Membrane-bound base-exchange reactions in animal tissues.

The calcium-stimulated incorporation of ethanolamine, L-serine and choline into rabbit synaptosomal phospholipids in vitro has been investigated. The synaptosomal membranes were prelabelled in vitro in their choline-, ethanolamine- or serine-phosphoglycerides by base-exchange and then chasing experiments were performed by displacing the lipid-bound base by ethanolamine, choline or L-serine labelled with a different isotope. The results indicate that membrane phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine are substrates for the exchange with all the three mentioned bases. A very small phospholipid pool (0.5-2% of the total available pool) is active in the calcium-dependent exchange between membrane phosphatidylcholine or phosphatidylethanolamine and free bases, whereas the pool of exchanging phosphatidylserine is sensibly larger (2-9%). In another series of experiments the effect of the base-exchange reaction upon the production of cyclic-AMP at the level of rat brain synaptic membranes has been examined. An exchange with ethanolamine produces a significant decrease of the NaF-stimulated production of the cyclic nucleotide, whereas it increases the noradrenaline-induced production. With some exceptions, the exchange with L-serine produces opposite effects. The possible physiological importance of phospholipid pool at the synaptosomal level is discussed.