Distinctive selectivity for docosatetraenoic acid incorporation by Ehrlich ascites tumor cells

Three polyunsaturated fatty acids were incorporated into the lipids of Ehrlich ascites cells to examine the selectivity of the synthetic enzymes for chain length and the number of double bonds. Arachidonic acid (20:4n-6) and timnodonic acid (20:5n-6) showed approximately similar patterns of incorporation into neutral acylglycerols, choline glycerophospholipids and ethanolamine glycerophospholipids. Further division of the lipids into diacyl and alkylacyl categories established that 20:4n-6 was incorporated at relatively lower amounts in the triacylglycerols and in the alkylacyl and alkenylacyl forms of the glycerophospholipids. The pattern of incorporation of radioactive 20:5n-3 into the phospholipids was similar to that for 20:4n-6, but there was no evidence for a selectively restrained incorporation into the triacylglycerols. The 22-carbon acid (22:4n-6) differed greatly from its shorter homolog (20:4n-6) in being incorporated in much greater amounts into triacylglycerols than into phospholipids. It did not appear to be present at an appreciably higher relative amount in either the alkyl diacylglycerols or in the ether-containing choline glycerophospholipids. Nevertheless, it occurred with exceptionally high specific activities in the alkylacyl and alkenylacyl forms of ethanolamine glycerophospholipids in a manner that suggests marked discrimination by the cytidine-mediated ethanolamine phosphotransferase. This distinctive selectivity provides evidence for special role for a 22-carbon acid in lipid metabolism.