Comparison of osmotic fragility, fraction of reduced RBC membrane thiol, and reducing equivalent recycling in normal and SSRBCs after oxidant exposure. (A) NaCl-induced osmotic lysis at baseline and after oxidant exposure (HX/XO) for normal and SSRBCs; half-maximal effective concentration (EC₅₀) increased for SSRBCs alone (n = 3-10). *P < .05, mean ± SEM, is plotted. (B) Free (reduced) membrane thiol for normal and SSRBCs at baseline and after oxidant exposure (HX/XO; n = 6-8). *P < .05 for % loss, normal versus sickle cell. (C) GSH and NADPH E_hc for normal and SSRBCs at baseline and during oxidant exposure (n = 4-6). *P < .05 for normal versus sickle cell (baseline); +P < .05 for normal versus sickle cell (HX/XO exposed comparison); #P < .05 for normal versus sickle cell (HX/XO exposed comparison). At baseline, note that relative to normal RBCs, SS membrane thiol redox poise favors oxidation and GSH and NADPH reducing power is diminished (ie, less negative E_hc). Antioxidant system failure in SSBRCs is evidenced by the rate and degree to which GSH and NADPH reducing power is lost under O₂⁻ loading. Moreover, after oxidant exposure, SSRBCs suffer membrane thiol depletion and lose resilience to osmotic fragility (ie, rightward shift).