The relative ability of glucose and ascorbate to glycate and crosslink lens proteins in vitro. off

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.

Protein posttranslational modification (PTM) by glycation: Role in lens aging and age-related cataractogenesis

Crystallins, the most prevalent lens proteins, have no turnover throughout the entire human lifespan. These long-lived proteins are susceptible to post-synthetic modifications, including oxidation and glycation, which are believed to be some of the primary mechanisms for age-related cataractogenesis. Thanks to high glutathione (GSH) and ascorbic acid (ASA) levels as well as low oxygen content, the human lens is able to maintain its transparency for several decades. Aging accumulates substantial changes in the human lens, including a decreased glutathione concentration, increased reactive oxygen species (ROS) formation, impaired antioxidative defense capacity, and increased redox-active metal ions, which induce glucose and ascorbic acid degradation and protein glycation. The glycated lens crystallins are either prone to UVA mediated free radical production or they attract metal ion binding, which can trigger additional protein oxidation and modification. This vicious cycle is expected to be exacerbated with older age or diabetic conditions. ASA serves as an antioxidant in the human lens under reducing conditions to protect the human lens from damage, but ASA converts to the pro-oxidative role and causes lens protein damage by ascorbylation in high oxidation or enriched redox-active metal ion conditions. This review is dedicated in honor of Dr. Frank Giblin, a great friend and superb scientist, whose pioneering and relentless work over the past 45 years has provided critical insight into lens redox regulation and glutathione homeostasis during aging and cataractogenesis.

The homeostatic role of hydrogen peroxide, superoxide anion and nitric oxide in the vasculature

Reactive oxygen and nitrogen species are produced in a wide range of physiological reactions that, at low concentrations, play essential roles in living organisms. There is a delicate equilibrium between formation and degradation of these mediators in a healthy vascular system, which contributes to maintaining these species under non-pathological levels to preserve normal vascular functions. Antioxidants scavenge reactive oxygen and nitrogen species to prevent or reduce damage caused by excessive oxidation. However, an excessive reductive environment induced by exogenous antioxidants may disrupt redox balance and lead to vascular pathology. This review summarizes the main aspects of free radical biochemistry (formation, sources and elimination) and the crucial actions of some of the most biologically relevant and well-characterized reactive oxygen and nitrogen species (hydrogen peroxide, superoxide anion and nitric oxide) in the physiological regulation of vascular function, structure and angiogenesis. Furthermore, current preclinical and clinical evidence is discussed on how excessive removal of these crucial responses by exogenous antioxidants (vitamins and related compounds, polyphenols) may perturb vascular homeostasis. The aim of this review is to provide information of the crucial physiological roles of oxidation in the endothelium, vascular smooth muscle cells and perivascular adipose tissue for developing safer and more effective vascular interventions with antioxidants.

Vitamin C and the Lens: New Insights into Delaying the Onset of Cataract

Cataracts or clouding of the lens is the leading cause of blindness in the world. Age and diabetes are major risk factors, and with an increasing aging and diabetic population, the burden of cataracts will grow. Cataract surgery is an effective way to restore vision; however, alternatives to cataract surgery are required to reduce the looming cataract epidemic. Since it is well established that oxidative damage plays a major role in the etiology of cataracts, antioxidants have been promoted as therapies to delay and/or prevent cataracts. However, many antioxidant interventions including vitamin C have produced mixed results as anti-cataract therapies. Progress has been made towards our understanding of lens physiology and the mechanisms involved in the delivery and uptake of antioxidants to the lens which may guide future studies aimed at addressing some of the inconsistencies seen in previous animal and human studies. Of interest is the potential for vitamin C based supplements in delaying the onset of cataracts post vitrectomy which occurs in up to 80% of patients within two years. These targeted approaches are required to reduce the burden of cataract on hospitals and improve the quality of life of our aging and diabetic population.

Associations Between Dietary Antioxidant Intake and Markers of Atherosclerosis in Middle-Aged Women From North-Western Algeria

Background: The role of several dietary antioxidants in preventing the development and the progression of atherosclerosis has recently aroused considerable interest. Although they are not yet conclusive, most of the existing suggestions support this hypothesis.

Objective: The aim of the present work was to investigate the intake of dietary antioxidant nutrients in relation to atherogenic indices in a group of Algerian middle aged women with and without type 2 diabetes.

Methods: A cross-sectional study was conducted on a group of middle-aged women from the north western region of Algeria. Anthropometric and biochemical parameters were measured. Dietary intake was assessed using a validated 3-days food record. Atherogenic indices -total cholesterol-to-high-density lipoprotein cholesterol ratio (TC/HDL) and apolipoprotein (apo) B-to-apo A1 ratio, were calculated. Associations between antioxidants dietary intake and atherogenic indices were examined using logistic regressions.

Results: 95 women with type 2 diabetes were compared to 93 non-diabetic ones. Statistical differences (p < 0.05) were revealed for body weight, height, body mass index (BMI), glycosylated hemoglobin (HbA1c) and total cholesterol levels. Furthermore, significant differences were noted for vitamin C, E and copper dietary intakes. The TC/HDL ratio was significantly associated to the highest quartiles of vitamin C in all patients; 3.519[2.405–4.408], p = 0.009 and in non-diabetic women; 3.984[1.775–7.412], p = 0.020, respectively. The odd ratios of vitamin E intakes were about 2.425[2.017–5.715], p = 0.012 in all patients and 1.843[1.877–2.731], p = 0.019 in non-diabetic group, respectively. However, the Apo B/Apo A1 ratio was more correlated to the highest quartiles of zinc and copper in non-diabetic group; OR = 0.059[0.006–0.572], p = 0.015 and 0.192[0.048–0.766], p = 0.019, respectively.

Conclusion: The estimated risk of atherosclerosis measured through the TC/HDL ratio was correlated to vitamins antioxidant intake, while the probable risk assessed by the Apo B/Apo A1 ratio was more associated to the mineral profile.

The structural alteration and aggregation propensity of glycated lens crystallins in the presence of calcium: Importance of lens calcium homeostasis in development of diabetic cataracts

The imbalance of the calcium homeostasis in the lenticular tissues of diabetic patients is an important risk factor for development of cataract diseases. In the current study, the impact of elevated levels of calcium ions were investigated on structure and aggregation propensity of glycated lens crystallins using gel electrophoresis and spectroscopic assessments. The glycated proteins indicated significant resistance against calcium-induced structural insults and aggregation. While, glycated crystallins revealed an increased conformational stability; a slight instability was observed for these proteins upon interaction with calcium ions. Also, in the presence of calcium, the proteolytic pattern of native crystallins was altered and that of glycated protein counterparts remained almost unchanged. According to results of this study it is suggested that the structural alteration of lens crystallins upon glycation may significantly reduce their calcium buffering capacity in eye lenses. Therefore, under chronic hyperglycemia accumulation of this cataractogenic metal ion in the lenticular tissues may subsequently culminate in activation of different pathogenic pathways, leading to development of lens opacity and cataract diseases.

Extraction, purification and characterization of the crystallin protein of cataractous eye lens nucleus

The purpose of this study is to separate and identify the crystallin protein present in the nucleus of a human cataractous eye lens. Cataractous lenses were collected from different eye hospitals from patients of different etiologies with ages between 40 and 80 years. Lens nucleus proteins were extracted into four fractions on the basis of their solubility in different media by applying a reported method. These fractions were buffer-soluble proteins (PS), urea-soluble proteins (PU), yellow fraction proteins (PY) and insoluble proteins (PI). All three soluble fractions were subjected to HPLC and GPC analysis. Both HPLC and GPC analysis showed that each fraction contains α-, β- and γ-crystallins, a major class of protein present in the lenses of vertebrates. Various chromatographic parameters including precision, accuracy and linearity have been evaluated. Studies of water-insoluble crystallins using sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE) have demonstrated extreme homogeneity with evidence of major components with molecular masses of 18-70 kDa, similar to the crystallin of the water-soluble portion. The method was found to be suitable for the analysis of various isomers of crystallin protein present in human cataractous eye lens nuclei. The detailed results of the GPC are discussed. This study provides the first HPLC and GPC analysis of a human cataractous eye lens nucleus.

