Katsumi Mera

Effect of reactive‐aldehydes on the modification and dysfunction of human serum albumin

Advanced glycation end products (AGEs) are generated not only from glucose but also from several aldehydes such as methylglyoxal, glyoxal, and glycolaldehyde. The aim of the present study was to investigate the effect of several aldehydes on human serum albumin (HSA) in terms of the physicochemical properties and formation of AGE structures. HSA modified with methylglyoxal, generated by the glycolysis pathway and degradation of the Schiff base, showed the highest increase in the molecular weight and net negative charge, whereas glucose modification caused a small increase in the molecular weight even incubation for after 4 weeks. N(epsilon)-(carboxymethyl)lysine (CML), N(epsilon)-(carboxyethyl)lysine (CEL), and imidazolone increased in modified HSA in correlation with their lysine and arginine modification, whereas high amounts of GA-pyridine was detected in HSA modified with glycolaldehyde. Furthermore, the binding ability of HSA to warfarin and ketoprofen was more effectively decreased by methylglyoxal modification than the other aldehydes. These results indicated that changes in the physicochemical properties and the formation of AGE structures are highly dependent on the aldehydes.

Quantitative evaluation of the role of cysteine and methionine residues in the antioxidant activity of human serum albumin using recombinant mutants

The importance of cysteine (Cys) and methionine (Met) residues for the antioxidant activity of human serum albumin (HSA) was investigated using recombinant HSA mutants, in which Cys34 and/or the six Met residues had been mutated to Ala. The scavenging activities of the mutants against five reactive oxygen and nitrogen species were evaluated by a chemiluminescence assay, electron paramagnetic resonance spectroscopy, or a HPLC‐flow reactor assay. Our results showed that the contributions of Cys34 and the Met residues to the antioxidant activity of HSA were 61% and 29% against O2 •−, 68% and 61% against H2O2, 38% and 6% against HO•, 36% and 13% against HOCl, and 51% and 1% against •NO, respectively. Thus, the findings propose in a direct way that Cys34 plays a more important role than the Met residues in the antioxidant activity of HSA.

Effect of intravenous iron administration frequency on AOPP and inflammatory biomarkers in chronic hemodialysis patients: A pilot study

OBJECTIVES Intravenous iron administration (IVIR) is effective for correcting anemia in hemodialysis (HD) patients, but it also enhances the generation of hydroxyl radicals. Previously we demonstrated that IVIR increases oxidized serum albumin levels in HD patients. However, the effect of IVIR frequencies on the oxidative stress has never been studied before. Therefore, we compared the two IVIR schedules recommended by the Japanese Society for Dialysis Therapy guideline 2004 by measuring oxidized albumin in chronic HD patients. DESIGN AND METHODS Twenty-two HD patients were divided into two IVIR protocol groups (group I: 40 mg of iron 3 times a week for 4 weeks, group II: 40 mg of iron once a week for 3 months). These protocols differ in IVIR frequency, but receive the same amount of iron (total 520 mg). We compared these two regimens by determining the levels of hemoglobin, serum ferritin, advanced oxidation protein products (AOPP), and oxidized albumin at 0, 4, 8, 12, 16, and 20 weeks. RESULTS Both patient groups resulted in a similar and significant increase in hemoglobin levels, whereas group I markedly induced AOPP and oxidation of serum albumin than group II at 4 weeks (P<0.05). AOPP and oxidation of serum albumin was also gradually declined by 20 weeks, while the oxidized albumin and AOPP in group II was not significantly changed during the entire experimental period. Transferrin saturation and serum ferritin levels were also increased in group I compared with group II at 4 weeks (P<0.001). In addition, we found a strong positive correlation between oxidized albumin and serum ferritin levels (r=0.615, P<0.05), suggesting the possibility that the accumulation of iron stores has a causative role in the progression of oxidative stress in HD patients treated with IVIR. CONCLUSIONS The results of this study indicate that lower frequency IVIR protocol is recommended to reduce IVIR-induced oxidative stress in HD patients.

