4 - Vacant Heme Catalyzes NO Yield from Nitrosopersulfide

Abstract

When nitrosothiols react with excess hydrogen sulfide, H 2 S, they form several intermediates including nitrosopersulfide. The stability and importance of this species has been debated. While some data suggest nitrosopersulfidecan be a relatively stable source of NO activity, others suggest that the species degrades too quickly. In our hands, the species is relatively stable in isolation. The prevalence of iron-containing proteins throughout the human body adds importance to determining the interaction of ferrous- and ferric-iron containing proteins with nitrosopersulfide. Studying these interactions can also provide information about the potential in vivo stability and spontaneous reactivity of this species. We have used time-resolved electron paramagnetic resonance and UV-Vis absorption spectroscopy to study the reactions of nitrosopersulfide with heme proteins. We find that in the absence of heme proteins, nitrosopersulfide is relatively stable. Iron-nitrosyl hemoglobin is formed when nitrosopersulfide is reacted with unliganded hemoglobin of varying oxidation states, suggesting NO formation from nitrosopersulfide. However, iron-nitrosyl hemoglobin yields from the reaction of ferriheme and nitrosopersulfide is greater than that of ferrous heme and nitrosopersulfide. Little to no nitrosyl hemoglobin or methemoglobin is formed when nitrosopersulfide - is reacted with liganded ferrous heme. Although these data confirm the release of NO, they suggest a vacant heme is necessary to facilitate a direct heme-nitrosopersulfide reaction to form substantial NO. These data also suggest that the ferric iron in methemoglobin potentiates nitrosopersulfidereactivity. These results could potentially impact NO and sulfide bioavailability and reactivity. Supported by NIH grant HL058091