p-hydroxyphenylacetaldehyde is the major product of L-tyrosine oxidation by activated human phagocytes - A chloride-dependent mechanism for the conversion of free amino acids into reactive aldehydes by myeloperoxidase

Reactive aldehydes generated during lipid peroxidation have been implicated in the pathogenesis of atherosclerosis as well as other inflammatory diseases, A potential catalyst for such reactions is myeloperoxidase, a hemeprotein secreted by activated phagocytes. We now report that activated neutrophils utilize the myeloperoxidase-H2O2-chloride system to convert L-tyrosine to p-hydroxyphenylacetaldehyde. Production of p-hydroxyphenylacetaldehyde was nearly quantitative at physiological concentrations of L-tyrosine and chloride, Aldehyde generation required myeloperoxidase, H2O2, L-tyrosine, and chloride ion; it was inhibited by the H2O2 scavenger catalase and by the heme poisons azide and cyanide. Phorbol ester- and calcium ionophore-stimulated human neutrophils likewise generated p-hydroxyphenylacetaldehyde from L-tyrosine by a pathway inhibited by azide, cyanide, and catalase, Aldehyde production accounted for 75% of H2O2 generated by optimally stimulated neutrophils at plasma concentrations of L-tyrosine and chloride, Collectively, these results indicate that activated phagocytes, under physiological conditions, utilize myeloperoxidase to execute the chloride dependent conversion of L-tyrosine to the lipid-soluble aldehyde, p-hydroxyphenylacetaldehyde, in near quantitative yield, Moreover, like aldehydes derived from lipid peroxidation, amino acid-derived aldehydes may exert potent biological effects in vascular lesions and other sites of inflammation.