Characteristics of lysophospholipase activity expressed by cytosolic phospholipase A 2

Evidence has accumulated to suggest that a wide variety of mammalian cells and tissues express a cytosolic phospholipase A 2 with arachidonoyl preference (cPLA 2 ). Purified rabbit platelet‐derived cPLA 2 , as well as the human recombinant enzyme originally identified in the monocytic leukemic cell line U937, exhibit significant lysophospholipase activity. Several series of experiments indicated that a single protein mediated both activities. Treatment of the purified enzyme with p ‐bromophena‐cylbromide or an anti‐(rabbit platelet cPLA 2 ) monoclonal antibody, RHY‐5, suppressed the activity of phospholipase A 2 without any appreciable effect on lysophospholipase activity, suggesting that the domain(s) required for phospholipase A 2 activity may be located separately from that for lysophospholipase activity. Lysophospholipase activity was appreciably detected above the critical micellar concentration of the substrate. Lysophosphatidylcholine was also hydrolyzed efficiently when it was incorporated into liposomes made of dialkylphosphatidylcholine. The hydrolysis of lysophospholipid was dependent on the fatty acid bound at the sn 1 position; the relative rates of hydrolysis of 1‐oleoyllysophosphatidylcholine, 1‐palmitoyllysophosphatidylcholine, and 1‐stearoyllysophosphatidylcholine were 23, 8, and 1, respectively. A similar order of reactivity was observed with lysophospholipid incorporated into dialkylphosphatidylcholine liposomes. cPLA 2 may function not only as an arachidonate liberation enzyme but also as an enzyme responsible for degradation of certain molecular species of lysophospholipids formed in membranes.