Brain elongation of linoleic acid is a negligible source of the arachidonate in brain phospholipids of adult rats

The extent to which the adult brain can derive some of its arachidonic acid (AA) through internalized synthesis from linoleic acid (LA) is uncertain. Thus, we determined for plasma-derived LA in vivo rates for brain incorporation, beta-oxidation, and conversion to AA. Adult male unanesthetized rats, reared on a diet enriched in LA but low in AA, were infused intravenously for 5 min with 1-(14)C]LA. Timed arterial samples were collected until the animals were killed at 5 min and the brain was removed after microwaving. Within plasma lipids, >96% of radioactivity was in the form of unchanged 1-(14)C]LA, but (14)C]AA was insignificant (<0.2%). Eighty-six percent of brain radioactivity at 5 min was present as beta-oxidation products, whereas the remainder was mainly in 'stable' phospholipid or triglyceride as LA or AA (11 and <1%, respectively). Unesterified unlabeled LA rapidly enters brain from plasma, but its incorporation into brain total phospholipid and triglyceride, in the form of synthesized AA, is <1% of the amount that enters the brain. Thus, in rats fed even a diet containing low amounts of AA, the LA that enters brain is largely beta-oxidized, and is not a major source of AA in brain.