Binding of ethylenediamine to phosphatidylserine is inhibitory to Na+/K(+)-ATPase

Covalent linkage of ethylenediamine with the Na+/K(+)-ATPase complex from rabbit kidney outer medulla by the use of the water-soluble carbodiimide, N-ethyl,N'-(3-dimethylaminopropyl)carbodiimide, resulted in a 73% reaction with phosphatidylserine and only 27% with carboxylic groups in the proteic component of the enzyme. Condensation products from the reaction between phosphatidylserine and ethylenediamine, N-(O-phosphatidylseryl)ethylenediamine, N,N'-bis(O-phosphatidylseryl)ethylenediamine and its intermediary product O-phosphatidyl-[N,N'-bis(seryl)]ethylenediamine, were synthesised. Symmetrically substituted ethylenediamine was the most likely condensation product of ethylenediamine with endogenous phosphatidylserine. The synthesised lipids were incorporated in proteoliposomes containing Na+/K(+)-ATPase and only the addition of the phospholipid phosphatidylcholine. The ratio of phospholipid to protein was 52 (w/w). These proteoliposomes were perforated by the addition of 0.5% cholate and both the Na(+)-dependent phosphorylation level and its dependence on Na+, Mg2+ and ATP were measured. Phosphatidylcholine alone increased the half-maximal activation concentration for Na+ ([Na+]0.5) from 0.2 to 1-2 mM, for Mg2+ from 0.1 to 0.8 microM and for ATP from 0.02 to 0.3 microM. The Ki for K+ (in the absence of Na+) was unaffected: 12.8 microM vs. 12.5 microM in the non-reconstituted system. Replacing 10 mol% of phosphatidylcholine by phosphatidylethanolamine: or phosphatidylserine had no significant effect on [Na+]0.5: 1.1 and 0.7 mM, respectively. Replacing 5 mol% phosphatidylcholine by the bis(phosphatidylseryl) substituent of ethylenediamine further increased [Na+]0.5 to 13.7 mM, while half-maximal activation concentrations for Mg2+ and ATP were unaltered. The mono-phosphatidylseryl derivatives of ethylenediamine, each 5 mol%, also increased [Na+]0.5, but to a lesser extent (3.2-3.8 mM). In addition to their competitive effects, the phosphatidylseryl-substituted ethylenediamine compounds exerted a slowly-increasing non-competitive inhibition, not only in phosphorylation, but also in overall ATPase activity, which was reduced, although not abolished, by exogenous protein (bovine serum albumin). A detergent-like action in the usual sense is unlikely since liposomes containing these lipids remained intact. These studies prove that phospholipids are not only required for optimal activity of this transport enzyme, but in excess or in compositions deviating from the normal, may also be inhibitory.