Secreted Phospholipase A₂ Specificity on Natural Membrane Phospholipids

The secreted phospholipase A₂ (sPLA₂) family contains 10 catalytically active isoforms. Current in vitro biochemical studies have shown that individual sPLA₂s have distinct substrate selectivity in terms of the polar head groups or sn-2 fatty acids of their substrate phospholipids. Importantly, transgenic or knockout mice for distinct sPLA₂s display nonoverlapping phenotypes, arguing that they do act on different phospholipid substrates and mobilize unique lipid metabolites in vivo. In an effort to comprehensively understand lipid metabolism driven by individual sPLA₂s under pathophysiological conditions, we took advantages of mass spectrometric lipidomics technology to monitor the spatiotemporal changes in phospholipids (substrates) and products (fatty acids, lysophospholipids, and their metabolites) in tissues or cells of sPLA₂-transgenic or knockout mice. The in vivo lipidomic data were compared with the in vitro activity of recombinant sPLA₂s toward phospholipid mixtures extracted from the target tissues, cells, or extracellular membrane components on which sPLA₂s may intrinsically act. These approaches reveal that the overall tendency in in vitro assays using natural membranes is recapitulated in several in vivo systems, often with even more selective patterns of hydrolysis. In this chapter, we will summarize current understanding of the in vivo substrate specificity of sPLA₂s toward natural membrane phospholipids.