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

Abstract

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)×10⁶, (5.0±0.1)×10⁶ and (8.5±0.2)×10⁶ M⁻¹s⁻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 CO₃-^·/^·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.

Keywords::

• Bicarbonate
• lactoperoxidase
• peroxynitrite
• stopped-flow spectrophotometry

Declaration of interest: This work was supported by COST Action CM0603 and by the Ministry of Science and Higher Education, Grant DPN/N4/ COST2009.

This paper was first published online on Early Online on 19 November 2009.