Nitric oxide (NO) is an endogenously produced signaling molecule that regulates blood flow and platelet activation. However, intracellular and intravascular diffusion of NO are severely limited by scavenging reactions with hemoglobin, myoglobin, and other hemoproteins, raising unanswered questions as to how free NO can signal in hemoprotein-rich environments. We explored the hypothesis that NO could be stabilized as a ferrous heme-nitrosyl complex (Fe²⁺-NO, NO-ferroheme. Unexpectedly, we observed a rapid reaction of NO with labile ferric heme (Fe³⁺) and a reduced thiol to yield NO-ferroheme and a thiyl radical. This thiol-catalyzed reductive nitrosylation reaction occurs readily when hemin is solubilized in lipophilic environments, such as red blood cell membranes or bound to serum albumin. The resulting NO-ferroheme is stable, even in the presence of oxyhemoglobin, and potently inhibits platelet activation. NO-ferroheme albumin administered intravenously to mice results in dose-dependent vasodilation at low- to mid-nanomolar concentrations. In conclusion, we report the fastest rate of reductive nitrosylation observed to date to generate a NO-ferroheme molecule that resists oxidative inactivation, is soluble in cell membranes, and is transported intravascularly by albumin to promote potent vasodilation.  

Methods are described in the paper and preprint.

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