Potential inhibitory effect of highland barley protein hydrolysates on the formation of advanced glycation end-products (AGEs): A mechanism study

Advanced glycation end products (AGEs) can be caused during a glycoxidation reaction. This reaction is associated with complications of diabetes and the consequences of health problems. Therefore, we are exploring the prohibitory effect of highland barley protein hydrolysates (HBPHs) on AGE formation. Herein, first extracted the protein from highland barley with various pH conditions and then hydrolyzed using four different proteolytic enzymes (flavourzyme, trypsin, papain, pepsin) under different degrees of hydrolysis. We assessed three degrees of hydrolysates (lowest, middle, highest) of enzymes used to characterize the antioxidant activity and physicochemical properties. Among all the hydrolysates, flavourzyme-treated hydrolysates F-1, F-2, and F-3 indicated the high ability to scavenge DPPH (IC50 values of 0.97 %, 0.63 %, and 0.90 %), structural and functional properties. Finally, the inhibitory effect of the most active hydrolysates F-1, F-2, and F-3 against the AGEs formation was evaluated in multiple glucose-glycated bovine serum albumin (BSA) systems. Additionally, in a BSA system, F-3 exhibited the strong antiglycation activity, effectively suppressed the non-fluorescent AGE (CML), and the fructosamine level. Moreover, it decreased carbonyl compounds while also preventing the loss of thiol groups. Our results would be beneficial in the application of the food industry as a potential antiglycation agent for several chronic diseases.

Co-administration of "L-Lysine, Vitamin C, and Zinc" increased the antioxidant activity, decreased insulin resistance, and improved lipid profile in streptozotocin-induced diabetic rats

PURPOSE

We previously showed the beneficial effect of L-Lysine (Lys), a chemical chaperone, on reducing diabetic complications in diabetic rats and type 2 diabetic patients. Herein, we evaluated the effect of Lys co-administration with Vitamin C and Zinc (Lys+VC+Zn), in diabetic rats.

METHODS

The streptozotocin (50 mg/Kg) was injected into male adult Wistar rats to induce diabetes. Then, different groups of normal and diabetic rats were treated with Lys and Lys+VC+Zn for five months. So, there were 0.1 % Lys in the drinking water of both groups. The control groups received water alone. During the experiment, the body weight, and various parameters were determined in the blood, serum/plasma, and urine of the rats.

RESULTS

The determination of biochemical indexes confirmed diabetes induction and its complications in rats. Treatment with either Lys or Lys+VC+Zn resulted in reduced blood glucose and protein glycation (decreasing AGEs and HbA1c), increased insulin secretion, alleviated insulin resistance and HOMA-IR, improved lipid profile and HDL functionality (LCAT and PON1), enhanced antioxidant status (FRAP and AOPP), improved kidney function (decreased microalbuminuria, serum urea, and creatinine), and increased chaperone capacity (HSP70). Lys+VC+Zn showed better effects on these parameters than Lys alone.

CONCLUSIONS

The results of this study indicated that co-administration of Lys, a chemical chaperone, with two antioxidants (VC and Zn) potentiates its antidiabetic effects and prevent diabetic complications in rat model of diabetes.

Insight into the Effect of Methylglyoxal on the Conformation, Function, and Aggregation Propensity of α-Synuclein

It is well-known that people suffering from hyperglycemia have a higher propensity to develop Parkinson's disease (PD). One of the most plausible mechanisms linking these two pathologies is the glycation of neuronal proteins and the pathological consequences of it. α-Synuclein, a key component in PD, can be glycated at its fifteen lysine. In fact, the end products of this process have been detected on aggregated α-synuclein isolated from in vivo. However, the consequences of glycation are not entirely clear, which are of crucial importance to understand the mechanism underlying the connection between diabetes and PD. To better clarify this, we have here examined how methylglyoxal (the most important carbonyl compound found in the cytoplasm) affects the conformation and aggregation propensity of α-synuclein, as well as its ability to cluster and fuse synaptic-like vesicles. The obtained data prove that methylglyoxal induces the Lys-Lys crosslinking through the formation of MOLD. However, this does not have a remarkable effect on the averaged conformational ensemble of α-synuclein, although it completely depletes its native propensity to form soluble oligomers and insoluble amyloid fibrils. Moreover, methylglyoxal has a disrupting effect on the ability of α-synuclein to bind, cluster and fusion synaptic-like vesicles.

Novel inhibitory effect of black chokeberry (Aronia melanocarpa) from selected eight berries extracts on advanced glycation end-products formation and corresponding mechanism study

Numerous health hazards have been connected to advanced glycation end products (AGEs). In this investigation, using reaction models including BSA-fructose, BSA- methylglyoxal (MGO), and BSA-glyoxal (GO), we examined the anti-glycation potential of eight different berry species on AGEs formation. Our results indicate that black chokeberry (Aronia melanocarpa) exhibited the highest inhibitory effects, with IC50 values of 0.35 ± 0.02, 0.45 ± 0.03, and 0.48 ± 0.11 mg/mL, respectively. Furthermore, our findings suggest that black chokeberry inhibits AGE formation by binding to BSA, which alleviates the conformation alteration, prevents protein cross-linking, and traps reactive α-dicarbonyls to form adducts. Notably, three major polyphenols, including cyanidin-3-O-galactoside, cyanidin-3-O-arabinoside, and procyanidin B2 from black chokeberry, showed remarkably inhibitory effect on MGO/GO capture, and new adducts formation was verified through LC-MS/MS analysis. In summary, our research provides a theoretical basis for the use of berries, particularly black chokeberry, as natural functional food components with potential anti-glycation effects.

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.

Occurrence of dietary advanced glycation end-products in commercial cow, goat and soy protein based infant formulas

Thermal treatment is a key step during infant formula (IF) processing which causes protein glycation and formation of dietary advanced glycation end-products (dAGEs). This study aimed to evaluate the glycation degree in IF in relation to the ingredients of the formula. dAGEs concentrations have been determined by UPLC-MS/MS in a range of commercial cow-based, goat-based, and soy-based IF. Results indicated that the protein source, protein composition, and amount and type of carbohydrates determines the level of protein glycation in IFs. The investigated soy-based formula had significant higher concentrations of arginine and arginine-derived dAGEs than cow-based and goat-based formulas. IF containing hydrolyzed proteins had higher dAGEs concentrations than those containing intact proteins. Lactose-containing formula was more prone to glycation than those containing sucrose and maltodextrin. Data showed glycation degree in IF cannot be estimated by a single compound, but the complete picture of the dAGEs should be considered.

The antidiabetic effect of methanolic extract of Holarrhena pubescens seeds is mediated through multiple mechanisms of action

Holarrhena pubescens is widely used in Indian and Chinese medicine in the treatment of diabetes. The current work determined the oral hypoglycemic and antidiabetic effects of seed extract in rats. The probable mechanism of action was evaluated in-vitro by α - glucosidase inhibition, glucose metabolism in insulinoma (INS-1) cells to reflect secretion of insulin, and protein glycation inhibition. Its potential for herb-drug interaction was evaluated in the cytochrome P450 3A4 (CYP3A4) inhibition assay. The seed extract increased serum insulin levels and reduced serum blood glucose levels in the oral glucose tolerance test. It also reduced the serum glucose levels in streptozocin-induced diabetes. The extract also inhibited α -glucosidase enzyme activity and demonstrated that it can increase the secretion of insulin from INS to 1-rat insulinoma cell line cells in-vitro in a concentration-dependent manner. However, it had a very weak inhibitory effect on protein glycation and it did not affect the activity of CYP3A4. The results of the study showed that H. pubescens seed extract increases insulin secretion and inhibits glucose absorption both in-vivo and in-vitro with a weak protein glycation inhibitory effect. The herb is devoid of CYP3A4 inhibitory effect indicating that it may not have pharmacokinetic interaction with the drug metabolized by this enzyme.

Biochemical and Biophysical in Vitro Studies and Systematic Literature Review on the Antioxidant and Antiglycation Activities of Trazodone

BACKGROUND/AIMS

Trazodone is a selective serotonin reuptake inhibitor; however, other mechanisms of the drug's anti-depressive properties have also been postulated. Hence, the aim of the study was to perform a systematic review and assess antiglycoxidative properties of trazodone in in vitro models.

