Inhibition of Adenylyl Cyclase Activity in Brain Membrane Fractions by Arachidonic Acid and Related Unsaturated Fatty Acids

Pretreatment of mouse brain membranes with arachidonic acid (AA) and related unsaturated fatty acids at 30°C for 10 min decreased basal activity and isoproterenol/guanosine 5′-O-(3-thiotriphosphate) (GTP γ S)- and forskolin-stimulated activities of adenylyl cyclase to a level less than 5% of control. The presence of the carboxyl group on the fatty acids was essential for the inhibition, because no such inhibition was found with ethyl arachidonate or AA attached to diacylglycerols and phospholipids. The AA-mediated inhibition was observed when the activity was measured in the presence of Mn2+ or forskolin and was insensitive to pertussis toxin or guanosine 5′-O-(2-thiodiphosphate) (GDPβS), indicating a mechanism independent of GTP-binding proteins. In addition, the fact that stimulators of the adenylyl cyclase catalytic unit, ATP, GTP γ S and forskolin, when present during pretreatment, attenuate the inhibitory effect of AA may suggest that the catalytic unit is a target of AA. Bovine serum albumin suppressed the inhibition when present in the mixtures for pretreatment, but could not restore the adenylyl cyclase activity that had been reduced by AA, indicating an irreversible inhibition by AA. The effect of AA was found to be additive to P-site-mediated inhibition. The present study suggests the existence of another mechanism of regulation of adenylyl cyclase by unsaturated fatty acids.