Revisiting the nitrite reductase activity of hemoglobin with differential pulse voltammetry.

Nitric oxide (NO) is an omnipresent signalling molecule in all vertebrates. NO modulates blood flow and neural activity. Nitrite anion is one of the most important sources of NO. Nitrite is reduced to NO by various physiological mechanisms including reduction by hemoglobin in vascular system. In this study, nitrite reductase activity (NRA) of hemoglobin is reported using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in a wide potential window from +0.3 V to -1.3 V (vs. Ag/AgCl). To the best of our knowledge, a detailed look into NRA of hemoglobin is proposed here for the first time. Our results indicated two different regimes for reduction of nitrite by hemoglobin in its Fe(II) and Fe(I) states. Both reactions showed a reversible behaviour in the time scale of the experiments. The first reduction displayed a normal redox behaviour, while the latter one had the characteristics of a catalytic electro-reduction/oxidation. The reduction in Fe(II) state was selected as a tool for comparing the NRA of hemoglobin (Hb) and hemoglobin-S (Hb-S) under native-like conditions in a didodecyldimethyl ammonium bromide (DDAB) liquid crystal film. These investigations lay the prospects and guidelines for understanding the direct electrochemistry of hemoglobin utilizing a simplified mediator-free platform.