Mass Spectrometric Analysis of Glyoxal and Methylglyoxal-Induced Modifications in Human Hemoglobin from Poorly Controlled Type 2 Diabetes Mellitus Patients

Proteomics-based identification of biomarkers reflecting endogenous and exogenous exposure to the advanced glycation end product precursor methylglyoxal in SH-SY5Y human neuroblastoma cells

Methylglyoxal (MGO), a highly reactive precursor of advanced glycation end products, is endogenously produced and prevalent in various food products. This study aimed to characterize protein modifications in SH-SY5Y human neuroblastoma cells induced by MGO and identify potential biomarkers for its exposure and toxicity. A shot-gun proteomic analysis was applied to characterize protein modifications in cells incubated with and without exogenous MGO. Seventy-seven proteins were identified as highly susceptible to MGO modification, among which eight, including vimentin and histone H2B type 2-F, showing concentration-dependent modifications by externally added MGO, were defined as biomarkers for exogenous MGO exposure. Remarkably, up to 10 modification sites were identified on vimentin. Myosin light polypeptide 6 emerged as a biomarker for MGO toxicity, with modifications exclusively observed under cytotoxic MGO levels. Additionally, proteins like serine/threonine-protein kinase SIK2 and calcyphosin, exhibiting comparable or even higher modification levels in control compared to exogenous MGO-treated cells, were defined as biomarkers for endogenous exposure. Bioinformatics analysis revealed that motor proteins, cytoskeleton components, and glycolysis proteins were overrepresented among those highly susceptible to MGO modification. These results identify biomarkers for both endogenous and exogenous MGO exposure and provide insights into the cellular effects of endogenously formed versus externally added MGO.

Methylglyoxal in Cardiometabolic Disorders: Routes Leading to Pathology Counterbalanced by Treatment Strategies

Methylglyoxal (MGO) is the major compound belonging to reactive carbonyl species (RCS) responsible for the generation of advanced glycation end products (AGEs). Its upregulation, followed by deleterious effects at the cellular and systemic levels, is associated with metabolic disturbances (hyperglycemia/hyperinsulinemia/insulin resistance/hyperlipidemia/inflammatory processes/carbonyl stress/oxidative stress/hypoxia). Therefore, it is implicated in a variety of disorders, including metabolic syndrome, diabetes mellitus, and cardiovascular diseases. In this review, an interplay between pathways leading to MGO generation and scavenging is addressed in regard to this system's impairment in pathology. The issues associated with mechanistic MGO involvement in pathological processes, as well as the discussion on its possible causative role in cardiometabolic diseases, are enclosed. Finally, the main strategies aimed at MGO and its AGEs downregulation with respect to cardiometabolic disorders treatment are addressed. Potential glycation inhibitors and MGO scavengers are discussed, as well as the mechanisms of their action.

Mass Spectrometry Analysis of DNA and Protein Adducts as Biomarkers in Human Exposure to Cigarette Smoking: Acrolein as an Example

Acrolein is a major component in cigarette smoke and a product of endogenous lipid peroxidation. It is difficult to distinguish human exposure to acrolein from exogenous sources versus endogenous causes, as components in cigarette smoke can stimulate lipid peroxidation in vivo. Therefore, analysis of acrolein-induced DNA and protein adducts by the highly accurate, sensitive, and specific mass spectrometry-based methods is vital to estimate the degree of damage by this IARC Group 2A carcinogen. This Perspective reviews the analyses of acrolein-induced DNA and protein adducts in humans by mass spectrometry focusing on samples accessible for biomonitoring, including DNA from leukocytes and oral cells and abundant proteins from blood, i.e., hemoglobin and serum albumin.

