Relationships between base-exchange reaction and the microsomal phospholipid pool in the rat brain in vitro.

The calcium-stimulated incorporation of ethanolamine, choline and L-serine into rat brain microsomal phospholipids has been investigated. The membranes were prelabeled in vitro in their choline or serine phosphoglycerides by base-exchange and then chasing experiments were done by displacing the lipid-bound base by ethanolamine, choline, or L-serine labeled with a different isotope. The results indicate that membrane phosphatidylcholine is presumably a substrate for the exchange with all the three bases, whereas phosphatidylserine exchanges only with ethanolamine and L-serine but not with choline. A small phospholipid pool (3-7% of the total available pool) is active in the calcium-dependent exchange with choline, ethanolamine, and L-serine. When the microsomal membranes are prelabeled in vitro in their phosphatidylcholine moiety through the cytidine-dependent pathway and then chasing experiments are performed with the three nitrogenous bases, as above, the small phospholipid pool is hardly detectable. In view of these and other results (Gaiti et al., FEBS Letters 49:361 1975), it is suggested that at least two different pools of phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine might exist in rat brain microsomes.