IMPORTANCE OF PHOSPHOLIPASE-A2 IN PROSTAGLANDIN BIOSYNTHESIS

A model was devised to examine the cellular biochemical events which culminate in prostaglandin (PG) biosynthesis and release from tissues. Slices of guinea-pig spleen incubated in buffer containing [1-14C]arachidonic acid incorporate the label into cellular phospholipid and neutral lipid pools. The majority of incorporated radioactivity appeared in the lecithin fraction: smaller amounts were associated with neutral lipids (chiefly diglycerides) or remained unesterified. During incubation there was a small basal release of prostaglandins. When tissues were vibrated mechanically or shocked there was a loss of [1-14C]arachidonic acid from the phospholipid pools, a corresponding rise in the free substrate levels and an increase in the synthesis of 14C-labeled PGestradiol(E2). The synthesis of prostaglandins was blocked by indomethacin, and the loss of arachidonate from the phospholipid fraction of the cells was blocked by the anti-malarial drug mepacrine. During mechanical vibration or immunological challenge the labeled arachidonic acid released as a substrate for prostaglandin biosynthesis originated solely from the phospholipid fraction. Phospholipase is the key enzyme which mobilizes free fatty acids for prostaglandin biosynthesis during these types of cell injury. Spleen slices were also vibrated in the presence of labeled arachidonic acid without prior incorporation. This procedure increased prostaglandin biosynthesis several-fold, indicating that substrate availability is not the only requirement for stimulation of prostaglandin biosynthesis.