Highly unsaturated phospholipid molecular species of rat erythrocyte membranes: selective incorporation of arachidonic acid into phosphoglycerides containing polyunsaturation in both acyl chains.

This study describes for the first time the complete molecular species composition and turnover of [3H]arachidonic acid in various glycerophospholipid classes of rat erythrocytes, a model system that has been extensively used to investigate numerous membrane phenomena. Quantitative analysis of the individual molecular species of the choline, ethanolamine, serine, and inositol glycerophospholipid classes was possible by preparing their diradylglycerobenzoate derivatives that can be quantitated by on-line uv detection in conjunction with high-performance liquid chromatography; turnover of the molecular species containing arachidonate was evaluated in erythrocytes labeled with [3H]arachidonic acid. A unique observation was the significant amounts of 22:6-20:4, 20:4-20:4, and 18:2-20:4 species observed in the diacyl fractions of phosphatidylethanolamine and phosphatidylserine. Moreover, the analysis of the specific radioactivities of individual phospholipid species from erythrocytes incubated with [3H]arachidonic acid demonstrated a selective incorporation of arachidonic acid into the most highly unsaturated molecular species in all of the phospholipid classes examined. Although the 22:6-20:4, 20:4-20:4, and 18:2-20:4 species represented only 4.5% of the total mass of the diacyl phosphoglycerides, these species accounted for a major portion (37%) of the arachidonic acid incorporated into the phospholipids. These results demonstrate the existence of unique populations of phospholipid molecules in rat erythrocytes with a high degree of unsaturation that exhibit a very rapid metabolic turnover rate.