Kinetic studies of the reaction of heme-thiolate enzyme chloroperoxidase with peroxynitrite.

The kinetics of the reaction of chloroperoxidase with peroxynitrite was studied under neutral and acidic pH by stopped-flow spectrophotometry. Chloroperoxidase catalyzed peroxynitrite decay with the rate constant, k(c,) increasing with decreasing pH. The values of k(c) obtained at pH 5.1, 6.1 and 7.1 were equal to: (1.96+/-0.03)x10(6), (1.63+/-0.04)x10(6) and (0.71+/-0.01)x10(6)M(-1)s(-1), respectively. Chloroperoxidase was converted to compound II by peroxynitrite with pH-dependent rate constants: (12.3+/-0.4)x10(6) and (3.8+/-0.3)x10(6)M(-1)s(-1) at pH 5.1 and 7.1, respectively. After most of peroxynitrite had disappeared, the conversion of compound II into the ferric form of chloroperoxidase was observed. The recovery of the native enzyme was completed within 1s and 5s at pH 5.1 and 7.1, respectively. The possible reaction mechanisms of the catalytic decomposition of peroxynitrite by chloroperoxidase are discussed.