Phospholipid base-exchange reaction

Phosphatidylserine makes up 10% of phospholipids in mammals. This phospholipid is synthesized in a base-exchange reaction of serine with phosphatidylcholine or phosphatidylethanolamine. In the reaction, serine replaces choline or ethanolamine. 

Phospholipids cannot only be remodelled by base exchange reactions (Taki et aL, 1978) but also by reacylation reactions. The latter are extremely important in determining the final molecular species of lipid which is synthesized as well as allowing the participation of specific fatty acids in particular metabolic processes (e.g. release of arachidonate for prostaglandin formation, cf. van den Bosch, 1982). The entire subject of reacylation and molecular species turnover has been well reviewed by one of the major research workers in this field (Hill and Lands, 1970). Some recent discoveries are covered by Bell and Coleman (1980) and by Hawthorne and Ansell (1982). 

The incorporation of [ C]serine into phospholipids was catalyzed by a microsomal preparation from etiolated pea seedlings. This was apparently a base-exchange reaction (Vandor and Richardson, 1968) ... 

From the reported data, base-exchange reactions appear to be limited in nervous tissue to small pools of phospholipid molecules at membrane level, which are available for exchange. These pools normally constitute 5% of the total lipid molecules Ethanolamine exchanges in purified rat brain microsomal membranes with phosphatidyl serine (Ptd-Ser), phosphatidyl ethanolamine (Ptd-Etn) and phosphatidyl choline (Ptd-Cho) L-serine exchanges with Ptd-Etn, Ptd-Ser and Ptd-Cho and choline only with Ptd-Etn and Ptd-Cho. Similar pools have also been observed at the level of synaptosomal membranes from rabbit brain. 

It is apparent from Table 3 that only small pools of total membrane phospholipid participate in the exchange reaction in syn-aptosomes. On using -ethanolamine as a displacing base, the... 

Pinally, the methylation pathway could represent an enzymic system linked to the source of choline in brain. It is known that the choline moiety as such cannot pass into the brain from the blood stream, but rather a lipid-choline moiety, probably lysophosphati-dylcholine, might be involved in the transport of the base. The methylation pathway, however, can be visualized as another means of producing choline in brain, provided Ptd-ethanolamine is transported into the brain. Conversely, due to the noticeable presence in the brain tissue of free serine, a possible pathway for choline production in brain may be represented by an exchange reaction between free serine and endogenous brain phospholipids, 9 20 cessive decarboxylation of phosphatidyl serine to Ptd-ethanolamine in the same tissue, finally followed by the described methylation reactions to produce a definite pool of active Ptd-choline,... 

Vandor and Richardson (1968) found that ethanolamine could be incorporated into lipid when incubated with a microsomal fraction from etiolated pea (P. sativum L.) seedlings. The reaction had a pH optimum of 8.5 and required Ca + for optimal activity. There was no stimulation by ATP, CTP, or glycerol-3-P. This reaction appears to be the exchange of a free base for one bound to the phospholipid not only ethanolamine, but also choline and serine could be incorporated. 

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