Na/Mg exchanger activity as a function of intracellular Mg²⁺. Erythrocytes were loaded with variable Mg²⁺ concentrations as described in “Patients, materials, and methods.” (A) Hill plot analysis shows a V_max of 0.767 ± 0.25 mmol/10¹³ cell × h and a K_0.5 of 2.65 ± 0.4 mM for intracellular Mg²⁺ in Hb AA. (B) In Hb SS, the V_max was 3.77 ± 1.33 mmol/10¹³ cells × h and K_m of increase to 2.57 ± 0.56 mM with hyperbolic kinetic pattern. Values are expressed as mean ± SE of 3 different experiments in triplicate determinations.