Analysis of phospholipid peroxidation and protein lipoxidation products by LC-MS

Oxidized phospholipids (oxPLs) are produced by the action of free radicals and reactive oxidizing compounds on unsaturated phospholipids, forming a wide range of oxidized products including full-length species or chain-shortened species together with non-esterified breakdown products. These can be further divided into non-reactive versus electrophilic reactive molecules containing carbonyl groups. OxPLs have various biological activities that are thought to contribute to inflammatory-based diseases, although anti-inflammatory effects have also been reported. It is therefore important to be able to characterize the profile of oxPLs that occur in biological situations, and advanced liquid chromatography tandem mass spectrometry (LC-MSMS) techniques have become the method of choice for this purpose. High resolution mass spectrometry allows identification based on accurate mass of the oxidized products, but in complex samples there are advantages to using targeted approaches involving the detection of diagnostic fragment ions. Using such techniques, we have identified many different oxPLs in samples such as human plasma, parasitized red blood cells and LDL from control and diabetic patients. Moreover, reactive oxidized phospholipids are able to attack nucleophilic sites in proteins to form lipid-protein adducts in a process called lipoxidation, which is thought to contribute to the bioactivity of such oxPLs. While lipoxidation by small aldehydes such as acrolein and 4-hydroxynonenal is well established, using semi-targeted LC-MSMS approaches we have also shown the formation of adducts of reactive phospholipids with proteins, including ApoB-100. These methods are currently being developed further to help understand the occurrence of lipoxidation in biological samples and improve understanding of the mechanisms by which oxPLs may exert effects.