The effects of long chain fatty acids on sodium plus potassium ion-stimulated adenosine triphosphatase of rat brain.

Long chain fatty acids inhibit the Mg2+-dependent Na+ + K+-stimulated ATPase of rat brain microsomal membrane preparations without any significant effect on the “basic” Mg*+-stimulated component. The magnitude of inhibition increases with the chain length, reaching a maximum with myristate followed by a decline with higher fatty acids. Myristate (5 X 10e5 M) produces about 45 % inhibition of the Na+ + K+-stimulated ATPase. Unsaturated fatty acids are more inhibitory than their corresponding saturated congeners; e.g. at 5 X low6 M, oleate produces 66% inhibition compared with 15% by stearate. The inhibitory effects are not due to the binding of Mg2f by fatty acids as the effects are not reversed by increasing the Mgz+ concentration in the reaction. The inhibition is freely reversible and does not appear to be time dependent. Kinetic analysis of the action of myristate on the Naf + K+-stimulated ATPase indicates that the inhibition is uncompetitive with respect to ATP and competitive with respect to K+. The primary site of action of myristate appears to be an inhibition of K+ activation of the enzyme. The Na+ activation of the enzyme is also inhibited, but only when the concentration of Na+ is very low; in the presence of higher concentrations of Na+, the action of myristate can be interpreted as a second (in addition to Na+) competitive inhibitor of K+. I(+-stimulated #-nitrophenyl phosphatase is also inhibited in a competitive fashion by myristate. The myristate inhibition of the Na+ + K+-stimulated ATPase is as well elicited in the presence of ouabain or oligomycin. The effects of these mixed inhibitors are of a “cumulative” nature suggesting separate binding sites for each of the inhibitors.