Malondialdehyde-Induced Post-translational Modifications in Hemoglobin of Smokers by NanoLC-NSI/MS/MS Analysis

Malondialdehyde (MDA) is the most abundant α,β-unsaturated aldehyde generated from endogenous peroxidation of polyunsaturated fatty acids and is present in cigarette smoke. Post-translational modifications of blood hemoglobin can serve as biomarkers for exposure to chemicals. In this study, two types of MDA-induced modifications, the N-propenal and the dihydropyridine (DHP), were identified at multiple sites in human hemoglobin digest by the high-resolution mass spectrometry. The N-propenal and the DHP types of modification led to the increase of 54.0106 and 134.0368 amu, respectively, at the N-terminal and lysine residues. Among the 21 MDA-modified peptides, 14 with dose-response to MDA concentrations were simultaneously quantified in study subjects by the nanoflow liquid chromatography nanoelectrospray ionization tandem mass spectrometry under selected reaction monitoring (nanoLC-NSI-MS/MS-SRM) without prior enrichment. The results showed that the modifications of the N-propenal-type at α-Lys-11, α-Lys-16, α-Lys-61, β-Lys-8, and β-Lys-17, as well as the DHP-type at the α-N-terminal valine, are significantly higher in hemoglobin isolated from the blood of smokers than in nonsmoking individuals. This is the first report to identify and quantify multiple sites of MDA-induced modifications in human hemoglobin from peripheral blood. Our results suggest that the MDA-derived modifications on hemoglobin might represent valuable biomarkers for MDA-induced protein damage.

Diet and Obesity-Induced Methylglyoxal Production and Links to Metabolic Disease

The obesity rate in the United States is 42.4% and has become a national epidemic. Obesity is a complex condition that is influenced by socioeconomic status, ethnicity, genetics, age, and diet. Increased consumption of a Western diet, one that is high in processed foods, red meat, and sugar content, is associated with elevated obesity rates. Factors that increase obesity risk, such as socioeconomic status, also increase consumption of a Western diet because of a limited access to healthier options and greater affordability of processed foods. Obesity is a public health threat because it increases the risk of several pathologies, including atherosclerosis, diabetes, and cancer. The molecular mechanisms linking obesity to disease onset and progression are not well understood, but a proposed mechanism is physiological changes caused by altered lipid peroxidation, glycolysis, and protein metabolism. These metabolic pathways give rise to reactive molecules such as the abundant electrophile methylglyoxal (MG), which covalently modifies nucleic acids and proteins. MG-adducts are associated with obesity-linked pathologies and may have potential for biomonitoring to determine the risk of disease onset and progression. MG-adducts may also play a role in disease progression because they are mutagenic and directly impact protein stability and function. In this review, we discuss how obesity drives metabolic alterations, how these alterations lead to MG production, the association of MG-adducts with disease, and the potential impact of MG-adducts on cellular function.

Methylglyoxal-derived hemoglobin advanced glycation end products induce apoptosis and oxidative stress in human umbilical vein endothelial cells

The presence of excess glucose promotes hemoglobin glycation via the biochemical modification of hemoglobin by dicarbonyl products. However, the precise effects of Hb-AGEs in human umbilical vein endothelial cells (HUVECs) are not known to date. Therefore, we investigated the tentative effects of Hb-AGEs in HUVECs. Initially, we used the AGE formation assay to examine the selectivity of MGO toward various proteins. Among all proteins, MGO-Hb-AGEs formation was higher compared to the formation of other dicarbonyl-mediated AGEs. Our next data demonstrated that treatment with 0.5 mg/mL of Hb-AGEs-4w significantly reduced cell viability in HUVECs. Further, we evaluated the role of MGO in conformational and structural changes in Hb. The results showed that Hb demonstrated a highly altered conformation upon incubation with MGO. Moreover, Hb-AGEs-4w treatment strongly increased ROS production, and decreased mitochondrial membrane potential in HUVECs, and moderately reduced the expression of phosphorylated forms of p-38 and JNK. We observed that Hb-AGEs-4w treatment increased the number of apoptotic cells and the Bax/Bcl-2 ratio and cleaved the nuclear enzyme PARP in HUVECs. Finally, Hb-AGEs also inhibited migration and proliferation of HUVECs, thus be physiologically significant in endothelial dysfunction. Taken together, our data suggest that Hb-AGEs may play a critical role in inducing vascular endothelial cell damage. Therefore, this study may provide a plausible explanation for the potential Hb-AGEs in human endothelial cell dysfunction of diabetic patients.

