On the synthesis of plasmalogens

Abstract

The chemical synthesis is described of (rac)-trans-1-(n-hexadec-1′-enyloxy)-2-oleoylglycerol-3-phosphorylcholine (plasmalogen). This synthesis made use of a specific degradation of (rac)-trans-1-(n-hexadec-1′-enyloxy)-2,3-dioleoyl glycerol with pancreatic lipase (EC 3.1.1.3). This enzyme, which catalyses the hydrolysis of fatty acid ester bonds attached to the primary hydroxyl groups of glycerol, cannot split vinyl ether linkages, thereby yielding (rac)-trans-1-(n-hexadec-1′-enyloxy)-2-oleoyl glycerol. The latter compound was converted into a plasmalogen by a reaction with 2-bromoethyl-phosphoric acid dichloride and trimethylamine. A partial synthesis of cis-1-(n-alk-1′-enyloxy)-2-oleoyl-glycerol-3-phosphorylcholine was developed by application of this method to cis-1-(n-alk-1′-enyloxy)-2,3-dioleoyl glycerol. The preparation of cis-1-(n-alk-1′-enyloxy)-2,3-dioleoyl glycerol was made by acylation of cis-1-(n-alk-1′-enyloxy)glycerol obtained from ox-heart plasmalogen after degradation with phospholipase C (EC 3.1.4.3) and alkaline hydrolysis. The I.R. spectra of both plasmalogens were completely identical with each other and differed from the spectra of lecithins only by the presence of a vinyl ether absorption at 1660 cm−1. The N.M.R. spectrum of the acetylated synthetic (rac)-1-(n-hexadec-1′-enyloxy)glycerol as well as of the synthetic plasmalogen revealed a trans configuration of the vinyl ether linkage. Degradation with phospholipase A (EC 3.1.1.4) of the synthetic (rac)-plasmalogen gives a 50% conversion into free fatty acid and trans-1-(n-hexadec-1′-enyloxy)glycerol-3-phosphorylcholine while the partial synthetic enantiomeric plasmalogen was converted completely by this enzyme. The same degree of conversion was observed when both plasmalogens were degraded with phospholipase C to (rac)-trans-1-(n-hexadec-1′-enyloxy)-2-oleoyl glycerol respectively cis-1-(n-alk-1′-enyloxy)-2-oleoyl glycerol. When these plasmalogenic diglycerides were incubated with pancreatic lipase no further breakdown was observed indicating the absence of positional isomers. Both plasmalogens could be transformed by phospholipase D (EC 3.1.4.4) into 1-(n-alk-1′-enyloxy)-2-oleoyl-glycerol-3-phosphate. Purified pancreatic lipase did not degrade the plasmalogen.