Na/Mg exchanger activity as a function of extracellular Na⁺ in sickle and normal erythrocytes. The Mg²⁺ efflux activation was driven by external Na⁺ and followed a sigmoid pattern. (A) In Hb AA, analysis of this data indicates a V_max of 0.93 ± 0.22 mmol/10¹³ cells × h and K_0.5 of 64.2 ± 4.1 mM. (B) In Hb SS, K_m of 50.9 ± 2.1 mM and V_max of 3.15 ± 1.1 mmol/10¹³ cell × h. The values are mean ± SE of 9 experiments in triplicate determinations.