High-Performance affinity chromatographic studies of repaglinide and nateglinide interactions with normal and glyoxal- or methylglyoxal-modified human albumin serum

During diabetes human serum albumin (HSA), an important drug transport protein, can be modified by agents such as glyoxal (Go) and methylglyoxal (MGo) to form advanced glycation end-products. High-performance affinity microcolumns and zonal elution competition studies were used to compare interactions by the anti-diabetic drugs repaglinide and nateglinide with normal and Go- or MGo-modified HSA at Sudlow sites I and II of this protein. Both drugs had their strongest binding at Sudlow site II for the normal and modified forms of HSA. The association equilibrium constants at this site for repaglinide and nateglinide with normal HSA were 6.1 (± 0.2) × 10⁴ M^-1 and 7.1 (± 0.8) × 10⁵ M^-1, respectively, at pH 7.4 and 37⁰C; these values increased by up to 3.6-fold for repaglinide and decreased by up to 45-55 % for nateglinide when HSA was modified by Go or MGo at levels seen in prediabetes or diabetes. Both drugs were also found to bind at Sudlow site I, with association equilibrium constants at this site on normal HSA of 4.2 (± 0.3) × 10⁴ M^-1 for repaglinide and 5.0 (± 0.1) × 10⁴ M^-1 for nateglinide. The binding strength for repaglinide at Sudlow site I increased by 1.3- to 1.7-fold with the Go-modified HSA and decreased slightly (i.e., up to 19 %) for the MGo-modified HSA, while nateglinide showed only a small or insignificant change in binding with the same modified HSA samples. These results indicated that binding by repaglinide and nateglinide with HSA can be altered significantly by modification of this protein with Go or MGo, making these modifications of potential interest in the treatment of patients with these drugs during diabetes.

Carbonyl Stress in Red Blood Cells and Hemoglobin

The paper overviews the peculiarities of carbonyl stress in nucleus-free mammal red blood cells (RBCs). Some functional features of RBCs make them exceptionally susceptible to reactive carbonyl compounds (RCC) from both blood plasma and the intracellular environment. In the first case, these compounds arise from the increased concentrations of glucose or ketone bodies in blood plasma, and in the second-from a misbalance in the glycolysis regulation. RBCs are normally exposed to RCC-methylglyoxal (MG), triglycerides-in blood plasma of diabetes patients. MG modifies lipoproteins and membrane proteins of RBCs and endothelial cells both on its own and with reactive oxygen species (ROS). Together, these phenomena may lead to arterial hypertension, atherosclerosis, hemolytic anemia, vascular occlusion, local ischemia, and hypercoagulation phenotype formation. ROS, reactive nitrogen species (RNS), and RCC might also damage hemoglobin (Hb), the most common protein in the RBC cytoplasm. It was Hb with which non-enzymatic glycation was first shown in living systems under physiological conditions. Glycated HbA1c is used as a very reliable and useful diagnostic marker. Studying the impacts of MG, ROS, and RNS on the physiological state of RBCs and Hb is of undisputed importance for basic and applied science.

Potential Markers of Dietary Glycemic Exposures for Sustained Dietary Interventions in Populations without Diabetes

There is considerable interest in dietary and other approaches to maintaining blood glucose concentrations within the normal range and minimizing exposure to postprandial hyperglycemic excursions. The accepted marker to evaluate the sustained maintenance of normal blood glucose concentrations is glycated hemoglobin A1c (HbA1c). However, although this is used in clinical practice to monitor glycemic control in patients with diabetes, it has a number of drawbacks as a marker of efficacy of dietary interventions that might beneficially affect glycemic control in people without diabetes. Other markers that reflect shorter-term glycemic exposures have been studied and proposed, but consensus on the use and relevance of these markers is lacking. We have carried out a systematic search for studies that have tested the responsiveness of 6 possible alternatives to HbA1c as markers of sustained variation in glycemic exposures and thus their potential applicability for use in dietary intervention trials in subjects without diabetes: 1,5-anhydroglucitol (1,5-AG), dicarbonyl stress, fructosamine, glycated albumin (GA), advanced glycated end products (AGEs), and metabolomic profiles. The results suggest that GA may be the most promising for this purpose, but values may be confounded by effects of fat mass. 1,5-AG and fructosamine are probably not sensitive enough to the range of variation in glycemic exposures observed in healthy individuals. Use of measures based on dicarbonyls, AGEs, or metabolomic profiles would require further research into possible specific molecular species of interest. At present, none of the markers considered here is sufficiently validated and sensitive for routine use in substantiating the effects of sustained variation in dietary glycemic exposures in people without diabetes.

