Quantitation of the diacyl, alkylacyl, and alk-1-enylacyl subclasses of choline glycerophospholipids by chemical dephosphorylation and benzoylation.
A method for the determination of the relative amounts of the diacyl, alkylacyl, and alk-1-enylacyl subclasses of choline glycerophospholipids by benzoolysis is described. The procedure consists of simply heating the phospholipid with benzoic anhydride in the presence of boric acid for 5 h at 100 degrees C, followed by reaction with 4-dimethylaminopyridine for 2 h at room temperature and HPLC analysis of the lipidic products formed. With model compounds of the three subclasses it is shown that diacyl- and alkylacylglycerophosphocholines are completely dephosphorylated by this procedure, yielding quantitatively the corresponding diacyl- and alkylacylglycerobenzoates. The same procedure applied to a model alk-1-enylacylglycerophosphocholine gives only 53.5% of dephosphorylation, while the dephosphorylated products in turn are quantitatively converted into the corresponding acylglycerodibenzoates. The latter figure is shown to be fairly reproducible. The reduced dephosphorylation rate of plasmalogens appears to be due to complete disruption of the vinyl ether bond. The liberated fatty aldehyde gives rise to an addition product with benzoic anhydride, which was identified by gas chromatography-mass spectrometry. It is demonstrated that acyl migration occurring during the benzoolysis does not interfere with the HPLC separation of the glycerobenzoates and dibenzoates derived from the three distinct subclasses. Results of subclass determinations by benzoolysis of several natural diradylglycerophosphocholines are in good accordance with literature values. The agreement between the plasmalogen contents, determined by benzoolysis and by phosphorus determination following exposure to HCl and separation by thin-layer chromatography, is satisfactory. The reliability of the benzoolysis method is generally discussed.