Immunochemical detection of Nε-(carboxyethyl)lysine using a specific antibody

Methylglyoxal (MG) is generated through the Embden-Meyerhof and polyol pathways, and it rapidly reacts with proteins to form advanced glycation end products (AGE) such as N(epsilon)-(carboxyethyl)lysine (CEL). In the present study, polyclonal and monoclonal antibodies specific for CEL were prepared to estimate CEL content in aldehydes-modified proteins and the pathological localization in human kidneys. Polyclonal CEL-specific antibody was prepared by removing cross-reactive antibodies against N(epsilon)-(carboxymethyl) lysine (CML), one of the major AGE structures, using CML-conjugated affinity chromatography. Monoclonal CEL-specific antibody (CEL-SP) was obtained by immunization with CEL-bovine serum albumin, followed by successive screening according to CEL-RNase-positive but CML-RNase-negative criteria. A non-competitive ELISA showed that both the polyclonal and monoclonal CEL-specific antibodies significantly reacted with CEL-proteins but not with CML-proteins. A competitive ELISA also demonstrated that CEL-SP does not show cross-reactivity against CEL analogues such as CML, carboxymethylarginine (CMA) and S-carboxymethylcysteine (CMC), thus indicating that antibody is able to recognize the difference of one methyl group between carboxymethyl group and carboxyethyl group. Furthermore, CEL-SP significantly reacted with human serum albumin modified with MG but not with glyoxal or 3-deoxyglucosone, and its reactivity was highly correlated with the CEL content, which was determined by high performance liquid chromatography. Immunohistochemical studies using CEL-SP provided evidence that CEL-modified proteins accumulate in distal tubular epithelial cells of the diabetic rat. These results demonstrate that a specific antibody against CEL can be a powerful tool for detecting CEL both in vitro and in vivo.

Hypochlorous acid generates Nε-(carboxymethyl)lysine from Amadori products

Since the accumulation of Nε-(carboxymethyl)lysine (CML), a major antigenic advanced glycation end product, is implicated in tissue disorders in hyperglycemia and inflammation, the identification of the pathway of CML formation will provide important information regarding the development of potential therapeutic strategies for these complications. The present study was designed to measure the effect of hypochlorous acid (HOCl) on CML formation from Amadori products. The incubation of glycated human serum albumin (glycated-HSA), a model of Amadori products, with HOCl led to CML formation, and an increasing HOCl concentration and decreasing pH, which mimics the formation of these products in inflammatory lesions. CML formation was also observed when glycated-HSA was incubated with activated neutrophils, and was completely inhibited in the presence of an HOCl scavenger. These data demonstrated that HOCl-mediated CML formation from Amadori products plays a role in CML formation and tissue damage at sites of inflammation.

Glutaraldehyde is an effective cross-linker for production of antibodies against advanced glycation end-products

Immunohistochemical approaches have been widely used in the localization and quantification of advanced glycation end-products (AGEs). Traditional approaches for production of anti-AGE antibodies use cross-linkers such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) to conjugate the AGE antigen to the carrier protein. However, these approaches often fail to produce antibodies that are specific to the particular AGE of interest. In the present study, Nepsilon-(carboxymethyl)lysine (CML), a major antigenic AGE structure, was conjugated to human serum albumin (HSA) using various cross-linkers, including EDC, bis(sulfosuccinimidyl)suberate (BS3) and glutaraldehyde, to compare their efficiency for the production of epitope-specific antibodies. All of the cross-linkers tested were capable of conjugating CML to HSA, and each CML-conjugated HSA was recognized by previously characterized anti-CML antibody. However, only the use of glutaraldehyde as the cross-linker resulted in the production of a CML-specific monoclonal antibody, termed 2G11. 2G11 significantly recognized CML-modified HSA and peptide, whereas it did not recognize Nepsilon-(carboxyethyl)lysine (CEL)-modified HSA and peptide, indicating that 2G11 is highly specific to CML, and can distinguish the difference of a single methyl group between the two epitopes. To further demonstrate the use of glutaraldehyde, anti-AGE antibodies against CEL, S-(2-succinyl)cysteine and S-(carboxymethyl)cysteine were obtained by conjugation with glutaraldehyde. These studies demonstrate the efficacy of glutaraldehyde as a cross-linker for the production of antibodies against small molecules.

Natural compounds containing a catechol group enhance the formation of Nε-(carboxymethyl)lysine of the Maillard reaction

Inhibition of advanced glycation end-product (AGE) formation is a potential strategy for the prevention of clinical diabetes complications. Screening for new AGE inhibitors revealed several natural compounds that inhibited the formation of N(ε)-(carboxymethyl)lysine (CML), a major antigenic AGE structure, whereas natural compounds containing a catechol group, such as gallic acid and epicatechin, significantly enhanced CML formation. A similar enhancing effect was also observed by culturing THP-1 macrophages in the presence of catechol compounds. Although 4-methylcatechol significantly enhanced CML formation from glycated HSA (gHSA), a model for Amadori proteins, analogues of catechol such as 5-methylresorcinol and methylhydroquinone showed no enhancing effect. Even though 1mM 4-methylcatechol, epicatechin, and gallic acid significantly enhanced CML formation from gHSA, it was significantly inhibited by decreasing their concentration. The enhancing effect of 1mM catechol compounds was inhibited in the presence of the glutathione peroxidase system, thus demonstrating that hydrogen peroxide generated from catechol compounds plays an important role in the enhancement of CML formation. Furthermore, administration of 500mg/kg/day epicatechin to STZ-induced diabetic mice for 45days enhanced CML accumulation at the surface area of gastric epithelial cells in the stomach. This study provides the first evidence that high amounts of catechol-containing structures enhance oxidative stress, thus leading to enhanced CML formation, and this phenomenon may explain the paradoxical effect that some flavonoids have on redox status.

