Modulation of ATPase activities of human erythrocyte membranes by free fatty acids or phospholipase A₂

Summary

The artificial insertion of increasing amounts of unsaturated fatty acids into human erythrocyte membranes modulated ATPase activities in a biphasic manner, depending on the number and position of double bonds, their configuration, and the chain length. Uncharged long-chain fatty acid derivatives with double bonds and short-chain fatty acids were ineffective. Stearic acid stimulated Na⁺K⁺-ATPase only. Anionic and non-ionic detergents and α-lysophosphatidylcholine failed to stimulate ATPase activities at low, and inhibited them at high concentrations.

Mg²⁺-ATPase activity was maximally enhanced by a factor of 2 in the presence of monoenoic fatty acids; half-maximal stimulation was achieved at a molar ratio ofcis(trans)-configurated C18 acids/membrane phopholipid of 0.16 (0.26).

Na⁺K⁺-ATPase activity was maximally augmented by 20% in the presence of monoenoic C18 fatty acids at 37°C. Half-maximal effects were attained at a molar ratio oleic (elaidic) acid/phospholipid of 0.032 (0.075). Concentrations of free fatty acids which inhibited ATPase activities at 37°C were most stimulatory at reduced temperatures. AT 10°C, oleic acid increased Na⁺K⁺-ATPase activity fivefold (molar ratio 0.22).

Unsaturated fatty acids simulated the effect of calmodulin on Ca²⁺-ATPase of native erythrocyte membranes (i.e., increase ofV _max from 1.6 to 5 μmol PO ³⁻₄ ·phospholipid⁻¹·hr⁻¹, decrease of K ^′_Ca from 6 μm to 1.4–1.8 μm). Stearic acid decreasedK ^′_Ca (2 μm) only, probably due to an increase of negative surface charges.

A stimulation of Mg²⁺-ATPase, Na⁺K⁺-ATPase, and Ca²⁺-ATPase could be achieved by incubation of the membranes with phospholipase A₂.

An electrostatic segregation of free fatty acids by ATPases with ensuing alterations of surface charge densities and disordering of the hydrophobic environment of the enzymes provides an explanation of the results.