SPECIFIC ACTIVITY OF BRAIN PALMITOYL-COA POOL PROVIDES RATES OF INCORPORATION OF PALMITATE IN BRAIN PHOSPHOLIPIDS IN AWAKE RATS

In vivo rates of palmitate incorporation into brain phospholipids were measured in awake rats following programmed intravenous infusion of u nesterified [9,10-H-3] palmitate to maintain constant plasma specific activity. Animals were killed after 2-10 min of infusion by microwave irradiation and analyzed for tracer distribution in brain phospholipid and phospholipid precursor, i.e., brain unesterified palmitate and pa lmitoyl-CoA, pools. [9,10-H-3]Palmitate incorporation into brain phosp holipids was linear with time and rapid, with >50% of brain tracer in choline-containing glycerophospholipids at 2 min of infusion. However, tracer specific activity in brain phospholipid precursor pools was lo w and averaged only 1.6-1.8% of plasma unesterified palmitate specific activity. Correction for brain palmitoyl-CoA specific activity increa sed the calculated rate of palmitate incorporation into brain phosphol ipids (0.52 nmol/s/g) by similar to 60-fold. The results suggest that palmitate incorporation and turnover in brain phospholipids are far mo re rapid than generally assumed and that this rapid turnover dilutes t racer specific activity in brain palmitoyl-CoA pool owing to release a nd recycling of unlabeled fatty acid from phospholipid breakdown.