Oxidation of Threonylmethionine by Peroxynitrite. Quantification of the One-Electron Transfer Pathway by Comparison to One-Electron Photooxidation

Abstract

Peroxynitrite can modify methionine by one-and two-electron oxidation pathways. Here, we have quantified the extent of one-electron oxidation of threonylmethionine (Thr-Met) by peroxynitrite using a characteristic reaction according to which Thr-Met sulfur radical cations decompose via fragmentation of the Thr side chain, yielding acetaldehyde. The efficiencies, f acet,photo , for the formation of acetaldehyde from Thr-Met sulfur radical cations were obtained by means of one-electron photooxidation using triplet 4-carboxybenzophenone. Exact quantum yields for the formation of Thr-Met sulfur radical cations by triplet 4-carboxybenzophenone were obtained by laser flash photolysis and time-resolved UV spectroscopy. Acetaldehyde yields were measured for the reaction of peroxynitrite with Thr-Met, and division of these acetaldehyde yields by f acet,photo yielded the extents to which peroxynitrite reacted with Thr-Met via the one-electron transfer pathway. There was little one-electron oxidation of Thr-Met by peroxynitrite at pH 7.4, i.e., 1.5%, 1.8%, and 5.3% based on the total chemical conversion of Thr-Met for Thr-Met concentrations of 1 × 10 -3 , 5 × 10 -4 , and 1.75 × 10 -4 M, respectively. In all cases the major reaction product was the twoelectron oxidation product threonylmethionine sulfoxide. However, at pH 6.0, one-electron oxidation of Thr-Met showed a significantly higher efficiency of 14% for [Thr-Met] ) 1.75 × 10 -4 M. Under all experimental conditions the extent of one-electron oxidation increased with decreasing peptide concentration in agreement with a recently established mechanism according to which the one-electron oxidation of Met by peroxynitrite requires a unimolecular transformation of peroxynitrous acid to an excited species which is the ultimate one-electron oxidant.

Radical production from free and peptide‐bound methionine sulfoxide oxidation by peroxynitrite and hydrogen peroxide/iron(II)

“…Peroxynitrite [27,28] and hydrogen peroxide/iron(II)‐EDTA [11,12] are known to oxidize methionine to methionine sulfoxide among other products. The main radical adducts detected from methionine oxidation by these oxidants, however, were different from those produced from methionine sulfoxide oxidation (Fig.…”

Radiation Chemical Studies of Methionine in Aqueous Solution: Understanding the Role of Molecular Oxygen

“…It is gratifying to see that the disappearance of Met equals the sum of the identified products including the aldehyde, viz., for N 2 O/ O 2 (90:10), G(-1) ) 1.10 vs G(7) + G(12) + G(10) ) 1.09 µmol J -1 , and for air, G(-1) ) 0.78 vs G(7) + G(12) + G(10) ) 0.78. Table 1 10), it is about 62% higher. The main difference between N 2 O/O 2 and air or O 2 is that in the former the e aq -are efficiently transformed to HO • radical (eq 2), whereas in the latter two conditions, the e aq -are trapped by oxygen to give O 2 •-(eq 3).…”

Peroxynitrite: an endogenous oxidizing and nitrating agent

“…Peroxynitrite can modify methionine via one-electron and two-electron processes. The one-electron pathway occurs only to a lesser extent under acidic conditions and with low substrate concentration, leading to a methionine sulphide radical cation, which decomposes to methional and ethylene [59]. The major reaction forms the two-electron oxidation product methionine sulphoxide with a rate constant of 280 M − 1 s − 1 at pH 7.4 [60].…”