Analysis of Advanced Glycation Endproducts in Rat Tail Collagen and Correlation to Tendon Stiffening

Patellar tendon biomechanical and morphologic properties and their relationship to serum clinical variables in persons with prediabetes and type 2 diabetes

Tendon biomechanical properties and fibril organization are altered in patients with diabetes compared to healthy individuals, yet few biomarkers have been associated with in vivo tendon properties. We investigated the relationships between in vivo imaging-based tendon properties, serum variables, and patient characteristics across healthy controls (n = 14, age: 45 ± 5 years, body mass index [BMI]: 24 ± 1, hemoglobin A1c [HbA1c]: 5.3 ± 0.1%), prediabetes (n = 14, age: 54 ± 5 years, BMI: 29 ± 2; HbA1c: 5.7 ± 0.1), and type 2 diabetes (n = 13, age: 55 ± 3 years, BMI: 33 ± 2, HbA1c: 6.7 ± 0.3). We used ultrasound speckle-tracking and measurements from magnetic resonance imaging (MRI) to estimate the patellar tendon in vivo tangent modulus. Analysis of plasma c-peptide, interleukin-1β (IL-1β), IL-6, IL-8, tumor necrosis factor-α (TNF-α), adiponectin, leptin, insulin-like growth factor 1 (IGF-1), and C-reactive protein (CRP) was completed. We built regression models incorporating statistically significant covariates and indicators for the clinically defined groups. We found that tendon cross-sectional area normalized to body weight (BWN CSA) and modulus were lower in patients with type 2 diabetes than in healthy controls (p < 0.05). Our regression analysis revealed that a model that included BMI, leptin, high-density lipoprotein (HDL), low-density lipoprotein (LDL), age, and group explained ~70% of the variability in BWN CSA (R2 = 0.70, p < 0.001). For modulus, including the main effects LDL, groups, HbA1c, age, BMI, cholesterol, IGF-1, c-peptide, leptin, and IL-6, accounted for ~54% of the variability in modulus (R2 = 0.54, p < 0.05). While BWN CSA and modulus were lower in those with diabetes, group was a poor predicter of tendon properties when considering the selected covariates. These data highlight the multifactorial nature of tendon changes with diabetes and suggest that blood variables could be reliable predictors of tendon properties.

Early Stages of Ex Vivo Collagen Glycation Disrupt the Cellular Interaction and Its Remodeling by Mesenchymal Stem Cells-Morphological and Biochemical Evidence

Mesenchymal stem cells (MSCs), pivotal for tissue repair, utilize collagen to restore structural integrity in damaged tissue, preserving its organization through concomitant remodeling. The non-enzymatic glycation of collagen potentially compromises MSC communication, particularly upon advancing the process, underlying various pathologies such as late-stage diabetic complications and aging. However, an understanding of the impact of early-stage collagen glycation on MSC interaction is lacking. This study examines the fate of in vitro glycated rat tail collagen (RTC) upon exposure to glucose for 1 or 5 days in contact with MSCs. Utilizing human adipose tissue-derived MSCs (ADMSCs), we demonstrate their significantly altered interaction with glycated collagen, characterized morphologically by reduced cell spreading, diminished focal adhesions formation, and attenuated development of the actin cytoskeleton. The morphological findings were confirmed by ImageJ 1.54g morphometric analysis with the most significant drop in the cell spreading area (CSA), from 246.8 μm2 for the native collagen to 216.8 μm2 and 163.7 μm2 for glycated ones, for 1 day and 5 days, respectively, and a similar trend was observed for cell perimeter 112.9 μm vs. 95.1 μm and 86.2 μm, respectively. These data suggest impaired recognition of early glycated collagen by integrin receptors. Moreover, they coincide with the reduced fibril-like reorganization of adsorbed FITC-collagen (indicating impaired remodeling) and a presumed decreased sensitivity to proteases. Indeed, confirmatory assays reveal diminished FITC-collagen degradation for glycated samples at 1 day and 5 days by attached cells (22.8 and 30.4%) and reduced proteolysis upon exogenous collagenase addition (24.5 and 40.4%) in a cell-free system, respectively. The mechanisms behind these effects remain uncertain, although differential scanning calorimetry confirms subtle structural/thermodynamic changes in glycated collagen.

