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Simin Masoudi, Fiona Stapleton, Todd W Mitchell, Mark D P Willcox; Diurnal variations of human tear lipids. Invest. Ophthalmol. Vis. Sci. 2014;55(13):35.
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© ARVO (1962-2015); The Authors (2016-present)
To quantify diurnal variations of the molecular lipid composition of human tears.
Tears were collected in the morning and evening for 7-10 days from 30 healthy subjects with no ocular disease using fine glass capillary tubes. Lipids were extracted and analysed by chip-based nano-electrospray ionization tandem mass spectrometry. Lipid profiles of tears in the morning and evening were compared using linear mixed model.
116 lipid species were quantified representing eight lipid classes, namely lysophosphatidylcholines, lysophosphatidylethanolamines (LPE), phosphatidylcholines, phosphatidylethanolamines (PE), sphingomyelins, phosphatidylserines, cholesterol esters, and triacylglycerides. Individual lipid species were normalized with respect to total lipid in each sample. The level of all three species of LPE (species 16:0, 18:0, 18:1) increased (16:0, 1.14±0.87% am vs 1.50±1.05% pm, p=0.01; 18:0, 1.45±0.84% am vs 2.05 ±1.12% pm, p<0.001; 18:1, 1.95±1.31% am vs 2.62±1.42% pm, p=0.001). The tear levels of nine of twelve studied species of PE (comprising 76.2% of the total PE) decreased from morning to evening (the three most abundant molecular species of PE are: 36:2, 0.36±0.23% am vs 0.26±0.13% pm, p=0.03; 36:3, 0.25±0.15% am vs 0.21±0.10% pm, p=0.05; 38:4, 0.14±0.09 am vs 0.09±0.06% pm, p=0.001). LPE and PE constituted 5.4% of the total lipids. The level of the other lipid species did not change over the course of the day.
The overall lipid class profiles of normal tears showed that the levels of more than 94% of tear lipids remain unchanged during the day. However the level of phosphatidylethanolamine and its breakdown product lysophosphatidylethanolamine showed diurnal variation.
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