Inhibition of human red cell NaK-ATPase by magnesium and potassium

Abstract

The inhibition of NaK-ATPase (EC 3.6.1.3) from human red cells by Mg 2+ is markedly dependent on the relative concentrations of Na + and K +. Inhibition increases with increasing K + and decreases with increasing Na +. The inhibition appears to be a combined effect of Mg 2+ and K + at sites distinct from the sites at which these cations activate the enzyme. The kinetics of activation of the enzyme by Na + with inhibitory levels of Mg 2+ and K + are biphasic, indicating both low and high affinity Na + sites. At noninhibitory levels of Mg 2+ and K + only high affinity Na + sites are seen. The results are inconsistent with any model in which Mg 2+ and K + compete with Na + at a single site. A kinetic model is proposed to explain the mechanism of inhibition by Mg 2+ and K +.