METHODS

Trazodone's scavenging and chelating properties were measured with spectrophotometric method. The impact of the drug on carbonyl/oxidative stress was marked in the bovine serum albumin (BSA) model where sugars (glucose, fructose, galactose, ribose) and aldehydes (glyoxal and methylglyoxal) were used as glycation agents. Aminoguanidine and N-acetylcysteine (NAC) were applied as reference glycation/free radical inhibitors. Glycation biomarkers (kynurenine, N-formylkynurenine, dityrosine as well as advanced glycation end products contents) were assessed spectrofluorometrically. Concentrations of oxidation parameters (total thiols (TTs), protein carbonyls (PCs) and also advanced oxidation protein products (AOPPs) levels) were determined spectrophotometrically.

RESULTS

We demonstrated that trazodone poorly scavenged radicals (hydroxyl radical, nitric oxide, hydrogen peroxide and 2,2-diphenyl-1-picrylhydrazyl radical) and showed low ferrous ion chelating, unlike aminoguanidine and NAC. Sugars/aldehydes caused enhancement of glycation parameters, as well as a decrease of TTs and an increase of PCs and AOPPs levels compared to BSA incubated alone. Trazodone did not reduce oxidation parameters to the baseline (BSA) and significantly exacerbated glycation markers in comparison with both BSA and BSA+glycators. The content of glycation products was markedly lower in aminoguanidine and NAC than in trazodone. The molecular docking of trazodone to BSA revealed its very low affinity, which may indicate non-specific binding of trazodone, facilitating the attachment of glycation factors.

CONCLUSION

According to our findings, it may be concluded that trazodone poorly counteracts oxidation and intensifies glycation in vitro. A possible mechanism for antiglycoxidative effect of trazodone in vivo may be the enhancement of the body's adaptive response, as indicated by the results of our systematic review.

Penta-O-galloyl-β-d-glucose inhibits the formation of advanced glycation end-products (AGEs): A mechanistic investigation

Penta-O-galloyl-β-d-glucose (PGG) was prepared from tannic acid methanolysis products based on HSCCC, and its protective effects and mechanism on the glucose-induced glycation were investigated for the first time. PGG was confirmed to exhibit strong anti-AGEs effects in bovine serum albumin (BSA)-glucose (Glu) and BSA-methylglyoxal (MGO) glycation systems. It was showed that PGG could inhibit the AGEs formation by blocking glycated intermediates (fructosamine and α-dicarbonyl compounds), eliminating radicals, and chelating metal-ions. In-depth mechanism analysis proved that PGG could prevent BSA from glycation by hindering the accumulation of amyloid fibrils, stabilizing the BSA secondary structures, and binding the partial glycation sites. Furthermore, PGG exhibited a prominent trapping capacities on the reactive intermediate MGO by generating PGG-mono-MGO adduct. This research indicated that PGG could be an effective agent to block Glu/MGO-triggered glycation and offered new insights into PGG as a functional ingredient in food materials for preventing diabetic syndrome.

Effect of crowding, compartmentalization and nanodomains on protein modification and redox signaling - current state and future challenges

Biological milieus are highly crowded and heterogeneous systems where organization of macromolecules within nanodomains (e.g. membraneless compartments) is vital to the regulation of metabolic processes. There is an increasing interest in understanding the effects that such packed environments have on different biochemical and biological processes. In this context, the redox biochemistry and redox signaling fields are moving towards investigating oxidative processes under conditions that exhibit these key features of biological systems in order to solve existing paradigms including those related to the generation and transmission of specific redox signals within and between cells in both normal physiology and under conditions of oxidative stress. This review outlines the effects that crowding, nanodomain formation and altered local viscosities can have on biochemical processes involving proteins, and then discusses some of the reactions and pathways involving proteins and oxidants that may, or are known to, be modulated by these factors. We postulate that knowledge of protein modification processes (e.g. kinetics, pathways and product formation) under conditions that mimic biological milieus, will provide a better understanding of the response of cells to endogenous and exogenous stressors, and their role in ageing, signaling, health and disease.

Decreased methylglyoxal-mediated protein glycation in the healthy aging mouse model of ectopic expression of UCP1 in skeletal muscle

Mice with ectopic expression of uncoupling protein-1 (UCP1) in skeletal muscle exhibit a healthy aging phenotype with increased longevity and resistance to impaired metabolic health. This may be achieved by decreasing protein glycation by the reactive metabolite, methylglyoxal (MG). We investigated protein glycation and oxidative damage in skeletal muscle of mice with UCP1 expression under control of the human skeletal actin promoter (HSA-mUCP1) at age 12 weeks (young) and 70 weeks (aged). We found both young and aged HSA-mUCP1 mice had decreased advanced glycation endproducts (AGEs) formed from MG, lysine-derived N_ε(1-carboxyethyl)lysine (CEL) and arginine-derived hydroimidazolone, MG-H1, whereas protein glycation by glucose forming N_ε-fructosyl-lysine (FL) was increased ca. 2-fold, compared to wildtype controls. There were related increases in FL-linked AGEs, N_ε-carboxymethyl-lysine (CML) and 3-deoxylglucosone-derived hydroimidazolone 3DG-H, and minor changes in protein oxidative and nitration adducts. In aged HSA-mUCP1 mice, urinary MG-derived AGEs/FL ratio was decreased ca. 60% whereas there was no change in CML/FL ratio - a marker of oxidative damage. This suggests that, normalized for glycemic status, aged HSA-mUCP1 mice had a lower flux of whole body MG-derived AGE exposure compared to wildtype controls. Proteomics analysis of skeletal muscle revealed a shift to increased heat shock proteins and mechanoprotection and repair in HSA-mUCP1 mice. Decreased MG-derived AGE protein content in skeletal muscle of aged HSA-mUCP1 mice is therefore likely produced by increased proteolysis of MG-modified proteins and increased proteostasis surveillance of the skeletal muscle proteome. From this and previous transcriptomic studies, signaling involved in enhanced removal of MG-modified protein is likely increased HSPB1-directed HUWE1 ubiquitination through eIF2α-mediated, ATF5-induced increased expression of HSPB1. Decreased whole body exposure to MG-derived AGEs may be linked to increased weight specific physical activity of HSA-mUCP1 mice. Decreased formation and increased clearance of MG-derived AGEs may be associated with healthy aging in the HSA-mUCP1 mouse.

Crowding modulates the glycation of plasma proteins: In vitro analysis of structural modifications to albumin and transferrin and identification of sites of modification

Protein modification occurs in biological milieus that are characterized by high concentrations of (macro)molecules (i.e. heterogeneous and packed environments). Recent data indicate that crowding can modulate the extent and rate of protein oxidation, however its effect on other post-translational modifications remains to be explored. In this work we hypothesized that crowding would affect the glycation of plasma proteins. Physiologically-relevant concentrations of albumin (35 mg mL^-1) and transferrin (2 mg mL^-1) were incubated with methylglyoxal and glyoxal (5 μM-5 mM), two α-oxoaldehyde metabolites that are elevated in the plasma of people with diabetes. Crowding was induced by adding dextran or ficoll polymers. Electrophoresis, electron microscopy, fluorescence spectroscopy and mass spectrometry were employed to investigate the structural consequences of glycation under crowded conditions. Our data demonstrate that crowding modulates the extent of formation of transferrin cross-links, and also the modification pathways in both albumin and transferrin. Arginine was the most susceptible residue to modification, with lysine and cysteine also affected. Loss of 0.48 and 7.28 arginine residues per protein molecule were determined on incubation with 500 μM methylglyoxal for albumin and transferrin, respectively. Crowding did not influence the extent of loss of arginine and lysine for either protein, but the sites of modification, detected by LC-MS, were different between dilute and crowded conditions. These data confirm the relevance of studying modification processes under conditions that closely mimic biological milieus. These data unveil additional factors that influence the pattern and extent of protein modification, and their structural consequences, in biological systems.

iProtGly-SS: A Tool to Accurately Predict Protein Glycation Site Using Structural-Based Features

Posttranslational modification (PTM) is an important biological mechanism to promote functional diversity among the proteins. So far, a wide range of PTMs has been identified. Among them, glycation is considered as one of the most important PTMs. Glycation is associated with different neurological disorders including Parkinson and Alzheimer. It is also shown to be responsible for different diseases, including vascular complications of diabetes mellitus. Despite all the efforts have been made so far, the prediction performance of glycation sites using computational methods remains limited. Here we present a newly developed machine learning tool called iProtGly-SS that utilizes sequential and structural information as well as Support Vector Machine (SVM) classifier to enhance lysine glycation site prediction accuracy. The performance of iProtGly-SS was investigated using the three most popular benchmarks used for this task. Our results demonstrate that iProtGly-SS is able to achieve 81.61%, 93.62%, and 92.95% prediction accuracies on these benchmarks, which are significantly better than those results reported in the previous studies. iProtGly-SS is implemented as a web-based tool which is publicly available at http://brl.uiu.ac.bd/iprotgly-ss/ .