Deciphering the Nature of Caffeic Acid to Inhibit the HSA Aggregation Induced by Glyoxal

BACKGROUND

Under certain circumstances, the path for protein folding deviates and attains an alternative path forming misfolded states, which are the key precursors for protein aggregation. Protein aggregation is associated with variety of diseases and leads to the cytotoxicity. These protein aggregate related diseases have been untreated so far. However, extensive attempts have been applied to develop anti-aggregating agents as possible approaches to overcome protein aggregation. Different types of substances have been reported to halt or decrease the formation of ordered protein aggregates both in vitro and in vivo, such as polyphenols and metal ions.

OBJECTIVE

In the present study the in vitro aggregation of human serum albumin (HSA) by using a reactive dicarbonyl glyoxal has been investigated, simultaneously an attempt has been done to inhibit the glyoxal (GO) induced aggregation of (HSA) by caffeic acid (CA).

METHODS

Different methods have been employed to investigate the process, fluorescence spectroscopy, circular dichroism, cango red binding assay, thioflavin T dye binding, turbidimetric analysis, docking study and transmission electron microscopy.

RESULTS

Results have shown that elevated concentration of GO forms aggregates of HSA, and the activity of CA suggested the possibility of inhibiting the HSA aggregation at higher concentrations, and this compound was found to have an anti-aggregation property.

CONCLUSION

The present study explained that micro molar concentrations of CA inhibits the aggregation of HSA and showed pronounced anti-aggregation effect at increasing concentrations in the presence of GO which is elevated in diabetic and hyperglycaemia conditions.

Site-specific chemoproteomic profiling of targets of glyoxal

Aim: Advanced glycation end products (AGE) are the biomarkers of aging and diabetes which are formed via reactions between glycating agents and biomacromolecules. However, no proteomic study has been reported to systematically investigate the protein substrates of AGEs. Results: In this paper, we used an aniline-based probe to capture the glyoxal-imine intermediate which is the transition sate of glyoxal-derived AGEs. Combined with the tandem orthogonal proteolysis activity-based protein profiling strategy, we successfully identified 962 lysines modified by glyoxal. Conclusion: Enzymes in glycolysis are heavily modified by glyoxal and our biochemical experiments showed that glyoxal can significantly inhibit the activity of GAPDH and glycolysis. These data indicated that AGEs modifications may contribute to pathological processes through impairing the glycolytic process.

Stability of glyoxal- and methylglyoxal-modified hemoglobin on dried blood spot cards as analyzed by nanoflow liquid chromatography tandem mass spectrometry

Blood sampling by the dried blood spot (DBS) technique has become commonly applied in newborn screening. It is often used for analysis of small molecules, such as metabolites. Recently, DBS sampling has been applied for quantification of post-translational protein modifications. Glyoxal and methylglyoxal are two simple oxoaldehydes released from glycated proteins in the Maillard reaction. They are widely distributed in the environment (e.g. cigarette smoke) and found in foods and beverages. Glyoxal and methylglyoxal are shown to react with biomolecules including DNA and proteins. In this laboratory, we previously identified the sites of modification by these two oxoaldehydes in human hemoglobin and found that the extents of modification at certain sites of lysine and arginine residues are significantly higher in type 2 diabetes mellitus patients than in nondiabetic individuals. In this study, we examine the stability of these modifications of hemoglobin stored on DBS cards at room temperature or 4 °C in the ambient air. After hemoglobin was extracted from the DBS cards, it was digested by trypsin and analyzed by nanoflow liquid chromatography coupled with nanospray ionization tandem mass spectrometry. The results show that the extents of all these PTMs are stable within 14 and 21 days when stored on DBS at room temperature and at 4 °C, respectively. Extraction of globin from DBS cards is mostly advantageous for hemolytic blood samples. This assay is sensitive as only a quarter of a DBS card containing ca. 12 μL of blood is required. Thus, it is practically useful to measure the extents of glyoxal- and methylglyoxal-induced hemoglobin modifications from DBS cards.

