Mahtab U. Ahmed

Bio: Mahtab U. Ahmed is an academic researcher from University of South Carolina. The author has contributed to research in topics: Glycation & Amadori rearrangement. The author has an hindex of 8, co-authored 9 publications receiving 2377 citations. Previous affiliations of Mahtab U. Ahmed include Sewanee: The University of the South.

N-epsilon-(carboxyethyl)lysine, a product of the chemical modification of proteins by methylglyoxal, increases with age in human lens proteins.

Abstract: Advanced glycation end-products and glycoxidation products, such as Nepsilon-(carboxymethyl)lysine (CML) and pentosidine, accumulate in long-lived tissue proteins with age and are implicated in the aging of tissue proteins and in the development of pathology in diabetes, atherosclerosis and other diseases. In this paper we describe a new advanced glycation end-product, Nepsilon-(carboxyethyl)lysine (CEL), which is formed during the reaction of methylglyoxal with lysine residues in model compounds and in the proteins RNase and collagen. CEL was also detected in human lens proteins at a concentration similar to that of CML, and increased with age in parallel with the concentration of CML. Although CEL was formed in highest yields during the reaction of methylglyoxal and triose phosphates with lysine and protein, it was also formed in reactions of pentoses, ascorbate and other sugars with lysine and RNase. We propose that levels of CML and CEL and their ratio to one another in tissue proteins and in urine will provide an index of glyoxal and methylglyoxal concentrations in tissues, alterations in glutathione homoeostasis and dicarbonyl metabolism in disease, and sources of advanced glycation end-products in tissue proteins in aging and disease.

Quantification of malondialdehyde and 4-hydroxynonenal adducts to lysine residues in native and oxidized human low-density lipoprotein

Abstract: Malondialdehyde (MDA) and 4-hydroxynonenal (HNE) are major end-products of oxidation of polyunsaturated fatty acids, and are frequently measured as indicators of lipid peroxidation and oxidative stress in vivo. MDA forms Schiff-base adducts with lysine residues and cross-links proteins in vitro; HNE also reacts with lysines, primarily via a Michael addition reaction. We have developed methods using NaBH4 reduction to stabilize these adducts to conditions used for acid hydrolysis of protein, and have prepared reduced forms of lysine-MDA [3-(N epsilon-lysino)propan-1-ol (LM)], the lysine-MDA-lysine iminopropene cross-link [1,3-di(N epsilon-lysino)propane (LML)] and lysine-HNE [3-(N epsilon-lysino)-4-hydroxynonan-l-ol (LHNE)]. Gas chromatography/MS assays have been developed for quantification of the reduced compounds in protein. RNase incubated with MDA or HNE was used as a model for quantification of the adducts by gas chromatography/MS. There was excellent agreement between measurement of MDA bound to RNase as LM and LML, and as thiobarbituric acid-MDA adducts measured by HPLC; these adducts accounted for 70-80% of total lysine loss during the reaction with MDA. LM and LML (0.002-0.12 mmol/ mol of lysine) were also found in freshly isolated low-density lipoprotein (LDL) from healthy subjects. LHNE was measured in RNase treated with HNE, but was not detectable in native LDL. LM, LML and LHNE increased in concert with the formation of conjugated dienes during the copper-catalysed oxidation of LDL, but accounted for modification of < 1% of lysine residues in oxidized LDL. These results are the first report of direct chemical measurement of MDA and HNE adducts to lysine residues in LDL. LM, LML and LHNE should be useful as biomarkers of lipid peroxidative modification of protein and of oxidative stress in vitro and in vivo.

Lipoxidation products as biomarkers of oxidative damage to proteins during lipid peroxidation reactions

Abstract: Oxidative stress is implicated in the pathogenesis of numerous disease processes including diabetes mellitus, atherosclerosis, ischaemia reperfusion injury and rheumatoid arthritis. Chemical modification of amino acids in protein during lipid peroxidation results in the formation of lipoxidation products which may serve as indicators of oxidative stress in vivo. The focus of the studies described here was initially to identify chemical modifications of protein derived exclusively from lipids in order to assess the role of lipid peroxidative damage in the pathogenesis of disease. Malondialdehye (MDA) and 4-hydroxynonenal (HNE) are well characterized oxidation products of polyunsaturated fatty acids on low-density lipoprotein (LDL) and adducts of these compounds have been detected by immunological means in atherosclerotic plaque. Thus, we first developed gas chromatography-mass spectrometry assays for the Schiff base adduct of MDA to lysine, the lysine-MDA-lysine diimine cross-link and the Michael addition product of HNE to lysine. Using these assays, we showed that the concentrations of all three compounds increased significantly in LDL during metal-catalysed oxidation in vitro. The concentration of the advanced glycation end-product N epsilon-(carboxymethyl)lysine (CML) also increased during LDL oxidation, while that of its putative carbohydrate precursor the Amadori compound N epsilon-(1-deoxyfructose-1-yl)lysine did not change, demonstrating that CML is a marker of both glycoxidation and lipoxidation reactions. These results suggest that MDA and HNE adducts to lysine residues should serve as biomarkers of lipid modification resulting from lipid peroxidation reactions, while CML may serve as a biomarker of general oxidative stress resulting from both carbohydrate and lipid oxidation reactions.

