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D. Borchman, G. N. Foulks, M. C. Yappert, D. Tang, J. Bell; Human Tear Lipid Compositional, Structural and Functional Relationships Using Spectroscopy. Invest. Ophthalmol. Vis. Sci. 2008;49(13):82.
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Rapid tear break-up time is a common feature of aqueous deficient and evaporative dry eye, suggesting there may be a shared structural abnormality of the tear film that is responsible for the instability. Disruption of the normal lipid-protein-aqueous structure of the tear film is possibly this event. Understanding the normal lipid-protein-aqueous composition and structure of the tear film could lead the way to improved therapy of meibomian gland disease.
Lipid composition was measured using matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF/MS). Lipid structure was measured using FTIR. Changes in the anisotropy and fluorescence intensity of the tryptophan in lactoglobulin were used to quantify wax ester binding. Lactigobulin was used as a model for tear lipocalin. Rates of evaporation were measured in vitro gravimetrically.
Lipid Composition. By manipulating the cation and matrix composition of tear lipid samples for MALDI-TOF/MS analysis, we quantified and identified lipids such as: cholesterol, phospholipids, hydrocarbons and wax esters with a sensitivity of 9 pmoles. Differences between tear fluid and meibum lipid composition were detected. Lipid Structure. The hydrocarbon chain order measured was higher for 80% of the 25 subjects with meibum gland dysfunction compared to age matched controls. Function. Wax binding, mucin, lysozyme, lactoglobulin or wax alone at concentrations up to100 mg/ml did not significantly influence the rate of evaporation in vitro. In contrast, lipid layered on the surface of a tear protein mixture decreased the rate of evaporation by 27% reaching a plateau level above which adding 10 fold more lipid did not change the rate of evaporation. At 25oC, human reflex tears evaporated at a rate similar to phosphate buffered saline, 0.297 ± 0.004 mg/cm2/min. At 340C, in still air, the rate was 2.9 times higher than at 25oC. These estimated rates are lower than the levels found in real environments and based on our data we estimated that at 34oC, a wisp of dry air (20 SCFM) could evaporate all of the tear fluid in less than 3 seconds.
Meibum lipid compositional changes with age and meibomian gland dysfunction reflect changes in hydrocarbon chain conformation. Spectroscopic techniques are ideal to study the composition and structure of lipid from individual patients as well as to study lipid/protein interactions with age, sex and in samples from all classifications of dry eye.
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