Featured Article: Inhibition of diabetic cataract by glucose tolerance factor extracted from yeast

Diabetes leads to many complications; among them is the development of cataract. Hyperglycemia brings to increased polyol concentration in the lens, to glycation of lens proteins, and to elevated level of ROS (Reactive Oxygen Species) causing oxidative stress. The glucose tolerance factor (GTF) was found by several groups to decrease hyperglycemia and oxidative stress both in diabetic animals and humans. The aim of our study was to explore the damages induced by high glucose to the eye lens and to assess the protective effects of GTF both in vivo and in vitro The in vivo study included control healthy rats, streptozotocin (STZ) diabetic untreated rats, and STZ diabetic rats orally treated with 15 doses of GTF. The diabetic untreated rats developed cataracts, whereas the development of cataract was totally or partially prevented in GTF treated animals. In vitro studies were done on bovine lenses incubated for 14 days. Half of the lenses were incubated in normal glucose conditions, and half in high glucose conditions (450 mg%). To one group of the normal or high glucose condition GTF was added. The optical quality of all the lenses was measured daily by an automated scanning laser system. The control lenses, whether with or without GTF addition, did not show any reduction in their quality. High glucose conditions induced optical damage to the lenses. Addition of GTF to high glucose conditions prevented this damage. High glucose conditions affected the activity of aldose reductase and sodium potassium ATPase in lens epithelial cell. Addition of GTF decreased the destructive changes induced by high glucose conditions. The amount of soluble cortical lens proteins was decreased and structural changes were detected in lenses incubated in high glucose medium. These changes could be prevented when GTF was added to high glucose medium. Our findings demonstrate the anticataractogenic potential of GTF.

Ascorbic acid: new role of an age-old micronutrient in the management of periodontal disease in older adults

To review the new role of an age-old micronutrient - ascorbic acid - in the management of periodontal disease. Articles pertaining to the topic were searched in PubMed and other search engines from year 1974 to April 2014 with the following key words: "ascorbic acid," "ascorbate," "vitamin C," "periodontal disease," "gingivitis," "periodontitis," "anti-oxidants" and "elderly." Balanced nutrition is an essential factor in the elderly. Modification of nutritional requirement is important to overcome the effect of an unbalanced diet in older individuals as a result of several external and internal host-associated factors. Micronutrient requirements as aging advances could change, and require due attention. Ascorbic acid and its relationship with periodontal disease are very well known. However, recent changes in the concept of understanding the pathogenicity has led to a new path of therapeutic intervention with ascorbic acid in many chronic diseases. Oxidative stress with its associated burden might alter the disease process. In the era of "periodontal medicine," the impact of remote tissue changes on systemic disease has to be taken into serious consideration. Deficiency of nutritional impact on the host, with micronutrient vitamin C detailed in this review with sources, absorption, interaction and its relationship with systemic disease, and thereby the impact on periodontal disease. Ascorbic acid plays an important role in the aging process, and in the maintenance of periodontal health in the elderly.

Antioxidant micronutrients and cardiovascular risk in patients with diabetes: a systematic review

Background

Inverse associations between micronutrient intake and cardiovascular outcomes have been previously shown, but did not focus on diabetic patients.

Objective

To systematically review the role of micronutrients in the development/presence of cardiovascular outcomes in patients with diabetes.

Methods

We searched Medline, Embase, and Scopus (January/1949-March/2012) for observational studies that evaluated micronutrients and cardiovascular outcomes in patients with diabetes, and then selected and extracted the data (two independent reviewers).

Results

From the 15 658 studies identified, five were included, comprising three case-control and two cohorts, with a follow-up of 7-15 years. A meta-analysis was not performed due to the different antioxidant micronutrients (types and measurement methods) and outcomes evaluated. The micronutrients assessed were vitamin C intake in diet and/ or supplementation, chromium and selenium in toenail samples, and α-tocopherol and zinc in serum levels. Intake of > 300 mg of vitamin C through supplementation was associated with increased risk of cardiovascular disease, coronary artery disease (CAD), and stroke (RR 1.69-2.37). High levels of α-tocopherol in serum were associated with 30% lower CAD risk in another study (HR 0.71; 95%CI 0.53-0.94). Among minerals (zinc, selenium, and chromium), an inverse association between zinc and CAD was observed; levels lower than 14.1 μmol/L were associated with an increased risk for CAD (RR 1.70; 95%CI 1.21-2.38).

Conclusion

The information available on this issue is scarce. Further prospective studies are needed to elucidate the role of these nutrients in the cardiovascular risk of patients with diabetes.

Interaction of zinc, ascorbic acid, and folic acid in glycation with albumin as protein model

Using albumin as model, we conducted series of in vitro glycation experiments to examine role of zinc in glycation using glucose at 4-100 mg/ml, incubations at 37°C or 60°C, duration of 2 or 4 weeks and in presence of zinc or ascorbic acid (AA) or folic acid (FA). Modifications of bovine serum albumin (BSA) were examined by using fluorescence of advanced glycation end products (AGEs) and dityrosine, UV, and Fourier transformed infrared spectroscopy. Adding zinc (0 to 768.5 μmol/l) resulted in significant inhibition of albumin glycation by glucose with a linear fit, y = -0·0895x + 230·99(R² = 0·7676, p = 0·013). The glycation by fructose was greater than that of glucose with stronger inhibitory effect by zinc in fructose-glycation (t= -5.8, p=0.002). Addition of zinc significantly decreased fluorescence as seen in Zn + FA or Zn + AA sets as compared to sets of FA alone (p=0.00056) or AA alone (p=0.037). The fluorescence for dityrosine and AGE had a correlation of 0.897 (p<0.01). The data from fluorescence, UV, and FTIR spectra collectively suggested inhibitory effect of zinc in BSA glycation alone or in presence of FA and AA, showing new dimension for the protective action of zinc in hyperglycemic conditions.

Preventive effects of chebulic acid isolated from Terminalia chebula on advanced glycation endproduct-induced endothelial cell dysfunction

AIM OF THE STUDY

The aqueous extract of Terminalia chebular fruits was reported to have anti-hyperglycemia and anti-diabetic complication effects. The present study therefore investigated the protective mechanism of chebulic acid, a phenolcarboxylic acid compound isolated from the ripe fruits of Terminalia chebula against advanced glycation endproducts (AGEs)-induced endothelial cell dysfunction.

MATERIALS AND METHODS

To investigate the protective mechanism of chebulic acid against vascular endothelial dysfunction human umbilical vein endothelial cells (HUVEC) were treated with chebulic acid in the presence/absence of glyceraldehyde-related AGEs (glycer-AGEs).

RESULTS

HUVEC incubated with 100 μg/ml of glycer-AGEs had significantly enhanced reactive oxygen species formation, whereas the treatment of chebulic acid dose-dependently reduced glycer-AGE-induced formation to 108.2 ± 1.9% for 25 μM versus 137.8 ± 1.1% for glycer-AGEs treated alone. The transendothelial electrical resistance (TER) value of the glycer-AGEs group was dramatically decreased to 76.9 ± 2.2% compared to the control, whereas chebulic acid treatment prevented glycer-AGE-induced TER change with a value of 91.3 ± 5.3%. The incubation of confluent HUVEC with 100 μg/ml of glycer-AGEs for 24h remarkably increased the adhesion of human monocytic THP-1 cells compared to non-stimulated HUVEC. These increases in HUVEC adhesiveness were dose-dependently reduced by chebulic acid.