Advanced glycation end-products as mediators of the aberrant crosslinking of extracellular matrix in scarred liver tissue

The extracellular matrix of cirrhotic liver tissue is highly crosslinked. Here we show that advanced glycation end-products (AGEs) mediate crosslinking in liver extracellular matrix and that high levels of crosslinking are a hallmark of cirrhosis. We used liquid chromatography-tandem mass spectrometry to quantify the degree of crosslinking of the matrix of decellularized cirrhotic liver samples from patients and from two mouse models of liver fibrosis and show that the structure, biomechanics and degree of AGE-mediated crosslinking of the matrices can be recapitulated in collagen matrix crosslinked by AGEs in vitro. Analyses via cryo-electron microscopy and optical tweezers revealed that crosslinked collagen fibrils form thick bundles with reduced stress relaxation rates; moreover, they resist remodelling by macrophages, leading to reductions in their levels of adhesion-associated proteins, altering HDAC3 expression and the organization of their cytoskeleton, and promoting a type II immune response of macrophages. We also show that rosmarinic acid inhibited AGE-mediated crosslinking and alleviated the progression of fibrosis in mice. Our findings support the development of therapeutics targeting crosslinked extracellular matrix in scarred liver tissue.

Inhibition of advance glycation end products formation, gastrointestinal digestion, absorption and toxicity: A comprehensive review

Advanced glycation end-products (AGEs) are the final products of the non-enzymatic interaction between reducing sugars and amino groups in proteins, lipids and nucleic acids. In numerous diseases, such as diabetes, neuropathy, atherosclerosis, aging, nephropathy, retinopathy, and chronic renal illness, accumulation of AGEs has been proposed as a pathogenic mechanism of inflammation, oxidative stress, and structural tissue damage leading to chronic vascular issues. Current studies on the inhibition of AGEs mainly focused on food processing. However, there are few studies on the inhibition of AGEs during digestion, absorption and metabolism although there are still plenty of AGEs in our body with our daily diet. This review comprehensively expounded AGEs inhibition mechanism based on the whole process of digestion, absorption and metabolism by polyphenols, amino acids, hydrophilic colloid, carnosine and other new anti-glycation agents. Our study will provide a ground-breaking perspective on mediation or inhibition AGEs.

Significance of Type II Collagen Posttranslational Modifications: From Autoantigenesis to Improved Diagnosis and Treatment of Rheumatoid Arthritis

Collagen type II (COL2), the main structural protein of hyaline cartilage, is considerably affected by autoimmune responses associated with the pathogenesis of rheumatoid arthritis (RA). Posttranslational modifications (PTMs) play a significant role in the formation of the COL2 molecule and supramolecular fibril organization, and thus, support COL2 function, which is crucial for normal cartilage structure and physiology. Conversely, the specific PTMs of the protein (carbamylation, glycosylation, citrullination, oxidative modifications and others) have been implicated in RA autoimmunity. The discovery of the anti-citrullinated protein response in RA, which includes anti-citrullinated COL2 reactivity, has led to the development of improved diagnostic assays and classification criteria for the disease. The induction of immunological tolerance using modified COL2 peptides has been highlighted as a potentially effective strategy for RA therapy. Therefore, the aim of this review is to summarize the recent knowledge on COL2 posttranslational modifications with relevance to RA pathophysiology, diagnosis and treatment. The significance of COL2 PTMs as a source of neo-antigens that activate immunity leading to or sustaining RA autoimmunity is discussed.

