Distribution of arachidonic acid in choline- and ethanolamine-containing phosphoglycerides in subfractionated human neutrophils

Abstract

Human neutrophils were fractionated on Percoll gradients and the various subcellular fractions were analyzed for phospholipid and fatty acid composition. The results showed that plasma membranes and azurophilic granules were enriched with ethanolamine-(PE) relative to choline-(PC) containing phosphoglycerides. A remarkable degree of uniformity existed throughout the gradient with respect to the subclass composition of the subcellular PC and PE components. In each fraction 50-60% of the PC was diacyl, 40-45% was 1-O-alkyl-2-acyl (ether linked), and 2-5% was 1-O-alk-1'-enyl-2-acyl (plasmalogenic). For PE, 20-25% was diacyl, 7-12% ether linked, and 64-76% plasmalogenic. When neutrophils were incubated for 15 min with [1-14C]arachidonic acid and subfractionated most of the PC-associated label was intracellularly localized. A similar result was observed in PE, however, when the cells were allowed to stand for 2 h in fatty acid-free buffer following the 15 min of labeling and then subfractionated there was a sizable migration of [14C]arachidonate into plasma membrane PE. In all cases the diacyl subclass was labeled most heavily after 15 min but after an additional 2 h of incubation in fatty acid-free buffer there was a direct transfer of label to the ether- and plasmalogenic-linked PC and PE subclasses. It was also found that arachidonoyl-coenzyme A 1-acyl-lysophosphatide acyltransferase activity was inherent in all three major membrane types but was enriched in the endoplasmic reticulum/secondary granule fraction. Arachidonate consistently accounted for roughly 5% of the PC and 17% of the PE fatty chain composition in each subcellular fraction. These findings demonstrate that, despite the uniform arachidonate and PC and PE subclass composition within the various neutrophil subcellular fractions, the bulk of the PC- and PE-associated arachidonate is localized in intracellular membranes.