Figure 5.

Determination of the K _1/2 for sodium activation of ³²PO₄ influx. Sodium activation of ³²PO₄ flux into erythrocytes (top) and K562 cells (bottom) was determined. In these experiments, the total phosphate concentration was 0.3 mM. Both curves appear sigmoidal, although the data from K562 cells is equally well fit by a single site (Michaelis-Menten) equation. Using the Hill equation, the concentration of sodium required for half maximal activation of phosphate influx was 46 mM for erythrocytes and 34 mM for K562 cells. The extrapolated maximum flux was 277 μmol/[kg hemoglobin (Hgb) · h] in erythrocytes and 3,900 nmol/(g protein · min) in K562 cells. The Hill coefficient, N, was 1.9 for erythrocytes and 1.5 for K562 cells. When the Michaelis-Menten equation was used to fit the data from K562 cells, the V_max was 6,000 nmol/(g protein · min) and K _1/2 = 80 mM.