Competitive Inhibition and the Sodium Pump

Abstract

IN a recent article¹ Dr J. D. Robinson questions Priestland and Whittam's² conclusions (which extend previous findings by Sachs and Welt³ and Glynn and myself⁴) on the kinetics of the potassium activation of the sodium pump in red cells. Dr Robinson states that competitive inhibition cannot account for the fact that the effector–velocity curve for potassium is hyperbolic in the absence of sodium but sigmoid when sodium is present. This statement would be true if the transport system had to react with only one potassium ion in order to induce ion translocation or ATPase activity. The available experimental evidence suggests that more than one (probably two) potassium ions are involved in each transport cycle⁵. If the occupation of more than one site is necessary for ion transport there is no need a priori to postulate, as Robinson does, indirect interactions between separate sites to account for S-shaped activation curves³. Moreover, if in the absence of sodium the affinity of the transport system is such as to allow one site to be completely saturated with potassium even at low potassium concentrations, in a two site kinetics simple competitive inhibition can account for the change induced by sodium in the shape of the potassium-activation curve⁴.