CONCLUSIONS

The present study shows the effects of chebulic acid against the progression of AGE-induced endothelial cell dysfunction suggesting that this compound may constitute a promising intervention agent against diabetic vascular complications.

Anaerobic vs aerobic pathways of carbonyl and oxidant stress in human lens and skin during aging and in diabetes: A comparative analysis

The effects of anaerobic (lens) vs aerobic (skin) environment on carbonyl and oxidant stress are compared using de novo and existing data on advanced glycation and oxidation products in human crystallins and collagen. Almost all modifications increase with age. Methylglyoxal hydroimidazolones, carboxymethyllysine, and carboxyethyllysine are severalfold higher in lens than in skin and markedly increase upon incubation of lens crystallins with 5mM ascorbic acid. In contrast, fructose-lysine, glucosepane crosslinks, glyoxal hydroimidazolones, metal-catalyzed oxidation (allysine), and H(2)O(2)-dependent modifications (2-aminoapidic acid and methionine sulfoxide) are markedly elevated in skin, but relatively suppressed in the aging lens. In both tissues ornithine is the dominant modification, implicating arginine residues as the principal target of the Maillard reaction in vivo. Diabetes (here mostly type 2 studied) increases significantly fructose-lysine and glucosepane in both tissues (P<0.001) but has surprisingly little effect on the absolute level of most other advanced glycation end products. However, diabetes strengthens the Spearman correlation coefficients for age-related accumulation of hydrogen peroxide-mediated modifications in the lens. Overall, the data suggest that oxoaldehyde stress involving methylglyoxal from either glucose or ascorbate is predominant in the aging noncataractous lens, whereas aging skin collagen undergoes combined attack by nonoxidative glucose-mediated modifications, as well as those from metal-catalyzed oxidation and H(2)O(2).

Presbyopia and cataract: a question of heat and time

Not only are human lenses different in many ways from those of non-primates, they also undergo dramatic changes with age. These age-dependent alterations lead to perturbations in the properties of older lenses, and ultimately to disturbances in visual function, which typically become apparent at middle age. Recent data suggest that many, if not all, of these age-dependent features can be traced to the lack of macromolecular turnover in the lens and to the inexorable modifications to proteins and membrane components over a period of decades. Exposure of lenses to heat can reproduce many of these alterations, suggesting that long-term incubation at body temperature may be an important factor in aging the human lens. Two conclusions flow from this. Firstly, the human lens may be an ideal tissue for studying macromolecular aging in man. Secondly, it will be extremely challenging to examine the origin of human age-related conditions, such as presbyopia and nuclear cataract, using traditional laboratory animals. Characterising the unfolding and decomposition of long-lived macromolecules appears to provide the key to understanding the two most common human lens disorders: presbyopia and age-related nuclear cataract.

Vitaminas e minerais com propriedades antioxidantes e risco cardiometabólico: controvérsias e perspectivas

No processo celular de obtenção de energia, são gerados compostos chamados espécies reativas de oxigênio (ERO) que, em excesso, podem causar danos celulares. Estresse oxidativo resulta do desequilíbrio no estado de óxido-redução a favor da oxidação. Dos mecanismos de defesa antioxidante, participam enzimas endógenas e algumas vitaminas e minerais. A vitamina E encontra-se no plasma e na partícula de LDL, protegendo lipídeos da oxidação. Estudos observacionais relataram associação inversa entre ingestão de vitamina E e risco cardiometabólico (RCM). Entretanto, ensaios clínicos não comprovaram a eficácia de sua suplementação nos desfechos cardiometabólicos. A vitamina C participa do sistema de regeneração da vitamina E, mantendo o potencial antioxidante plasmático. Dados sobre os benefícios de sua suplementação na redução do risco cardiometabólico são inconclusivos. A atividade antioxidante dos carotenoides é responsável, em parte, por seu papel protetor contra doenças cardiovasculares e cânceres. A suplementação desse nutriente também não trouxe resultados consistentes no que se refere à redução do RCM. A participação do zinco e do selênio na defesa antioxidante vem sendo estudada mais recentemente, mas a sua suplementação em indivíduos com níveis séricos normais e ingestão adequada na dieta desses minerais não parece ser necessária. De um modo geral, há muita controvérsia sobre o papel desses micronutrientes no RCM. Estudos epidemiológicos sugerem que o consumo de substâncias antioxidantes provenientes da dieta ou dietas ricas em frutas e hortaliças diminui o RCM. Mais estudos são necessários antes de se recomendar o uso de antioxidantes isolados na forma de suplementos para tal finalidade.

Lens aging: effects of crystallins

The primary function of the eye lens is to focus light on the retina. The major proteins in the lens--alpha, beta, and gamma-crystallins--are constantly subjected to age-related changes such as oxidation, deamidation, truncation, glycation, and methylation. Such age-related modifications are cumulative and affect crystallin structure and function. With time, the modified crystallins aggregate, causing the lens to increasingly scatter light on the retina instead of focusing light on it and causing the lens to lose its transparency gradually and become opaque. Age-related lens opacity, or cataract, is the major cause of blindness worldwide. We review deamidation, and glycation that occur in the lenses during aging keeping in mind the structural and functional changes that these modifications bring about in the proteins. In addition, we review proteolysis and discuss recent observations on how crystallin fragments generated in vivo, through their anti-chaperone activity may cause crystallin aggregation in aging lenses. We also review hyperbaric oxygen treatment induced guinea pig and 'humanized' ascorbate transporting mouse models as suitable options for studies on age-related changes in lens proteins.

Glycation by ascorbic acid oxidation products leads to the aggregation of lens proteins

Previous studies from this laboratory have shown that there are striking similarities between the yellow chromophores, fluorophores and modified amino acids released by proteolytic digestion from calf lens proteins ascorbylated in vitro and their counterparts isolated from aged and cataractous lens proteins. The studies reported in this communication were conducted to further investigate whether ascorbic acid-mediated modification of lens proteins could lead to the formation of lens protein aggregates capable of scattering visible light, similar to the high molecular aggregates found in aged human lenses. Ascorbic acid, but not glucose, fructose, ribose or erythrulose, caused the aggregation of calf lens proteins to proteins ranging from 2.2 x 10(6) up to 3.0 x 10(8 )Da. This compared to proteins ranging from 1.8 x 10(6) up to 3.6 x 10(8 )Da for the water-soluble (WS) proteins isolated from aged human lenses. This aggregation was likely due to the glycation of lens crystallins because [U-(14)C] ascorbate was incorporated into the aggregate fraction and because NaCNBH(3), which reduces the initial Schiff base, prevented any protein aggregation. Reactions of ascorbate with purified crystallin fractions showed little or no aggregation of alpha-crystallin, significant aggregation of beta(H)-crystallin, but rapid precipitation of purified beta(L)- and gamma-crystallin. The aggregation of lens proteins can be prevented by the binding of damaged crystallins to alpha-crystallin due to its chaperone activity. Depending upon the ratios between the components of the incubation mixtures, alpha-crystallin prevented the precipitation of the purified beta(L)- and gamma-crystallin fractions during ascorbylation. The addition of at least 20% of alpha-crystallin by weight into glycation mixtures with beta(L)-, or gamma-crystallins completely inhibited protein precipitation, and increased the amount of the high molecular weight aggregates in solution. Static and dynamic light scattering measurements of the supernatants from the ascorbic acid-modified mixtures of alpha- and beta(L)-, or gamma-crystallins showed similar molar masses (up to 10(8 )Da) and hydrodynamic diameter (up to 80( )nm). These data support the hypothesis, that if the lens reducing environment is compromised, the ascorbylation of lens crystallins can significantly change the short range interactions between different classes of crystallins leading to protein aggregation, light scattering and eventually to senile cataract formation.

