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D Borchman, L Huang, D Tang, GR John, MC Yappert; The Composition, Structure and Protein Interactions of Human Tear Film Lipid . Invest. Ophthalmol. Vis. Sci. 2002;43(13):81.
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Purpose: Structural changes in the lipid tear film may play a role in the stability and rate of evaporation of tear fluid. The lipid tear film has yet to be described structurally at the molecular level. This study uses spectroscopic techniques that have been proven to be uniquely powerful for measuring lipid composition and molecular structure. Methods: Tear lipids were extracted from Schirmer strips that were used to collect lipids directly from the tear meniscus. Three types of samples were studied: anhydrous tear lipid; lipid exposed to human tear fluid; lipid exposed to tear fluid processed to remove mucins and proteins. Infrared, fluorescence and matrix-assisted laser desorption ionization-mass spectroscopes were used. Results: Lipid hydrocarbon chain order and sphingomylein were relatively higher and triacylglycerols lower in tear film lipids from younger individuals. Anhydrous lipid, a state similar to that found in the meibomian gland ducts, was 36 % ordered. The head group phosphate moieties were hydrogen bonded. With the addition of tear fluid, lipid-lipid interactions increased, as evident by a dramatic 13C increase in the lipid phase transition temperature. Fluorescence data showed that a substance(s), in the tear fluid, over 5 kDa, interacted with the lipid head group region but not the hydrocarbon chain region and, upon binding with the lipid, excluded water from the tear fluid - tear film lipid interface. Conclusion: The data suggest that upon leaving the meibomian ducts, the lipids undergo molecular structural changes that would be expected to decrease the rate of evaporation and increase the stability of the tear lipid layer. A substance in the tear fluid binds to the tear film lipid expelling water an event expected to enhance the stability and barrier function of the lipid film. Tear lipids from younger individuals were more ordered than those from older individuals as a consequence of lipid compositional differences. Spectroscopic data provide valuable insight on the molecular details that affect tear film stability and changes related to age and dry eye symptoms.
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