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)-ω-hydroxy-FAs (OAHFAs) were observed. The main detected OAHFAs were based on very long-chain ω-hydroxy-FA (C_30:1, C_32:1, and C_34:1) acylated through their ω-hydroxyls by a C_18:1-FA. Due to their amphiphilic anionogenic nature, OAHFAs may be responsible for stabilization of the tear fi lm 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.