Photosensitizing Activity of Advanced Glycation Endproducts on Tryptophan, Glucose 6-phosphate Dehydrogenase, Human Serum Albumin and Ascorbic Acid Evaluated at Low Oxygen Pressure†

A comparative study of the photosensitizing activity of advanced glycation endproducts (AGEs) prepared by incubating glucose (Glc), threose (Threo) and ascorbate (AH-) in the presence of lysine (Lys) was performed. Photochemical activity was evaluated under low oxygen pressure with the aim to simulate the conditions of the eye lens. AGE-sensitized tryptophan and AH- photodecomposition and glucose 6-phosphate dehydrogenase inactivation were studied. In all systems, glucose-derived AGEs showed the highest photosensitizing efficiency, followed by ascorbate and threose. The presence of different sensitizers in glycation products mixtures was investigated. For this purpose, Trp decomposition quantum yields were determined at 344 and 367 nm. The values obtained at 344 nm are between three and six times higher than those observed at 367 nm, confirming the presence of at least two compounds with different photosensitizing activities in the mixtures. The chemiluminescence associated with the AGE-mediated oxidation of free Trp and Trp residues in human serum albumin was also studied, and a good correlation between the emission of light and the extent of Trp decomposition was found. In conclusion, it is demonstrated that glucose derived AGEs, which can be formed in vivo in the eye lens of diabetic patients and are accumulated in elderly lenses, have a higher photosensitizing efficiency, at low oxygen pressure, than those arising from ascorbate and threose. This high efficiency is especially significant when proteins are employed as photochemical targets, indicating that protein-sensitizer interaction and the local environment around the sensitizers play an important role.

Isolation, purification and characterization of histidino-threosidine, a novel Maillard reaction protein crosslink from threose, lysine and histidine

We isolated a novel acid-labile yellow chromophore from the incubation of lysine, histidine and d-threose and identified its chemical structure by one and two-dimensional NMR spectroscopy combined with LC-tandem mass spectrometry. This new cross-link exhibits a UV absorbance maximum at 305 nm and a molecular mass of 451 Da. The proposed structure is 2-amino-5-(3-((4-(2-amino-2-carboxyethyl)-1H-imidazol-1-yl)methyl)-4-(1,2-dihydroxyethyl)-2-formyl-1H-pyrrol-1-yl)pentatonic acid, a cross-link between lysine and histidine with addition of two threose molecules. It was in part deduced and confirmed through synthesis of the analogous compound from n-butylamine, imidazole and d-threose. We assigned the compound the trivial name histidino-threosidine. Systemic incubation revealed that histidino-threosidine can be formed in low amounts from fructose, glyceraldehyde, methylglyoxal, glycolaldehyde, ascorbic acid, and dehydroascorbic acid, but at a much higher yield with degradation products of ascorbic acid, i.e. threose, erythrose, and erythrulose. Bovine lens protein incubated with 10 and 50 mM threose for two weeks yielded 560 and 2840 pmol/mg histidino-threosidine. Histidino-threosidine is to our knowledge the first Maillard reaction product known to involve histidine in a crosslink.

Advanced glycation end products and receptors in Fuchs' dystrophy corneas undergoing Descemet's stripping with endothelial keratoplasty

PURPOSE

To describe the histopathologic features of Descemet's membrane (DM) obtained from Fuchs' endothelial corneal dystrophy (FECD) corneas undergoing Descemet's stripping with endothelial keratoplasty (DSEK) and to assess the presence of advanced glycation end products (AGEs) and their receptors in FECD endothelium and DM.

DESIGN

Prospective observational case series.

PARTICIPANTS

Five eyes of 5 patients undergoing DSEK for FECD and 4 normal control eyebank corneas.

METHODS

Descemet's membrane and corneal endothelium from FECD patients undergoing DSEK were assessed with hematoxylin-eosin staining and immunohistochemistry for AGEs, receptor of AGEs (RAGE), and galectin 3 (AGE-R3).

MAIN OUTCOME MEASURES

Histopathologic abnormalities and presence of AGEs, RAGE, and AGE-R3 in DSEK specimens.

RESULTS

Histopathologic assessment of DSEK specimens from FECD patients disclosed thickening and nodularity of DM and loss of endothelial cells. Immunohistochemical staining of FECD DM for AGE, RAGE, and AGE-R3 showed an abundance of AGEs in the anterior portion of DM, mild positivity for RAGE, and moderate positivity for AGE-R3.

CONCLUSIONS

Tissue quality after DSEK is sufficient to allow detailed histopathologic analysis. The presence of AGEs, RAGE, and AGE-R3 in DM and corneal endothelium of FECD patients supports a link between accumulation of AGEs, oxidative stress, and corneal endothelial cell apoptosis in the pathogenesis of FECD.

LC-MS display of the total modified amino acids in cataract lens proteins and in lens proteins glycated by ascorbic acid in vitro

We previously reported chromatographic evidence supporting the similarity of yellow chromophores isolated from aged human lens proteins, early brunescent cataract lens proteins and calf lens proteins ascorbylated in vitro [Cheng, R. et al. Biochimica et Biophysica Acta 1537, 14-26, 2001]. In this paper, new evidence supporting the chemical identity of the modified amino acids in these protein populations were collected by using a newly developed two-dimensional LC-MS mapping technique supported by tandem mass analysis of the major species. The pooled water-insoluble proteins from aged normal human lenses, early stage brunescent cataract lenses and calf lens proteins reacted with or without 20 mM ascorbic acid in air for 4 weeks were digested with a battery of proteolytic enzymes under argon to release the modified amino acids. Aliquots equivalent to 2.0 g of digested protein were subjected to size-exclusion chromatography on a Bio-Gel P-2 column and four major A330nm-absorbing peaks were collected. Peaks 1, 2 and 3, which contained most of the modified amino acids were concentrated and subjected to RP-HPLC/ESI-MS, and the mass elution maps were determined. The samples were again analyzed and those peaks with a 10(4) - 10(6) response factor were subjected to MS/MS analysis to identify the daughter ions of each modification. Mass spectrometric maps of peaks 1, 2 and 3 from cataract lenses showed 58, 40 and 55 mass values, respectively, ranging from 150 to 600 Da. Similar analyses of the peaks from digests of the ascorbylated calf lens proteins gave 81, 70 and 67 mass values, respectively, of which 100 were identical to the peaks in the cataract lens proteins. A total of 40 of the major species from each digest were analyzed by LC-MS/MS and 36 were shown to be identical. Calf lens proteins incubated without ascorbic acid showed several similar mass values, but the response factors were 100 to 1000-fold less for every modification. Based upon these data, we conclude that the majority of the major modified amino acids present in early stage brunescent Indian cataract lens proteins appear to arise as a result of ascorbic acid modification, and are presumably advanced glycation end-products.

