De novo biosynthesis of docosahexaenoyl-phosphatidic acid in bovine retinal microsomes.

Lysophosphatidic acid stimulated several-fold the formation of docosahexaenoyl-phosphatidic acid from 14C-labeled docosahexaenoic acid (22:6 (n-3] in the bovine retina. 1-Palmitoyl- and 1-oleoyl-sn-glycerol 3-phosphate were the preferred acceptors. Most of the activity was localized in the 105 000 X g microsomal fraction. Despite the very high content of 22:6 in the phospholipids of photoreceptor membranes, only about 1% of the microsomal activity was found in discs isolated from rod outer segments. The newly synthesized docosahexaenoyl-phosphatidic acid was further metabolized to diacylglycerols, triacylglycerols, phosphatidylcholine and phosphatidylserine. The de novo synthesis of docosahexaenoyl-phosphatidylcholine was stimulated by 1 mM CDPcholine. Lysophosphatidic acid and lysophosphatidylcholine up to 50 microM do not compete with each other for 22:6 in the formation of their respective diacylated lipids. This suggests that this fatty acid is introduced into phosphatidic acid and phosphatidylcholine via different acylation systems. We conclude that, in addition to the deacylation-acylation cycle, there is also an active pathway for the acylation of 22:6 into glycerolipids during the de novo biosynthesis of phosphatidic acid.