Phloroglucinol prevents albumin glycation as well as diminishes ROS production, glycooxidative damage, nitrosative stress and inflammation in hepatocytes treated with high glucose

The treatment of diabetes mellitus aftermaths became one of medicine's most significant therapeutical and financial issues in the XXI century. Most of which are related to protein glycation and oxidative stress caused by long lasting periods of hyperglycemia. Thus, even within a venerable one, searching for new drugs, displaying anti-glycation and anti-oxidative properties seem useful as an additive therapy of diabetes. In this paper, we assessed the anti-glycating properties of phloroglucinol, a drug discovered in the XIX century and still used in many countries for its antispasmodic action. Herewith, we present its effect on protein glycation, glycoxidation, and oxidative damage in an albumin glycation/oxidation model and HepG2 cells treated with high glucose concentrations. The phloroglucinol showed the strongest and the widest protective effect within all analyzed antiglycating (aminoguanidine, pioglitazone) and anti-oxidative (vitamin C, GSH) agents. To the very best of our knowledge, this is the first study showing the properties of phloroglucinol in vitro what once is proven in other models might deepen its clinical applications.

Quantitation of glycated albumin by isotope dilution mass spectrometry

BACKGROUND

Glycated albumin is considered an alternative glycemic indicator in certain situations where HbA1c does not accurately reflect glycemic status. These patient cases are usually associated with decreased erythrocyte lifespan, gestational diabetes, or end-stage renal disease. The aim of our study was to develop an assay for absolute quantitation of glycated albumin based on isotope dilution liquid chromatography-mass spectrometry.

METHODS

The plasma samples were reduced/alkylated, spiked with isotope-labeled standards RQIKKQTALV(D₈)E and RQIKK(fructosyl)QTALV(D₈)E and enzymatically digested by Glu-C. The samples were analyzed on an LC-MS system. Two MRM transitions (M³⁺ → (b₉-3H₂O)²⁺ and M³⁺ → (b₁₀-3H₂O)²⁺ or M³⁺ → b₉²⁺ and M³⁺ → b₁₀²⁺) were used for each peptide, then the percentage of glycation (MS GA%) was calculated.

RESULTS

The comparison study demonstrated a good linear correlation between our LC-MS/MS and Lucica method with r² = 0.95. The intra-day CV for the low HbA1c sample was 2.2%, while CV for the high HbA1c sample was 0.64%. Inter-day CV for low HbA1c sample was 5.6%, while the CV for the high HbA1c sample was 5.7%. We found the LLOQ to be 0.12 nmol/ml for the non-glycated and glycated peptide. No interference from hemoglobin was observed up to 500 mg/dL concentration.

CONCLUSIONS

This is the first implementation of isotope dilution LC-MS assay for glycated albumin with simultaneously quantitation of glycated and non-glycated peptides. The method includes a simple sample preparation and has demonstrated a good analytical performance.

Dietary polyphenols inhibit plasma protein arabinosylation: Biomolecular interaction of genistein and ellagic acid with serum albumins

The mode of interaction of polyphenolic compounds like genistein (GTN) and ellagic acid (EGA) with human and bovine serum albumin (HSA and BSA, respectively) was found to differ significantly. Stern-Volmer (SV) analysis of the fluorescence quenching data revealed that the binding strength of EGA (1.9 ± 0.09 × 10⁵ M^-1) to HSA is about one order of magnitude higher than GTN (2.24 ± 0.06 × 10⁴ M^-1). While the static quenching of HSA fluorescence was found to proceed through simple Stern-Volmer (SV) mechanism, a quenching sphere-of-action model was indispensable for BSA. Temperature dependent fluorescence along with a series of other biophysical experiments and ensemble docking calculation revealed that EGA and GTN bind to the serum proteins primarily through the entropy driven process. The α-helical content and the microenvironment near Trp residue of HSA and BSA did not show any appreciable change due to the binding of either GTN or EGA. Interestingly, both GTN and EGA were found to inhibit the formation of advanced glycated end (AGE) product of serum proteins up to the extent of 70-90% within 12-24 h. Relatively moderate binding propensity along with the anti-glycation ability of the polyphenols confirmed that GTN and EGA can be used either as an alternative or towards development of suitable drugs in the prevention of many diabetic-related complications.

fruits: Fractionated extracts as inhibitors of protein glycation and oxidative/nitrative damage of human plasma components

Sorbus aucuparia L. is a source of edible fruits appreciated for their nutritional and medicinal properties. In this work some bioactivity mechanisms were evaluated, which might be connected with the traditional application of rowanberries in cardiovascular complications of diabetes. With the use of a panel of chemical and biological in vitro models the rowanberry extracts were proved to significantly inhibit the formation of advanced glycation end products, neutralise multiple oxidants generated in vivo, increase the non-enzymatic antioxidant capacity of human plasma and protect plasma components (proteins and lipids) against oxidative/nitrative damage at in vivo-relevant levels (1-5 µg/mL). Moreover, the extracts were found safe in cytotoxicity tests on the peripheral blood mononuclear cells. The comprehensive phytochemical profiling of the extracts (RP/HILIC-UHPLC-PDA-ESI-MS³, HPLC-PDA, and UV-spectrophotometric methods) led to the identification of 51 phenolics, including caffeic and ferulic acids pseudodepsides (34 compounds, prevailing isomers of chlorogenic acid and cynarin, total content up to 269.4 mg/g), flavonols (mostly quercetin glycosides, up to 5.8 mg/g), flavan-3-ol derivatives (proanthocyanidin oligomers and polymers, up to 17.0 mg/g), and simple phenolic acids. The experiments on model constituents of the extracts and correlation studies were used to evaluate contribution of polyphenols to the observed effects. Taking into account the possible additive and synergistic effects, the co-occurrence of various compounds was indicated as partly responsible for biological activity of the fruits. Considering both the composition and activity parameters, the methanol-water (1:1, v/v) extract and its concentrated phenolic fractions appeared to be the most advantageous for biological application.

Anacardium humile St. Hil as a novel source of antioxidant, antiglycation and α-amylase inhibitors molecules with potential for management of oxidative stress and diabetes

ETHNOPHARMACOLOGICAL RELEVANCE

The substantial increase in diabetes cases worldwide has been a major public health problem, and the use of medicinal plants can be considered an interesting alternative to control the disease and its complications. Anacardium humile St. Hill. (Anacardiaceae) is a typical plant from the Brazilian savanna, popularly known for its antidiarrheal, expectorant, antidiabetic and anti-inflammatory properties, however, few studies have fully described its biological properties. This study aimed to investigate in vitro and ex vivo the antioxidant and antiglycation potential of A. humile ethanolic extract, its organic fractions and three isolated molecules (quercetin, catechin and gallic acid), their capacity to inhibit the glycolytic enzyme α-amylase, as well as their cytotoxic effects against RAW264.7 macrophages.

MATERIAL AND METHODS

The ethanolic extract of A. humile, its organic fractions and three isolated molecules (catechin, quercetin and gallic acid) were tested for their antioxidant (ORAC, FRAP and DPPH) and antiglycation (BSA/Fructose, BSA/Methylglyoxal, Arginine/Methylglyoxal and Lysine/Methylglyoxal) capacities, and also for its potential to inhibit the enzyme α-amylase. Additionally, bioactive compounds present in the A. humile leaves fractions were elucidated by an HPLC-ESIMS/MS analysis.

