NO Scavenging through Reductive Nitrosylation of Ferric Mycobacterium tuberculosis and Homo sapiens Nitrobindins

Abstract

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 Mycobacterium tuberculosis and Homo sapiens nitrobindins (Mt-Nb(III) and Hs-Nb(III), respectively). The conversion of Mt-Nb(III) and Hs-Nb(III) to Mt-Nb(II)-NO and Hs-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., ^NOk_on ≥ 1.0 × 10⁶ M^-1 s^-1). The pseudo-first-order rate constant for the reductive nitrosylation of Mt-Nb(III) and Hs-Nb(III) (i.e., k) is not linearly dependent on the NO concentration but tends to level off, with a rate-limiting step (i.e., k_lim) whose values increase linearly with [OH^-]. This indicates that the conversion of Mt-Nb(III) and Hs-Nb(III) to Mt-Nb(II)-NO and Hs-Nb(II)-NO, respectively, is limited by the OH^--based catalysis. From the dependence of k_lim on [OH^-], the values of the second-order rate constant k_OH- for the reductive nitrosylation of Mt-Nb(III)-NO and Hs-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.

Keywords: NO scavenging; heme-protein; nitrobindin; reductive nitrosylation.

Scheme 1

Mechanism of reductive nitrosylation of Mt-Nb(III) and Hs-Nb(III).

Figure 1

Reductive nitrosylation of Mt-Nb(III) at 20.0 °C. (A) Difference optical absorption spectrum of Mt-Nb(III) minus Mt-Nb(II)-NO (static difference optical absorption spectrum; continuous line) and of Mt-Nb(III)-NO minus Mt-Nb(II)-NO (kinetic difference optical absorption spectrum; circles) at pH 7.8. (B) Time course of reductive nitrosylation of Mt-Nb(III) at pH 8.2 (a), 8.7 (b), and 9.2 (c). Values of the rate constant k were calculated according to Equation (1): 7.3 × 10⁻³ s⁻¹ (a), 2.2 × 10⁻² s⁻¹ (b), and 6.9 × 10⁻² s⁻¹ (c). The Mt-Nb(III) concentration was 4.1 × 10⁻⁶ M. The NO concentration was 5.0 × 10⁻⁴ M. (C) Dependence of Logk (s⁻¹) on Log[NO] (M) at pH 7.8 (circles), 8.2 (triangles), 8.7 (diamonds), and 9.2 (squares). The continuous lines were calculated according to Equation (3) with the following parameters: pH 7.8: K = 5.7 (±0.6) × 10⁻⁵ M and k_lim = 2.5 (±0.3) × 10⁻³ s⁻¹; pH 8.2: K = 5.2 (±0.6) × 10⁻⁵ M and k_lim = 8.1 (±0.9) × 10⁻³ s⁻¹; pH 8.7: K = 4.8 (±0.5) × 10⁻⁵ M and k_lim = 2.4 (±0.3) × 10⁻² s⁻¹; pH 9.2: K = 5.0 (±0.6) × 10⁻⁵ M and k_lim = 7.8 (±0.9) × 10⁻² s⁻¹. (D) Dependence of k_lim on the OH⁻ concentration. The continuous line was calculated according to Equation (4) with k_OH− = 4.9 (±0.5) × 10³ M⁻¹ s⁻¹. Where not shown, the error bars were smaller than the symbol.

Figure 2

Reductive nitrosylation of Hs-Nb(III) at 20.0 °C. (A) Difference optical absorption spectrum of Hs-Nb(III) minus Hs-Nb(II)-NO (static difference optical absorption spectrum; continuous line) and of Hs-Nb(III)-NO minus Hs-Nb(II)-NO (kinetic difference optical absorption spectrum; circles) at pH 8.0. (B) Time course of reductive nitrosylation of Hs-Nb(III) at pH 8.0 (a), 8.6 (b), and 9.2 (c). Values of the rate constant k were calculated according to Equation (1): 6.1 × 10⁻³ s⁻¹ (a), 2.3 × 10⁻² s⁻¹ (b), and 8.8 × 10⁻² s⁻¹ (c). The Hs-Nb(III) concentration was 3.5 × 10⁻⁶ M. The NO concentration was 5.0 × 10⁻⁴ M. (C) Dependence of Logk (s⁻¹) on Log[NO] (M) at pH 8.0 (circles), 8.6 (triangles), 8.9 (diamonds), and 9.2 (squares). The continuous lines were calculated according to Equation (3) with the following parameters: pH 8.0: K = 4.4 (±0.5) × 10⁻⁵ M and k_lim = (6.7 (±0.7) × 10⁻³ s⁻¹; pH 8.6: K = 5.6 (±0.7) × 10⁻⁵ M and k_lim = 2.8 (±0.3) ×10⁻² s⁻¹; pH 8.9: K = 5.6 (±0.7) × 10⁻⁵ M and k_lim = 5.3 (±0.6) × 10⁻² s⁻¹; pH 9.2: K = 5.7 (±0.6) × 10⁻⁵ M and k_lim = 1.1 (±0.2) × 10⁻¹ s⁻¹. (D) Dependence of k_lim on the OH⁻ concentration. The continuous line was calculated according to Equation (4) with k_OH− = 6.9 (±0.7) × 10³ M⁻¹ s⁻¹. Where not shown, the error bars were smaller than the symbol.