Total oxylipin concentrations in NIST SRM 1950 plasma compared to fresh and commercially obtained human plasma

Oxylipins are oxidized PUFAs such as hydroxy-PUFAs and epoxy-PUFAs. Total oxylipins are commonly analyzed following base hydrolysis as nonesterified oxylipins by means of LC-MS/MS. For that, a standard operating procedure has been developed, which was successfully tested in an international interlaboratory comparison. Here, we report the concentrations of total oxylipins in National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1950 plasma determined by LC-MS/MS analyses in different laboratories, which are similar to those in other freshly prepared human plasma samples. Hydroxy-PUFA of the long-chain PUFA arachidonic acid, DHA, and EPA occur between 1 and 75 nmol/l (0.3-24 ng/ml) and linoleic acid-derived hydroxy-PUFA up to 400 nmol/l (120 ng/ml). The concentrations of arachidonic acid-, DHA-, and EPA-derived epoxy-PUFA range between 2 and 20 nmol/l (0.6-6 ng/ml) and of epoxy-linoleic acid up to 100 nmol/l (32 ng/ml). We also report 70 oxylipins, which cannot be detected (∼<0.5 nmol/l; 0.2 ng/ml), such as multiple hydroxylated PUFA, including so-called specialized proresolving mediators (except for 10,17-dihydroxy-DHA), and most isoprostanes. If the plasma is prepared or stored inappropriately, oxylipins are formed by autoxidation, leading to artificially high levels of total oxylipins. This is shown for the concentrations in different commercially available plasma pools of unknown origin, being 100-1000-fold higher. The concentrations provided here for NIST SRM 1950 plasma and freshly prepared plasma thus inform which concentration range of total oxylipins should be expected in human plasma from healthy subjects and increase the utility of NIST SRM 1950 for analytical quality control protocols.