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Antti Rantamaki, Tuulikki Seppanen-Laakso, Matej Oresic, Matti Jauhiainen, Juha M. Holopainen; Human Tear Fluid Lipidome: from Composition to Function. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3728. doi: https://doi.org/.
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The human tear fluid lipidome remains only partly known. Here we have explored this with an emphasis to identify the major polar and neutral lipids of the aqueous tear fluid. The physiological significance of the lipidome is also addressed by showing that neutral lipids cannot spread without the help of polar lipids.
Tear fluid was collected from healthy human subjects. The samples were centrifuged and only the aqueous solutions were pooled. The collected tears were subjected to thin layer chromatographic, enzymatic, and mass spectrometric analysis to reveal the lipidome. To address the physiological aspect of the lipidome, spreading of olive oil drops on blank hydrophilic mica sheets and polar egg yolk phosphatidylcholine (eggPC) coated mica was compared by following the changes in contact angle. Olive oil and eggPC were used as simple model substances representing the neutral and polar tear fluid lipids, respectively. The blank mica surface represented the air-water interface.
Based on enzymatic analysis, the concentration of choline-containing lipids, triglycerides, and cholesterol esters were ~70, 10, and 35 µM, respectively. Mass spectrometric analysis of the tear fluid showed that phosphatidylethanolamine and phosphatidylcholine were the two most common polar lipids detected comprising more than half of all identified lipids. Interestingly, ~40 mol% of the lipids were ether-lipids. Lysophosphatidylcholine and lysophosphatidic acid comprised ~15% and sphingolipids ~5% of the lipids, respectively. Approximately 20-30% of all lipids were neutral lipids (triglycerides and cholesterol esters). Most of the polar and neutral lipids were unsaturated. Compared to a blank mica surface, the spreading of neutral olive oil on eggPC-coated mica was faster, more stable, and the final contact angle was significantly smaller.
We provide evidence that polar lipids are the most common lipid species in the aqueous human tear fluid. Furthermore, we provide a physiological rationale for the observed lipid composition. The results open insights into the functional role of lipids in the tear fluid and also aids in providing new means to understand and treat diseases of the ocular surface.
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