Lack of allosterically controlled intramolecular transfer of nitric oxide from the heme to cysteine in the β subunit of hemoglobin

Abstract

The SNO-Hb hypothesis holds that hemebound nitric oxide (NO) present in the β subunits of T-state hemoglobin (Hb) will be transferred to the β-93 cysteine upon conversion to R-state Hb, thereby forming SNO-Hb. A deficiency in the ability of Hb to facilitate this intramolecular transfer has recently been purported to play a role in pulmonary hypertension and sickle cell disease. We prepared deoxygenated Hb samples with small amounts of heme-bound NO and then oxygenated the samples. Electron paramagnetic resonance (EPR) spectroscopy was used to (1) determine the concentration of iron nitrosyl Hb (Fe-NO Hb), (2) show that the NO is evenly distributed among α and β subunits, and (3) show that the Hb undergoes a change in its quaternary state (T to R) upon oxygenation. We did not observe a decrease in the concentration of Fe-NO Hb on oxygenation, which is inconsistent with the prediction of the SNO-Hb hypothesis. © 2006 by The American Society of Hematology.