Chlorination by the Myeloperoxidase-H 2 O 2 -CI - Antimicrobial System at Acid and Neutral pH

Abstract

Summary

The oxidation of Cl^- ion by H₂O₂ yielding Cl⁺ is catalyzed by myelope-roxidase (MPO) prepared from human PMN. Both H₂O₂ and CI^- in this reaction serve as substrates. Affinity (apparent) studies between MPO and CI^- and MPO and H₂O₂ were carried out. Both (apparent) affinities vary according to pH. The higher the pH, the less the affinity for Cl^-. In contrast, the affinity between MPO and H₂O₂ increases with pH. The dissociation constant of the MPO-CI^- enzyme substrate complex is affected by the second substrate, H₂O₂. The effect of H₂O₂ on the MPO-C1^- affinity is competitive. Likewise, the MPO-H₂O₂ affinity is decreased with increasing Cl^- concentration. This influence is also competitive. From these interactions, it may be concluded that the optimal pH for chlori-nation is dependent on the [H₂O₂]/[C1^-] ratio. The relationship of pH, H₂O₂, and CI^- concentrations may be expressed by the following formula: pH = log [CI^-] x560/ [H₂O₂]. Using the formula, it can be shown that optimal chlorination may occur between pH 4.0 and 7.4.

We wish to thank Paul Wallace and Gerry Heatley for technical assistance, Roberta Lemon for preparing the manuscript, and George Daynes for photography. This investigation was supported by Grant CA-1567 awarded by the National Cancer Institute, DHEW, and USPHS Grant HD-1805, NIH.