BASE‐EXCHANGE REACTIONS FOR THE SYNTHESIS OF PHOSPHOLIPIDS IN NERVOUS TISSUE: THE INCORPORATION OF SERINE AND ETHANOLAMINE INTO THE PHOSPHOLIPIDS OF ISOLATED BRAIN MICROSOMES^1,2

Abstract

Abstract— The calcium‐dependent incorporation of l‐[3‐3H]serine and [1,2‐14C]ethanol‐amine into the phospholipid of isolated subcellular fractions from chick brain was studied and the properties of incorporation were examined. The microsomal fraction was found to possess the highest rate of incorporation and was able to convert under optimal conditions about 120 nmol of labelled serine and 220 nmol of ethanolamine/g of fresh brain microsomes/h. The requirement for Ca2+ ion appeared to be absolute. Mg2+ ion caused a gradual reduction in the existing enzymic activity, only when pre‐incubated with microsomes and labelled bases before adding Ca2+ ion. The incorporation of serine and ethanolamine was actively inhibited by Hg2+, Co2+, Cu2+ and Mn2+ ions, and was abolished by ethylenediamine tetra‐acetate treatment. Ethanolamine, but not choline, inositol or carnitine, competitively inhibited serine incorporation, while d‐serine had slight effect. Conversely, l‐serine inhibited competitively the incorporation of ethanolamine. The greater part of the incorporated serine (85 per cent) was localized in microsomal phosphatidylserine, while a small percentage was found in phosphatidylethanolamine. Similarly, 90 per cent of the incorporated ethanolamine was confined to phosphatidylethanolamine and a small percentage was found in the plasmalogen derivative. The mechanism of serine and ethanolamine incorporation was investigated. The results are compared with those published for similar mammalian and non‐mammalian systems.

Effect of Cytidine on Membrane Phospholipid Synthesis in Rat Striatal Slices

“…The failure of exogenous cytidine to increase tissue PS levels is consistent with the fact that PS, unlike PC or PE, is not synthesized by the CDP-choline (or CDPethanolamine) pathway but by the exchange of serine with the head groups of other phospholipids (Porcellati et al ., 1971 ;Holbrook and Wurtman, 1988) .…”

Exchange between free- and phospholipidalcoholic amines in microsomal membranes of Acanthamoeba palestinensis.

“…However, in rat liver, phosphatidylserine as well as lecithin has been reported to exchange bases with free choline, ethanolamine and L-serine; while phosphatidylethanolamine was reported to serve as a substrate for exchange with free ethanolamine as well as L-serine (12,13). Similar reactions have been observed in brain tissue (11). Thus, in addition to supplying a means for incorporation of L-serine into phospholipids, exchange reactions represent a potential mechanism for modulating the most common polar groups of membrane phospholipids.…”

Asymmetric synthesis of ethanolamine phospholipids in chicken brain microsomes, through the cytidine pathway

“…Microsomes were obtained from adult chicken brain as in [6] and incubated with CDP [W]ethanolamine as in [7]. The reaction was stopped by addition of Ca* + (final cont.…”