The role of mass spectrometry in studies of glycation processes and diabetes management

In the last decade, mass spectrometry has been widely employed in the study of diabetes. This was mainly due to the development of new, highly sensitive, and specific methods representing powerful tools to go deep into the biochemical and pathogenetic processes typical of the disease. The aim of this review is to give a panorama of the scientifically valid results obtained in this contest. The recent studies on glycation processes, in particular those devoted to the mechanism of production and to the reactivity of advanced glycation end products (AGEs, AGE peptides, glyoxal, methylglyoxal, dicarbonyl compounds) allowed to obtain a different view on short and long term complications of diabetes. These results have been employed in the research of effective markers and mass spectrometry represented a precious tool allowing the monitoring of diabetic nephropathy, cardiovascular complications, and gestational diabetes. The same approaches have been employed to monitor the non-insulinic diabetes pharmacological treatments, as well as in the discovery and characterization of antidiabetic agents from natural products. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 38:112-146, 2019.

Profiling of Methylglyoxal Blood Metabolism and Advanced Glycation End-Product Proteome Using a Chemical Probe

Methylglyoxal (MG) is quantitatively the most important precursor to advanced glycation end-products (AGEs), and evidence is accumulating that it is also a causally linked to diabetes and aging related diseases. Living systems primarily reside on the glyoxalase system to detoxify MG into benign d-lactate. The flux to either glycation or detoxification, accordingly, is a key parameter for how well a system handles the ubiquitous glyoxal burden. Furthermore, insight into proteins and in particular their individual modification sites are central to understanding the involvement of MG and AGE in diabetes and aging related diseases. Here, we present a simple method to simultaneously monitor the flux of MG both to d-lactate and to protein AGE formation in a biological sample by employing an alkyne-labeled methylglyoxal probe. We apply the method to blood and plasma to demonstrate the impact of blood cell glyoxalase activity on plasma protein AGE formation. We move on to isolate proteins modified by the MG probe and accordingly can present the first general inventory of more than 100 proteins and 300 binding sites of the methylglyoxal probe on plasma as well as erythrocytic proteins. Some of the data could be validated against a number of in vivo and in vitro targets for advanced glycation previously known from the literature; the majority of proteins and specific sites however were previously unknown and may guide future research into MG and AGE to elucidate how these are functionally linked to diabetic disease and aging.

Correlation between Glyoxal-Induced DNA Cross-Links and Hemoglobin Modifications in Human Blood Measured by Mass Spectrometry

Glyoxal is an oxoaldehyde generated from the degradation of glucose-protein conjugates and from lipid peroxidation in foods and in vivo, and it is also present in the environment (e.g., cigarette smoke). The major endogenous source of glyoxal is glucose autoxidation, and the glyoxal concentrations in plasma are higher in diabetic patients than in nondiabetics. Glyoxal reacts with biomolecules forming covalently modified DNA and protein adducts. We previously developed sensitive and specific assays based on nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS) for quantification of DNA cross-linked adducts (dG-gx-dC and dG-gx-dA) and for hemoglobin adducts derived from glyoxal. In this study, we isolated and analyzed both leukocyte DNA and hemoglobin from the blood of diabetic patients and compared the adduct levels with those from nondiabetic subjects using the modified assays. The results indicated that the extents of glyoxal-induced hemoglobin modifications on α-Lys-11, α-Arg-92, β-Lys-17, and β-Lys-66 were statistically higher in diabetic patients than nondiabetics and they correlated with HbA1c significantly. Moreover, the levels of dG-gx-dC in leukocyte DNA correlated positively with the extents of globin modification at α-Lys-11 and β-Lys-17, while levels of dG-gx-dA correlated with those at α-Lys-11 and α-Arg-92 in nonsmoking subjects. Comparing the levels and the correlation coefficients of these hemoglobin and DNA adducts including or excluding smokers, it appears that smoking is not a major contributor to glyoxal-induced adduction of hemoglobin and leukocyte DNA. To the best of our knowledge, this is one of the few reports of positive correlation between DNA and protein adducts of the same compound (glyoxal) in the blood from the same subjects. Because of the high abundance of hemoglobin in blood, the results indicate that quantification of glyoxal-modified peptides in hemoglobin might serve as a dosimetry for glyoxal and a practical surrogate biomarker for assessing glyoxal-induced DNA damage and its prevention.

