The permeability of the human red blood cell to sulfate ions

Sulfate permeability was measured at Donnan equilibrium as a function of three variables, the sulfate, chloride, and hydrogen ion concentration in the medium. The data were used for a quantitative evaluation of a number of typical predictions of a fixed charge model of the ion permeable regions of the red blood cell membrane. It could be shown that more than 1,000-fold variations of sulfate flux, $J_{SO_4 } $ , could be represented as a function of a single variable SO _4m ²⁻ , the sulfate concentration in the membrane. SO _4m ²⁻ was calculated from the measured values of all three variables by means of a previously published equation (Passow,Progress in Biophysics and Molecular Biology, vol. 19, pt. II, pp. 425–467, 1969). In this equation, two constants can be arbitrarily chosen:Ā, the sum of the charged and uncharged forms of dissociable fixed charges, andK, the dissociation constant of the fixed charges. For the present calculations, the previously obtained valuesĀ=2.5 andK=1·10⁻⁹ were used. The resulting relationship between $J_{SO_4 } $ and SO _4m ²⁻ was found to obey the equation $$J_{SO_4 } = c_I \cdot \frac{{SO_{4m}^{2 - } }}{{c_{II} + SO_{4m}^{2 - } }}e^{aSO_{4m}^{2 - } } $$ wherec _I=1.62·10⁻⁹,c _II=2.3·10⁻²,a=4.94 gave the best fit for data obtained at 27°C. The exponential increase of $J_{SO_4 } $ with SO _4m ²⁻ suggests that there exists a cooperative facilitation of sulfate flux with increasing SO _4m ²⁻ . Measurements of the apparant activation energy of sulfate flux yielded a value of 32.7 Kcal/mole. This value was independent of the pH at which the measurements were made.