Strecker-Type Degradation of Phenylalanine Initiated by 4-Oxo-2-alkenals in Comparison to That Initiated by 2,4-Alkadienals, 4,5-Epoxy-2-alkenals, or 4-Hydroxy-2-nonenal

The conversion of phenylalanine to phenylacetaldehyde as a consequence of its reaction with 4-oxo-2-alkenals was studied both to characterize the reaction pathway and to compare the reactivities and kinetic constants of oxoalkenals with those of other lipid oxidation products: 2,4-alkadienals, 4,5-epoxy-2-alkenals, and 4-hydroxy-2-nonenal. Oxoalkenals produced the Strecker aldehyde through imine formation, which was then decarboxylated and hydrolyzed. In the course of the reaction the lipid was converted into an unsaturated hydroxylamine that eventually cycled to 2-alkylpyrrole. The Ea of phenylacetaldehyde formation in the presence of oxoalkenals was 55-64 kJ/mol. This Ea was similar to the Ea determined for the other tertiary lipid oxidation products assayed (58-67 kJ/mol), but higher than the Ea determined for alkadienals (28-38 kJ/mol). However, this difference in Ea only correlated with the amount of phenylacetaldehyde produced at 37 °C. At higher temperatures, 4-oxo-2-nonenal was the lipid-derived carbonyl compound that produced the highest amount of the Strecker aldehyde, therefore pointing to this oxoalkenal as the most efficient Strecker aldehyde forming compound derived from lipids. For this reason, oxoalkenals should be expected to play a significant role in reactions in which Strecker aldehydes are recognized intermediates, as occurs in the formation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP).