Phospholipid and fatty acid specificity of endothelial lipase: potential role of the enzyme in the delivery of docosahexaenoic acid (DHA) to tissues.

Docosahexaenoic acid (DHA; 22:6 n-3) is an essential fatty acid required for the normal function of several tissues, especially the brain. Previous studies suggested that lysophosphatidylcholine (lysoPC) is a preferred carrier of DHA to the brain, although the pathways of the formation of DHA-containing lysophospholipids in plasma have not been delineated. We propose that endothelial lipase (EL), a phospholipase A1 that plays an important role in the metabolism of high density lipoproteins, may be responsible for the generation of DHA lysophospholipids in plasma. Here we studied the substrate specificity of EL using deuterium-labeled phospholipids with different polar head groups, as well as DHA-enriched natural phospholipids to test this hypothesis. Glycerol-stabilized phospholipids were treated with recombinant EL, and the products were analyzed by liquid chromatography/electrospray ionization mass spectrometry. EL showed the polar head group specificity in the order of phosphatidylethanolamine>phosphatidylcholine>phosphatidylserine>phosphatidic acid. Within the same phospholipid class, the enzyme showed preference for the species containing DHA at the sn-2 position, and was inactive in the hydrolysis of phospholipids containing an ether linkage. Since EL is known to be secreted by the cells of blood-brain barrier, we suggest that it plays an important role in the delivery of DHA lysophospholipid carriers to the brain.