Formation and signaling by an NO-ferroheme complex provides a novel pathway for intravascular and intracellular NO signaling

Nitric oxide (NO) regulates vascular function through the activation of soluble guanylate cyclase in vascular smooth muscle and platelets. NO signaling is mediated by paracrine diffusion from endothelial cells to smooth muscle. Many questions relate to how intracellular, paracrine, and endocrine diffusion is possible in hemoglobin/myoglobin-rich environments. This diffusion is constrained by fast reactions of NO with both oxygenated and deoxygenated ferric hemoproteins and by radical-radical reactions with superoxide and hydrogen peroxide. Extensive data now supports the hypothesis that red blood cells support NO-dependent signaling through nitrite reductase reaction pathways of nitrite with deoxygenated hemoglobin and the red blood cell expressed "endothelial" NOS. Diffusion of NO outside of a red blood cell should be limited by scavenging reactions of NO with hemoglobin and multiple scavenging reactions in endothelium and smooth muscle with reactive oxygen species, however, this signaling is observed in preclinical and clinical studies. We propose a potential solution to this paradox based on a recent discovery of a rapid reaction pathway between NO, oxidized heme, and reduced thiols. The reaction of NO with ferric heme in the presence of reduced thiols rapidly generates an NO-ferroheme complex and a thiol radical, with electron transfer from the thiol to the NO-ferric heme complex. This reaction is first-order in NO and produces an NO-ferroheme complex. The NO-ferroheme is stable in red cell membrane and bound to albumin, and promotes potent vasodilation in vivo. Furthermore, this species is not scavenged by hemoglobin or myoglobin. We propose a novel signaling paradigm whereby NO-ferroheme can form via nitrite reduction or red cell eNOS and can transfer a stable NO-ferroheme between membrane, albumin, and smooth muscle, where the NO-ferroheme can activate soluble guanylate cyclase. This new signaling molecule offers a potential explanation for NO signaling in a heme- and reactive oxygen species-rich cellular environment.