Role of hydrogen sulfide in catalyzing the formation of NO-ferroheme

We recently demonstrated a rapid reaction between labile ferric heme and nitric oxide (NO) in the presence of reduced glutathione (GSH) or other small thiols in a process called thiol-catalyzed reductive nitrosylation, yielding a novel signaling molecule, labile nitrosyl ferrous heme (NO-ferroheme), which we and others have shown can regulate vasodilation and platelet homeostasis. Red blood cells (RBCs) contain high concentrations of GSH, and NO can be generated in the RBC via nitrite reduction and/or RBC endothelial nitric oxide synthase (eNOS) so that NO-ferroheme could, in principle, be formed in the RBC. NO-ferroheme may also form in other cells and compartments, including in plasma, where another small and reactive thiol species, hydrogen sulfide (H₂S/HS^-), is also present and may catalyze NO-ferroheme formation akin to GSH. Here, we compare the reactivity of GSH and hydrogen sulfide with hemin in physiologically relevant media, including human serum albumin (HSA) and RBC membranes. Strikingly, hydrogen sulfide demonstrated a second-order rate constant over 10 times higher than GSH. We propose that the increased solubility of H₂S vs GSH in lipophilic environments - where labile heme is most readily found - and the increased steric hindrance of the bulkier GSH account for the faster reaction kinetics observed with hydrogen sulfide. Our findings suggest that the hydrogen sulfide-catalyzed reductive nitrosylation reaction produces thionitrous acid (HSNO), which readily undergoes further reactions with excess hydrogen sulfide to form nitrosopersulfide (SSNO^-) and polysulfides. These results suggest a common theme in thiol-catalyzed reductive nitrosylation of labile ferric heme that could play an important role in NO signaling.