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Hydroxylation of phenylalanine by the hypoxanthine-xanthine oxidase system.

When phenylalanine was treated with hypoxanthine and xanthine oxidase in citrate buffer (pH 5.5), p-tyrosine, m-tyrosine and o-tyrosine were identified as hydroxylated products. Addition of superoxide dismutase or catalase to this system prevented the hydroxylation, implying that superoxide radical and hydrogen peroxide were essential intermediates in the hydroxylation reaction. Chemical scavengers of hydroxyl radical (·OH) prevented the hydroxylation. In particular, 50 mM potassium iodide completely prevented the hydroxylation. The addition of lactoferrin or Fe3+ accelerated the hydroxylation of phenylalanine. These results indicate that the hydroxylation of phenylalanine is caused by ·OH formed in a hypoxanthine-xanthine oxidase system.