Mycobacterium leprae GlbO has been proposed to represent merging of both O-2 uptake/transport and scavenging of nitrogen reactive species. Peroxynitrite reacts with M. leprae GlbO(II)-NO leading to GlbO(III) via the GlbO(III)-NO species. The value of the second order rate constant for GlbO(III)-NO formation is > 1 x 10(8) M-1 s(-1) in the absence and presence of CO2 (1.2 x 10(-3) M). The CO2-independent value of the first order rate constant for GlbO(III)-NO denitrosylation is (2.5 +/- 0.4) x 10(1) s(-1). Furthermore, peroxynitrite reacts with GlbO(II)O-2 leading to GlbO(III) via the GlbO(IV)=O species. Values of the second order rate constant for GlbO(IV)=O formation are (4.8 +/-0.5) x 10(4) and (6.3 +/- 0.7) x 10(1) M-1 s(-1) in the absence and presence of CO2 (=1.2 x 10(-3) M), respectively. The value of the second order rate constant for the peroxynitrite-mediated GlbO(IV)=O reduction (=(1.5 +/- 0.2) x 10(4) M-1 S-1) is CO2-independent. These data argue for a role of GlbO in the defense of M. leprae against nitrosative stress. (c) 2006 Elsevier Inc. All rights reserved.