Product distribution with 4-CH3-phenylalanine as substrate for the aromatic amino acid hydroxylases: 4-HOCH2-phenylalanine (dark gray), 3-CH3-4-HO-phenylalanine (light gray), and 3-HO-4-CH3-phenylalanine (white). Reactions were carried out and the products analyzed by HPLC as described in ref 6. Conditions: 25 mM sodium phosphate buffer, pH 7.0, 30μM ferrous ammonium sulfate, 1.2 mM 4-CH3-phenylalanine, 10 μM enzyme, and 150 μM 6-methyltetrahydropterin at 30 °C.