Novel Pyridinium Cross-Link Structures Derived from Glycolaldehyde and Glyoxal

Short-chained α-hydroxycarbonyl compounds such as glycolaldehyde (GA) and its oxidized counterpart glyoxal (GX) are known as potent glycating agents. Here, a novel fluorescent lysine-lysine cross-link 1-(5-amino-5-carboxypentyl)-3-(5-amino-5-carboxy-pentylamino)pyridinium salt (meta-DLP) was synthesized and its structure unequivocally proven by ¹H NMR, ¹³C-NMR attached proton test, and 2D NMR. Further characterization of chemical properties and mechanistic background was obtained in comparison to the known monovalent protein modification 2-ammonio-6-(3-oxidopyridinium-1-yl)hexanoate (OP-lysine). Identification and quantitation in various sugar incubations with N²-t-Boc-lysine revealed a novel alternative formation pathway for both advanced glycation end products (AGEs) by the interplay of both carbonyl compounds, GA and GX, which was confirmed by isotope labeling experiments. The concentration of pyridinium AGEs was about 1000-fold lower compared to the well-established N⁶-carboxymethyl lysine. However, pyridinium AGEs were shown to lead to the photosensitized generation of singlet oxygen in irradiation experiments, which was verified by the detection of 3,3'-(naphthalene-1,4-diyl)-dipropionate endoperoxide. Furthermore, meta-DLP was identified in hydrolyzed potato chip proteins by collision-induced dissociation mass spectrometry after HPLC enrichment.

Glycation Alters the Fatty Acid Binding Capacity of Human Serum Albumin

Glycation significantly alters the physicochemical and biofunctional properties of proteins in foods and in vivo. In the present study, human serum albumin (HSA) as the major transporter of fatty acids was modified with glyoxal under physiological conditions. Reversibly albumin-bound glyoxal was removed, and advanced glycation end products were quantitated by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The total modification of protein-bound lysine and arginine residues reached up to 4.2 and 9.6%, respectively. The impact of these modifications on the transport capacity of long-chain fatty acids was characterized by spin-labeled fatty acid probes via electron paramagnetic resonance spectroscopy. With increasing degree of glycation, the equivalence of the seven binding sites of native HSA with a dissociation constant of 0.74 ± 0.09 μM was set off with only the three high-affinity sites 2, 4, and 5 remaining (0.46 ± 0.07 μM). The other four sites were shifted to low affinities with significantly higher dissociation constants (1.32 ± 0.35 μM). Tryptic peptide mapping enabled us to relate these findings to molecular changes at specific binding sites. Modification hotspots identified were lysine 351, 286, 159 and arginine 144, 485, 117. Further investigation of plasma protein samples of uremic patients vs healthy controls gave first insights into the in vivo situation.

Analysis of Glyoxal- and Methylglyoxal-Derived Advanced Glycation End Products during Grilling of Porcine Meat

The reaction of the N⁶-amino group of lysine residues and 1,2-dicarbonyl compounds during Maillard processes leads to advanced glycation end products (AGEs). In the present work, we deliver a comprehensive analysis of changes of carbohydrates, dicarbonyl structures, and 11 AGEs during the grilling of porcine meat patties. While raw meat contained mainly glyoxal-derived N⁶-carboxymethyl lysine (CML), grilling led to an increase of predominantly methylglyoxal-derived AGEs N⁶-carboxyethyl lysine (CEL), N⁶-lactoyl lysine, methylglyoxal lysine dimer (MOLD), and methylglyoxal lysine amide (MOLA). Additionally, we identified and quantitated a novel methylglyoxal-derived amidine compound N¹,N²-di-(5-amino-5-carboxypentyl)-2-lactoylamidine (methylglyoxal lysine amide, MGLA) in heated meat. Analysis of carbohydrates suggested that approximately 50% of the methylglyoxal stemmed from the fragmentation of triosephosphates during the heat treatment. Surprisingly, N⁶-lactoyl lysine was the major AGE, and based on model incubations, we propose that approximately 90% must be explained by the nonenzymatic acylation of lysine through S-lactoylglutathione, which was quantitated for the first time in meat herein.

