Analyses of cholesterol and derivatives in ocular tissues using LC-MS/MS methods

Cholesterol is considered one of the most abundant sterols present in mammals, amphipathic in nature, and a key constituent of the cell membrane. Its unique chemical structure consisting of four linked hydrocarbon rings, with an aliphatic chain on one end and a hydroxyl group on the other, confers it the ability to form hydrogen bonds with other lipid classes, like phospholipids or sphingolipids head groups. However, due to its hydrophobic moiety, the fatty acid chain, free cholesterol (FC) it is difficult to be analyzed via electrospray ionization (ESI). The method presented in this chapter to identify and quantify free cholesterol and cholesteryl esters (CE) is based on the chemical derivatization of the sample, strategy devised to avoid the problematic of ESI. However, relevant mention should be made to an alternative separation protocol, which uses ultrahigh performance liquid chromatography and in-source collision-induced dissociation to achieve a simultaneous quantification of FC, CE, and triglycerides (TG).