Oxidative stress and inflammatory conditions result in the formation of carbonyl-containing lipid derivatives. These reactive carbonyl groups can interact with amine groups on proteins via the formation of Schiff base adducts, modifications that can lead to changes to the proteins structure and influence overall activity. Phospholipase A2 (PLA2) modified with 1-palmitoyl-2-(9'oxononanoyl)-sn-glycero-3-phosphocholine (PoxnoPC) forms Schiff bases that trigger an increase in protein activity, possibly through changes in enzyme conformation, protein-protein interaction, or peptide-protein heteroligomerisation (Code et al., 2010). However, the site of carbonyl modification that is responsible for the change in PLA2 activity remains unknown, so further studies were required to address this question. In this study, PLA2 from Apis mellifera modified with physiological concentrations of PoxnoPC (15μM) was analysed by mass spectrometry. Fragmentation data was used to identify the nature and sites of modification in the modified PLA2. No modifications containing PoxnoPC were found in the protein; however, modifications containing 9-oxononanoic acid (ON) were found on a number of lysine residues including Lys14 and Lys66. These residues are located in the vicinity of the catalytic site, the calcium binding site, and the carboxyl terminal end loop of the PLA2 structure, suggesting that PLA2 catalysed the breakdown of PoxnoPC to ON, although it is not known whether this occurred before or after adduct formation. The modifications of lysine residues found could account for the generation of highly reactive protofibril intermediates that increase the enzyme's overall activity (Code et al., 2010) which supports the need for detailed quantitative mass spectrometry and 3D mapping studies to complement these findings. References: Code, C. et al., 2010. Biochim Biophys Acta 1798 (8); 1593-1600. Acknowledgments: AR was supported by a Marie Curie Intra-European Fellowship within the 7th European Community Framework Program (IEF 255076). This work is supported by the EPSRC (EP/I017887) and Aston University.