Modulation of guanosine triphosphatase activity of G proteins by arachidonic acid in rat Leydig cell membranes.

Previous results from our group have indicated that arachidonic acid decrease cAMP production through a modification of heterotrimeric G proteins. In the present study, we have characterized the high affinity GTPase activity present in Leydig cell membranes and its regulation by fatty acids. The high-affinity GTPase activity, measured as [gamma32P] GTP hydrolysis rate, was both time and protein concentration dependent. Arachidonic acid elicited a dose-dependent inhibition of enzyme activity with an IC50 = 26.7+/-1.1 microM. The existence of only two double bonds in linoleic acid is reflected by a decrease in its inhibitory activity (IC50 = 34+/-2.3 microM). Saturated fatty acids showed no effect at this level. The kinetic analysis as interpreted by Lineweaver-Burk plots, indicated that 50 microM arachidonic acid had no effect on the apparent affinity for GTP, but resulted in a 40% decreases in the maximal velocity of the reaction. Arachidonic acid modulation of GTPase activity was not attenuated by blocking eicosanoid metabolism with inhibitors of 5'-lipoxygenase, cyclooxygenase, or epoxygenase P-450. The addition of arachidonic acid to pertussis toxin-treated membranes had no effect on the enzyme activity, indicating that arachidonic acid does not modify the GTPase activity present in Galphas protein. However, ADP-ribosylation with cholera toxin followed by arachidonic acid treatment led to a further 40% inhibition when compared with cholera toxin treatment alone. These results allowed us to postulate that arachidonic acid inhibits the GTPase activity of Gi protein family. To further analyze the mechanism of arachidonic acid inhibition of GTPase activity, the effect of arachidonic acid on the [35S]GTPgammaS binding was studied. No effect of this fatty acid on GTP binding was found. Combining our previous results with those found here, we can conclude that arachidonic acid maintains Gi proteins in their active state, which in turn inhibit adenylate cyclase and results in decrease cAMP levels.