Formation of a hydroxyl radical by the myeloperoxidase-NADH-oxygen system

When phenylalanine was incubated with myeloperoxidase (MPO) and NADH in citrate buffer (pH 4.5), o-, m-, and p-tyrosines were identified as hydroxylated products. Tyrosine formation was dependent on the reaction time and MPO concentration. No significant quantities of tyrosines were formed on if MPO was omitted and inactivated MPO was added instead of active MPO. The tyrosine formation by the MPO-NADH system was greatly reduced under anaerobic conditions, and significantly inhibited by hydroxyl radical scavengers. Superoxide dismutase was a potent inhibitor, but catalase was less effective. Even though the superoxide radical (O2-)-producing ability of the MPO-NADH system was about 29% of that of the hypoxanthine-xanthine oxidase system, under the experimental conditions employed, the rate of tyrosine formation from phenylalanine by two systems was found to be a similar. The above results suggest that the formation of a hydroxyl radical (OH.) may occur in the MPO-NADH system under aerobic conditions and a superoxide radical may be involved in the OH. formation, with MPO promoting the OH. formation from O2-.