Oxidation of Methionine by Human Polymorphonuclear Leukocytes

Min-Fu Tsan; Jasmine W. Chen

doi:10.1172/JCI109756

Oxidation of Methionine by Human Polymorphonuclear Leukocytes

Min-Fu Tsan and Jasmine W. Chen

Published May 1, 1980 - More info

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Abstract

Studies of the photosensitized oxidation have demonstrated that photodynamic oxidation of methionine is mediated by singlet oxygen (¹O₂). In this study, we demonstrated that phagocytosing human polymorphonuclear leukocytes (PMN), but not resting PMN, oxidized both intracellular and extracellular methionine to methionine sulfoxide. N-ethylmaleimide, which inhibits phagocytosis and cellular metabolism, inhibited the oxidation of methionine. Neutrophils from patients with chronic granulomatous disease did not oxidize methionine even in the presence of phagocytosis. The oxidation of methionine by phagocytosing normal PMN was inhibited by ¹O₂ quenchers, (1.4-diazabicyclo-[2,2,2]-octane, tryptophan, NaN₃), myeloperoxidase (MPO) inhibitors (NaN₃, KCN) and catalase. In contrast, superoxide dismutase, ethanol, and mannitol had no effect. Furthermore, ¹O₂ quenchers did not interfere with the production of superoxide (O₂⁻) by phagocytosing PMN. The combination of catalase and SOD did not enhance the inhibition of methionine by phagocytosing PMN. On the other hand, deuterium oxide stimulated the oxidation of methionine by PMN almost 200%.

H₂O₂ at high concentrations oxidized methionine to methionine sulfoxide. However, when similar amounts of H₂O₂ were added to human PMN, they did not oxidize methionine. In contrast, when H₂O₂, at concentrations too low to oxidize methionine by itself, was added to the granular fraction, but not the soluble fraction, they oxidized methionine to methionine sulfoxide. The oxidation of methionine by the combination of H₂O₂ and granular fractions was inhibited by ¹O₂ quenchers and MPO inhibitors, but it was stimulated by deuterium oxide. Removal of chloride anion also prevented the oxidation of methionine by the granular fractions.

Our results suggest that the oxidation of methionine by phagocytosing PMN is dependent on the MPO-mediated antimicrobial system (MPO-H₂O₂-Cl⁻). They also suggest, but do not prove that the oxidation of methionine is mediated by ¹O₂. Oxidation of methionine may be one of the mechanisms that human PMN damage microorganisms.

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