Paradoxical Impact of Antioxidants on Post-Amadori Glycoxidation

Abstract

The inhibition of post-Amadori advanced glycation end product (AGE) formation by three different classes of AGE inhibitors, carbonyl group traps, chelators, and radical-trapping antioxidants, challenge the current paradigms that: 1) AGE inhibitors will not increase the formation of any AGE product, 2) transition metal ions are required for oxidative formation of AGE, and 3) screening AGE inhibitors only in systems containing transition metal ions represents a valid estimate of potential in vivo mechanisms. This work also introduces a novel multifunctional AGE inhibitor, 6-dimethylaminopyridoxamine (dmaPM), designed to function as a combined carbonyl trap, metal ion chelator, and radicaltrapping antioxidant. Other AGE inhibitors including pyridoxamine, aminoguanidine, o-phenylenediamine, dipyridoxylamine, and diethylenetriaminepentaacetic acid were also examined. The results during uninterrupted and interrupted ribose glycations show: 1) an unexpected increase in the yield of pentosidine in the presence of radical-trapping phenolic antioxidants such as Trolox and dmaPM, 2) significant formation of N ⑀ -carboxymethyllysine (CML) in the presence of strong chelators and phenolic antioxidants, which implies that there must be nonradical routes to CML, 3) prevention of intermolecular cross-links with radical-trapping inhibitors, and 4) that dmaPM shows excellent inhibition of AGE. Glucose glycations reveal the expected inhibition of pentosidine and CML with all compounds tested, but in a buffer free of trace metal ions the yield of CML in the presence of radical-trapping antioxidants was between the metal ion-free and metal ion-containing controls. Protein molecular weight analyses support the conclusion that Amadori decomposition pathways are constrained in the presence of metal ion chelators and radical traps.Nonenzymatic protein glycation by reducing sugars, such as glucose or ribose, is a complicated cascade of condensations, rearrangements, fragmentations, and oxidative modifications that lead to a plethora of compounds collectively called advanced glycation end products (AGEs) 1 (1). AGE products slowly build up and contribute to the age-dependent chemical modification of long-lived tissue proteins. Once formed, AGE products may prevent normal protein function and recognition or stimulate potentially detrimental interactions in signaling pathways (2, 3). Glycation reactions are believed to have a significant role in the progression of diabetic complications largely because of elevated levels of glucose. Indeed, the formation of AGE has been increasingly implicated in pathogenesis of diabetic complications, atherosclerosis, and Alzheimer's disease (4 -6). Inhibitors that act to reduce AGE formation may prove useful for limiting nonenzymatic protein modification associated in the pathology of these and other chronic agerelated diseases. The reaction of amino compounds, such as lysine, with reducing sugars is known as the Maillard reaction (7, 8). The "classical" or Hodge pathway begins with reversible formation of...

Vitamin B₆ Metabolism in Chronic Kidney Disease – Relation to Transsulfuration, Advanced Glycation and Cardiovascular Disease

“…Although we performed only correlation tests, our study suggests a link between the vitamin B 6 status and advanced glycation in humans beyond treatment with PM. The negative correlation found for RBC-PMP with pentosidine in CKD stage 2–4 seems to reflect experimental findings [11,30,31]. The absence of this relation in HD might be related to changes in dialysis treatment.…”

“…PM decreased the formation of both pentosidine and CML [31]. Although PM has already passed phase 2 studies in patients with type 1 and 2 diabetes and nephropathy [32], a distinct relationship of the vitamin B 6 status with AGE concentrations or AGE-related pathologies in humans has not been tested so far.…”

Inhibition of Glycosylation Processes: the Reaction between Pyridoxamine and Glucose

“…This latter group includes compounds capable of trapping radical species (e.g., vitamin C) [4], and metal-chelating agents (e.g., aminoguanidine and pyridoxine). In fact, metal ions have been found to be effective in vivo catalysts for electron transfers in the enaminol intermediates detected during the formation of Amadori compounds, which produce superoxides and a-dicarbonyl compounds that facilitate glycosylation [23].…”

“…However, it should be noted that compound 4 cannot be used at high concentrations for therapeutic purposes, as it can block amino groups in human proteins [25]. Also, compound 5 can chelate species that boost oxidation [23], and has been found to possess an inhibitory capacity 3.6 times lower than that of pyridoxamine (1) [43]. b ) The value for hemoglobin was based on the number of lysine residues, which mainly react with glucose [41].…”

Sequestering Agents of Intermediate Reactive Aldehydes as Inhibitors of Advanced Lipoxidation End‐Products (ALEs)

“…The radical trapping ability of dmaPM was then studied in an in vitro model, based on ABAP (2,2 -azobis(2-amidinopropane) dihydrochloride) as radical inducer and on the fluorescent protein allophycocyanin as substrate: dmaPM was significantly effective, with a potency similar to that of the well-known radical scavenger Trolox, whereas PM had the opposite effect, and thus increased the oxidation rate of the substrate. This new carbonyl-trapping and radical-trapping agent, also owing to its metal-ion chelating properties, showed excellent inhibition of AGE formation in vitro (Culbertson et al, 2003b). Hence dmaPM as a multifunctional agent seems to have great promise, but further studies are required to demonstrate its efficacy in vivo.…”