Peroxynitrite-mediated formation of free radicals in human plasma: EPR detection of ascorbyl, albumin-thiyl and uric acid-derived free radicals

Formation of peroxynitrite by the fast reaction between nitric oxide and superoxide anion may represent a critical control point in cells producing both species, leading to either down-regulation of the physiological effects of superoxide anion and nitric oxide by forming an inert product, nitrate, or to potentiation of their toxic effects by oxidation of nearby molecules by peroxynitrite. (The term peroxynitrite is used to refer to the sum of all possible forms of peroxynitrite anion and peroxynitrous acid unless otherwise specified.) In this report we demonstrate that, in spite of all the antioxidant defences present in human plasma, its interaction with peroxynitrite leads to generation of free radical intermediates such as (i) the ascorbyl radical, detected by direct EPR, (ii) the albumin-thiyl radical, detected by spin-trapping experiments with both N-tert-butyl-α-phenylnitrone and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and (iii) a uric acid-derived free radical, detected as the DMPO radical adduct in plasma whose thiol groups were previously blocked with 5,5-dithiobis-(2-nitrobenzoic acid). The identity of the latter adduct was confirmed by parallel experiments demonstrating that it is not detectable in plasma pretreated with uricase, whereas it is formed in incubations of peroxynitrite with uric acid. Peroxynitrite-mediated oxidations were also followed by oxygen consumption and ascorbate and plasma-thiol depletion. Our results support the view that peroxynitrite-mediated one-electron oxidation of biomolecules may be an important event in its cytotoxic mechanism. In addition, the data have methodological implications by providing support for the use of EPR methodologies for monitoring both free radical reactions and ascorbate concentrations in biological fluids.