THE SELECTIVE USE OF STEAROYL-POLYUNSATURATED MOLECULAR-SPECIES OF PHOSPHATIDYLCHOLINE AND PHOSPHATIDYLETHANOLAMINE FOR THE SYNTHESIS OF PHOSPHATIDYLSERINE

In rat liver microsomes, [H-3]serine was incorporated primarily into t he two most abundant molecular species of microsomal phosphatidylserin e (PS), 18:0/20:4 and 18:0/22:6, by Ca2+-dependent base exchange. The pattern of PS molecular species synthesized was very similar to the sp ecies composition of PS and markedly different from the species compos ition of either microsomal precursor, phosphatidylcholine (PC) or phos phatidylethanolamine (PE). The data indicated that the enrichment of r at liver PS in mainly three fatty acids - stearic, arachidonic, and do cosahexaenoic acids, occurred by (1) the preference by PS synthases fo r the stearoyl-polyunsaturated molecular species, 18:0/20:4 and 18:0/2 2:6, of PC and PE and (2) a discrimination against the use of the palm itoyl-polyunsaturated species, 16:0/20:4 and 16:0/22:6, and the stearo yl-diunsaturated species, 18:0/18:2. The preferential use of the two s pecies of PC and PE, based on their acyl chain content and not on thei r relative abundance, demonstrates that an individual molecular specie s can be selected out of the total pool for a defined function.