Release of arachidonic acid from 1-alkyl-2-acyl-sn-glycero-3-phosphocholine, a precursor of platelet-activating factor, in rat alveolar macrophages

Platelet activating factor and the bioactive metabolites of arachidonic acid are secreted by alveolar macrophages in response to stimulation by phagocytic agents or calcium ionophore. We have previously shown a deacylation-acetylation sequence in the formation of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) from alkylacyl-(long chain)-GPC (Albert, D.H. and Snyder, F. (1983) J. Biol. Chem. 258, 97-102). This sequence may be an important source of 20:4 during inflammatory reactions since, in alveolar macrophages, the ether lipid precursor of PAF represents 35% of the choline glycerophospholipids and has a much higher content (35%) of 20:4 in the sn-2 position than does diacyl-GPC (17%). Alveolar macrophages prelabeled with 14C-labeled fatty acids (16:0, 18:1, 18:2 and 20:4) and 1-3H]alkyllyso-GPC were used to study the release of fatty acids from ether-linked and diacyl phospholipids. Each of these fatty acids was incorporated primarily into the choline glycerophospholipids of alveolar macrophages. The release of 20:4 from macrophage phospholipids was increased by treatment of the labeled cells with the calcium ionophore A23187 (2 microM) or zymosan (1 mg/ml), whereas the release of 16:0, 18:1 and 18:2 was not increased above control levels by either stimuli. Although more of the labeled 20:4 is released from the diacyl-GPC (50% of the total released), substantial amounts (44%) of 20:4 are derived from alkylacyl-GPC after incubating the stimulated cells for 60 min. The loss of 20:4 continued from the diacyl species throughout the incubation period studied, whereas a slower net release of 20:4 lost from the alkylacyl-GPC fraction was evident after 2 h. We conclude that the deacylation-reacylation cycle is an important aspect of the metabolism of 20:4 and alkylacyl-GPC during inflammatory stimulation of alveolar macrophages and that the deacylation of this ether-linked phospholipid (which is the first step in the formation of PAF) is responsible for a significant amount of the 20:4 released.