OXIDATION OF BROMIDE BY THE HUMAN-LEUKOCYTE ENZYMES MYELOPEROXIDASE AND EOSINOPHIL PEROXIDASE - FORMATION OF BROMAMINES

Myeloperoxidase and eosinophil peroxidase catalyzed the oxidation of b romide ion by hydrogen peroxide (H2O2) and produced a brominating agen t that reacted with amine compounds to form bromamines, which are long -lived oxidants containing covalent nitrogen-bromine bonds. Results we re consistent with oxidation of bromide to an equilibrium mixture of h ypobromous acid (HOBr) and hypobromite ion (OBr-). Up to 1 mol of brom amine was produced per mole of H2O2, indicating that bromamine formati on prevented the reduction of HOBr/OBr- by H2O2 and the loss of oxidiz ing and brominating activity. Bromamines differed from HOBr/OBr- in th at bromamines reacted slowly with H2O2, were not reduced by dimethyl s ulfoxide, and had absorption spectra similar to those of chloramines, but shifted 36 nm toward higher wavelengths. Mono- and di-bromo deriva tives (RNHBr and RNHBr(2)) of the beta-amino acid taurine were relativ ely stable with half-lives of 70 and 16 h at pH 7, 37 degrees C. The m ono-bromamine was obtained with a 200-fold excess of amine over the am ount of HOBr/OBr- and the di-bromamine at a 2:1 ratio of HOBr/OBr- to the amine. In the presence of physiologic levels of both bromide (0.1 mM) and chloride (0.1 M), myeloperoxidase and eosinophil peroxidase pr oduced mixtures of bromamines and chloramines containing 6 +/- 4% and 88 +/- 4% bromamine. In contrast, only the mono-chloramine derivative (RNHCl) was formed when a mixture of hypochlorous acid (HOCl) and hypo chlorite ion (OCl-) was added to solutions containing bromide and exce ss amine. The rapid formation of the chloramine prevented the oxidatio n of bromide by HOCl/OCl-, and the chloramine did not react with bromi de within 1 h at 37 degrees C. The results indicate that when enzyme c atalyzed bromide or chloride oxidation took place in the presence of a n amine compound at 10 mM or higher, bromamines were not produced in s econdary reactions such as the oxidation of bromide by HOCl/OCl- and t he exchange of bromide with chlorine atoms of chloramines. Therefore, the amount of bromamine produced by myeloperoxidase or eosinophil pero xidase was equal to the amount of bromide oxidized by the enzyme. Brom ide was preferred over chloride as the substrate for both enzymes.