Effects of iron nitrosylation on sickle cell hemoglobin solubility

One mechanism by which nitric oxide (NO) has been proposed to benefit patients with sickle cell disease is by reducing intracellular polymerization of sickle hemoglobin (HbS). In this study we have examined the ability of nitric oxide to inhibit polymerization by measuring the solubilizing effect of iron nitrosyl sickle hemoglobin (HbS-NO). Electron paramagnetic resonance spectroscopy was used to confirm that, as found in vivo, the primary type of NO ligation produced in our partially saturated NO samples is pentacoordinate alpha-nitrosyl. Linear dichroism spectroscopy and delay time measurements were used to confirm polymerization. Based on sedimentation studies we found that, although fully ligated (100% tetranitrosyl) HbS is very soluble, the physiologically relevant, partially ligated species do not provide a significant solubilizing effect. The average solubilizing effect of 26% NO saturation was 0.045; much less than the 0.15 calculated for the effect of 26% oxygen saturation. Given the small amounts of NO-ligated hemoglobin achievable through any kind of NO therapy, we conclude that NO therapy does not benefit patients through any direct solubilizing effect.