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
Hk. Wasner et al., 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, Acta diabetologica, 33(2), 1996, pp. 126-138
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.