Gl. Squadrito et al., STOPPED-FLOW KINETIC-STUDY OF THE REACTION OF ASCORBIC-ACID WITH PEROXYNITRITE, Archives of biochemistry and biophysics, 322(1), 1995, pp. 53-59
A new mechanism is presented for the oxidation of ascorbate by peroxyn
itrite. Our mechanism involves the reaction of ascorbate both with gro
und-state peroxynitrous acid (HOONO) and with a reactive intermediate
(HOONO); the reactive intermediate is postulated to be formed in the
decay of HOONO to form nitrate. At physiological pH, the ascorbate mon
oanion (AH(-)) is the predominant ascorbate species. The plot of the o
bserved rate constant for peroxynitrite decay (k(obs)) vs AH(-) for th
e reaction of peroxynitrite with AH shows two regions, one linear and
one curved. In the linear region, which involves high AH(-) concentrat
ions, the reaction is dominated by the bimolecular reaction between HO
ONO and AH(-). At lower AH(-) concentrations, this bimolecular reactio
n slows and reactions with both HOONO and HOONO produce the observed
curvature, Analysis of the data leads to the estimation of the ratio o
f rate constants for the reaction of AH(-) with HOONO (k(2)*) and the
decay of HOONO to nitrate (k(N)), giving the value of k(2)/k(N) = 31
58 +/- 505 M(-1); and of the rate constant (k(2)) for the reaction bet
ween AH- and HOONO, k(2) = 236 +/- 14 M(-1) s(-1). Ascorbate displays
higher selectivity for HOONO than does methionine or 2-keto-4-thiomet
hylbutanoic acid, two substrates whose reactivity toward HOONO and HOO
NO has previously been reported. The biological relevance of the reac
tion of ascorbate with peroxynitrite is discussed in terms of the rate
constants and the concentrations of AH(-) typically found in biologic
al systems; ascorbate may react with HOONO, although the reaction wit
h ground-state HOONO probably is too slow to occur in vivo. (C) 1995 A
cademic Press, Inc.