Human tear film and meibum. Very long chain wax esters and (O-acyl)-omega-hydroxy fatty acids of meibum.

Human meibum was targetly analyzed for the presence of intact wax esters (WEs) and related compounds by means of reverse-phase HPLC in combination with ion trap mass spectrometry. The major detected WEs were based on C(18:n) (n = 1-4) unsaturated FAs ranking in the following order of abundance: C(18:1)>C(18:2)>C(18:3)>C(18:4). The major fatty alcohols (FAls) found in WE were of saturated nature and varied from C(18:0) to C(28:0). The three most abundant species were C(18:1)-FA esters of C(24:0), C(25:0), and C(26:0)-FAl. Typically, a major compound based on C(18:1)-FA and a saturated FAl was accompanied by a few related compounds based on a C(18:2), C(18:3), and C(18:4)-FA. Contrary to previous reports, no epoxy-WEs or epoxy-FAs were detected in fresh and 1-year-old meibum samples. More than 20 (O-acyl)-omega-hydroxy-FAs (OAHFAs) were observed. The main detected OAHFAs were based on very long-chain omega-hydroxy-FA (C(30:1), C(32:1), and C(34:1)) acylated through their omega-hydroxyls by a C(18:1)-FA. Due to their amphiphilic anionogenic nature, OAHFAs may be responsible for stabilization of the tear film lipid layer by creating an interface between the vast pool of strictly nonpolar lipids of meibum (WEs, cholesteryl esters, etc.) and the aqueous subphase beneath it, a role previously attributed to phospholipids.