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Seher Yuksel, Nita Bhat, Amber Wilkerson, Anne McMahon, Jadwiga Wojtowicz, Igor A Butovich; Side-by-side comparison of branching and elongation patterns of cholesteryl esters of human and mouse meibum. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4175.
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Branching of lipids strongly influences their packing, melting, and fluidity. As human meibum is formed predominantly of wax esters (40%, w/w, WE) and cholesteryl esters (30%, CE), the effect of branching on properties of meibum and its physiological behavior could be high. Previously, we evaluated the composition of intact WE of human and mouse meibum using gas chromatography-mass spectrometry and demonstrated their extensive branching. However, another major class of meibum lipids – CE – remained insufficiently characterized. Our goal was to evaluate branching and elongation patterns of intact CE of human and mouse meibum.
Meibum samples were collected from normal human subjects (n=8) and C57BL/6J wild-type mice (n=12). Intact CE were analyzed using liquid chromatography (LC) and mass spectrometry (MS). Authentic chemical standards of straight-chain saturated (SCE) and unsaturated CE (UCE) were used to detect and quantitate them in meibum samples. The compounds were identified using their LC retention times and a common analytical ion m/z 369.3521 (M–fatty acid+H)+, as well as compound-specific ions (M+H)+, (M+NH4)+, (M+K)+, (M–cholesterol)−, and their fragments generated in MS/MS experiments.
The overall balance of major UCE to SCE was found to be, respectively, 24% and 76% (mol/mol) in humans, and 21% and 79% – in mice. All tested UCE (C16:1 to C32:1) were based on straight, even numbered mono- and di-unsaturated fatty acids (FA). In both species, the majority of SCE were branched (73%). Straight chain SCE with even-numbered FA chains (C12:0 to C32:0) were found to comprise ≤3% of total CE, while their branched counterparts gave 41%. Odd numbered SCE (from C11:0 to C27:0) comprised about 32% of all CE and were almost exclusively branched. Other members of the homolog series of SCE and UCE, and other structural isomers were present in trace amounts, if any, and were below the limit of detection and/or quantitation.
The majority of CE of human and mouse meibum are either branched or unsaturated. A large quantity of CE had odd-numbered FA residues. These features are known to decrease the packing order and lower melting points of lipids, compared with their saturated counterparts with straight chain, even-numbered FA residues. The balance between these features may have a strong impact on rheological and physiological characteristics of meibum in vivo.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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