Find in Library

Ferric nitrobindins (Nbs) selectively bind NO and catalyze the conversion of peroxynitrite to nitrate. In this study, we show that NO scavenging occurs through the reductive nitrosylation of ferric <i>Mycobacterium tuberculosis</i> and <i>Homo sapiens</i> nitrobindins (<i>Mt</i>-Nb(III) and <i>Hs</i>-Nb(III), respectively). The conversion of <i>Mt</i>-Nb(III) and <i>Hs</i>-Nb(III) to <i>Mt</i>-Nb(II)-NO and <i>Hs</i>-Nb(II)-NO, respectively, is a monophasic process, suggesting that over the explored NO concentration range (between 2.5 × 10⁻⁵ and 1.0 × 10⁻³ M), NO binding is lost in the mixing time (i.e., ^NO<i>k</i>_on ≥ 1.0 × 10⁶ M⁻¹ s⁻¹). The pseudo-first-order rate constant for the reductive nitrosylation of <i>Mt</i>-Nb(III) and <i>Hs</i>-Nb(III) (i.e., <i>k</i>) is not linearly dependent on the NO concentration but tends to level off, with a rate-limiting step (i.e., <i>k</i>_lim) whose values increase linearly with [OH⁻]. This indicates that the conversion of <i>Mt</i>-Nb(III) and <i>Hs</i>-Nb(III) to <i>Mt</i>-Nb(II)-NO and <i>Hs</i>-Nb(II)-NO, respectively, is limited by the OH⁻-based catalysis. From the dependence of <i>k</i>_lim on [OH⁻], the values of the second-order rate constant <i>k</i>_OH− for the reductive nitrosylation of <i>Mt</i>-Nb(III)-NO and <i>Hs</i>-Nb(III)-NO were obtained (4.9 (±0.5) × 10³ M⁻¹ s⁻¹ and 6.9 (±0.8) × 10³ M⁻¹ s⁻¹, respectively). This process leads to the inactivation of two NO molecules: one being converted to HNO₂ and another being tightly bound to the ferrous heme-Fe(II) atom.