Quantitation of Oxylipins in Biological Samples, Focusing on Plasma, and Urine

Oxylipins are a large family of oxygenated fatty acids generated by enzymes that include lipoxygenases (LOX), cyclooxygenases (COX), and cytochrome P450s. They are formed in almost all cell types and organs and play essential roles in health and disease. Many were discovered in the 1980s, including prostaglandins (PGs), thromboxane, and leukotrienes (LTs). Nowadays, the analysis of these lipids is generally performed using targeted LC-MS/MS methods, which can quantify down to levels of around 1 pg on column, depending on the specific platform. Robust scheduled assays that can analyze up to 150–200 lipids are increasing in popularity but require a very high degree of technical competence and ongoing monitoring to ensure the accuracy of their result. Close attention to peak quality and isomer identification is required. ELISAs also exist, but there are considerations relating to specificity when applied to complex biological samples. Oxylipins are metabolized by β-oxidation, and many of their tetranor and dinor compounds can be readily detected in urine. Robust assays for some are in clinical use for inflammatory disease and cancer. Last, some are found esterified to phospholipids (PLs), glycerides, and sterol esters. Quantitative assays for these are less established because of the lack of synthetic standards, but this is a growing area. This chapter will outline the basic principles of oxylipin analysis, focusing on LC-MS/MS, presenting the current state of the art in relation to how to approach establishing such methods in the laboratory for plasma and urine.