Phosphatidylcholine removal from brain lipid extracts expands lipid detection and enhances phosphoinositide quantification by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry

The utilization of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the analytical detection and quantification of phosphoinositides and other lipids in lipid extracts from biological samples was explored. Since phosphatidylcholine species in crude extracts have been shown to cause ion suppression of the MS signals for other lipids, a minicolumn of a silica gel cation exchanger was used to adsorb the cationic lipids including the phosphatidylcholine species from the chloroform phase of fetal and adult murine brain extracts. In positive ion mode, lipid peaks that had been completely suppressed in the crude extract became readily detectable and quantifiable in the flow-through fraction from the column. In negative ion mode, improved sensitivity made it possible to readily detect and measure phosphatidylinositol-4,5-bisphosphate (PIP(2)) which had been only marginally detectable before the fractionation. By incorporating an internal standard into the samples, the relative MALDI-TOF MS signals obtained for increasing concentrations of mammalian phosphatidylinositol (PtdIns) increased linearly with correlation coefficients >0.95. Using strong cation exchange minicolumn treated extracts, the levels of PtdIns and PIP(2) in adult and fetal murine brains were measured and compared. The removal of cationic lipids from the chloroform-methanol murine brain extracts resulted in improved overall detection of neutral and anionic lipids and quantification of phosphoinositides by MALDI-TOF MS.