Kinetics of chlorination of monochlorodimedone by myeloperoxidase

The phagocyte-derived enzyme myeloperoxidase has been recently implicated i n the pathogenesis of atherosclerosis, because it catalyzes the reaction of hydrogen peroxide with chloride ions to give the highly toxic oxidant hypo chlorous acid. The aim of this study was to determine the dependence of thi s reaction on the concentration of hydrogen peroxide and of the enzyme by m eans of the photometric monochlorodimedone assay. The initial rate of hypoc hlorous acid formation increased less than proportionally with increasing m yeloperoxidase concentrations. Variation of the concentration of hydrogen p eroxide had a biphasic effect, with an optimal concentration of hydrogen pe roxide. Above this concentration enzyme destruction is apparently predomina nt. The progress curves of hypochlorous acid formation showed two distinct maxima. it was concluded that hypochlorous acid not only reacts with monoch lorodimedone but also with the amino groups of myeloperoxidase to form inte rmediary chloramines that may further chlorinate monochlorodimedone. This w as supported by the kinetics in the presence of the amino compound glycine, a competitive substrate for chlorination by hypochlorous acid. In the pres ence of high concentrations of glycine the progress curve rises continuousl y, yielding a greatly increased concentration of chlorinating species, eith er hypochlorous acid or chloramines. We concluded that glycine protects mye loperoxidase against hypochlorous acid-induced self-destruction.