Abstract

Effects of K+, ethanol and norepinephrine on the binding kinetics of ouabain to (Na+,K+)-adenosine triphosphatase in beef brain microsomes were examined. K+ reduced the rate and apparent affinity for ouabain binding markedly. Whereas ethanol and norepinephrine themselves inhibited ouabain binding slightly, they stimulated binding in the presence of K+. Norepinephrine enhanced the effect of ethanol. Dissociation of ouabain was biphasic, with fast and slow components corresponding to high and low apparent affinity. About 65% of the enzyme had high affinity, regardless of conditions. Norepinephrine and ethanol had differential effects on the rate of dissociation from high and low affinity enzyme, however. Alpha receptor blockade generally prevented the effects of norepinephrine. These results show that, although norepinephrine and ethanol have a modest effect on the amount of enzyme that can bind ouabain, their main effect on (Na+,K+)-adenosine triphosphatase is to antagonize the binding of K+ to its allosteric site that inhibits ouabain binding. The data support the hypothesis that ouabain binds rapidly to a K+-insensitive form of phosphorylenzyme or to its dephosphorylated analog and dissociates rapidly from E1.