The relative utilization of eicosapentaenoic and arachidonic acids by the 1-acylgycerophosphorylcholine and the 1-acylglycerophosphorylinositol acyltransferase pathways in human platelet microsomes

The relative entry of eicosapentaenoic acid (EPA) and arachidonic acid (AA) via 1-acylglycerophosphorylcholine and 1-acylglycerophosphorylinositol acyltransferase activity into phosphatidylcholine (PC) and phosphatidylinositol (PI), respectively, was studied in human platelets. A known mixture of [¹⁴C]EPA and [³H]AA along with an acyl-CoA generating system and lyso-acceptor phospholipids were incubated with a platelet mixed membrane preparation. The radioactivities in the products were determined following lipid extraction and thin-layer chromatographic separations. The [³H]AA/[¹⁴C]EPA ratios in the products, PC and PI, indicated some preference of both acyltransferase pathways for EPA over AA at 50 μM exogenously-added lysoPC or lysoPI. Altering the concentration of lysoPC (10–50 μM) did not significantly affect the selectivity for EPA. In contrast, reducing the concentration of lysoPI from 50 μM to 10 μM changed the selectivity of the acyltransferase pathway from favouring EPA to slightly favouring AA. The presence of a mixture of lysoPC and lysoPI resulted in a partitioning of EPA more towards PC and less towards PI which may account, in some part, for the greater AA/EPA mass ratio observed in the PI from human subjects consuming fish oil. The magnitude of the differences between PC and PI in their AA/EPA mass ratios observed in vivo, however, cannot be explained by the selectivity demonstrated herein by the acyltransferase pathways.