Fluorescence from the Maillard Reaction and its Potential Applications in Food Science

The chemistry of the Maillard reaction involves a complex set of steps, and its interpretation represents a challenge in basic and applied aspects of Food Science. Fluorescent compounds have been recognized as important early markers of the reaction in food products since 1942. However, the recent advances in the characterization of fluorophores' development were observed in biological and biomedical areas. The in vivo non-enzymatic glycosylation of proteins produces biological effects, promoting health deterioration. The characteristic fluorescence of advanced glycosylation end products (AGEs) is similar to that of Maillard food products and represents an indicator of the level of AGE-modified proteins, but the structure of the fluorescent groups is, typically, unknown. Application of fluorescence measurement is considered a potential tool for addressing key problems of food deterioration as an early marker or index of the damage of biomolecules. Fluorophores may be precursors of the brown pigments and/or end products. A general scheme of the Maillard reaction is proposed in this article, incorporating the pool concept. A correct interpretation of the effect of environmental and compositional conditions and their influences on the reaction kinetics may help to define the meaning of fluorescence development for each particular system.

Meniscal material properties are minimally affected by matrix stabilization using glutaraldehyde and glycation with ribose

Knee meniscus replacement holds promise, but current allografts are susceptible to biodegradation. Matrix stabilization with glutaraldehyde, a crosslinking agent used clinically to fabricate cardiovascular bioprostheses, or with glycation, a process of crosslinking collagen with sugars such as ribose, is a potential means of rendering tissue resistant to such degradation. However, stabilization should not significantly alter meniscal material properties, which could disturb normal function in the knee. Our objective was to evaluate the effects of glutaraldehyde- and glycation-induced matrix stabilization on the material properties of porcine meniscus. Normal untreated meniscus specimens were tested in confined compression at one of three applied stresses (0.069, 0.208, 0.347 MPa), subjected to either a glutaraldehyde or glycation stabilization treatment, and then re-tested to measure changes in tissue aggregate modulus, permeability, and compressive strain at equilibrium. Changes in these properties significantly increased with glutaraldehyde concentration and exposure time to ribose. One glutaraldehyde and three glycation treatments did not alter aggregate modulus or compressive strain at equilibrium compared to controls (p > 0.10). However, all treatments increased permeability by at least 108% compared to controls (p < 0.001). This study reveals a dose-dependent relationship between meniscal material properties and certain stabilization conditions and identifies treatments that minimally affect these properties. Further research is necessary to determine whether these treatments prevent enzymatic degradation before and after surgical implantation in the knee.

Age-related nuclear cataract—oxidation is the key

Age is by far the biggest risk factor for cataract, and it is sometimes assumed that cataract is simply an amplification of this aging process. This appears not to be the case, since the lens changes associated with aging and cataract are distinct. Oxidation is the hallmark of age-related nuclear (ARN) cataract. Loss of protein sulfhydryl groups, and the oxidation of methionine residues, are progressive and increase as the cataract worsens until >90% of cysteine and half the methionine residues are oxidised in the most advanced form. By contrast, there may be no significant oxidation of proteins in the centre of the lens with advancing age, even past age 80. The key factor in preventing oxidation seems to be the concentration of nuclear glutathione (GSH). Provided that nuclear GSH levels can be maintained above 2 mm, it appears that significant protein oxidation and posttranslational modification by reactive small molecules, such as ascorbate or UV filter degradation products, is not observed. Adequate coupling of the metabolically-active cortex, the source of antioxidants such as GSH, to the quiescent nucleus, is crucial especially since it would appear that the cortex remains viable in old lenses, and even possibly in ARN cataract lenses. Therefore it is vital to understand the reason for the onset of the lens barrier. This barrier, which becomes apparent in middle age, acts to impede the flow of small molecules between the cortex and the nucleus. The barrier, rather than nuclear compaction (which is not observed in human lenses), may contribute to the lowered concentration of GSH in the lens nucleus after middle age. By extending the residence time within the lens centre, the barrier also facilitates the decomposition of intrinsically unstable metabolites and may exacerbate the formation of H(2)O(2) in the nucleus. This hypothesis, which is based on the generation of reactive oxygen species and reactive molecules within the nucleus itself, shifts the focus away from theories for cataract that postulated a primary role for oxidants generated outside of the lens. Unfortunately, due to marked variability in the lenses of different species, there appears at present to be no ideal animal model system for studying human ARN cataract.

Does supplemental vitamin C increase cardiovascular disease risk in women with diabetes?

BACKGROUND

Vitamin C acts as a potent antioxidant; however, it can also be a prooxidant and glycate protein under certain circumstances in vitro. These observations led us to hypothesize that a high intake of vitamin C in diabetic persons might promote atherosclerosis.

OBJECTIVE

The objective was to examine the relation between vitamin C intake and mortality from cardiovascular disease.

DESIGN

We studied the relation between vitamin C intake and mortality from total cardiovascular disease (n = 281), coronary artery disease (n = 175), and stroke (n = 57) in 1923 postmenopausal women who reported being diabetic at baseline. Diet was assessed with a food-frequency questionnaire at baseline, and subjects initially free of coronary artery disease were prospectively followed for 15 y.

RESULTS

After adjustment for cardiovascular disease risk factors, type of diabetes medication used, duration of diabetes, and intakes of folate, vitamin E, and beta-carotene, the adjusted relative risks of total cardiovascular disease mortality were 1.0, 0.97, 1.11, 1.47, and 1.84 (P for trend < 0.01) across quintiles of total vitamin C intake from food and supplements. Adjusted relative risks of coronary artery disease were 1.0, 0.81, 0.99, 1.26, and 1.91 (P for trend = 0.01) and of stroke were 1.0, 0.52, 1.23, 2.22, and 2.57 (P for trend < 0.01). When dietary and supplemental vitamin C were analyzed separately, only supplemental vitamin C showed a positive association with mortality endpoints. Vitamin C intake was unrelated to mortality from cardiovascular disease in the nondiabetic subjects at baseline.

CONCLUSION

A high vitamin C intake from supplements is associated with an increased risk of cardiovascular disease mortality in postmenopausal women with diabetes.

Ascorbic acid increases the severity of spontaneous knee osteoarthritis in a guinea pig model

OBJECTIVE

To determine whether ascorbic acid might be of benefit for the treatment of spontaneous osteoarthritis (OA) when administered over a long period of time.

METHODS

We investigated the effects of 8 months' exposure to low, medium, and high doses of ascorbic acid on the in vivo development of histologic knee OA in the male Hartley guinea pig. The low dose represented the minimum amount needed to prevent scurvy. The medium dose was the amount present in standard laboratory guinea pig chow and resulted in plasma levels comparable with those achieved in a person consuming 200 mg/day (5 fruits and vegetables daily). The high dose was the amount shown in a previous study of the guinea pig to slow the progression of surgically induced OA.

RESULTS

We found an association between ascorbic acid supplementation and increased cartilage collagen content but, in contrast to findings in a previous study of surgically induced OA in the guinea pig, ascorbic acid worsened the severity of spontaneous OA. Active transforming growth factor beta (TGF beta) was expressed in marginal osteophytes, whose size and number were significantly increased with increasing intake of ascorbic acid. Synovial fluid levels of cartilage oligomeric matrix protein, a biomarker of cartilage turnover, corroborated the histologic findings.

CONCLUSION

Ascorbic acid has been shown to activate latent TGF beta. Prolonged intraarticular exposure to TGF beta has been shown to cause OA-like changes. We found expression of active TGF beta in osteophytes, a prominent feature of the joint histology seen in association with ascorbic acid treatment. Thus, the deleterious effects of prolonged ascorbic acid exposure may be mediated in part by TGF beta. This worsening of OA with ascorbic acid supplementation suggests that ascorbic acid intake should not be supplemented above the currently recommended dietary allowance (90 mg/day for men and 75 mg/day for women).