Inhibition of Advanced Glycation End Product Formation in Rat Tail Tendons by Polydatin and p-Coumaric acid: an In Vitro Study

Advanced glycation end products (AGEs) formed through non-enzymatic glycosylation between a protein and sugar molecule are highly harmful to the human body. In hyperglycemic patients, AGE formation is more due to high glucose circulating in the blood, causing inter and intra molecular cross-linking of collagen leading to reduction of collagen elasticity. This cross-linked collagen develops resistance to matrix metalloproteinases leading to impaired collagen turnover. The aim of this work is to determine the anti-glycation effects of polydatin and p-coumaric acid in preventing collagen cross-linking by incubating rat tail tendons (RTTs) as collagen source in high glucose concentration (50 mM) for a week. The RTTs were then characterized for tensile strength, cross-linking efficiency, circular dichroism spectrometry, collagen, glucose, and aldehyde contents. Electrophoresis was carried out to evaluate the level of cross-linking in collagen and the results confirmed the ability of the drugs in preventing complex intermolecular cross-link formation induced by non-enzymatic glycosylation. CD data showed alteration in the secondary structure of collagen where AGE formation had occurred. More collagen was extracted by pepsin from RTTs treated with glucose alone (6.88 mg/10 mg tendon) when compared with drug-treated groups (4.25, 2.56 mg/10 mg tendon for polydatin and p-coumaric acid, respectively). Tensile strength (20.66% and 18.95%), cross-linking percentage (32.5% and 29.84%), and glucose content (2.3 and 1.8 mg/100 mg) of drug-treated groups were similar to the positive control (19.07%, 30.13%, and 2.61 mg/100 mg) thus proving the anti-glycation potential of the drugs. Hence, both polydatin and p-coumaric acid could play a pivotal role in preventing AGE formation.

The Putative Role of Methylglyoxal in Arterial Stiffening: A Review

BACKGROUND

Arterial stiffening is a hallmark of vascular ageing and a consequence of many diseases including diabetes mellitus. Methylglyoxal (MGO), a highly reactive α-dicarbonyl mainly formed during glycolysis, has emerged as a potential contributor to the development of arterial stiffness. MGO reacts with arginine and lysine residues in proteins to form stable advanced glycation endproducts (AGEs). AGEs may contribute to arterial stiffening by increased cross-linking of collagen within the extracellular matrix (ECM), by altering the vascular structure, and by triggering inflammatory and oxidative pathways. Although arterial stiffness is mainly determined by ECM and vascular smooth muscle cell function, the effects of MGO and MGO-derived AGEs on these structures have not been thoroughly reviewed to date.

METHODS AND RESULTS

We conducted a PubMed search without filtering for publication date which resulted in 16 experimental and 22 clinical studies eligible for inclusion. Remarkably, none of the experimental and only three of the clinical studies specifically mentioned MGO-derived AGEs. Almost all studies reported an association between arterial stiffness and AGE accumulation in the arterial wall or increased plasma AGEs. Other studies report reduced arterial stiffness in experimental models upon administration of AGE-breakers.

CONCLUSIONS

No papers published to date directly show an association between MGO or MGO-derived AGEs and arterial stiffening. The relevance of the various underlying mechanisms is not yet clear, which is particularly due to methodological challenges in the detection of MGO and MGO-derived AGEs at the molecular, intra- and pericellular, and structural levels, as well as in challenges in the assessment of intrinsic arterial wall properties at ECM- and tissue levels.

