The selective use of stearoyl-polyunsaturated molecular species of phosphatidylcholine and phosphatidylethanolamine for the synthesis of phosphatidylserine.

In rat liver microsomes, [3H]serine was incorporated primarily into the two most abundant molecular species of microsomal phosphatidylserine (PS), 18:0/20:4 and 18:0/22:6, by Ca(2+)-dependent base exchange. The pattern of PS molecular species synthesized was very similar to the species composition of PS and markedly different from the species composition of either microsomal precursor, phosphatidylcholine (PC) or phosphatidylethanolamine (PE). The data indicated that the enrichment of rat liver PS in mainly three fatty acids--stearic, arachidonic, and docosahexaenoic acids, occurred by (1) the preference by PS synthases for the stearoyl-polyunsaturated molecular species, 18:0/20:4 and 18:0/22:6, of PC and PE and (2) a discrimination against the use of the palmitoyl-polyunsaturated species, 16:0/20:4 and 16:0/22:6, and the stearoyl-diunsaturated species, 18:0/18:2. The preferential use of the two species of PC and PE, based on their acyl chain content and not on their relative abundance, demonstrates that an individual molecular species can be selected out of the total pool for a defined function.