Selective acylation of alkyllysophospholipids by docosahexaenoic acid in Ehrlich ascites cells.

Ehrlich ascites cells were cultured with 1-O-[3H]alkylglycero-3-phosphoethanolamine (1-[3H]alkyl-GPE) or 1-O-[3H]alkylglycero-3-phosphocholine (1-[3H]alkyl-GPC) to reveal the selective retention of polyunsaturated fatty acids at second position of ether-containing phospholipids. Although small percentages of the lysophospholipids were degraded into long-chain alcohol, both alkyllyso-GPE and -GPC were acylated at the rate of approximately 2 nmol/30 min per 10(7) cells. Alkylacylacetylglycerols were prepared from the acylated products by phospholipase C treatment, acetylation and TLC, and fractionated according to the degree of unsaturation by AgNO3-TLC. The distribution of the radioactivity among the subfractions indicated that both alkyllysophospholipids were mainly esterified by docosahexaenoic acid and to a somewhat lesser extent by arachidonic acid. The selectivity for docosahexaenoic acid in the esterification of 1-alkyl-GPE was much stronger than in that of 1-alkyl-GPC. Although acyl-CoA: 1-alkyl-glycerophosphoethanolamine acyltransferase activity of Ehrlich cell microsomes with arachidonoyl-CoA and docosahexaenoyl-CoA as acyl donors was negligible compared with the acyl-CoA:1-alkyl-glycerophosphocholine acyltransferase activity, a significant amount of 1-alkyl-GPE was acylated in the microsomes without exogenously added acyl-CoA. HPLC analysis revealed that docosahexaenoic acid and arachidonic acid were mainly esterified by the microsomal transferase. Acylation of 1-alkyl-GPC with docosahexaenoic acid and arachidonic acid was also observed in the absence of added acyl-CoA, but the activity was lower than that for 1-alkyl-GPE. Although the source of the acyl donor in the acylation has not been determined, the acylation is probably due to the direct transfer of acyl groups between intact phospholipids. The above results provided the first evidence that the lysophospholipid acyltransferase system including the transacylase activity participates in the selective retention of docosahexaenoic acid in intact cells and a cell free system.