Analysis of human serum lipoprotein lipid composition using MALDI-TOF mass spectrometry

This study used matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) to identify all lipid classes in human serum lipoproteins. After the major lipoproteins classes were isolated from serum by ultracentrifugation, the lipids were extracted and mixed with 2,5-dihydroxybenzoic acid (2,5-DHB) dissolved in Folch's solution (chloroform/methanol 2: 1, v/v). MALDI-TOF MS analysis of the samples identified phospholipids (PLs), lysophospholipids (lysoPLs), sphingolipids (SLs), triglycerides (TGs), cholesteryl esters (CEs), and free cholesterol; it also showed the characteristics of individual fatty acid chains in serum lipids. MALDI-TOF MS allowed analysis of strongly hydrophobic and non-polar molecules such as CEs and TGs as well as hydrophilic molecules such as phospholipids. Direct analysis of fatty acids was not possible. The concentrations of lipids were not consistent with the ion peak intensities, since the extent of polarity affected the ionization characteristics of the molecules. However, lipid molecules with similar molecular structures but various fatty acid chains, such as phosphatidylcholine (PCs), were analyzed quantitatively by MALDI-TOF MS. Quantitative measurement of cholesterol was possible with the use of an internal standard. This study shows that MALDI-TOF MS can be used for direct investigation and quantitative analysis of the phospholipid composition of serum lipoproteins.