Uric acid oxidation by peroxynitrite: Multiple reactions, free radical formation, and amplification of lipid oxidation

Uric acid has been considered to be an efficient scavenger of peroxynitrite but the reaction between urate and peroxynitrite has been only partially c haracterized. Also, previous studies have indicated that urate may increase peroxynitrite-mediated oxidation of low density lipoprotein (LDL). Here, w e examined the reaction between urate and peroxynitrite by combining kineti c, oxygen consumption, spin trapping, and product identification studies; i n parallel, are tested the effect of urate upon peroxynitrite-mediated lipi d oxidation. Our results demonstrated that urate reacts with peroxynitrite with an apparent second order rate constant of 4.8 x 10(2) M-1.s(-1) in a c omplex process, which is accompanied by oxygen consumption and formation of allantoin, alloxan, and urate-derived radicals. The main radical was ident ified as the aminocarbonyl radical by the electrospray mass spectra of its 5,5-dimethyl-l-pyrroline N-oxide adduct. Mechanistic studies suggested that urate reacts with peroxynitrous acid and with the radicals generated from its decomposition to form products that can further react with peroxynitrit e anion. These many reactions may explain the reported efficiency of urate in inhibiting some peroxynitrite-mediated processes. Production of the amin ocarbonyl radical, however, may propagate oxidative reactions. We demonstra ted that this radical is likely to be the species responsible for the effec ts of urate in amplifying peroxynitrite-mediated oxidation of liposomes and LDL, which was monitored by the formation of lipid peroxides and thiobarbi turic acid-reactive substances. The aminocarbonyl radical was not detectabl e during urate attack by other oxidants and consequently it is unlikely to be responsible for all previously described prooxidant effects of uric acid . (C) 1999 Academic Press.