Lipids of human meibum: Mass-spectrometric analysis and structural elucidation

Igor A. Butovich, Eduardo Uchiyama, and James P. McCulley

Ophthalmology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9057

Corresponding Author: igor.butovich{at}utsouthwestern.edu

Abstract The purpose of this study was to structurally characterize the major lipid species present in human meibomian gland secretions (MGS) of individual subjects by means of ion trap atmospheric pressure ionization mass-spectrometric analysis (API MSn). The samples of MGS and authentic lipid standards were analyzed using direct infusion HPLC with API MSn detection (HPLC API MSn). Multiple precursor ions were observed in the positive ion mode. Among those, previously identified cholesterol (Chl; m/z 369; [M-H2O+H]+) and oleic acid (OA; m/z 283; [M+H]+) were found. The other major compounds of the general molecular formula CnH2n-2O2, were consistent with wax esters (WE) with OA as fatty acyl component. HPLC-API MSn analysis revealed the presence of large quantities of Chl esters (Chl-E) in all of the tested samples. Less than 0.1% (w/w) of oleamide was detected in human MGS. Common phospholipids and ceramides were not present among the major MGS lipids. Phosphocholine-based lipids were found in MGS in quantities less than 0.01% (w/w), if any. This ratio is two orders of magnitude lower than reported earlier. These observations suggest that MGS are a major source of nonpolar lipids of WE and Chl-E families for the tear film lipid layer, but not of its previously reported (phospho)lipid, ceramide, and fatty acid amide components.

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