ONE-ELECTRON AND 2-ELECTRON OXIDATIONS OF METHIONINE BY PEROXYNITRITE

Peroxynitrite is stable, but its acid, HOONO, either rearranges to for m nitrate or oxidizes nearby biomolecules. We report here the reaction s of HOONO with methionine and the methionine analog 2-keto-4-thiometh ylbutanoic acid (KTBA). These oxidations proceed by two competing mech anisms. The first yields the sulfoxide; the second-order rate constant s, k(2), for this process for methionine and KTBA are 181 +/- 8 and 27 7 +/- 11 M(-1).s(-1), respectively, at pH 7.4 and 25 degrees C. In the second mechanism, methionine or KTBA undergoes a one-electron oxidati on that ultimately gives ethylene. We propose that the one-electron ox idant is an activated form of peroxynitrous acid, HOONO, that is form ed in a steady state mechanism. The ratios of the second-order rate co nstants for the ethylene-producing reaction (k(2)()) and the first-or der rate constant to produce nitric acid (k(N)) for methionine and KTB A, k(2)()/k(N), are 1250 +/- 290 and 6230 +/- 1390 M(-1), respectivel y. Both eerie and peroxydisulfate ions also oxidize KTBA to ethylene, confirming a one-electron transfer mechanism. The yields of neither Me tSO nor ethylene are affected by several hydroxyl radical scavengers, suggesting that a unimolecular homolysis of HOONO to HO. and (NO2)-N-. is not involved in these reactions. HOONO gives hydroxyl radical-lik e products from various substrates but displays more selectivity than does the hydroxyl radical; thus, HOONO is incompletely trapped by typ ical HO. scavengers. However, a mechanism involving dissociation of HO ONO to caged radicals cannot be ruled out at this time.