Diabetes with heart failure increases methylglyoxal modifications in the sarcomere, which inhibit function

Patients with diabetes are at significantly higher risk of developing heart failure. Increases in advanced glycation end products are a proposed pathophysiological link, but their impact and mechanism remain incompletely understood. Methylglyoxal (MG) is a glycolysis byproduct, elevated in diabetes, and modifies arginine and lysine residues. We show that left ventricular myofilament from patients with diabetes and heart failure (dbHF) exhibited increased MG modifications compared with nonfailing controls (NF) or heart failure patients without diabetes. In skinned NF human and mouse cardiomyocytes, acute MG treatment depressed both calcium sensitivity and maximal calcium-activated force in a dose-dependent manner. Importantly, dbHF myocytes were resistant to myofilament functional changes from MG treatment, indicating that myofilaments from dbHF patients already had depressed function arising from MG modifications. In human dbHF and MG-treated mice, mass spectrometry identified increased MG modifications on actin and myosin. Cosedimentation and in vitro motility assays indicate that MG modifications on actin and myosin independently depress calcium sensitivity, and mechanistically, the functional consequence requires actin/myosin interaction with thin-filament regulatory proteins. MG modification of the myofilament may represent a critical mechanism by which diabetes induces heart failure, as well as a therapeutic target to avoid the development of or ameliorate heart failure in these patients.

A Stable Isotope Dilution Nanoflow Liquid Chromatography Tandem Mass Spectrometry Assay for the Simultaneous Detection and Quantification of Glyoxal-Induced DNA Cross-Linked Adducts in Leukocytes from Diabetic Patients

Glyoxal (gx) is a bifunctional electrophile capable of cross-linking DNA. Although it is present in foods and from the environment, endogenous formation of glyoxal occurs through metabolism of carbohydrates and oxidation of lipids and nucleic acids. Plasma concentrations of glyoxal are elevated in in diabetes mellitus patients compared to nondiabetics. The most abundant 2'-deoxyribonucleoside adducts cross-linked by glyoxal are dG-gx-dC, dG-gx-dA, and dG-gx-dG. These DNA cross-links can be mutagenic by damaging the integrity of the DNA structure. Herein, we developed a highly sensitive and specific assay for the simultaneous detection and quantification of the dG-gx-dC and dG-gx-dA cross-links based on stable isotope dilution (SID) nanoflow liquid chromatography nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS) under the highly selected reaction monitoring mode and using a triple quadrupole mass spectrometer. The entire assay procedure involved addition of the stable isotope standards [¹⁵N₅]dG-gx-dC and [¹⁵N₅]dG-gx-dA as internal standards, enzyme hydrolysis to release the cross-links as nucleosides, enrichment by a reversed-phase solid-phase extraction column, and nanoLC-NSI/MS/MS analysis. The detection limit is 0.19 amol for dG-gx-dC and 0.89 amol for dG-gx-dA, which is 400 and 80 times more sensitive, respectively, than capillary LC-NSI/MS/MS assay of these adducts. The lower limit of quantification was 94 and 90 amol for dG-gx-dC and dG-gx-dA, respectively, which is equivalent to 0.056 and 0.065 adducts in 10⁸ normal nucleotides in 50 μg of DNA. In type 2 diabetes mellitus (T2DM) patients (n = 38), the levels of dG-gx-dC and dG-gx-dA in leukocyte DNA were 1.94 ± 1.20 and 2.10 ± 1.77 in 10⁸ normal nucleotides, respectively, which were significantly higher than those in nondiabetics (n = 39: 0.83 ± 0.92 and 1.05 ± 0.99 in 10⁸ normal nucleotides, respectively). Excluding the factor of smoking, an exogenous source of glyoxal, levels of these two cross-linked adducts were found to be significantly higher in nonsmoking T2DM patients than in nonsmoking control subjects. Furthermore, the levels of dG-gx-dC and dG-gx-dA correlated with HbA1c with statistical significance. To our best knowledge, this is the first report of the identification and quantification of glyoxal-derived cross-linked DNA adducts in human leukocyte DNA and their association with T2DM. This SID nanoLC-NSI/MS/MS assay is highly sensitive and specific and it requires only 50 μg of leukocyte DNA isolated from 2-3 mL of blood to accurately quantify these two cross-linked adducts simultaneously. Our assay thus provides a useful biomarker for the evaluation of glyoxal-derived DNA damage.