A Hypothesis: Fructosamine-3-Kinase-Related-Protein (FN3KRP) Catalyzes Deglycation of Maillard Intermediates Directly Downstream from Fructosamines

Non-enzymatic glycation (a.k.a. Maillard reaction) is a series of random spontaneous reactions between reducing sugars and amines, resulting in the formation of irreversible advanced glycation endproducts (AGE's). In food chemistry, this process is beneficial by contributing to the flavor, aroma, texture, and appearance of cooked foods. In vivo, however, Maillard reaction is deleterious because uncontrolled modification and crosslinking of biological macromolecules impairs their function. Consequently, chronic hyperglycemia of diabetes mellitus, for instance, leads to increased non-enzymatic glycation and diverse, multi-organ pathologies of diabetic complications. Based on the fact that toxic compounds, such as free radicals, are detoxified in vivo by specific defense mechanisms, one would expect to find mechanisms to control glucose toxicity as well. Thus far, only one such enzyme, fructosamine-3-kinase (FN3K), has been characterized. It operates intracellularly by catalyzing ATP-dependent removal of Maillard adducts, D-fructoselysines, from proteins, thereby reducing the Maillard reaction flux from glucose to AGE's. When FN3K was isolated, a closely related but distinct protein copurified with it. Unlike FN3K, however, this enzyme, fructosamine-3-kinase-related protein (FN3KRP), does not phosphorylate D-fructoselysines but it does phosphorylate several other (non-physiological) substrates. Interestingly, the distribution of FN3KRP in nature appears to be nearly universal whereas that of FN3K is limited to endotherms. In this article, it is suggested that the function of FN3KRP is deglycation of Maillard adducts downstream from fructoselysines. Such a mechanism, if proven correct, would be valuable given reports on apparent correlations between FN3KRP and some chronic conditions and/or diseases, such as a recent publication which proposes that the FN3KRP gene may be a longevity gene.

AGE/RAGE in diabetic kidney disease and ageing kidney

Diabetic kidney disease (DKD) is the primary cause of chronic kidney disease that inevitably progress to end-stage kidney disease. Intervention strategies such as blood glucose control is effective for preventing DKD, but many patients with DKD still reach end-stage kidney disease. Although comprehensive mechanisms shed light on the progression of DKD, the most compelling evidence has highlighted that hyperglycemia-related advanced glycation end products (AGEs) formation plays a central role in the pathogenesis of DKD. Pathologically, accumulation of AGEs-mediated receptor for AGEs (RAGE) triggers oxidative stress and inflammation, which is the major deleterious effect of AGEs in host and intestinal microenvironment of diabetic and ageing conditions. The activation of AGEs-mediated RAGE could evoke nicotinamide adenine dinucleotide phosphate oxidase-induced reactive oxygen and nitrogen species production and subsequently give rise to oxidative stress in DKD and ageing kidney. Therefore, targeting RAGE with its ligands mediated oxidative stress and chronic inflammation is considered as an additional intervention strategy for DKD and ageing kidney. In this review, we summarize AGEs/RAGE-mediated oxidative stress and inflammation signaling pathways in DKD and ageing kidney, discussing opportunities and challenges of targeting at AGEs/RAGE-induced oxidative stress that could hold the promising potential approach for improving DKD and ageing kidney.

Novel Amidine Protein Cross-Links Formed by the Reaction of Glyoxal with Lysine

One crucial aspect of the Maillard reaction is the formation of reactive α-dicarbonyl structures like glyoxal, which are prone toward further reactions with proteins, e.g., the N⁶-amino group of lysine. The initially formed labile glyoxal-imine was previously established as a key intermediate in the formation of the advanced glycation end products N⁶-carboxymethyl lysine (CML), glyoxal lysine amide (GOLA), glyoxal lysine dimer (GOLD), and N⁶-glycolyl lysine (GALA). Here, we introduce a novel amidine cross-link structure N¹,N²-bis-(5-amino-5-carboxypentyl)-2-hydroxy-acetamidine (glyoxal lysine amidine, GLA), which is formed exclusively from glyoxal through the same isomerization cascade. After independent synthesis of the authentic reference standard, we were able to quantitate this cross-link in incubations of 40 mM N²-t-Boc-lysine with glyoxal and various sugars (40-100 mM) under mild conditions (pH 7.4, 37 °C) using an HPLC-MS/MS method. Furthermore, incubations of proteins (6 mg/mL) with 50 mM glyoxal confirmed the cross-linking by GLA, which was additionally identified in acidic hydrolyzed proteins of butter biscuits after HPLC enrichment.

