Biosynthesis of the Endogenous Cyclic Adenosine Monophosphate (AMP) Antagonist, Prostaglandylinositol Cyclic Phosphate (cyclic PIP), from Prostaglandin E and Activated Inositol Polyphosphate in Rat Liver Plasma Membranes

The endogenous cyclic adenosine monophosphate (AMP) antagonist, cyclic PIP, has been identified as a prostaglandylinositol cyclic phosphate. It inhibits protein kinase A 100% and activates protein serine phosphatase about sevenfold. It is biosynthesized by an enzyme of the plasma membrane when the assay mixture contains adenosine triphosphate (ATP), Mg2+, prostaglandin E and a novel inositol polyphosphate, which cannot be substituted by commercially available inositol phosphates. This novel inositol polyphosphate is a very labile compound. On anion exchange chromatography it elutes in the range of ATP, which may indicate the presence of three phosphate groups. It adsorbs on charcoal, which suggests the presence of a hydrophobic component, possibly a guanosine. Pyrophosphates obtained from inositol 1,4- and inositol 2,4-bisphosphate are accepted by cyclic PIP synthetase for the synthesis of cyclic PIP. The biosynthesis is characterized by enzyme kinetic parameters like dependence on time, enzyme and substrate concentration. The pH optimum of the enzyme is in the range 7.5-8. The enzyme functions optimally with prostaglandin E and poorly with prostaglandin A as the substrate. The presence of fluoride in the assay causes a three- to fourfold increase in cyclic PIP synthesis, which may be correlated with activation via G proteins. These data support previous reports on the chemical structure and action of cyclic PIP. With respect to the possible isomers of cyclic PIP, these indicate that it is most likely the C4-hydroxyl group of the inositol which binds the C15-hydroxyl group of prostaglandin E. A model of hormone-stimulated synthesis of cyclic PIP is proposed: phospholipase A2 and phospholipase C, activated by G proteins upon alpha-adrenergic stimulation, liberate either unsaturated fatty acids or inositol phosphates, which are transformed to prostaglandins and to novel inositol polyphosphate with an energy-rich bond. The cyclic PIP synthetase combines these two substrates to cyclic PIP.