Preparation, chromatography and mass spectrometry of cholesteryl ester and glycerolipid‐bound aldehydes

To identify lipid ester core aldehydes (aldehydes still bound to parent lipid molecules) among the secondary products of peroxidation, we have prepared reference standards of cholesteryl esters, diacylglycerols and the common glycerophospholipids (GPL) containing an aldehyde function. We subjected the unsaturated lipid esters to hydroxylation with osmium tetroxide and to carbon‐carbon bond cleavage with periodic acid. The resulting aldehyde esters (mainly 5‐oxovalerates and 9‐oxononanoates of cholesterol and the glycerolipids) were isolated and purified by thin‐layer chromatography (TLC) and reverse‐phase high‐performance liquid chromatography (HPLC). Liquid chromatography/mass spectrometry (LC/MS) was used to identify the cholesteryl and other neutral lipid ester aldehydes as the dinitrophenylhydrazones (DNPH). The choline (CGPL) and ethanolamine (EGPL) glycerophospholipids containing core aldehydes were identified by fast atom bombardment mass spectrometry (FAB‐MS) and by gas chromatography/mass spectrometry (GC/MS) of the aldehyde‐containing diacylglycerol moieties following dephosphorylation with phospholipase C and preparation of the methoximes (MOX) and the trimethylsilyl (TMS) ethers. The lipid esters containing the C5 and C9 aldehydes are chemically stable compounds with excellent chromatographic properties yielding mass spectra characteristic of the parent compounds. The overall yield of the core aldehydes was 10–20% of the unsaturated starting material.