In vitro and in vivo inhibition of myeloperoxidase with 5-fluorouracil

OBJECTIVE: Myeloperoxidase (MPO) exists in neutrophils and has an important bactericidal role. During phagocytosis, MPO catalyzes a peroxidative reaction using chloride ion and hydrogen peroxide (H2O2) as substrate. The aim of the present study was to investigate whether 5-fluorouracil (5-FU), a chemotherapeutic agent, has a direct inhibitory effect on MPO and to evaluate some properties of this inhibition. METHODS: The inhibitory effect of 5-FU on MPO was studied in rat tissue, human leukocytes, and leukocytes from cancer patients under 5-FU therapy. MPO was solubilized in a detergent-containing buffer. MPO activity was measured spectrophotometrically through the oxidation of a synthetic substrate tetramethyl benzidine in the presence of H2O2. RESULTS: 5-FU inhibited tissue-associated MPO activity in a dose-dependent but not time-dependent manner with an IC50 value of 0.6 mg/ml. 5-FU also inhibited MPO activity in isolated human leukocytes in a dose-dependent manner, and the IC50 value was 0.75 mg/ml. Using this 5-FU concentration, the inhibitory effect was monitored at different substrate concentrations. Leukocyte MPO activities of patients receiving 5-FU therapy were compared before treatment and after the first, second, and third administration cycles. 5-FU treatment resulted in a significant decrease in leukocyte MPO activity, and repeated 5-FU treatment caused additional decrease. CONCLUSION: Our data showed that 5-FU directly inhibited the MPO activity of human leukocytes in vitro and in vivo. We concluded that, the patients treated with 5-FU should be intensively followed for the risk of infections.