Two kinds of informations about arachidonic acid (AA) metabolism in platelet phospholipids
(PL) have been obtained from the use of purified phospholipases: 1) Beside the determination
of PL sidedness in the plasma membrane, non-lytic degradation by phospholipase A2
+ sphingomyelinase C showed that only 6 % of the total platelet AA is localized in
the outer surface of the plasma membrane. This heterogeneous distribution is actually
a consequence of PL asymmetry, since sphingomyelin and phosphatidylcholine, which
predominate in membrane outer leaflet, contain only traces or relatively lower amounts,
respectively, of AA than the internal lipids. It is further shown that incubating
platelets with free AA specifically labels the large internal pool of AA, whereas
the small external pool is renewed by a direct exchange of phosphatidylcholine with
plasma lipoproteins. This offers a doublelabelling method allowing to explore the
exact role of each AA pool.
2) Platelet aggregation by Clostridium welchii phospholipase C produces the same metabolic changes (accumulation of phosphatidic
and lysophosphatidic acids) as those induced by thrombin. These observations have
led to describe the existence of a cytosolic phosphatidylinositol-specific phospholipase
C and a membrane-bound diglyceride lipase. Both enzymes, coupled to diglyceride− (and
monoglyceride−) kinase(s), could achieve AA release and (lyso) phosphatidic acid accumulation.
Some properties of these enzymes (subcellular localization, calcium and pH dependence,
positional specificity) will be presented.