RESULTS

The analysis showed relevant antioxidant activity of DCM (1264.85 ± 76.90 μM Trolox eq/g ORAC; 216.71 ± 1.04 μM Trolox eq/g FRAP and 3.03 ± 0.08 IC₅₀ μg/mL IC₅₀ DPPH) and EtOAc (1300.11 ± 33.04 ORAC, 236.21 ± 23.86 FRAP and 3.03 ± 0.14 μg/mL IC₅₀ DPPH) fractions and also of the isolated molecules, mainly gallic acid (1291.19 ± 8.41 μM Trolox eq/g ORAC, 1103.52 ± 31.48 μM Trolox eq/g FRAP and 0.78 ± 0.11 μg/mL IC₅₀ DPPH). Concerning the antiglycation activity, all samples inhibited over 88% in the BSA-FRU method. In the BSA-MGO and ARG-MGO methods, the Hex, DCM, EtOAc fractions and the isolated molecule catechin stood out. However, in the LYS-MGO model, only the isolated molecules showed significant results. In α-amylase assay, all fractions, for exception Hex, presented notable inhibition capacity with low IC₅₀ values, especially DCM, EtOAc, ButOH and H₂O (IC₅₀ 0.56 ± 0.10, 0.84 ± 0.01, 0.74 ± 0.03 and 0.79 ± 0.06 μg/mL, respectively). Tests using hepatic tissue showed a notorious capacity of the DCM, AcOEt and ButOH fractions, as well as of the isolated molecules to inhibit lipid peroxidation and ROS production, and also to preserve thiol groups. Molecules of great antioxidant potential were found in our samples, such as kaempferol, quercetin, catechin, gallic acid and luteolin.

CONCLUSION

A. humile extract and its organic fractions showed promising antioxidant and antiglycation potential and a prominent capacity to inhibit the α-amylase enzyme. Hence, this study presents new results and stimulates further research to elucidate the biological properties of A. humile and its capacity to manage DM and its complications.

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.

flower extract on methylglyoxal-induced protein glycation and oxidative damage to DNA

BACKGROUND

Methylglyoxal (MG) is a highly reactive dicarbonyl precursor for the formation of advanced glycation end products (AGEs) associated with age-related diseases, including diabetes and its complications. Clitoria ternatea L. flower has been reported to possess antioxidant and antiglycating properties. Evidence indicates that the extract of Clitoria ternatea L. flower inhibits fructose-induced protein glycation and oxidative damage to bovine serum albumin (BSA). However, there is no evidence to support the inhibitory effect of CTE against MG-mediated protein glycation and oxidative damage to protein and DNA. Therefore, the aim of the present study was to investigate whether C. ternatea flower extract (CTE) prevents MG-induced protein glycation and oxidative DNA damage.

METHODS

The formation of fluorescent AGEs in BSA was evaluated using spectrofluorometer. The protein carbonyl and thiol group content were used for detecting protein oxidation. DNA strand breakage in a glycation model comprising of MG, lysine and Cu²⁺ or a free radical generator 2,2'-azobis(2-methylpropionamidine) dihydrochloride (AAPH) systems was investigated using gel electrophoresis. Generation of superoxide anions and hydroxyl radicals in the MG/lysine system was assessed by the cytochrome c reduction assay and thiobarbituric acid reactive substances assay, respectively. High performance liquid chromatography (HPLC) was used to measure the MG-trapping ability.

RESULTS

In the BSA/MG system, CTE (0.25-1 mg/mL) significantly inhibited the formation of fluorescent AGEs and protein oxidation by reducing protein carbonyl content as well as preventing the protein thiol depletion. The concentration of CTE at 0.125-1 mg/mL prevented oxidative DNA cleavage in MG/lysine and AAPH systems associated with the inhibition of superoxide anion and hydroxyl radical formation. It also directly trapped MG in a concentration-dependent manner, ranging from 15 to 43%.

CONCLUSIONS

The study findings suggest that the direct carbonyl trapping ability and the free radical scavenging activity of CTE are the underlying mechanisms responsible for the prevention of protein glycation and oxidative DNA damage.

The impact of technical failures on recombinant production of soluble proteins in Escherichia coli: a case study on process and protein robustness

Technical failures lead to deviations in process parameters that can exceed studied process boundaries. The impact on cell and target protein is often unknown. However, investigations on common technical failures might yield interesting insights into process and protein robustness. Recently, we published a study on the impact of technical failures on an inclusion body process that showed high robustness due to the inherent stability of IBs. In this follow-up study, we investigated the influence of technical failures during production of two soluble, cytosolic proteins in E. coli BL21(DE3). Cell physiology, productivity and protein quality were analyzed, after technical failures in aeration, substrate supply, temperature and pH control had been triggered. In most cases, cell physiology and productivity recovered during a subsequent regeneration phase. However, our results highlight that some technical failures lead to persistent deviations and affect the quality of purified protein.

Antiglycation and antitumoral activity of Tribulus terrestris dry extract

OBJECTIVE

Investigation of the antiglycation and antitumoral potential of standardized and saponins-enriched extracts of Tribulus terrestris herbal medicine.

MATERIALS AND METHODS

The procedures for the evaluation of the antiglycation activity of the standardized (TtSE) and saponins-enriched (TtEE) extracts of T. terrestris were: determination of relative mobility in electrophoresis (RME), free amino groups using OPA method and advanced glycation end-products (AGEs) fluorescence. Antioxidant activity was determined by DPPH radical scavenging test. In vitro antitumor activity of TtSE and TtEE was evaluated in human tumor cell lines.

RESULTS

The results were obtained by antiglycation tests (RME, OPA method and AGEs fluorescence determination), using BSA as protein and ribose as glycation agent, and antioxidant assay (DPPH test); it was verified that both extracts of T. terrestris have antiglycation and antioxidant activity. In addition, the extracts were able to induce death of more than 50% of human tumor cell lines.

CONCLUSION

The present study showed that standardized and saponins-enriched extracts of T. terrestris herbal medicine present antiglycation and antioxidant and antiproliferative action in human tumor cells lines. The saponins-enriched extract proved a greater antiglycation and antioxidant activity in comparison to the standardized type.

Glycation of soy proteins leads to a range of fractions with various supramolecular assemblies and surface activities

Dry and subsequent wet heating were used to glycate soy proteins with dextran or glucose, followed by fractionation based on size and solubility. Dry heating led to protein glycation (formation of furosine, Nε-(carboxymethyl)-l-lysine, Nε-(carboxyethyl)-l-lysine, and protein-bound carbonyls) and aggregation (increased particle size); while subsequent wet heating induced partial unfolding and de-aggregation. The measurable free amino group content of soy proteins changed from 0.77 to 0.14, then to 0.62 mmol/g upon dry and subsequent wet heating; this non-monotonic evolution is probably due to protein structural changes, and shows that this content should be interpreted with caution as a glycation marker. After both heating steps, the smaller-sized water-soluble fractions showed higher surface activity than the larger insoluble ones, and dextran conjugates exhibited a higher surface activity than their glucose counterparts. We thereby achieved a comprehensive understanding of the properties of various fractions in plant protein fractions, which is essential when targeting applications.

Rosmarinic acid restrains protein glycation and aggregation in human serum albumin: Multi spectroscopic and microscopic insight - Possible Therapeutics Targeting Diseases

Protein aggregation and glycation are directly associated with many pathological conditions including several neurodegenerative disorders. This study investigates the potential of naturally occurring plant product, Rosmarinic acid (RA), to inhibit the glycation and aggregation process. In this study, we report that varying concentrations of methylglyoxal (MG) induce advanced glycation end products (AGEs) and aggregates formation in HSA in vitro on day 6 and day 8, respectively. AGEs specific fluorescence confirmed the formation of AGEs in HSA in the presence of MG and further characterized the inhibitory potential of RA. It was found that the presence of RA prevented AGEs formation in vitro. Further, aggregates of HSA were characterized employing multi spectroscopic and microscopic techniques and RA was found to inhibit this process. This study proposes that RA could be a potential natural molecule to treat disorders where AGEs and aggregates of proteins play a pivotal role.

Action of metformin therapy against advanced glycation, oxidative stress and inflammation in type 2 diabetes patients: 3 months follow-up study

BACKGROUND

Persistence hyperglycemia results in the formation of advanced glycation end products (AGEs) by non-enzymatic glycation. AGEs and their receptor RAGE play an important role in generation of inflammatory molecules and oxidative stress. Metformin regulates insulin responsive gene and helps to achieve glycemic control however, no extensive study reported about its role against glycation induced oxidative stress and vascular inflammation. Therefore, present work focused on clinical relevance of three months metformin therapy in type 2 diabetes mellitus patients against glycation induced oxidative stress and vascular inflammation.

