Dual Regulation of Nitric Oxide by Cobalamin: Spectroscopic and Structural Evidence for Scavenging and Reversible Nitrosyl Formation

Abstract Nitric oxide (NO) is a critical signaling molecule in vascular biology. Still, its interactions with cobalt-containing corrinoids, such as cobalamin (Vitamin B₁₂), are poorly understood compared to its well-known iron-heme chemistry. Using a multi-spectroscopic approach, we demonstrate that cobalamin species dynamically regulate NO bioavailability through two distinct mechanisms. First, NO rapidly and irreversibly oxidizes reduced cobalamin [Cb(II)] to Cb(III) via a one-electron transfer, establishing a potent NO-scavenging pathway shown in Fig. 1(graphical abstract ). Second, the oxidized hydroxocobalamin [Cb(III)] forms a reversible cobalt-nitrosyl adduct ([Cb(III)–NO]³⁺), characterized by a ν(NO) FTIR band at 1700 cm⁻¹ and a labile EPR signal, indicating its role as a transient NO reservoir. In porcine aortic endothelial cells, we observed a dynamic redox cycle in which extracellular Cb(III) is partially reduced to Cb(II), enabling the continuous modulation of NO flux. These findings reveal that cobalamins function as bifunctional redox modulators—both scavenging and storing NO—providing a new mechanistic framework for their therapeutic potential in nitrosative stress and vascular pathologies.