Pharmacological prevention of diabetic cataract

Cataract--opacification of the lens--is closely related to diabetes as one of its major late complications. This review deals with three molecular mechanisms that may be involved in the development of diabetic cataract: nonenzymatic glycation of eye lens proteins, oxidative stress, and activated polyol pathway in glucose disposition. Implications resulting from these mechanisms for possible pharmacological interventions to prevent diabetic cataract are discussed. The article reviews research on potential anticataract agents, including glycation inhibitors, antioxidants, and aldose reductase inhibitors. Information on possible benefits of putative anticataract agents comes from a variety of approaches, ranging from laboratory experiments, both in vitro and in vivo, to epidemiological studies in patients.

Characterization and detection of lysine–arginine cross-links derived from dehydroascorbic acid

Covalently cross-linked proteins are among the major modifications caused by the advanced Maillard reaction. So far, the chemical nature of these aggregates is largely unknown. L-dehydroascorbic acid (DHA, 5), the oxidation product of L-ascorbic acid (vitamin C), is known as a potent glycation agent. Identification is reported for the lysine-arginine cross-links N6-[2-[(4-amino-4-carboxybutyl)amino]-5-(2-hydroxyethyl)-3,5-dihydro-4H-imidazol-4-ylidene]-L-lysine (9), N6-[2-[(4-amino-4-carboxybutyl)amino]-5-(1,2-dihydroxyethyl)-3,5-dihydro-4H-imidazol-4-ylidene]-L-lysine (11), and N6-[2-[(4-amino-4-carboxybutyl)amino]-5-[(1S,2S)-1,2,3-trihydroxypropyl]-3,5-dihydro-4H-imidazol-4-ylidene]-L-lysine (13). The formation pathways could be established starting from dehydroascorbic acid (5), the degradation products 1,3,4-trihydroxybutan-2-one (7, L-erythrulose), 3,4-dihydroxy-2-oxobutanal (10, L-threosone), and L-threo-pentos-2-ulose (12, L-xylosone) were proven as precursors of the lysine-arginine cross-links 9, 11, and 13. Products 9 and 11 were synthesized starting from DHA 5, compound N6-[2-[(4-amino-4-carboxybutyl)amino]-5-[(1S,2R)-1,2,3-trihydroxypropyl]-3,5-dihydro-4H-imidazol-4-ylidene]-L-lysine (16) via the precursor D-erythro-pentos-2-ulose (15). The present study revealed that the modification of lysine and arginine side chains by DHA 5 is a complex process and could involve a number of reactive carbonyl species.

Advanced glycation endproducts as UVA photosensitizers of tryptophan and ascorbic acid: consequences for the lens

Upon aging, the lens accumulates brown fluorophores, mainly derived from the Maillard reaction between vitamin C oxidation products and crystallins lysine residues. At the same time, the concentration of UVA filters decreases, allowing some radiation to be absorbed by lenticular advanced glycation endproducts (AGEs). This paper quantifies the photosensitizing activity of AGEs at various oxygen pressures, and compares it to that of lenticular riboflavin (RF). Solutions containing the sensitizer and the substrates tryptophan (Trp) and ascorbate (AH(-)) were irradiated at 365 nm. We show that the AGEs-photosensitized Trp oxidation rate increases with AGEs concentration and is optimal at 5% oxygen, the pressure in the lens. By contrast, for AH(-), the photooxidation rate increases with oxygen concentration. Despite the higher quantum yield of RF-depending reactions, its low concentration as compared to that of AGEs in aging lenses induces significantly higher Trp and AH(-) photodegradation rates with AGEs than with RF. As ascorbate is more rapidly photodegraded than Trp, the antioxidant competitively protects Trp from oxidation up to 1 mM, although not absolutely. We conclude that in the aging lens, AH(-) exerts a strong UVA protecting activity, but does not impede some Trp residue to be photodegraded proportionally to the AGEs concentration.

Increased levels of advanced glycation end products in human cataractous lenses

PURPOSE

To investigate the occurrence of advanced glycation end products (AGEs) formed oxidatively (pentosidine and N(epsilon)-carboxymethyl lysine [CML]) or nonoxidatively (imidazolone) in human lenses and the relation of AGEs to lens coloration, cataract type, and patients' diabetic state.

SETTING

Departments of Ophthalmology and Internal Medicine III, University of Jena, Jena, Germany.

METHODS

Pentosidine, CML, and imidazolone concentrations were measured in the water-soluble protein fraction of 44 cataractous lenses (from 24 nondiabetic and 20 diabetic donors) and 6 noncataractous control lenses.

RESULTS

Pentosidine, CML, and imidazolone were higher in cataractous lenses than in control lenses (pentosidine, 3.7 pmol/mg +/- 5.3 (SD) and 1.9 +/- 1.7 pmol/mg, respectively; CML, 3.0 +/- 2.2 nmol/mg and 1.3 +/- 0.7 nmol/mg, respectively; imidazoline, 80.4 +/- 93.3 AU/mg and 19.6 +/- 18.5 AU/mg, respectively). Among the cataractous lenses, the highest AGE concentrations were found in mature cataracts, with a statistically significant increase in CML. The AGE content increased relative to the intensity of brown coloration of the lens; the brown coloration also indicated the highest rise of imidazolone compared to pentosidine and CML. Lenses from diabetic donors had generally similar pentosidine values and elevated CML and imidazolone levels compared to lenses from nondiabetic donors. The pentosidine, CML, and imidazolone levels in the lenses correlated significantly with one another but not with patient age.

CONCLUSION

Advanced glycation end products formed oxidatively and nonoxidatively occurred to a higher degree in cataractous lenses than in noncataractous lenses. The strong relationship between the lenses' AGE content, color/opacity, and the state of the cataract may indicate that advanced glycation plays a pivotal role in cataract formation.

Isolation and characterization of a new advanced glycation endproduct of dehydroascorbic acid and lysine

Proteins are subject of posttranslational modification by sugars and their degradation products in vivo. The process is often referred as glycation. L-Dehydroascorbic acid (DHA), an oxidation product of L-ascorbic acid (vitamin C), is known as a potent glycation agent. A new product of modification of lysine epsilon -amino group by DHA was discovered as a result of the interaction between Boc-Lys and dehydroascorbic acid. The chromatographic and spectral analyses revealed that the structure of the product was 1-(5-ammonio-5-carboxypentyl)-3-oxido-4-(hydroxymethyl)pyridinium. The same compound was isolated from DHA modified calf lens protein after hydrolysis and chromatographic separation. The study confirmed that L-erythrulose is an important intermediate of modification of proteins by DHA. The structure of the reported product and in vitro experiments suggested that L-erythrulose could further transform to L-threose, L-erythrose and glycolaldehyde under conditions similar to physiological. The present study revealed that the modification of epsilon -amino groups of lysine residues by DHA is a complex process and could involve a number of reactive carbonyl species.

Inhibition of ascorbic acid-induced modifications in lens proteins by peptides

The effects of three dipeptides L-phenylalanyl-glybine, glycyl-L-phenylalanine,and aspartame (L-aspartyl-L-phenylalanine, methyl ester) as inhibitors of the ascorbic acid-induced modifications in lens proteins were studied. Their efficiency was compared to that of two known inhibitors--aminoguanidine and carnosine. The tested dipeptides diminished protein carbonyl content by 32-58% and most moderated the formation of chromophores, as measured by the absorbency at 325 nm of the glycated proteins. The appearance of non-tryptophan fluorescence (excitation 340 nm/emission 410 nm) was observed for proteins glycated with ascorbic acid. All of the dipeptides examined, as well as aminoguanidine, decreased this glycation-related fluorescence. The potential inhibitors prevented the intensive formation of very high molecular weight aggregates. A competitive mechanism of their inhibitory effect was proposed, based on the reactivity of individual substances toward ascorbic acid. These findings indicate that they have a potential for use as alternatives for aminoguanidine as an anti-glycation agent.

