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 phosph atase 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 cann ot be substituted by commercially available inositol phosphates. This novel inositol polyphosphate is a very labile compound. On anion excha nge chromatography it elutes in the range of ATP, which may indicate t he presence of three phosphate groups. It adsorbs on charcoal, which s uggests the presence of a hydrophobic component, possibly a guanosine. Pyrophosphates obtained from inositol 1,4- and inositol 2,4-bisphosph ate are accepted by cyclic PIP synthetase for the synthesis of cyclic PIP. The biosynthesis is characterized by enzyme kinetic parameters li ke dependence on time, enzyme and substrate concentration. The pH opti mum of the enzyme is in the range 7.5-8. The enzyme functions optimall y with prostaglandin E and poorly with prostaglandin A as the substrat e. The presence of fluoride in the assay causes a three- to fourfold i ncrease in cyclic PIP synthesis, which may be correlated with activati on via G proteins. These data support previous reports on the chemical structure and action of cyclic PIP. With respect to the possible isom ers of cyclic PIP, these indicate that it is most likely the C4-hydrox yl group of the inositol which binds the C15-hydroxyl group of prostag landin E. A model of hormone-stimulated synthesis of cyclic PIP is pro posed: phospholipase A2 and phospholipase C, activated by G proteins u pon alpha-adrenergic stimulation, liberate either unsaturated fatty ac ids or inositol phosphates, which are transformed to prostaglandins an d to novel inositol polyphosphate with an energy-rich bond. The cyclic PIP synthetase combines these two substrates to cyclic PIP.