Catalytic scavenging of peroxynitrite by lactoperoxidase in the absence and presence of bicarbonate.

The kinetics of the reaction of lactoperoxidase with peroxynitrite was studied under neutral and acidic pH. Lactoperoxidase catalyses peroxynitrite decay with the rate constant, k(c), increasing with decreasing pH. The values of k(c) obtained at pH 7.1, 6.1 and 5.1 are (1.9+/-0.1)x10(6), (5.0+/-0.1)x10(6) and (8.5+/-0.2)x10(6) M(-1)s(-1), respectively. This tendency means that peroxynitrous acid is the species involved in the reaction with the catalytic centre of lactoperoxidase. Lactoperoxidase is also able to scavenge peroxynitrite in the presence of bicarbonate with the rate constant identical, within experimental error, to that measured in the absence of bicarbonate. It is thus concluded that CO(3)-(.)/(.)NO(.2) radicals formed in the system do not inactivate LPO. The mechanism of the catalytic scavenging of peroxynitrite by LPO is proposed. The physiological relevance of this reaction is discussed.