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I. A. Butovich, E. Uchiyama, S. Agee, L. Mendiola, J. P. McCulley; Structural Analysis of the Nonpolar Lipids Present in the Human Meibomian Gland Secretions Using Ion Trap Mass Spectrometry. Invest. Ophthalmol. Vis. Sci. 2007;48(13):441.
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© ARVO (1962-2015); The Authors (2016-present)
To qualitatively characterize the major lipid species present in human meibomian gland secretions (MGS) of individual subjects by means of atmospheric pressure ionization ion trap mass-spectrometric analysis (API MSn).
The samples of MGS and authentic lipid standards were analyzed using HPLC-API MSn and direct infusion API MSn. The lipid samples were analyzed in both the negative and the positive ion modes. To determine structures of the major components, their precursor ions were isolated and subjected to further sequential fragmentation. Authentic chemical standards and/or their close analogs were used to verify the structures of the MGS lipids.
Free cholesterol (Chl), its esters (Chl-E) and oleic acid (OA) were consistently found in MGS. The other major compounds fit the general molecular formula CnH2n-2O2 (subset 1; n=36-48; all M+H+). Their structures were elucidated in sequential fragmentation API MSn experiments. The compounds were found to be OA-based wax esters (WE). Accompanying them were two homologous series of WE which fit the molecular formulas CnH2n-4O2 (subset 2; linoleic acid-based WE; n=38-46; M+H+) and CnH2nO2 (subset 3; stearic acid-based WE; n=38-46; M+H+). Only very low amounts of triglycerols were found to be present in MGS. Contrary to the previous reports, no diglycerols, monoglycerols, or ceramides were detected.
These observations suggest that MGS are a major source of nonpolar lipids of WE, Chl, and Chl-E families for the tear film lipid layer (TFLL). The major WE have OA in their structures, followed by stearic acid and linoleic acid in terms of their abundance.
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