A uremic toxin, 3-carboxy-4-methyl-5-propyl-2-furanpropionate induces cell damage to proximal tubular cells via the generation of a radical intermediate.

3-Carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF), a furan fatty acid uremic toxin (UT) and a substrate for organic ion transporters, contributes to the accumulation of CMPF in renal tubular cells. Although oxidative stress induced by UTs has been proposed as a mechanism of its toxicity in chronic kidney disease, little information is available regarding the redox property of CMPF and its relation to renal cell damage. The findings herein show that CMPF enhances the production of reactive oxygen species (ROS) in HK-2 cells in the presence of angiotensin II (A-II), an inducer of O(2)(·-). When iron is also present, CMPF and A-II induce the Fenton reaction, resulting in a further increase in ROS production. Such CMPF-induced oxidative stress increases TGF-β1 secretion in HK-2 cells, and a positive correlation between CMPF-induced ROS production and the secretion of active TGF-β1 was observed. CMPF caused a reduction in cell viability which was negatively correlated with intracellular ROS production. These negative effects of CMPF in HK-2 cells were completely suppressed by probenecid, an inhibitor of organic anion transport. Interestingly, in vitro ROS assays indicate that CMPF directly interacts with superoxide anion radicals (O(2)(·-)) and peroxy radicals (LOO) to produce CMPF radicals. The subsequent interaction of CMPF radicals with dissolved oxygen leads to the overproduction of O(2)(·-). Based on these findings, we conclude that CMPF, which accumulates in the renal cells, appears to play a prominent role as a pro-oxidant which subsequently leads to renal cellular damage via the overproduction of O(2)(·-).

Usefulness of antibodies for evaluating the biological significance of AGEs.

Polyclonal and monoclonal antibodies have been widely applied to demonstrate the presence of advanced glycation end products (AGEs) in vivo. However, our previous study showed that monoclonal anti‐AGE antibody (6D12) and polyclonal anti‐Nɛ‐(carboxymethyl)lysine (CML) antibody recognize not only CML but also Nɛ‐(carboxyethyl)lysine (CEL), thus indicating that we should pay attention to the specificity of the antibodies. As a result, we prepared specific monoclonal antibodies against CML, CEL, Nω‐(carboxymethyl)arginine (CMA), and S‐(carboxymethyl)cysteine (CMC). Our immunochemical study using anti‐CMA antibody demonstrated that the CMA content increased in a time‐dependent manner when collagen was incubated with glucose, indicating that immunological quantification using the specific antibody is especially useful for measuring an acid‐labile AGE structure, such as CMA. Monoclonal antibody is also applied to identify a novel biological marker in pathological lesions. We prepared antibody libraries against proteins modified with aldehydes, such as glyoxal, methylglyoxal, and glycolaldehyde (GA), and one antibody, GA5, which specifically reacts with the GA‐modified protein that is recognized in human atherosclerotic lesions. Following successive high‐performance liquid chromatography purification, the GA5‐reactive compound was isolated and its chemical structure was found to be 3‐hydroxy‐4‐hydroxymethyl‐1‐(5‐amino‐5‐carboxypentyl) pyridinium cation, which was named GA‐pyridine. Taken together, these results demonstrate that a specific antibody is a powerful tool for analyzing novel biomarkers, formation pathways, and the efficacy of AGE inhibitors.

Immunological detection of Nω-(carboxymethyl)arginine by a specific antibody

N ω‐(carboxymethyl)arginine (CMA) is an acid‐labile advanced glycation end product (AGE) that was discovered in enzymatic hydrolysate of glycated collagen. Subsequently, CMA was also detected in human serum, and its level in patients with diabetes was found to be higher than in people without the disease. However, the histological localization of CMA and its pathophysiological significance remains poorly understood. Here, to address this issue, we developed a monoclonal antibody specific for CMA. This antibody reacted with CMA and CMA‐protein adduct, whereas it did not cross‐react with its analogues, such as Nɛ‐(carboxymethyl)lysine and S‐(carboxymethyl)cysteine, indicating that the antibody specifically recognizes CMA. Upon immunohistochemical analysis, a significant CMA immnoreactivity was found in atherosclerotic lesions, whereas no such immunoreactivity was observed in normal regions. This suggests that the accumulation of CMA in tissue proteins may contribute to the pathophysiologies associated with aging and age‐related diseases.