METHODS

Among recruited 40 medicated-naive type 2 diabetes mellitus patients, 31 patients were continued with metformin therapy. Biomarkers of plasma protein glycation (fructosamine, protein carbonyls, β-amyloid) antioxidants and oxidative stress markers (GSH, catalase, NO, PON-1, AOPP, LPO; RAGE isoforms (sRAGE, esRAGE); inflammatory markers (IL-6, TNF-α) were determined at baseline and after 3-months of treatment. The expression profile of membrane RAGE, NF-κB, CML was studied in PBMNCs and GLUT-1 in erythrocyte ghost by western blotting.

RESULTS

Metformin showed maximum percent declined from baseline to three months therapy in levels of fructosamine, β-amyloid, sRAGE, inflammatory cytokines (IL-6, TNF-α) and percent increment in esRAGE and antioxidants levels. It showed reduced levels of IL-6 and TNF-α by declining expression of CML, membrane RAGE and NF-κB in type 2 diabetes mellitus patients after three months therapy.

CONCLUSIONS

First report in Indian diabetes mellitus patients, where metformin showed effective inhibition against glycation and receptor mediated cellular inflammation. However, these findings need to be tested in a randomized trial.

Recent advances of ultrasound-assisted Maillard reaction

Maillard reaction (MR) is one of the most important chemical reactions in the food science domain with a long history of more than 100 years. As for ultrasound-assisted MR (US-MR), it has gradually drawn attention in a recent decade. Purpose of this paper is to provide a systematic review on recent advances of US-MR in model systems, glycation of protein, and food processing. Fundamental studies on simple MR model systems (i.e. reducing sugar and amino acid) have reported a promoted generation of colored and volatile MR products (MRPs). Critical steps influenced by US and possible mechanisms have been elucidated simultaneously. Other studies focused on modification of proteins which undergoes a glycation between proteins and saccharides as the initial stage of MR. Since the MR rate is extremely low in the presence of protein and saccharide, US becomes a promising mean of promoting the glycation. As a result, a number of functional properties of glycated protein obtained by US are significantly promoted, which extend their utilization in the food industry. The rest of studies reviewed in this article are concentrated on applying US to process real foods. Many attributes changed during US-assisted processing are induced by MR. Positive aspects brought by the promoted US-MR include enhanced antioxidant capacity and organoleptic properties (e.g. desirable color, low bitterness, enhanced flavor, etc.), as well as inhibited hazards (e.g. advanced glycation end-products, acrylamide, etc.) formed in the processed foods. Meanwhile, the promoted MR by US may also inevitably bring some negative aspects to the processed foods due to unfavored yellowish/browning colors, off-flavors and hazard components.

Influence of ultrasound pretreatment on the subsequent glycation of dietary proteins

The influence of ultrasound treatment on the subsequent glycation process of proteins is controversial. Glycation behaviors of bovine serum albumin (BSA), β-lactoglobulin (β-Lg) and β-casein (β-CN) after ultrasound pretreatment (UP) were compared by both evaluating glycation kinetics and analyzing structural changes of proteins. UP resulted in both unfolding and aggregation behavior in protein samples, which altered the accessibility of the Lys and Arg. Five cycles of UP up-regulated the glycation degree of BSA and β-Lg, possibly due to the unfolding behavior induced by UP, which exposed additional glycation sites. In contrast, 30 cycles of UP induced a dramatic increase (by 97.9 nm) in particle size of BSA, thus burying portions of glycation sites and suppressing the glycation process. Notably, UP had minimal influence on glycation kinetics of β-CN, due to its intrinsic disordered structure. Based on proteomics analysis, the preference of Lys and Arg during glycation was found to be changed by UP in BSA and β-Lg. Four, 3 and 3 unique carboxyethylated lysine residues were identified in glycated BSA after 0, 5 and 30 cycles of UP, respectively. This study suggests that the protein glycation can be affected by UP, depending on the ultrasonication duration and native structure of the protein.

Differential effect of biophenols on attenuation of AGE-induced hemoglobin aggregation

Despite most studied activities of natural biophenols rely on antioxidant properties, little clues explored their key structural components with regard to opposing action on glycation-induced aggregation. Herein, human hemoglobin (hHb)/fructose system used to decipher if structural peculiarities of two biophenols "chlorogenic acid (CGA) and curcumin (CUR)" are effective toward AGEs-bridged aggregate formation. Suppression in amyloid cross-β formation was monitored by CD spectroscopy, fluorescence microscopy, ANS and AGE fluorescence. Reduction in molten globule structure of modified-Hb by CGA was corroborated with helix structure, thiol group and lysine residues content estimation for native, glycated and biophenols treated samples. ThT and Congo red assays showed the cross-β breaking properties of CGA. Molecular docking outcomes revealed the positioning of CGA/CUR is driven by "aromatic interactions" with Trp β₁180 and Tyr α₂540. These interactions are modulated by the structural constraints such as number of hydroxyl groups and their methylation status directing the biophenols to the amyloidogenic core. The results are applicable to formulation of small-molecule nutraceuticals for treatment of conformational diseases.

Comprehensive Analysis of Protein Glycation Reveals Its Potential Impacts on Protein Degradation and Gene Expression in Human Cells

Glycation as a type of non-enzymatic protein modification is related to aging and chronic diseases, especially diabetes. Global analysis of protein glycation will aid in a better understanding of its formation mechanism and biological significance. In this work, we comprehensively investigated protein glycation in human cells (HEK293T, Jurkat, and MCF7 cells). The current results indicated that this non-enzymatic modification was not random, and protein at the extracellular regions and the nucleus were more frequently glycated. Systematic and site-specific analysis of glycated proteins allowed us to study the effect of the primary sequences and secondary structures of proteins on glycation. Furthermore, nearly every enzyme in the glycolytic pathway was found to be glycated and a possible mechanism was proposed. Many glycation sites were also previously reported as acetylation and ubiquitination sites, which strongly suggested that this non-enzymatic modification may disturb protein degradation and gene expression. The current results will facilitate further studies of protein glycation in biomedical and clinical research.

Glycation-induced modification of tissue-specific ECM proteins: A pathophysiological mechanism in degenerative diseases. Biochim Biophys Acta Gen Subj. 2019;1863:129411. [PMID: 31400438 DOI: 10.1016/j.bbagen.2019.08.004]

BACKGROUND

Glycation driven generation of advanced glycation end products (AGEs) and their patho-physiological role in human degenerative diseases has remained one of the thrust areas in the mainstream of disease biology. Glycation of extracellular matrix (ECM) proteins have deleterious effect on the mechanical and functional properties of tissues. Owing to the adverse pathophysiological concerns of glycation, there is a need to decipher the underlying mechanisms.

SCOPE OF REVIEW

AGE-modified ECM proteins affect the cell in the vicinity by altering protein structure-function, matrix-matrix or matrix-cell interaction and by activating signalling pathway through receptor for AGE. This review is intended for addressing the AGE-induced modification of tissue-specific ECM proteins and its implication in the pathogenesis of various organ-specific human ailments.

MAJOR CONCLUSIONS

The glycation affects the canonical cell behaviour due to alteration in the interaction of glycated ECM with receptors like integrins and discodin domain, and the signalling cues generated subsequently affect the downstream signalling pathways. Consequently, the variation of structural and functional properties of tissues due to matrix glycation helps in the initiation or progression of the disease condition.

GENERAL SIGNIFICANCE

This review offers comprehensive knowledge about the remodelling of glycation induced ECM and tissue-specific pathological concerns. As glycation of ECM affects the normal tissues and cell behaviour, the scientific discourse may also provide cues for developing candidate drugs that may help in attenuating the adverse effects of AGEs and perhaps open a research window of tailoring novel strategies for the management of glycation induced human degenerative diseases.