Manipulation of remodeling pathways to enhance the mechanical properties of a tissue engineered blood vessel

There is a current need for a small diameter vascular graft due to the limited supply of autogenous grafts and the failure of synthetic grafts due to thrombosis and/or intimal hyperplasia. The use of living cells and tissues to fabricate a small diameter graft (i.e., tissue engineered blood vessel, TEBV) could be useful given the endothelialization potential and biocompatibility benefits of such a graft. However, while sufficient strength has been attained in a TEBV, coordinate compliance has yet to be fine-tuned. In this study we investigate the effects of biological response modifiers, retinoic acid (RA) and ascorbic acid (AA) on TEBV biomechanics as a function of time and subsequently correlate observed RA/AA induced changes in TEBV mechanics with alterations in smooth muscle cell (SMC) biochemistry. TEBVs were constructed using a fibrillar type I collagen network populated by human aortic smooth muscle cells (AoSMC). Following construction this TEBV was treated with 0.3 mM AA and 0.1 mM RA (concentrations found to induce changes in VSMC phenotype). Ultimate tensile stress (UTS), rate of relaxation (RR) and elastic efficiency (EE) of RA/AA treated and untreated TEBVs were measured following 1, 7, 15, 30, 45, and 60 days of treatment. At corresponding time points, the effect of these treatments on collagen and elastin protein synthesis and mRNA expression was examined. RA/AA treated TEBV strength increased and stiffness decreased compared to controls as a function of time. Relative collagen synthesis in treated TEBVs exceeded control levels by nearly two-fold at 15 and 30 days of incubation. RA/AA treated collagen gene expression followed a similar trend. Relative elastin synthesis was also greater in treated TEBVs as compared to untreated TEBVs at 15 and 30 days of incubation and correspondingly elastin mRNA expression was significantly elevated at 15 days of incubation. These data provide evidence that RA/AA treated TEBVs exhibit mechanical properties which more closely mimic those of a native vessel than their untreated counterparts and that changes in extracellular matrix composition and matrix gene expression in the presence of RA/AA treatment may play an important role in the development of said mechanical properties.

A role for tertiary structure in the generation of antigenic diversity and molecular association of the Tams1 polypeptide in Theileria annulata

The major merozoite-piroplasm surface antigen (mMPSA) of Theileria annulata, Tams1, is known to be antigenically diverse. The possession of variable N-linked glycosylation sites and removal of monoclonal antibody 5E1 reactivity by mild periodate treatment suggested, previously, that divergent epitopes may be conferred by secondary modification. This study has shown that monoclonal antibody 5E1 and polyspecific antisera raised against the native protein react against divergent amino acid epitopes that are dependent on a molecular conformation that is sensitive to periodate. Therefore, no experimental evidence exists to confirm the sequence prediction that Tams1 undergoes N-linked glycosylation. Data is also presented indicating that the conformation of the antigen results in presentation of divergent regions on the external surface of the molecule, while conserved regions are more likely to be internal and hidden. In addition, non-reducing SDS-PAGE analysis demonstrated that Tams1 can undergo molecular association to form homo-dimers, trimers and multimers. The potential influence of tertiary structure and inter-molecular association on Tams1 diversity and function is discussed.

Accumulation of advanced glycation endproducts reduces chondrocyte-mediated extracellular matrix turnover in human articular cartilage

OBJECTIVE

The prevalence of osteoarthritis (OAs) increases with age and coincides with the accumulation of advanced glycation endproducts (AGEs) in articular cartilage, suggesting that accumulation of glycation products may be involved in the development of OA. This study was designed to examine the effects of accumulation of AGEs on the turnover of the extracellular matrix of human articular cartilage.

DESIGN

Chondrocyte mediated cartilage degradation (GAG release, colorimetric) was measured in human articular cartilage of donors aged 19-82 years (N=30, 4-day culture). In addition, to mimic the age-related increase in AGE levels in vitro, cartilage was cultured in the absence or presence of glucose, ribose or threose. Cartilage degradation and proteoglycan synthesis ((35)SO(2)(-4) incorporation) were measured and related to the degree of cartilage AGE levels (fluorescence at 360/460 nm).

RESULTS

Chondrocyte-mediated degradation of articular cartilage (i.e. GAG release) decreased with increasing age of the cartilage donor (r=-0.43, P< 0.02). In vitro incubation of cartilage with glucose, ribose or threose resulted in a range of AGE levels that was highly correlated to the chondrocyte-mediated cartilage degradation (r=-0.77, P< 0.001, N=26). In addition, in these in vitro glycated cartilage samples, a decrease in proteoglycan synthesis was observed at increasing AGE levels (r=-0.54, P< 0.005, N=25).

CONCLUSIONS

This study shows that an increase in AGE levels negatively affects the proteoglycan synthesis and degradation of articular cartilage. In combination, these two effects reduce the turnover of the cartilage and thereby the maintenance and repair capacity of the tissue. By this mechanism, the age-related increase in cartilage AGE levels may contribute to the development of OA.

Similarity of the yellow chromophores isolated from human cataracts with those from ascorbic acid-modified calf lens proteins: evidence for ascorbic acid glycation during cataract formation

Chromatographic evidence supporting the similarity of the yellow chromophores isolated from aged human and brunescent cataract lenses and calf lens proteins ascorbylated in vitro is presented. The water-insoluble fraction from early stage brunescent cataract lenses was solubilized by sonication (WISS) and digested with a battery of proteolytic enzymes under argon to prevent oxidation. Also, calf lens proteins were incubated with ascorbic acid for 4 weeks in air and submitted to the same digestion. The percent hydrolysis of the proteins to amino acids was approximately 90% in every case. The content of yellow chromophores was 90, 130 and 250 A(330) units/g protein for normal human WISS, cataract WISS and ascorbate-modified bovine lens proteins respectively. Aliquots equivalent to 2.0 g of digested protein were subjected to size-exclusion chromatography on a Bio-Gel P-2 column. Six peaks were obtained for both preparations and pooled. Side by side thin-layer chromatography (TLC) of each peak showed very similar R(f) values for the long wavelength-absorbing fluorophores. Glycation with [U-(14)C]ascorbic acid, followed by digestion and Bio-Gel P-2 chromatography, showed that the incorporated radioactivity co-eluted with the A(330)-absorbing peaks, and that most of the fluorescent bands were labeled after TLC. Peaks 2 and 3 from the P-2 were further fractionated by preparative Prodigy C-18 reversed-phase high-performance liquid chromatography. Two major A(330)-absorbing peaks were seen in peak 2 isolated from human cataract lenses and 5 peaks in fraction 3, all of which eluted at the same retention times as those from ascorbic acid glycated calf lens proteins. HPLC fractionation of P-2 peaks 4, 5 and 6 showed many A(330)-absorbing peaks from the cataract WISS, only some of which were identical to the asorbylated proteins. The major fluorophores, however, were present in both preparations. These data provide new evidence to support the hypothesis that the yellow chromophores in brunescent lenses represent advanced glycation endproducts (AGEs) probably due to ascorbic acid glycation in vivo.

