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‐ 3 H]serine and [1,2‐ 14 C]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 Ca 2+ ion appeared to be absolute. Mg 2+ ion caused a gradual reduction in the existing enzymic activity, only when pre‐incubated with microsomes and labelled bases before adding Ca 2+ ion. The incorporation of serine and ethanolamine was actively inhibited by Hg 2+ , Co 2+ , Cu 2+ and Mn 2+ 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.