Investigating the preventive effects of baicalin and gallocatechin against glyoxal-induced cystatin aggregation

Several mammalian proteins form pathological deposits under nonphysiological conditions that are associated with many degenerative diseases. Protein aggregation is associated with aging, as well as a variety of diseases, including cystic fibrosis, amyotrophic lateral sclerosis (ALS), and hypertrophic cardiomyopathy. There is a lack of any potential anti-amyloidogenic agents and therapeutics till date. Polyphenols have been accredited with myriad biological effects. An analysis of the effects of natural agents like baicalin (BC) and gallocatechin (GC) on aggregation process can open new avenues for the treatment of protein misfolding diseases. Thus, investigation of the effects of these flavonoids on Buffalo Heart Cystatin (BHC) aggregation induced by a reactive metabolic dialdehyde, glyoxal (GO), was taken up. Results have shown that elevated concentration of GO forms aggregates of BHC, which was characterized by an increase in the ANS fluorescence intensity, an increase in ThT fluorescence intensity, red shift in Congo red absorbance, negative ellipticity peak at 217 nm in the far-UVCD and BHC aggregates displaying by TEM. Using fluorescence spectroscopic analysis with Thioflavin T, CD and electron microscopic studies, anti-aggregation effects of polyphenols, BC and GC were analyzed. The study showed that BC and GC produced concentration-dependent anti-aggregation effects with GC producing a more pronounced effect than BC. The study proposed a mechanistic approach assuming structural constraints and specific aromatic interactions of polyphenols with sheets of BHC aggregates.

Effect of Cu(ii) on in vitro glycation of human serum albumin by methylglyoxal: a LC-MS-based proteomic approach

It has been reported that glycation of human serum albumin (HSA) changes its capability for copper binding whereas the increase of free copper might have an impact on protein glycation - a key process in diabetes progression. In this work, proteomic analysis of non-glycated HSA and HSA glycated with methylglyoxal (MGo) in the absence or in the presence of Cu(ii) (0.1; 1.0; 5.0 mg Cu L^-1) has been undertaken. Trypsin hydrolysates were subjected to capillary HPLC-ESI-QTOF-MS and MS/MS. Raw data were analyzed using two proteomic platforms: MaxQuant () and ProteinScape (Bruker). Considering seven MGo-derived modifications, the sequence coverage was 98% for non-modified HSA and ≥93% for HSA incubated with MGo or MGo + Cu(ii). Peptide mapping yielded 76 identical peptides in all samples though important differences were found between non-modified HSA and protein glycated with or without Cu(ii). Overall, 46 peptides with residues from 1 to 3 modified were detected/sequenced; the MGo-derived modifications found were: hydroimidazolone, argpyrimidine, N^ε-carboxyethyl-lysine and S-carboxyethyl-cysteine; 39 modified sites were identified (22 on arginine, 12 on lysine, and 5 on cysteine) and among them, 27 were common for ProteinScape and MaxQuant. The count of the modified peptides and the comparative analysis of their abundance in different samples indicated that Cu(ii) at physiological and sub-physiological concentrations inhibited HSA glycation as compared to the glycation of the Cu-devoid protein; at higher concentrations (5 mg Cu L^-1), this inhibitory effect tends to be inverted. The results obtained suggest that increased protein glycation might be associated with Cu-deficiency and with excessive Cu(ii) concentrations, calling for more detailed studies performed on real-world samples with a strict control of copper concentration.

Na+/K+-ATPase interaction with methylglyoxal as reactive metabolic side product

Proteins are subject to oxidative modification and the formation of adducts with a broad spectrum of reactive species via enzymatic and non-enzymatic mechanisms. Here we report that in vitro non-enzymatic methylglyoxal (MGO) binding causes the inhibition and formation of MGO advanced glycation end-products (MAGEs) in Na⁺/K⁺-ATPase (NKA). Concretely, MGO adducts with NKA amino acid residues (mainly Arg) and N^ε-(carboxymethyl)lysine (CML) formation were found. MGO is not only an inhibitor for solubilized NKA (IC₅₀=91±16μM), but also for reconstituted NKA in the lipid bilayer environment, which was clearly demonstrated using a DPPC/DPPE liposome model in the presence or absence of the NKA-selective inhibitor ouabain. High-resolution mass spectrometric analysis of a tryptic digest of NKA isolated from pig (Sus scrofa) kidney indicates that the intracellular α-subunit is naturally (post-translationally) modified by MGO in vivo. In contrast to this, the β-subunit could only be modified by MGO artificially, and the transmembrane part of the protein did not undergo MGO binding under the experimental setup used. As with bovine serum albumin, serving as the water-soluble model, we also demonstrated a high binding capacity of MGO to water-poorly soluble NKA using a multi-spectral methodology based on electroanalytical, immunochemical and fluorimetric tools. In addition, a partial suppression of the MGO-mediated inhibitory effect could be observed in the presence of aminoguanidine (pimagedine), a glycation suppressor and MGO-scavenger. All the results here were obtained with the X-ray structure of NKA in the E1 conformation (3WGV) and could be used in the further interpretation of the functionality of this key enzyme in the presence of highly-reactive metabolic side-products, glycation agents and generally under oxidative stress conditions.

