Reaction of myeloperoxidase with its product HOCI

The reaction of human myeloperoxidase with its product, hypochlorous acid was investigated using both rapid‐scan spectrophotometry and the stopped‐flow technique. In the reaction of myeloperoxidase with hypochlorous acid a primary compound is found with propersties similar to that of compound I and which is converted into compound II. The primary reaction is strongly pH‐dependent. At pH 7.2 the reaction is too fast to be measured but at higher pH values it is possible to determine the apparent second‐order rate constant. Its value decreases to about 2 × 107 M−1· s−1 at pH 8.3 and to 2.3 (±0.4) × 106 M−1· s−1 at pH 9.2, respectively. The dissociation constant for the formation of the primary compound is 25.7 (± 15.3) μM at pH9.2 and about 2.5μM at pH 8.3. The apparent second‐order rate constant for the formation of compound II is hardly affected by pH and varies between 2 to 5 × 104 M−1· s−1 at pH 10.2 and pH 8.3 respectively.Reaction of myeloperoxidase with hypochlorous acid also resulted in irreversible partial bleaching of the chromophore. Chloride, which is a substrate of the enzyme not only protects myeloperoxidase against bleaching by hypochlorous acid but also competitively i