Cell cytotoxity and anti-glycation activity of taxifolin-rich extract from Japanese larch, Larix kaempferi

The larches, the Larix genus of plants are known as a natural source of taxifolin (dihydroquercetin), and extracts of its taxifolin rich xylem are used in dietary supplements to maintain health. In the present study, to assess biological activities of a methanol extract of the Japanese larch, Larix kaempferi (LK-ME), the effects of LK-ME on cell viability, inflammatory cytokine expression, and glycation were investigated. The effects of taxifolin which is known to be a main compound of LK-ME, and its related flavonoids, quercetin and luteolin were also examined. The results show that taxifolin exhibits lower growth inhibition activity and lesser induction activity of inflammatory cytokines in a human monocyte derived cell line, THP-1 cells, while in vitro anti-glycation activities of taxifolin were inhibiting at comparable levels to those of quercetin and luteolin. The growth inhibition and the cytokine induction activities, and the anti-glycation effects of LK-ME are assumed to have properties similar to taxifolin. The results of high performance liquid chromatography (HPLC) analysis indicated that taxifolin was detected as the main peak of LK-ME at the absorbance of 280 nm, and the concentration of taxifolin was measured as 3.12 mg/ml. The actual concentration of taxifolin in LK-ME is lower than the concentration estimated from the IC₅₀ values calculated by the results of glycation assays, suggesting that other compounds contained in LK-ME are involved in the anti-glycation activity.

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.

Acceleration of protein glycation by oxidative stress and comparative role of antioxidant and protein glycation inhibitor

Hyperglycemia in diabetes causes protein glycation that leads to oxidative stress, release of cytokines, and establishment of secondary complications such as neuropathy, retinopathy, and nephropathy. Several other metabolic disorders, stress, and inflammation generate free radicals and oxidative stress. It is essential to study whether oxidative stress independently enhances protein glycation leading to rapid establishment of secondary complications. Oxidative stress was experimentally induced using rotenone and Fenton reagent for in vivo and in vitro studies, respectively. Results showed significant increase in the rate of modification of BSA in the form of fructosamine and protein-bound carbonyls in the presence of fenton reagent. Circular dichroism studies revealed gross structural changes in the reduction of alpha helix structure and decreased protein surface charge was confirmed by zeta potential studies. Use of rotenone demonstrated enhanced AGE formation, ROS generation, and liver and kidney tissue glycation through fluorescence measurement. Similar findings were also observed in cell culture studies. Use of aminoguanidine, a protein glycation inhibitor, demonstrated reduction in these changes; however, a combination of aminoguanidine along with vitamin E demonstrated better amelioration. Thus, oxidative stress accelerates the process of protein glycation causing gross structural changes and tissue glycation in insulin-independent tissues. Use of antioxidants and protein glycation inhibitors in combination are more effective in preventing such changes and could be an effective therapeutic option for preventing establishment of secondary complications of diabetes.

Changes in the phenolic compositions of Elaeagnus umbellata and Sambucus lanceolata after in vitro gastrointestinal digestion and evaluation of their potential anti-diabetic properties

In this work, the phenolic composition of E. umbellata leaves and berries is reported. Berries were rich in flavonols, whereas leaves presented abundant flavonols and ellagitannins. Then, the enzyme-inhibitory properties, anti-glycation and antioxidant activities of E. umbellata and Sambucus lanceolata (its phenolic profile has been already established in a previous work) were tested by several in vitro assays and compared. The simulated gastrointestinal digestion resulted in a decrease on their phenolic composition. Nevertheless, both species still had the ability to inhibit aldose reductase activity and protein glycation and scavenge free radicals at the end of the process. Thus, the potential beneficial effects of E. umbellata and S. lanceolata seems to be kept to some extent after passage throughout the digestive system. Altogether, this study provides further insight into investigation of these species as dietary sources of bioactive compounds to lower the risk of type-2 diabetes and obesity.

Analysis of Protein Glycation Using Phenylboronate Acrylamide Gel Electrophoresis

Carbohydrate modification of proteins adds complexity and diversity to the proteome. However, undesired carbohydrate modifications also occur in the form of glycation, which have been implicated in diseases such as diabetes, Alzheimer's disease, autoimmune diseases, and cancer. The analysis of glycated proteins is challenging due to their complexity and variability. Numerous analytical techniques have been developed that require expensive specialized equipment and complex data analysis. In this chapter, we describe two easy-to-use electrophoresis-based methods that will enable researchers to detect, identify, and analyze these posttranslational modifications. This new cost-effective methodology will aid the detection of unwanted glycation products in processed foods and may lead to new diagnostics and therapeutics for age-related chronic diseases.

Structural effects of methylglyoxal glycation, a study on the model protein MNEI

The reaction of free amino groups in proteins with reactive carbonyl species, known as glycation, leads to the formation of mixtures of products, collectively referred to as advanced glycation endproducts (AGEs). These compounds have been implicated in several important diseases, but their role in pathogenesis and clinical symptoms' development is still debated. Particularly, AGEs are often associated to the formation of amyloid deposits in conformational diseases, such as Alzheimer's and Parkinson's disease, and it has been suggested that they might influence the mechanisms and kinetics of protein aggregation. We here present the characterization of the products of glycation of the model protein MNEI with methylglyoxal and their effect on the protein structure. We demonstrate that, despite being an uncontrolled process, glycation occurs only at specific residues of the protein. Moreover, while not affecting the protein fold, it alters its shape and hydrodynamic properties and increases its tendency to fibrillar aggregation. Our study opens the way to in deep structural investigations to shed light on the complex link between protein post-translational modifications, structure, and stability.

Aldo-ketoreductase 1 (AKR1) improves seed longevity in tobacco and rice by detoxifying reactive cytotoxic compounds generated during ageing

BACKGROUND

Maintenance of seed viability is an important factor for seedling vigour and plant establishment. Lipid peroxidation mediated reactive carbonyl compounds (RCC's) and non-enzymatic modifications of proteins through Maillard and Amadori products reduce seed viability and seedling vigour.

RESULTS

In this study, the relevance of RCCs on genotypic variation in rice seed viability and overexpression of an aldo-ketoreductase (AKR1) enzyme that detoxify cytotoxic compounds to improve seed viability and vigour was studied. Physiological and biochemical approaches were integrated to quantify the variation in seed viability and seedling vigour in rice genotypes after exposing to ageing treatment. AKR1 was overexpressed in a susceptible rice genotype and tobacco to study the relevance of reduced RCC's on seed viability and seedling vigour. The glycation and lipid peroxidation compounds accumulated after accelerated ageing treatments in rice genotypes. The accumulation of malondialdehyde, methyl glyoxal, Maillard and Amadori products affected the seed viability and germination as they showed a significant negative relationship. The transgenic rice and tobacco seeds expressing AKR1 showed lower levels of cytotoxic compounds and glycation products that resulted in improved seed viability and seedling vigour in rice and tobacco.

CONCLUSIONS

The study demonstrates that, reactive cytotoxic compounds affect the seed viability during storage. Detoxification of reactive cytotoxic compounds by Aldo-keto reductases is one of the mechanisms to improve the seed longevity during storage.

Glycated lysine-141 in haptoglobin improves the diagnostic accuracy for type 2 diabetes mellitus in combination with glycated hemoglobin HbA1c and fasting plasma glucose

BACKGROUND

Recent epidemiological studies indicate that only 30-50% of undiagnosed type 2 diabetes mellitus (T2DM) patients are identified using glycated hemoglobin (HbA1c) and elevated fasting plasma glucose (FPG) levels. Thus, novel biomarkers for early diagnosis and prognosis are urgently needed for providing early and personalized treatment.

METHODS

Here, we studied the glycation degrees of 27 glycation sites representing nine plasma proteins in 48 newly diagnosed male T2DM patients and 48 non-diabetic men matched for age (range 35-65 years). Samples were digested with trypsin and enriched for glycated peptides using boronic acid affinity chromatography. Quantification relied on mass spectrometry (multiple reaction monitoring) using isotope-labelled peptides as internal standard.

RESULTS

The combination of glycated lysine-141 of haptoglobin (HP K141) and HbA1c provided a sensitivity of 94%, a specificity of 98%, and an accuracy of 96% to identify T2DM. A set of 15 features considering three glycation sites in human serum albumin, HP K141, and 11 routine laboratory measures of T2DM, metabolic syndrome, obesity, inflammation, and insulin resistance provided a sensitivity of 98%, a specificity of 100%, and an accuracy of 99% for newly diagnosed T2DM patients.

CONCLUSIONS

Our studies demonstrated the great potential of glycation sites in plasma proteins providing an additional diagnostic tool for T2DM and elucidating that the combination of these sites with HbA1c and FPG could improve the diagnosis of T2DM.