Analysis of Chlorination, Nitration, and Nitrosylation of Tyrosine and Oxidation of Methionine and Cysteine in Hemoglobin from Type 2 Diabetes Mellitus Patients by Nanoflow Liquid Chromatography Tandem Mass Spectrometry

The post-translational modification (PTM) of proteins by endogenous reactive chlorine, nitrogen, and oxygen species is implicated in certain pathological conditions, including diabetes mellitus. Evidence showed that the extents of modifications on a number of proteins are elevated in diabetic patients. Measuring modification on hemoglobin has been used to monitor the extent of exposure. This study develops an assay for simultaneous quantification of the extent of chlorination, nitration, and oxidation in human hemoglobin and to examine whether the level of any of these modifications is higher in poorly controlled type 2 diabetic mellitus patients. This mass spectrometry-based assay used the bottom-up proteomic strategy. Due to the low amount of endogenous modification, we first characterized the sites of chlorination at tyrosine in hypochlorous acid-treated hemoglobin by an accurate mass spectrometer. The extents of chlorination, nitration, and oxidation of a total of 12 sites and types of modifications in hemoglobin were measured by nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry under the selected reaction monitoring mode. Relative quantification of these PTMs in hemoglobin extracted from blood samples shows that the extents of chlorination at α-Tyr-24, nitration at α-Tyr-42, and oxidation at the three methionine residues are significantly higher in diabetic patients (n = 19) than in nondiabetic individuals (n = 18). After excluding the factor of smoking, chlorination at α-Tyr-24, nitration at α-Tyr-42, and oxidation at the three methionine residues are significantly higher in the nonsmoking diabetic patients (n = 12) than in normal nonsmoking subjects (n = 11). Multiple regression analysis performed on the combined effect of age, body-mass index (BMI), and HbA1c showed that the diabetes factor HbA1c contributes significantly to the extent of chlorination at α-Tyr-24 in nonsmokers. In addition, age contributes to oxidation at α-Met-32 significantly in all subjects and in nonsmokers. These results suggest the potential of using chlorination at α-Tyr-24-containing peptide to evaluate protein damage in nonsmoking type 2 diabetes mellitus.

An LC/ESI-SRM/MS method to screen chemically modified hemoglobin: simultaneous analysis for oxidized, nitrated, lipidated, and glycated sites

Proteins are continuously exposed to various reactive chemical species (reactive oxygen/nitrogen species, endogenous/exogenous aldehydes/epoxides, etc.) due to physiological and chemical stresses, resulting in various chemical modifications such as oxidation, nitration, glycation/glycoxidation, lipidation/lipoxidation, and adduct formation with drugs/chemicals. Abundant proteins with a long half-life, such as hemoglobin (Hb, t 1/2 63 days, ∼150 mg/mL), are believed to be major targets of reactive chemical species that reflect biological events. Chemical modifications on Hb have been investigated mainly by mechanistic in vitro experiments or in vivo/clinical experiments focused on single target modifications. Here, we describe an optimized LC/ESI-SRM/MS method to screen oxidized, nitrated, lipidated, and glycated sites on Hb. In vivo preliminary results suggest that this method can detect simultaneously the presence of oxidation (+16 Da) of α-Met(32), α-Met(76), β-Met(55), and β-Trp(15) and adducts of malondialdehyde (+54 Da) and glycation (+162 Da) of β-Val(1) in a blood sample from a healthy volunteer. Graphical Abstract Screening chemical modifications on hemoglobin.