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T.J. Millar, P. Mudgil; Penetration of Human Tear Lipocalin Into a Meibomian Lipid Layer . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5599.
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Lipocalin is a major tear lipoprotein that transports and releases lipids from the ocular surface to the outer meibomian lipid layer. It is also surface active and therefore it is possible that it could penetrate the meibomian lipid layer and contribute to lowering the surface tension of the tear film. To determine if this is likely, we have compared the penetration of apolipocalin and hololipocalin extracted from human tears with the penetration of lysozyme into films of meibomian lipids and phospholipids held at different surface pressures.
A lipid film of meibomian lipids, phosphatidylcholine (PC), phosphatidylglycerol (PG), phosphatidylserine (PS), or phosphatidylethanolamine (PE) was spread onto the surface of a buffered aqueous subphase in a Langmuir trough and compressed to predetermined pressures. Proteins were injected into the subphase and the area of the film was held constant. The penetration of protein into the surface film was measured by changes in surface pressure, and by epifluorescence microscopy of films doped with a small amount of fluorescently tagged lipid and fluorescently tagged protein.
Lysozyme and apolipocalin at concentrations 100 times less than normally occur in tears, readily penetrated a meibomian lipid layer and PS, PG and PE films. Hololipocalin showed moderate penetration only after extended time. All proteins were unable to penetrate PC films held at modest pressure (10mN/m) unless much higher concentrations of protein were used. Epifluorescence showed that the pattern of mixing of proteins and lipids at the surface varied depending upon the lipid at the surface, and that protein stabilized the lipid film as indicated by a marked reduction of mass movement of the lipid films.
Lipocalin compared with lysozyme seems to have little capability of penetrating the meibomian lipid layer. This is likely to be due to the stability of hololipocalin. With lipids bound into its core, hololipocalin presents a hydrophilic outer surface and conformational stability which effectively traps it in the aqueous layer.Therefore, unlike other tear proteins, lipocalin is unlikely to be directly contributing to lowering the surface tension of the tear film.
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