Reaction of Ascorbate with Lysine and Protein Under Autoxidizing Conditions: Formation of N-epsilon(carboxymethyl)lysine by Reaction Between Lysine and Products of Autoxidation of Ascorbate

Abstract: N epsilon-(Carboxymethyl)lysine (CML) has been identified as a product of oxidation of glucose adducts to protein in vitro and has been detected in human tissue proteins and urine [Ahmed, M. U., Thorpe, S. R., & Baynes, J. W. (1986) J. Biol. Chem. 261, 4889-4894; Dunn, J. A., Patrick, J. S., Thorpe, S. R., & Baynes, J. W. (1989) Biochemistry 28, 9464-9468]. In the present study we show that CML is also formed in reactions between ascorbate and lysine residues in model compounds and protein in vitro. The formation of CML from ascorbate and lysine proceeds spontaneously at physiological pH and temperature under air. Kinetic studies indicate that oxidation of ascorbic acid to dehydroascorbate is required. Threose and N epsilon-threuloselysine, the Amadori adduct of threose to lysine, were identified in the ascorbate reaction mixtures, suggesting that CML was formed by oxidative cleavage of N epsilon-threuloselysine. Support for this mechanism was obtained by identifying CML as a product of reaction between threose and lysine and by analysis of the relative rates of formation of threuloselysine and CML in reactions of ascorbate or threose with lysine. The detection of CML as a product of reaction of ascorbate and threose with lysine suggests that other sugars, in addition to glucose, may be sources of CML in proteins in vivo. The proposed mechanism for formation of CML from ascorbate is an example of autoxidative glycosylation of protein and suggests that CML may also be an indicator of autoxidative glycosylation of proteins in vivo.

Role of Oxidative Stress in Development of Complications in Diabetes

Abstract: N epsilon-(carboxymethyl)lysine, N epsilon-(carboxymethyl)hydroxylysine, and the fluorescent cross-link pentosidine are formed by sequential glycation and oxidation reactions between reducing sugars and proteins. These compounds, termed glycoxidation products, accumulate in tissue collagen with age and at an accelerated rate in diabetes. Although glycoxidation products are present in only trace concentrations, even in diabetic collagen, studies on glycation and oxidation of model proteins in vitro suggest that these products are biomarkers of more extensive underlying glycative and oxidative damage to the protein. Possible sources of oxidative stress and damage to proteins in diabetes include free radicals generated by autoxidation reactions of sugars and sugar adducts to protein and by autoxidation of unsaturated lipids in plasma and membrane proteins. The oxidative stress may be amplified by a continuing cycle of metabolic stress, tissue damage, and cell death, leading to increased free radical production and compromised free radical inhibitory and scavenger systems, which further exacerbate the oxidative stress. Structural characterization of the cross-links and other products accumulating in collagen in diabetes is needed to gain a better understanding of the relationship between oxidative stress and the development of complications in diabetes. Such studies may lead to therapeutic approaches for limiting the damage from glycation and oxidation reactions and for complementing existing therapy for treatment of the complications of diabetes.

Advanced glycation end-products: a review.

Abstract: Advanced glycation end-products are a complex and heterogeneous group of compounds that have been implicated in diabetes related complications At present it is not known if they are the cause or the consequence of the complications observed We discuss the chemistry of advanced glycated end-product formation and their patho-biochemistry particularly in relation to the diabetic microvascular complications of retinopathy, neuropathy and nephropathy as well as their role in the accelerated vasculopathy observed in diabetes The concept of carbonyl stress as a cause for advanced glycated end-product toxicity is mentioned We discuss alterations in the concentrations of advanced glycated end-products in the body, particularly in relation to changes occurring with age, diabetes and its complications such as nephropathy Problems relating to current methods of advanced glycated end-product detection and measurement are highlighted including the lack of a universally established method of detection or unit of measurement Agents used for the treatment of advanced glycated end-product accumulation are reviewed, with an emphasis on the results of the recent phase III trials using aminoguanidine and diabetes related complications

Advanced glycation end products: sparking the development of diabetic vascular injury.

Abstract: Advanced glycation end products (AGEs) are proteins or lipids that become glycated after exposure to sugars. AGEs are prevalent in the diabetic vasculature and contribute to the development of athe...