Collagen Biosynthesis, Processing, and Maturation in Lung Ageing

In addition to providing a macromolecular scaffold, the extracellular matrix (ECM) is a critical regulator of cell function by virtue of specific physical, biochemical, and mechanical properties. Collagen is the main ECM component and hence plays an essential role in the pathogenesis and progression of chronic lung disease. It is well-established that many chronic lung diseases, e.g., chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) primarily manifest in the elderly, suggesting increased susceptibility of the aged lung or accumulated alterations in lung structure over time that favour disease. Here, we review the main steps of collagen biosynthesis, processing, and turnover and summarise what is currently known about alterations upon lung ageing, including changes in collagen composition, modification, and crosslinking. Recent proteomic data on mouse lung ageing indicates that, while the ER-resident machinery of collagen biosynthesis, modification and triple helix formation appears largely unchanged, there are specific changes in levels of type IV and type VI as well as the two fibril-associated collagens with interrupted triple helices (FACIT), namely type XIV and type XVI collagens. In addition, levels of the extracellular collagen crosslinking enzyme lysyl oxidase are decreased, indicating less enzymatically mediated collagen crosslinking upon ageing. The latter contrasts with the ageing-associated increase in collagen crosslinking by advanced glycation endproducts (AGEs), a result of spontaneous reactions of protein amino groups with reactive carbonyls, e.g., from monosaccharides or reactive dicarbonyls like methylglyoxal. Given the slow turnover of extracellular collagen such modifications accumulate even more in ageing tissues. In summary, the collective evidence points mainly toward age-induced alterations in collagen composition and drastic changes in the molecular nature of collagen crosslinks. Future work addressing the consequences of these changes may provide important clues for prevention of lung disease and for lung bioengineering and ultimately pave the way to novel targeted approaches in lung regenerative medicine.

Pathways of Non-enzymatic Lysine Acylation

Posttranslational protein modification by lysine acylation is an emerging mechanism of cellular regulation and fine-tunes metabolic processes to environmental changes. In this review we focus on recently discovered pathways of non-enzymatic lysine acylation by reactive acyl-CoA species, acyl phosphates, and α-dicarbonyls. We summarize the metabolic sources of these highly reactive intermediates, demonstrate their reaction mechanisms, give an overview of the resulting acyl lysine modifications, and evaluate the consequences for cellular regulatory processes. Finally, we discuss interferences between lysine acylation and lysine ubiquitylation as a potential molecular mechanism of dysregulated protein homeostasis in aging and related diseases.

Comprehensive Analyses of Carbohydrates, 1,2-Dicarbonyl Compounds, and Advanced Glycation End Products in Industrial Bread Making

The technology of bread making is characterized by three major steps: dough mixing, proofing, and baking. To follow the course of Maillard processes in an authentic food matrix, the complete manufacturing process of wheat bread rolls was assessed along all production steps with the quantitation of sugars, furfurals, 1,2-dicarbonyl compounds, and advanced glycation end products (AGEs). As a result, the AGE profile was significantly enlarged to more than 12 structures, and comprehensive mechanistic insights were provided. The analyses of five major German bread types including wheat, brown, rye bread, pumpernickel, and crispbreads led to AGE contents of 69-149 mg/kg bread or 984-1857 mg/kg protein. Major lysine protein modifications were carboxymethyl, carboxyethyl, and formyl lysine and pyrraline. Arginine was mainly modified by methylglyoxal (MGO) to give imidazolinones. A major part of MGO was confirmed to stem from microbial metabolism.

