Aromatic Hydroxylation of Phenylalanine as an Assay for Hydroxyl Radicals

Highly reactive hydroxyl radicals (OH·) attack the amino acid phenylalanine to give ortho-, meta-, and para-tyrosines. An improved HPLC-based separation of these products is described. Both D- and L-phenylalanine are attacked by OH·, although only the latter is a substrate for the enzyme phenylalanine hydroxylase. Use of this improved aromatic hydroxylation assay to detect OH· is illustrated by measuring OH· in ozonized aqueous solutions at pH 7.4, and by showing that blood from some premature babies appears to be capable of catalyzing OH· formation.