Phosphorylation of (Na+ + K+)-ATPase; stimulation and inhibition by substituted and unsubstituted amines

(1) In view of a previously established stimulation of steady-state phosphorylation of (Na+ + K+)-ATPase by imidazole and its inhibition by tris(hydroxymethyl)aminomethane, the effect of (structure, chemical composition and charge of) a number of primary, secondary and tertiary amines (including imidazole derivatives) has been investigated. (2) Primary amines are predominantly inhibitory and diamines are more inhibitory than monoamines. The strongest inhibition is exerted by ethylenediamine (I50 in 50 mM imidazole = 25 microM, vs. 60 mM for n-propylamine). Increasing the distance between the two amino groups from 3.7 to 8.7 A increases the I50 180-fold. The optimal distance of 3-4 A indicates a similar distance between two ligand(presumably Na+)-binding sites on the enzyme. (3) Screening or substitution of the central N-atom decreases inhibition by the nitrogen compound. Triple substitution by propyl or allyl groups leads to maximal activation, amounting to about 90% of the Na+-activation level. Triethyl substitution gives suboptimal activation and tributyl substitution leads to inhibition. Substitution by polar or negatively charged carboxyl groups diminishes or even abolishes inhibition and also diminishes or abolishes activation. (4) Although occasionally positive charge is not required for inhibition, it is prerequisite for activation. Within certain families of compounds (e.g., ethylenediamine and imidazole derivatives) inhibition or activation increases with pKa, hence with positive charge. (5) The above data are interpreted in terms of inhibition, which is competitive to Na+, being governed by Coulomb interaction. Activation, on the other hand, is predominantly determined by lipophilic (van der Waals or pi-pi electron) interactions, excluding water from the phosphorylation site, hence decreasing phosphoenzyme hydrolysis and increasing the phosphoenzyme level. The requirement of charge (though hidden by substitution) implies weak additional electrostatic interaction.