Metabolic conversion of phosphatidylserine via phosphatidylethanolamine into phosphatidylcholine in rat brain

Exogenous, liposomal [14C]phosphatidylserine, and that synthesized from [14C]serine, were very slowly metabolized in cortex and hippocampus slices of rat brain; phosphatidylethanolamine (PE) formed from phosphatidylserine (PS) was not methylated to phosphatidylcholine (PC) for up to 6 hours of incubation. Among homogenates prepared from 7 separate brain regions, the cerebellum showed the highest, and the striatum and pons the lowest rate of PS synthesis and its further decarboxylation to PE; in all of these regions the stepwise methylation of PE to PC was very low. Isolated microsomal and mitochondrial fractions of whole rat brain, mixed together and incubated with [14C]serine and S-adenosylmethionine, displayed a high level of newly synthesized mitochondrial PE, and a low level of methylated PC in the microsomes. Moreover, PE formed in brain microsomes by the base exchange reaction was converted into PC in an insignificant range. These data show a limited activity for sequential methylation of PE into PC in rat brain, and suggest that it is probably not caused by the slow movement of mitochondrial PE.