The Selective Incorporation of l*C-Glycerol into Different Species of Phosphatidic Acid, Phosphatidylethanolamine, and Phosphatidylcholine*

A solubilized preparation from pigeon liver microsomes produced four species of phosphatidate when incubated with a mixture of linoleoyland stearoyl-CoA and sn-glycero-3phosphate. The appearance of distearoyl (15%) and dilinoleoyl (42%) species indicates that either acid may acylate both positions 1 and 2. I-Stearoyl-Z-linoleoylglycero-3-phosphate (21%) was no more abundant than l-linoleoyl-2-stearoylglycero-3-phosphate (22%), indicating that the cell-free system was not selective in esterifying these two acids. Slices of rat liver provided results in marked contrast to those above in that small amounts (1%) of disaturated phosphatidates were produced, and essentially none (3%) of the acids at position 2 was saturated. The major species produced by the slices were predominantly (75%) monoene and diene types formed by the esterification of oleate and linoleate, respectively, at position 2 with saturated acids at position 1. Choline and ethanolamine phosphoglycerides also contained disproportionately greater amounts of newly synthesized, radioactive diene species than would be expected from the endogenous abundance of these types. Endogenous species of diacylglycerophosphorylethanolamine include monoene (2%), diene (19%), tetraene (48%), and hexaene (19%), whereas the isotopic content of the newly synthesized species was approximately 14, 28, 19, and 20%, respectively.