Determination of substrate preference in phosphatidylserine decarboxylation by liquid chromatography-electrospray ionization mass spectrometry

A method has been developed to determine the substrate preference in phosphatidylserine decarboxylation (PSD), the process by which phosphatidylserine is converted to phosphatidylethanolamine (PE) in the mitochondria. The in vitro assay utilized liposomes containing deuterium-labeled PS molecular species incubated with liver and brain cortex mitochondria, and the conversion of PS to the corresponding PE species was monitored by electrospray ionization mass spectrometry in conjunction with reversed-phase liquid chromatography. Employing this approach we were able to establish for the first time that there exists a substrate preference in PSD in liver (18:0,18:1 > or = 18:0,22:6 > 18:0,20:4-PS) and brain cortex (18:0,22:6 > 18:0,18:1 > 18:0,20:4-PS). The observed PSD molecular species preference, however, did not reflect the mitochondrial PE profile, suggesting that selectivity in other processes such as de novo PE synthesis, intracellular transport of phospholipid molecules, or remodeling by deacylation-reacylation may be important contributors in maintaining a specific lipid profile in mitochondria.