Acetyl radical production by the methylglyoxal-peroxynitrite system: a possible route for L-lysine acetylation

Methylglyoxal is an α-oxoaldehyde putatively produced in excess from triose phosphates, aminoacetone, and acetone in some disorders, particularly in diabetes. Here, we investigate the nucleophilic addition of ONOO(-), known as a potent oxidant and nucleophile, to methylglyoxal, yielding an acetyl radical intermediate and ultimately formate and acetate ions. The rate of ONOO(-) decay in the presence of methylglyoxal [k(2,app) = (1.0 ± 0.1) × 10(3) M(-1) s(-1); k(2) ≈ 1.0 × 10(5) M(-1) s(-1)] at pH 7.2 and 25 °C was found to be faster than that reported with monocarbonyl substrates (k(2) < 10(3) M(-1) s(-1)), diacetyl (k(2) = 1.0 × 10(4) M(-1) s(-1)), or CO(2) (k(2) = 3-6 × 10(4) M(-1) s(-1)). The pH profile of the methylglyoxal-peroxynitrite reaction describes an ascendant curve with an inflection around pH 7.2, which roughly coincides with the pK(a) values of both ONOOH and H(2)PO(4)(-) ion. Electron paramagnetic resonance spin trapping experiments with 2-methyl-2-nitrosopropane revealed concentration-dependent formation of an adduct that can be attributed to 2-methyl-2-nitrosopropane-CH(3)CO(•) (a(N) = 0.83 mT). Spin trapping with 3,5-dibromo-4-nitrosobenzene sulfonate gave a signal that could be assigned to a methyl radical adduct [a(N) = 1.41 mT; a(H) = 1.35 mT; a(H(m)) = 0.08 mT]. The 2-methyl-2-nitrosopropane-CH(3)CO(•) adduct could also be observed by replacement of ONOO(-) with H(2)O(2), although at much lower yields. Acetyl radicals could be also trapped by added L-lysine as indicated by the presence of (ε)N-acetyl-L-lysine in the spent reaction mixture. This raises the hypothesis that ONOO(-)/H(2)O(2) in the presence of methylglyoxal is endowed with the potential to acetylate proteins in post-translational processes.