Potential Phospholipid Source(s) of Arachidonate Used for the Synthesis of Leukotrienes by the Human Neutrophil

The present study has employed two approaches to address the question of whether there are specific phospholipid sources of arachidonate used for leukotriene biosynthesis in the human neutrophil. Firstly, g.c.-m.s. analysis indicated that arachidonate was lost from all major arachidonate-containing phospholipid subclasses during cell activation with ionophore A23187. On a molar basis, the rank order of breakdown among the three major phospholipids was: 1-alk-1-enyl-2-arachidonoyl-sn-glycero-3-phosphoethanolamine greater than 1-alkyl-2-arachidonoyl-sn-3-phosphocholine greater than 1-acyl-2-arachidonyl-sn-3-phosphoinositol. Leukotrienes released into the supernatant fluid accounted for only 10-35% of the total arachidonate depletion. Phospholipid sources were also identified in labelling experiments where the specific radioactivity of arachidonate in phospholipid subclasses, as well as leukotrienes produced during cell activation, was measured. The specific radioactivity of arachidonate within 1-acyl-linked molecular species of phosphatidylcholine and phosphatidylinositol was initially high relative to the leukotrienes and decreased rapidly with stimulation. By contrast, the specific radioactivity of arachidonate in all three subclasses of phosphatidylethanolamine, 1-acyl, 1-alkyl, and 1-alk-1-enyl, was 3-5-fold below that of the leukotrienes throughout cell activation. Of the six major arachidonate-containing subclasses, only in the case of 1-O-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine did the specific radioactivity correlate well with that of leukotriene B4 and 20-hydroxyleukotriene B4. These data strongly suggest that 1-ether-linked phospholipids are an important source of arachidonate used for leukotriene biosynthesis.