Stochastic non-enzymatic modification of long-lived macromolecules - A missing hallmark of aging

Damage accumulation in long-living macromolecules (especially extracellular matrix (ECM) proteins, nuclear pore complex (NPC) proteins, and histones) is a missing hallmark of aging. Stochastic non-enzymatic modifications of ECM trigger cellular senescence as well as many other hallmarks of aging affect organ barriers integrity and drive tissue fibrosis. The importance of it for aging makes it a key target for interventions. The most promising of them can be AGE inhibitors (chelators, O-acetyl group or transglycating activity compounds, amadorins and amadoriases), glucosepane breakers, stimulators of elastogenesis, and RAGE antagonists.

Enzymatic and non-enzymatic crosslinks found in collagen and elastin and their chemical synthesis

This review summarized the enzymatic and non-enzymatic crosslinks found in collagen and elastin and their organic synthesis.

Methylglyoxal, a Highly Reactive Dicarbonyl Compound, in Diabetes, Its Vascular Complications, and Other Age-Related Diseases

The formation and accumulation of methylglyoxal (MGO), a highly reactive dicarbonyl compound, has been implicated in the pathogenesis of type 2 diabetes, vascular complications of diabetes, and several other age-related chronic inflammatory diseases such as cardiovascular disease, cancer, and disorders of the central nervous system. MGO is mainly formed as a byproduct of glycolysis and, under physiological circumstances, detoxified by the glyoxalase system. MGO is the major precursor of nonenzymatic glycation of proteins and DNA, subsequently leading to the formation of advanced glycation end products (AGEs). MGO and MGO-derived AGEs can impact on organs and tissues affecting their functions and structure. In this review we summarize the formation of MGO, the detoxification of MGO by the glyoxalase system, and the biochemical pathways through which MGO is linked to the development of diabetes, vascular complications of diabetes, and other age-related diseases. Although interventions to treat MGO-associated complications are not yet available in the clinical setting, several strategies to lower MGO have been developed over the years. We will summarize several new directions to target MGO stress including glyoxalase inducers and MGO scavengers. Targeting MGO burden may provide new therapeutic applications to mitigate diseases in which MGO plays a crucial role.

Carbamylation and glycation compete for collagen molecular aging in vivo

Tissue aging is a complex phenomenon involving molecular aging of matrix proteins, which mainly results from their progressive alteration by nonenzymatic post-translational modifications (NEPTMs) such as glycation and carbamylation. These two reactions, which correspond to the binding of reactive metabolites (i.e. reducing sugars and urea-derived cyanate, respectively) on amino groups of proteins, occur during aging and are amplified in various chronic diseases such as diabetes mellitus or chronic renal disease (CKD). Since these reactions target the same functional groups, they can reciprocally compete for protein modification. Determining which NEPTM is predominant in tissues is necessary to better understand their role in the development of long-term complications of chronic diseases. For that purpose, two different murine models were used for reproducing such a competitive context: a CKD-diabetic mice model and a cyanate-consuming mice model. The competition has been evaluated by quantifying glycation and carbamylation products by LC-MS/MS in skin and aorta total extracts as well as in skin type I collagen. The results showed that the simultaneous enhancement of glycation and carbamylation reactions resulted in a decrease of the formation of glycation products (especially Amadori products) whereas the concentrations of homocitrulline, a carbamylation product, remained similar. These results, which have been obtained in both tissues and in purified skin type I collagen, suggest that carbamylation takes precedence over glycation for the modification of tissue proteins, but only in pathological conditions favouring these two NEPTMs. While glycation has been considered for a long time the predominant NEPTM of matrix proteins, carbamylation seems to also play an important role in tissue aging. The existence of competition between these NEPTMs must be taken into account to better understand the consequences of molecular aging of matrix proteins in tissue aging.