Ferulic acid prevents methylglyoxal-induced protein glycation, DNA damage, and apoptosis in pancreatic β-cells

Methylglyoxal (MG) can react with amino acids of proteins to induce protein glycation and consequently the formation of advanced glycation end-products (AGEs). Previous studies reported that ferulic acid (FA) prevented glucose-, fructose-, and ribose-induced protein glycation. In this study, FA (0.1-1 mM) inhibited MG-induced protein glycation and oxidative protein damage in bovine serum albumin (BSA). Furthermore, FA (0.0125-0.2 mM) protected against lysine/MG-mediated oxidative DNA damage, thereby inhibiting superoxide anion and hydroxyl radical generation during lysine and MG reaction. In addition, FA did not have the ability to trap MG. Finally, FA (0.1 mM) pretreatment attenuated MG-induced decrease in cell viability and prevented MG-induced cell apoptosis in pancreatic β-cells. The results suggest that FA is capable of protecting β-cells from MG-induced cell damage during diabetes.

Influence of storage and heating on protein glycation levels of processed lactose-free and regular bovine milk products

Thermal treatment preserves the microbiological safety of milk, but also induces Maillard reactions modifying for example proteins. The purpose of this study was evaluating the influence of consumer behaviors (storage and heating) on protein glycation degrees in bovine milk products. Lactosylation and hexosylation sites were identified in ultra-high temperature (UHT), lactose-free pasteurized, and lactose-free UHT milk (ULF) and infant formula (IF) using tandem mass spectrometry (electron transfer dissociation). Overall, 303 lactosylated and 199 hexosylated peptides were identified corresponding to 170 lactosylation (31 proteins) and 117 hexosylation sites (25 proteins). In quantitative terms, storage increased lactosylation up to fourfold in UHT and IF and hexosylation up to elevenfold in ULF and threefold in IF. These levels increased additionally twofold when the stored samples were heated (40°C). In conclusion, storage and heating appear to influence protein glycation levels in milk at similar or even higher degrees than industrial processing.

Monitoring protein glycation by electrospray ionization (ESI) quadrupole time-of-flight (Q-TOF) mass spectrometer

In this study electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometry was used to investigate protein glycation. The glycated species of cytochrome C, lysozyme, and β-casein formed during glycation with d-glucose were identified and monitored in binary systems heated at 70°C under dry and aqueous conditions. Cytochrome C had multiple charges in non-glycated state, primarily changing from +13 to +17 positive charges, whereas β-casein had charge states up to +30. Upon heating with glucose at 70°C in aqueous state, attachment of one glucose molecule onto proteins was observed in each charge state. However, heating in dry state caused much more glucose attachment, leading to the formation of multiple glycoforms of proteins. By using ESI-QTOF-MS technique, formation of glycated cytochrome C containing up to 12 glucose moieties were observed, while glycated species containing 6 and 8 glucose moieties were observed for lysozyme and β-casein, respectively in various heating conditions.

Cyanidin-3-rutinoside attenuates methylglyoxal-induced protein glycation and DNA damage via carbonyl trapping ability and scavenging reactive oxygen species

BACKGROUND

Advanced glycation end-products (AGEs) play a significant role in the development and progression of vascular complication in diabetes. Anthocyanin has been recently reported to possess antiglycating activity. This study aimed to determine whether a naturally occurring anthocyanin, cyanidin-3-rutinoside (C3R) inhibits methylglyoxal (MG) induced protein glycation and oxidative protein and DNA damage.

METHODS

C3R (0.125-1 mM) was incubated with bovine serum albumin and MG (1 mM) for 2 weeks. The formation of fluorescent AGEs was measured by using spectrofluorometer and thiol group content were used to detect protein oxidative damage. Gel electrophoresis was used to determine whether C3R (0.125-1 mM) reduced DNA strand breakage in a glycation model comprising lysine, MG and/or Cu(2+). The generation of superoxide anions and hydroxyl radicals were detected by the cytochrome c reduction assay and the thiobarbituric acid reactive substances assay. MG-trapping capacity was assessed by high performance liquid chromatography (HPLC).

RESULTS

C3R (0.25-1 mM) reduced the formation of fluorescent AGEs and depleted protein thiol groups in bovine serum albumin mediated by MG. At 1 mM C3R inhibited oxidative DNA damage in the glycation model (p < 0.05) and at 0.5-1 mM prevented Cu(2+) induced DNA strand breakage in the presence of lysine and MG. The findings showed that C3R reduced the formation of superoxide anion and hydroxyl radicals during the glycation reaction of MG with lysine. C3R directly trapped MG in a concentration and time dependent manner (both p < 0.001).

CONCLUSIONS

These findings suggest that C3R protects against MG-induced protein glycation and oxidative damage to protein and DNA by scavenging free radicals and trapping MG.

Strong inhibition of the polyol pathway diverts glucose flux to protein glycation leading to rapid establishment of secondary complications in diabetes mellitus

BACKGROUND

Polyol pathway and protein glycation are implicated in establishing secondary complications in diabetes. Their relative contribution to the process needs to be evaluated. It is essential to understand why some aldose reductase inhibitors (ARIs) trials are successful while some have failed and to study their effect on protein glycation.

METHODS

Aldose reductase (AR) was assayed using xylose as substrate; protein glycation was evaluated using total and specific fluorescence, fructoseamine and protein bound carbonyl content (PCO) measurements. Long term studies were carried out on streptozotocin induced diabetic rats for evaluation of urine parameters, tissue fluorescence. Anti-cataract action was studied by lens culture studies.

RESULTS

Epalrestat, a commercial ARI was also found to possess potent glycation inhibitory action. Long term experiments revealed strong protein glycation with higher concentration of citronellol (ARI) demonstrating shift in glucose flux. Treatment with epalrestat and limonene revealed improved urine parameters and tissue fluorescence. Lens culture studies revealed cataract formation at higher inhibition of AR while no lens opacity was observed at lower citronellol concentration and with limonene and epalrestat.

CONCLUSION

Strong inhibition of AR shifts the glucose flux to protein glycation causing damage. ARIs possessing protein glycation inhibition are more useful in amelioration of secondary complications.

Effect of addition of thermally modified cowpea protein on sensory acceptability and textural properties of wheat bread and sponge cake

This paper investigates the sensory acceptability and textural properties of leavened wheat bread and sponge cake fortified with cow protein isolates that had been denatured and glycated by thermal treatment. Defatted cowpea flour was prepared from cow pea beans and the protein isolate was prepared (CPI) and thermally denatured (DCPI). To prepare glycated cowpea protein isolate (GCPI) the cowpea flour slurry was heat treated before isolation of the protein. CPI was more susceptible to thermal denaturation than GCPI as determined by turbidity and sulphydryl groups resulting in greater loss of solubility. This is attributed to the higher glycation degree and higher carbohydrate content of GCPI as demonstrated by glycoprotein staining of SDS PAGE gels. Water absorption of bread dough was significantly enhanced by DCPI and to a larger extent GCPI compared to the control, resulting in softer texture. CPI resulted in significantly increased crumb hardness in baked bread than the control whereas DCPI or GCPI resulted in significantly softer crumb. Bread fortified with 4% DCPI or GCPI was similar to control as regards sensory and textural properties whereas 4% CPI was significantly different, limiting its inclusion level to 2%. There was a trend for higher sensory acceptability scores for GCPI containing bread compared DCPI. Whole egg was replaced by 20% by GCPI (3.5%) in sponge cake without affecting the sensory acceptability, whereas CPI and DCPI supplemented cakes were significantly different than the control.

Identification and quantification of bovine protein lactosylation sites in different milk products

The microbiological safety of milk is typically guaranteed by thermal treatments, such as pasteurization and ultra high temperature (UHT) treatment, whereas infant formula (IF) is often produced at even harsher conditions including a drying process. Thermal treatments have raised concerns, as they may denature proteins and initiate protein modifications. Previous studies identified already many lactosylation sites in milk and showed that the lactosylation degree of some proteins correlates to thermal treatment conditions. Here, we studied the glycation degrees of 124 lactosylation sites in 28 bovine milk proteins in raw milk, three brands of pasteurized milk, three brands of UHT milk, and five brands of IF. Whereas, the glycation degree of many lactosylation sites increased from raw milk, to pasteurized milk, UHT milk, and IF, several modification sites showed a different behavior indicating that global measures do not correctly reflect the reactivity of distinct sites. Interestingly, the glycation degrees varied considerably among the brands of UHT milk and IF indicating that specific production processes of a company have to be considered and not only the classification of milk as pasteurized or UHT. Thus, proper adjustments of the technical processes should allow reducing the lactosylation levels in both UHT milk and IF.

