F-induced changes and its reversal by ITP in membrane turbidity and adenylate cyclase activity of chick brain microsomes

The activation of adenylate cyclase by NaF was dependent on the previous incubation time and the concentration of F-. The activation by F- was irreversible and Mg2+ was required for the maximum effect. Turbidity of microsome suspension was also greatly increased by F- plus Mg2+. These effects on adenylate cyclase and membrane turbidity were specific for F- and F- saturation curves for both were similar, though Mg2+-saturation curves for both were dissimilar. The increase in turbidity induced by F- plus Mg2+ was rapidly reversed by ATP, GTP, ITP, UTP and CTP. However, ITP only, among all the triphospho-nucleotides tested, reversed the activity of adenylate cyclase previously activated by NaF plus MgC12. The activity of the enzyme reversed by ITP was not, however, re-enhanced by the presence of NaF in the assay medium. These results suggest the possiblity that F- induces a change in the membrane structure itself, and this change can be reversed by incubation with ITP. Consequently, adenylate cyclase may be conformed either to an activated or an unactivated state.