The fate of dietary advanced glycation end products in the body: from oral intake to excretion

Advanced glycation end products (AGEs), which are closely associated with various chronic diseases, are formed through the Maillard reaction when aldehydes react with amines in heated foods or in living organisms. The fate of dietary AGEs after oral intake plays a crucial role in regulating the association between dietary AGEs and their biological effects. However, the complexity and diversity of dietary AGEs make their fate ambiguous. Glycated modifications can impair the digestion, transport and uptake of dietary AGEs. High and low molecular weight AGEs may exhibit individual differences in their distribution, metabolism and excretion. Approximately 50-60% of free AGEs are excreted after dietary intake, whereas protein-bound AGEs exhibit a limited excretion rate. In this article, we summarize several AGE classification criteria and their abundance in foods, and in the body. A standardized static in vitro digestion method is strongly recommended to obtain comparable results of AGE digestibility. Sophisticated hypotheses regarding the intestinal transportation and absorption of drugs, as well as calculated physicochemical parameters, are expected to alleviate the difficulties determining the digestion, transport and uptake of dietary AGEs. Orally supplied AGEs with low or high molecular weights must be supported by well-defined amounts in investigations of excretion. Furthermore, unequivocal evidence should be obtained regarding the degradation and metabolism products of dietary AGEs.

Analysis of Chemically Labile Glycation Adducts in Seed Proteins: Case Study of Methylglyoxal-Derived Hydroimidazolone 1 (MG-H1)

Seeds represent the major source of food protein, impacting on both human nutrition and animal feeding. Therefore, seed quality needs to be appropriately addressed in the context of viability and food safety. Indeed, long-term and inappropriate storage of seeds might result in enhancement of protein glycation, which might affect their quality and longevity. Glycation of seed proteins can be probed by exhaustive acid hydrolysis and quantification of the glycation adduct N^ɛ-(carboxymethyl)lysine (CML) by liquid chromatography-mass spectrometry (LC-MS). This approach, however, does not allow analysis of thermally and chemically labile glycation adducts, like glyoxal-, methylglyoxal- and 3-deoxyglucosone-derived hydroimidazolones. Although enzymatic hydrolysis might be a good solution in this context, it requires aqueous conditions, which cannot ensure reconstitution of seed protein isolates. Because of this, the complete profiles of seed advanced glycation end products (AGEs) are not characterized so far. Therefore, here we propose the approach, giving access to quantitative solubilization of seed proteins in presence of sodium dodecyl sulfate (SDS) and their quantitative enzymatic hydrolysis prior to removal of SDS by reversed phase solid phase extraction (RP-SPE). Using methylglyoxal-derived hydroimidazolone 1 (MG-H1) as a case example, we demonstrate the applicability of this method for reliable and sensitive LC-MS-based quantification of chemically labile AGEs and its compatibility with bioassays.

Protocatechuic and 3,4-Dihydroxyphenylacetic Acids Inhibit Protein Glycation by Binding Lysine through a Metal-Catalyzed Oxidative Mechanism

The mechanism of inhibition of advanced glycation end product (AGE) formation by protocatechuic acid and 3,4-dihydroxyphenylacetic acid (DHPA) has been studied using a widespread applied in vitro model system composed of bovine serum albumin (BSA) and supraphysiological glucose concentrations. Protocatechuic acid and DHPA inhibited the formation of Amadori compounds, fluorescent AGEs (IC₅₀ = 62.1 ± 1.4 and 155.4 ± 1.1 μmol/L, respectively), and N^ε-(carboxymethyl)lysine (IC₅₀ = 535.3 ± 1.1 and 751.2 ± 1.0 μmol/L, respectively). BSA was pretreated with the two phenolic acids, and the formation of BSA-phenolic acid adducts was estimated by nanoflow liquid chromatography-electrospray ionization-quadrupole time-of-flight mass spectrometry. Results showed that the tested phenolic acids bound key sites of glycation in BSA through a metal-catalyzed oxidative mechanism. The antiglycative activity mechanism involved the formation of BSA-phenolic acid adducts, and it is unlikely that this occurs in vivo. These results raise the problem to design in vitro models closer to physiological conditions to reach biologically sound conclusions.