SIGNIFICANCE

It is well established that thermal treatment of milk triggers protein modifications, such as lactosylation of lysine residues in several proteins, although the extent of lactosylation has not been quantitatively compared for a broad panel of protein lactosylation sites among different commercial products. The current study extends previous reports by relatively quantifying 124 confirmed lactosylation sites in 28 bovine milk proteins including several low abundant proteins. Whereas, glycation is generally assumed to be an unspecific chemical reaction with the modification degrees depending on the protein and sugar concentrations, we could show that each protein and even each lactosylation site in a given protein is differently affected by thermal processes indicating that the global lactosylation degrees will not allow predicting the influence of a technical process on individual proteins and lactosylation sites. Additionally, we could show that brands of each milk product differ significantly in their glycation degrees with UHT milk brands for example spanning the whole range from the relatively low lactosylation degree of pasteurized milk to the rather high lactosylation degree of IF. Similar differences were obtained for IF that generally showed the highest glycation degree. The targeted quantification approach established and validated here will be useful to reveal technical processing steps that trigger individual lactosylation sites and thus can help to prevent such unwanted reactions. Even slight changes of the technical processes might allow reducing the lactosylation degree of milk proteins significantly without challenging the microbiological safety or affecting consumer behavior.

Analysis of free drug fractions by ultrafast affinity extraction: interactions of sulfonylurea drugs with normal or glycated human serum albumin

Ultrafast affinity extraction and a multi-dimensional affinity system were developed for measuring free drug fractions at therapeutic levels. This approach was used to compare the free fractions and global affinity constants of several sulfonylurea drugs in the presence of normal human serum albumin (HSA) or glycated forms of this protein, as are produced during diabetes. Affinity microcolumns containing immobilized HSA were first used to extract the free drug fractions in injected drug/protein mixtures. As the retained drug eluted from the HSA microcolumn, it was passed through a second HSA column for further separation and measurement. Items that were considered during the optimization of this approach included the column sizes and flow rates that were used, and the time at which the second column was placed on-line with the HSA microcolumn. This method required only 1.0 μL of a sample per injection and was able to measure free drug fractions as small as 0.09-2.58% with an absolute precision of ±0.02-0.5%. The results that were obtained indicated that glycation can affect the free fractions of sulfonylurea drugs at typical therapeutic levels and that the size of this effect varies with the level of HSA glycation. Global affinity constants that were estimated from these free drug fractions gave good agreement with those predicted from previous binding studies or determined through a reference method. The same approach could be utilized with other drugs and proteins or modified binding agents of clinical or pharmaceutical interest.

Protective ability of phenolics from white grape vinification by-products against structural damage of bovine serum albumin induced by glycation

Grape skins recovered from white grape vinification processes were studied as possible anti-glycation agents. Total phenolics were characterised by the Folin Ciocalteu assay, proanthocyanidins by depolymerisation with n-butanol/HCl, flavonols by HPLC-DAD, reducing capacity by ferric ion reducing antioxidant power assay (FRAP) and anti-glycation activity by a bovine serum albumin (BSA)/fructose model system. Structural modifications of BSA were investigated by 2D isoelectric focusing sodium dodecyl sulfate polyacrylamide gel electrophoresis (IEF/SDS-PAGE) and fluorescence measurements. Both pI and Mr. of BSA were modified upon glycation reaction. These changes attributable to the involvement of free amino groups in Maillard-type reactions were inhibited by the white grape skin extracts. The anti-glycation activity ranged between 250 and 711mmol aminoguanidine Eq/kg. These results raise the interest in the potential health benefits of by-products of white grape vinification that could have a secondary use as an ingredient for new functional foods targeting wellbeing of diabetic and elderly people.

hemisphaerica

Oxidative stress and protein glycation play pivotal roles in the pathophysiology of diabetes mellitus and its vascular complications. The present study aimed to investigate the anti-glycation properties of essential oils obtained from different parts of Juniperus communis subsp. hemisphaerica. The branchlets of male tree (BMT) and branchlets of female (BFT) tree, and fruits of J. communis subsp. hemisphaerica were extracted using steam distillation method. The oils were phytochemically analyzed using gas chromatography-mass spectrometry. Anti-glycation properties were evaluated using hemoglobin and insulin glycation assays. Overall, 18 volatile components were identified in the J. communis subsp. hemisphaerica oils, amounting to 82.1%, 100.0% and 96.4% of the BMT, BFT and fruit oils, respectively. Promising inhibitory activity was observed from all concentrations of the tested oils in the hemoglobin and insulin glycation assays. The inhibitory activities peaked to 89.9% (BFT oil; 200 μg mL(-1)) and 81.0% (BFT oil; 600 μg mL(-1)) in the hemoglobin and insulin glycation assays, respectively. The evidence from this study suggests that essential oils obtained from the fruits and branchlets of J. communis subsp. hemisphaerica possess anti-glycation properties. These activities may find implication for the prevention and treatment of diabetic complications.

Lipid peroxidation and the total antioxidant status in the pathogenesis of age related and diabetic cataracts: a study on the lens and blood

BACKGROUND

Cataract is one of the major causes of a visual impairment, which eventually leads to blindness. An oxidative damage to the lens proteins is a major factor which leads to cataract formation. Therefore, we intended to study the relationship between the biochemical markers of oxidative stress and various forms of cataracts.

METHODS

We examined the lenses and the sera of 120 subjects who were aged 50 to 80 years, who were distributed in two groups, viz. the study group (90 patients) and the control group (30 subjects). The oxidative stress was assessed by estimating the lipid peroxidation product in the form of thiobarbituric acid reactive substances (TBARS), the antioxidant status by measuring the levels of vitamin E and the total antioxidant capacity (TAC). The study group patients were further divided into those with nuclear cataracts (30 patients), cortical cataracts (30 patients), and diabetic cataracts (30 patients).

RESULTS

In this study, it was found that the levels of TBARS in the study group were significantly high (p<0.001), whereas the TAC (p<0.001) and the vitamin E (p<0.001) levels were significantly low, both in the lenses and the blood of the study group as compared to those of the control group.

CONCLUSION

Thus, the present study suggests that an imbalance between the oxygen free radicals and the antioxidants may lead to lipid peroxidation in the lens. Also, the elevated levels of glucose in the diabetic cataracts lead to the auto-oxidation of glucose and a non-enzymatic glycation of the lens protein. Thereby, the high molecular weight proteins aggregate in the cataract.

Detection of oxidized and glycated proteins in clinical samples using mass spectrometry--a user's perspective

BACKGROUND

Proteins in human tissues and body fluids continually undergo spontaneous oxidation and glycation reactions forming low levels of oxidation and glycation adduct residues. Proteolysis of oxidised and glycated proteins releases oxidised and glycated amino acids which, if they cannot be repaired, are excreted in urine.

SCOPE OF REVIEW

In this review we give a brief background to the classification, formation and processing of oxidised and glycated proteins in the clinical setting. We then describe the application of stable isotopic dilution analysis liquid chromatography-tandem mass spectrometry (LC-MS/MS) for measurement of oxidative and glycation damage to proteins in clinical studies, sources of error in pre-analytic processing, corroboration with other techniques - including how this may be improved - and a systems approach to protein damage analysis for improved surety of analyte estimations.

MAJOR CONCLUSIONS

Stable isotopic dilution analysis LC-MS/MS provides a robust reference method for measurement of protein oxidation and glycation adducts. Optimised pre-analytic processing of samples and LC-MS/MS analysis procedures are required to achieve this.

GENERAL SIGNIFICANCE

Quantitative measurement of protein oxidation and glycation adducts provides information on level of exposure to potentially damaging protein modifications, protein inactivation in ageing and disease, metabolic control, protein turnover, renal function and other aspects of body function. Reliable and clinically assessable analysis is required for translation of measurement to clinical diagnostic use. Stable isotopic dilution analysis LC-MS/MS provides a "gold standard" approach and reference methodology to which other higher throughput methods such as immunoassay and indirect methods are preferably corroborated by researchers and those commercialising diagnostic kits and reagents. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.