Amino acids as modulators of lipoxygenase oxidation mechanism. The identification and structural characterization of spin adducts intermediates by electron spin resonance and tandem mass spectrometry

Antioxidant activity is displayed by amino acids, such as tryptophan (Trp), tyrosine (Tyr) and histidine (His) in the spontaneous oxidation of linoleic acid (LA). In addition, when Trp was incubated with soybean lipoxygenase (LOX 1) and LA, a modulating effect was observed. The elucidation of the reaction pathways was achieved through the identification, by electron spin resonance (ESR) spectroscopy, of α-[4-pyridyl 1-oxide]-N-t-butyl nitrone (POBN) adducts with the selected amino acids. The latter, when electrosprayed (ESI) were detected in the gas-phase as radical cations. They were structurally characterized by tandem mass spectrometry (MS/MS) through collision-induced dissociation (CID) of the adducts. The kinetic data obtained from selected model systems suggested a reversible radical scavenging activity of tryptophan on the intermediate dienyl radical formed in the lipids lipoxygenase cascade.