Advanced glycation end products (AGEs), formed during Maillard or browning reactions by nonenzymatic glycation and oxidation (glycoxidation) of proteins, have been implicated in the pathogenesis of several diseases, including diabetes and uremia. AGEs, such as pentosidine and carboxymethyllysine, are markedly elevated in both plasma proteins and skin collagen of uremic patients, irrespective of the presence of diabetes. The increased chemical modification of proteins is not limited to AGEs, because increased levels of advanced lipoxidation end products (ALEs), such as malondialdehydelysine, are also detected in plasma proteins in uremia. The accumulation of AGEs and ALEs in uremic plasma proteins is not correlated with increased blood glucose or triglycerides, nor is it determined by a decreased removal of chemically modified proteins by glomerular filtration. It more likely results from increased plasma concentrations of small, reactive carbonyl precursors of AGEs and ALEs, such as glyoxal, methylglyoxal, 3-deoxyglucosone, dehydroascorbate, and malondialdehyde. Thus, uremia may be described as a state of carbonyl overload or "carbonyl stress" resulting from either increased oxidation of carbohydrates and lipids (oxidative stress) or inadequate detoxification or inactivation of reactive carbonyl compounds derived from both carbohydrates and lipids by oxidative and nonoxidative chemistry. Carbonyl stress in uremia may contribute to the long-term complications associated with chronic renal failure and dialysis, such as dialysis-related amyloidosis and accelerated atherosclerosis. The increased levels of AGEs and ALEs in uremic blood and tissue proteins suggest a broad derangement in the nonenzymatic biochemistry of both carbohydrates and lipids.

Recent studies have demonstrated a marked increase in the level of advanced glycation end products (AGEs) in the plasma, skin and amyloid fibrils of hemodialysis (HD) patients. The presence of AGEs in (beta2m) forming amyloid fibrils has been established in a previous immunochemical study relying on a monoclonal anti-AGE antibody. In the present study, Western blot analysis and immunohistochemistry reveal that the epitope recognized by this antibody is N epsilon-(carboxymethyl)lysine (CML) and that CML is one of the AGE structures present in amyloid fibrils. Thus, two AGE structures, CML and pentosidine, are now recognized in dialysis-related amyloidosis. AGE accumulation in uremia is not accounted for by elevated glucose levels. Since CML and pentosidine formation are closely linked to oxidative processes, we tested the hypothesis that a high oxidative stress enhanced AGE formation in HD patients. We focused on ascorbic acid (AA) because AA is easily oxidized under oxidative stress and its oxidized form (oxiAA) is a source of CML and pentosidine. In vitro incubation of beta2m with AA under atmospheric oxygen resulted in: (1) the rapid appearance of characteristic physicochemical properties of AGEs (brown color, fluorescence, polymerization tendency); (2) the transformation of beta2m into AGE-modified beta2m recognized by a specific monoclonal antibody; and (3) the accelerated formation of CML in beta2m and beta2m-peptide, recognized by mass spectrometry. A similar in vitro incubation of human serum albumin disclosed a parallel production of pentosidine measured by high-performance liquid chromatographic assay. In HD patients, the degree of AA oxidation, assessed as the ratio of oxiAA to total ascorbate, was more than twice as high as that of normal subjects (0.87 +/- 0.16 vs. 0.35 +/- 0.11, P < 0.0001), suggesting the presence of an increased oxidative stress. Interestingly, plasma level of oxiAA was correlated with the plasma levels of protein linked (P < 0.01, r2 = 0.25) and free (P < 0.05, r2 = 0.22) pentosidine. Altogether these results demonstrate that AGE, that is, CML and pentosidine, production is accelerated under oxidative stress, even in the absence of glucose. They suggest that, in uremia, CML and pentosidine production is determined both by an increased oxidative stress and the availability of precursors such as oxiAA. Finally, both CML and pentosidine contribute to the AGEs present in dialysis-related amyloid fibrils.

Abstract. The implication of advanced glycation end products (AGE) in the pathogenesis of atherosclerosis and of diabetic and uremic complications has stimulated a search for AGE inhibitors. This study evaluates the AGE inhibitory potential of several well-tolerated hypotensive drugs. Olmesartan, an angiotensin II type 1 receptor (AIIR) antagonist, as well as temocaprilat, an angiotensin-converting enzyme (ACE) inhibitor, unlike nifedipine, a calcium blocker, inhibit in vitro the formation of two AGE, pentosidine and N ⑀ -carboxymethyllysine (CML), during incubation of nonuremic diabetic, nondiabetic uremic, or diabetic uremic plasma or of BSA fortified with arabinose. This effect is shared by all tested AIIR antagonists and ACE inhibitors. On an equimolar basis, they are more efficient than aminoguanidine or pyridoxamine. Unlike the latter two compounds, they do not trap reactive carbonyl precursors for AGE, but impact on the production of reactive carbonyl precursors for AGE by chelating transition metals and inhibiting various oxidative steps, including carbon-centered and hydroxyl radicals, at both the pre-and post-Amadori steps. Their effect is paralleled by a lowered production of reactive carbonyl precursors. Finally, they do not bind pyridoxal, unlike aminoguanidine. Altogether, this study demonstrates for the first time that widely used hypotensive agents, AIIR antagonists and ACE inhibitors, significantly attenuate AGE production. This study provides a new framework for the assessment of families of AGE-lowering compounds according to their mechanisms of action.Advanced glycation and oxidation irreversibly modify proteins over the years and thus contribute to aging phenomena (1). Their local or generalized acceleration is associated with atherosclerosis (2-6) as well as with various diabetic (7-10) and uremic complications (11-13). Inhibition of advanced glycation end products (AGE) formation has thus become a therapeutic goal.Aminoguanidine, the first AGE inhibitor discovered in 1986 (14), and (Ϯ)-2-isopropylidenehydrazono-4-oxo-thiazolidin-5-ylacetanilide (OPB-9195) (15) are both hydrazine-derivatives. They inhibit in vitro the formation of AGE, pentosidine (16), and N ⑀ -carboxymethyllysine (CML) (17) from a variety of individual precursors, such as ribose, glucose, and ascorbate, as well as that of advanced lipoxidation end products (ALE), malondialdehyde-lysine and 4-hydroxynonenal-protein adduct (18), from arachidonate (19). They also inhibit pentosidine generation in diabetic and uremic plasma incubated for 4 wk (20).As expected, both compounds correct several biologic effects that are associated with AGE formation. In murine thymocyte and fibroblasts, they inhibit the phosphorylation of tyrosine residues of a number of intracellular proteins induced by cell surface Schiff base formation (21). Given to diabetic animal models, such as Otsuka-Long-Evans-Tokushima-Fatty (OLETF) or streptozotocin-treated rats, they reduce urinary albumin excretion and improve glomerular morphology (15,22). Oral admi...

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