraction from brunescent cataracts, and 2.3 nmol h(-1)mg protein(-1)with calf lens proteins ascorbylated for 4 weeks in vitro. The activity in the human lens proteins was observed in both the water-soluble and water-insoluble fractions, and was completely dependent upon the presence of oxygen. The pH optimum curve for superoxide formation increased from pH 6.5 to 10 with both the cataract and ascorbylated proteins. The superoxide-generating activity in human lens was completely bound to a boronate affinity column, but only partially bound with the ascorbylated proteins. The superoxide anion produced by a 5 m m solution of purified N(epsilon)-fructosyl-lysine was barely detectable, and therefore, could not account for the superoxide formed by any of the lens protein preparations. Also, superoxide formation increased 10-fold at pH 8.8 with fructosyl-lysine, but only 1.3-1.8-fold with human lens proteins. The addition of copper-stimulated superoxide formation with glycated bovine serum albumin, but no stimulation was seen with cataractous proteins. Assays of specific compounds showed that catechol, hydroquinone, 3-OH kynurenine and 3-OH anthranylic acid exhibited the greatest activity for superoxide generation, but had a very short halflife. 2,3-Dihydroxypyridine and 4,5 dihydroxynaphthalene were one and two orders of magnitude less reactive. In long-term incubations at 37 degrees, cataractous proteins retained the potential to produce superoxide anion, losing only half of the initial activity after 6-7 days. Therefore, the water-insoluble fraction from aged human lenses and dark brown cataracts are potentially capable of generating >100 nmol mg protein(-1)and >170 nmol mg protein(-1)of superoxide anion respectively, likely due to the presence of advanced glycation endproducts in human lens proteins. This spontaneous generation of superoxide anion in vivo could account for a major portion of the oxidation of sulfur amino acids seen during aging and cataract formation.

The generation of superoxide anions in glycation reactions with sugars, osones, and 3-deoxyosones

Glycoxidation is a process whereby glycated proteins chemically generate oxygen free radicals. Superoxide anion formation was measured by the superoxide dismutase-dependent reduction of ferricytochrome C in glycation reactions at pH 7.0 in the absence of transition metal ions. Assays were linear over 1 h, and most activity was seen after a 2 d incubation of 5 mM L-threose and 10 mM alpha-N-acetyl-lysine (N-Ac-Lys) or 10 mg/mL RNase A. Trioses, tetroses and their corresponding osones and 3-deoxyosones had the highest activity (12-16 nmoles O.-2/hr/ml) with N-Ac-Lys. Osones and 3-deoxyosones alone generated considerable O.-2, whereas aldose sugars largely did not. Xylosone and 3-deoxyxylosone produced 6 and 10 nmoles O.-2/hr/ml respectively with N-Ac-Lys, however, xylose was inactive, as were glucose and fructose. Glycation assays with 3-deoxyglucosone and glyoxal showed no activity, however, methyl glyoxal generated 1.7 and 2.0 nmoles O.-2/hr/ml with N-Ac-Lys and N-Ac-Arg, respectively. Therefore, Amadori compounds composed of lysine and short chain sugars can rapidly generate superoxide anion in the absence of metal ions.