Photochemically modified alpha-crystallin: a model system for aging in the primate lens

The purpose of this study was to quantitatively study the changes that occur upon irradiation of 3-hydroxykynurenine (3-HK) in the presence of alpha-crystallin under conditions similar to those in the lens. The samples were prepared in 10 mM phosphate buffer at pH 7.4, bubbled with O2 or Ar and irradiated with 300-400 nm light. The amount of light absorbed by the samples (Iabs) was measured using azobenzene as an actinometer. Modifications to alpha-crystallin were monitored by ultraviolet-visible and fluorescence spectroscopy. Aerobic samples had increased absorption around 320 nm and above 400 nm while the 3-HK maximum at 368 nm decreased. The isolated modified protein showed that there was increased absorption throughout the spectrum. Changes in the anaerobic samples were similar to those of the aerobic but occurred more slowly. As irradiation time increased fluorescence emission of the isolated protein red shifted and quantum yields of fluorescence (phi f) were calculated at different irradiation time intervals by comparison to 3-HK. By comparing OD320/OD365 for the model system to values from primate lenses, Iabs can be correlated with age and transmission of the sample in the blue region of the spectrum and thus allows lenticular aging to be quantitated.

Novel approaches to the analysis of the Maillard reaction of proteins

The Maillard reaction comprises a complex network of reactions which has proven to be of great importance in both food science and medicine. The majority of methods developed for studying the Maillard reaction in food have focused on model systems containing amino acids and monosaccharides. In this study, a number of electrophoretic techniques, including two-dimensional gel electrophoresis and capillary electrophoresis, are presented. These have been developed specifically for the analysis of the Maillard reaction of food proteins, and are giving important insights into this complex process.

Inhibitors of advanced glycation end product-associated protein cross-linking

The reaction of lens proteins with sugars over time results in the formation of protein-bound advanced glycation end products (AGEs). The most damaging element of AGE formation may be the synthesis of protein-protein cross-links in long-lived proteins, such as collagen or lens crystallins. A quantitative cross-linking assay, involving the sugar-dependent incorporation of [U-(14)C]lysine into protein, was employed to determine the efficacy of a variety of potential cross-linking inhibitors. Reaction mixtures contained 5.0 mM L-threose, 2.5 microCi [(14)C]lysine (1.0 mCi/mmole), 5.0 mg/ml bovine lens proteins, 0-10 mM inhibitor and 1.0 mM DTPA in 100 mM phosphate buffer, pH 7.0. Of 17 potential inhibitors tested, 11 showed 50% inhibition or less at 10 mM. The dicarbonyl-reactive compounds 2-aminoguanidine, semicarbazide and o-phenylenediamine inhibited 50% at 2.0 mM, whereas 10 mM dimethylguanidine had no effect. Several amino acids failed to compete effectively with [(14)C]lysine in the cross-linking assay; however, cysteine inhibited 50% at 1.0 mM. This was likely due to the sulfhydryl group of cysteine, because 3-mercaptopropionic acid and reduced glutathione exhibited similar activity. Sodium metabisulfite had the highest activity, inhibiting 50% at only 0.1-0.2 mM. Protein dimer formation, as determined by SDS-PAGE, was inhibited in a quantitatively similar manner. The dicarbonyl-reactive inhibitors and the sulfur-containing compounds produced similar inhibition curves for [(14)C]lysine incorporation over a 3 week assay with 250 mM glucose. A much lesser effect was observed on either the incorporation of [(14)C]glucose, or on fluorophore formation (360/420 nm), suggesting that non-cross-link fluorophores were also formed. The inhibitor data were consistent with cross-linking by a dicarbonyl intermediate. This was supported by the fact that the inhibitors were uniformly less effective when the 5.0 mM threose was replaced by either 3.0 mM 3-deoxythreosone or 3.0 mM threosone.

The major in vivo modifications of the human water-insoluble lens crystallins are disulfide bonds, deamidation, methionine oxidation and backbone cleavage

This investigation of the water-insoluble crystallins from human lenses has used multiple chromatographic separations to obtain proteins of sufficient purity for mass spectrometric analysis. Each fraction was analysed to determine the molecular masses of the constituent proteins as well as peptides in tryptic digests of these proteins. The major components of the water-insoluble crystallins were identified as alphaA- and alphaB-crystallins. In addition, gammaS-, betaB1-, gammaD-, betaA3/A1- and betaB2-crystallins were found, in order of decreasing abundance. Although there was evidence of some backbone cleavage, the predominant forms of alphaA-, alphaB, betaB2-, gammaS- and gammaD-crystallins were the intact polypeptide chains. The major modifications distinguishing the water-soluble crystallins were increased disulfide bonding, oxidation of Met, deamidation of Gln and Asn and backbone cleavage. Of the many reactions hypothesized to lead to crystallin insolubility and cataract, these results most strongly support metal-catalysed oxidation, deamidation and truncation as initiators of conformational changes that favor aggregation.

Does vitamin C act as a pro-oxidant under physiological conditions?

Vitamin C readily scavenges reactive oxygen and nitrogen species and may thereby prevent oxidative damage to important biological macromolecules such as DNA, lipids, and proteins. Vitamin C also reduces redox active transition metal ions in the active sites of specific biosynthetic enzymes. The interaction of vitamin C with 'free', catalytically active metal ions could contribute to oxidative damage through the production of hydroxyl and alkoxyl radicals; whether these mechanisms occur in vivo, however, is uncertain. To examine this issue, we reviewed studies that investigated the role of vitamin C, both in the presence and absence of metal ions, in oxidative DNA, lipid, and protein damage. We found compelling evidence for antioxidant protection of lipids by vitamin C in biological fluids, animals, and humans, both with and without iron cosupplementation. Although the data on protein oxidation in humans are sparse and inconclusive, the available data in animals consistently show an antioxidant role of vitamin C. The data on vitamin C and DNA oxidation in vivo are inconsistent and conflicting, but some of the discrepancies can be explained by flaws in experimental design and methodology. These and other important issues discussed here need to be addressed in future studies of the role of vitamin C in oxidative damage.

A possible role for vitamin C in age-related cataract

While many experimental studies have shown a protective effect of vitamin C in age-related cataract, other studies have revealed contrasting roles for this nutrient. Oxidative damage in the lens can be prevented by vitamin C. However, a pro-oxidant effect of vitamin C through H2O2 generation has been suggested. Vitamin C has also been shown to play a role in protein glycation, which is observed in cataract formation. A protective effect of dietary energy restriction appears to be inversely related to plasma vitamin C levels in rodents. Moreover, conclusions from human epidemiological and intervention studies are not uniform. The available evidence suggests that maintenance of sufficient plasma vitamin C is needed to prevent oxidative damage in the lens. More research will be needed in order to confirm the relative importance of of the different roles of vitamin C in the eye lens.

A systematic approach to evaluate the modification of lens proteins by glycation-induced crosslinking

To systematically evaluate the modification of lens proteins by aldose and dicarbonyl sugars during the glycation process, the sugar-dependent incorporation of Lys and Arg, SDS-PAGE profile, amino acid analysis, and fluorophore formation (excitation 370 nm/emission 440 nm) were determined. Reaction mixtures with glycolaldehyde, glyceraldehyde, threose and 3-deoxythreosone showed the greatest extent of Lys crosslinking and fluorescence formation. An increase in fluorescence intensity, but a decrease in Lys and Arg crosslinking, was found with glyoxal, methylglyoxal, hydroxypyruvaldehyde and threosone. In addition glyoxal, methylglyoxal and hydroxypyruvaldehyde caused the specific loss of Arg residues in lens proteins. Reaction mixtures with xylose, xylosone, glucose, glucosone and 3-deoxyglucosone exhibited the least protein modifications; however, incubation with 3-deoxyxylosone resulted in extensive loss of Lys and Arg residues, a higher extent of Lys or Arg crosslinking and significant fluorophore formation. Each sugar exhibited unique characteristics in the modification of lens proteins by glycation. To validly compare the protein modifications occurring during glycation reactions, a systematic approach was employed to evaluate the potential role of aldose and dicarbonyl sugars in protein modification.