The mechanism of myeloperoxidase-catalysed oxidation of aminopyrine

Abstract

1. Myeloperoxidase catalysed the H₂O₂-supported oxidation of aminopyrine in the presence of Cl^-, generating the aminopyrine cation radical (AP⁺). The rate of AP⁺ formation was determined by monitoring the absorbance at 565 nm. The pH optimum of the reaction was around 5.0. The rate of AP⁺ formation increased with increasing concentration of aminopyrine. Inhibition by excess H₂O₂ was seen at pH 4.5–5.5.

2. When Cl^- was replaced by Br^-, the rate of AP⁺ formation increased significantly, and inhibition by H₂O₂ became less evident and was observed only at pH 5.5. The rate of chemical oxidation of aminopyrine by HOCI was much slower than that by Br₂.

3. Compared with the chlorination of monochlorodimedone (MCD), the reaction between aminopyrine and HOCl, produced by the enzymic peroxidation of CI^-, is rate limiting in the myeloperoxidase-catalysed oxidation of aminopyrine. The differences in kinetic behaviour between the myeloperoxidase-catalysed chlorination of MCD and oxidation of aminopyrine are explained by the low reactivity of HOCI towards aminopyrine.