Allosteric Interactions in (Na⁺ + K⁺)-ATPase

Abstract

SODIUM and potassium ions are both required to activate the (Na⁺ + K⁺)-dependent–ATPase and each inhibits activation by the other¹. The K⁺ activates the turnover of enzyme hyperbolically at low Na⁺ concentrations, but with increasing concentrations of Na⁺ the activation curve becomes sigmoid². Similarly at low concentrations of K⁺, the Na⁺ activation curve is hyperbolic but at high concentrations of K⁺ it becomes sigmoid¹. These data indicate the presence of at least two Na⁺ and/or K⁺ sites, respectively, but do not indicate the mechanism of interaction between these ions or sites³. Priestland and Whittam², and others, have suggested that direct competition between the cations for the activating sites can account for the sigmoidal activation curves in the presence of high concentrations of the other cation. Alternatively, Robinson⁴ has argued that the sigmoidal activation curves, which are consistent with the heterotropic effects of an allosteric activator⁵, are evidence for allosteric interactions in the (Na⁺ + K⁺)–ATPase. In support of this hypothesis he has presented evidence for a co-operativity in the K⁺ activation of the p-nitrophenyl-phosphatase activity of the (Na⁺ + K⁺)–ATPase4. As Garrahan⁶ has pointed out, however, these data are not conclusive.