Nitric oxide transport on sickle cell hemoglobin: Where does it bind?

We have recently reported that nitric oxide inhalation in individuals with sickle cell anemia increases the level of NO bound to hemoglobin, with the development of an arterial-venous gradient, suggesting delivery to the tiss ues. A recent model suggests that nitric oxide, in addition to its well-kno wn reaction with heme groups, reacts with the beta -globin chain cysteine 9 3 to form S-nitrosohemoglobin (SNO-Hb) and that SNO-Hb would preferentially release nitric oxide in the tissues and thus modulate blood flow. However, we have also recently determined that the primary NO hemoglobin adduct for med during NO breathing in normal (hemoglobin A) individuals is nitrosyl (h eme)hemoglobin (HbFe(II)NO), with only a small amount of SNO-Hb formation. To determine whether the NO is transported as HbFe(II)NO or SNO-Hb in sickl e cell individuals, which would have very different effects on sickle hemog lobin polymerization, we measured these two hemoglobin species in three sic kle cell volunteers before and during a dose escalation of inhaled NO (40, 60, and 80 ppm). Similar to our previous observations in normal individuals , the predominant species formed was HbFe(II)NO, with a significant arteria l-venous gradient. Minimal SNO-Hb was formed during NO breathing, a finding inconsistent with significant transport of NO using this pathway, but sugg esting that this pathway exists. These results suggest that NO binding to h eme groups is physiologically a rapidly reversible process, supporting a re vised model of hemoglobin delivery of NO in the peripheral circulation and consistent with the possibility that NO delivery by hemoglobin may be thera peutically useful in sickle cell disease.