Free Heme and the Polymerization of Sickle Cell Hemoglobin

Abstract

In search of novel control parameters for the polymerization of sickle cell hemoglobin (HbS), the primary pathogenic event of sickle cell anemia, we explore the role of free heme, which may be excessively released in sickle erythrocytes. We show that the concentration of free heme in HbS solutions typically used in the laboratory is 0.02-0.04 mole heme/mole HbS. We show that dialysis of small molecules out of HbS solutions arrests HbS polymerization. The addition of 100-260 μM of free heme to dialyzed HbS solutions leads to rates of nucleation and polymer fiber growth faster by two orders of magnitude than before dialysis. Toward an understanding of the mechanism of nucleation enhancement by heme, we show that free heme at a concentration of 66 μM increases by two orders of magnitude the volume of the metastable clusters of dense HbS liquid, the locations where HbS polymer nuclei form. These results suggest that spikes of the free heme concentration in the erythrocytes of sickle cell anemia patients may be a significant factor in the complexity of the clinical manifestations of sickle cell anemia. The prevention of free heme accumulation in the erythrocyte cytosol may be a novel avenue to sickle cell therapy.

Increased oxidative stress alters nucleosides metabolite levels in sickle cell anemia

“…The intracellular polymerization of HbS during deoxygenation is the primary pathogenetic event in SCD [31]. Polymerization can transform a normal RBC into a dense, inflexible blood cell and cause harmful pathophysical effects, including sickling, vaso-occlusion and ischemia-reperfusion injury [32,33]. These complications have cyclic nature which involve the generation of excessive ROS and promotion of oxidative stress [34].…”

Sustained treatment of sickle cell mice with haptoglobin increases HO‐1 and H‐ferritin expression and decreases iron deposition in the kidney without improvement in kidney function

“…There may be free haem within sickle red cells that is directly released as free haem (Uzunova, et al 2010). The red cell membrane may act as a sponge for free haem (Atamna and Ginsburg 1995), and the presence of haem-laden red cell microparticles can induce kidney vaso-occlusion(Camus, et al 2012).…”

“…Finally, the minimal organ damage improvement may be related to the fact that haem, not Hb, is what causes actual oxidative damage and that not all free haem may be derived from cell-free Hb. There may be free haem within sickle red cells that is directly released as free haem (Uzunova et al, 2010). The red cell membrane may act as a sponge for free haem (Atamna & Ginsburg, 1995), and the presence of haem-laden red cell microparticles can induce kidney vaso-occlusion (Camus et al, 2012).…”

Hemoglobin to Hematocrit Ratio

“…The pathophysiology behind this ratio has not been previously described. It may relate the HbS concentration within the red blood cell (RBC) with certain rheologic features of the blood and circulatory vessels known to affect polymerization and RBC sickling rates [27, 28]. Nonetheless, this MCHC proxy describes the propensity of the abnormal hemoglobin to polymerize, driving the RBC sickling and vaso-occlusion that underlie osteonecrosis in SCD.…”