Tear lipocalins: Potential lipid scavengers for the corneal surface

PURPOSE. To investigate the dynamic effect of tear lipocalins (TLs), the ma jor lipid-binding protein in tears, at aqueous-cornea and lipid-aqueous int erfaces, and their potential contribution to surface tension in the tear fi lm. METHODS. Human apo- and holo-TLs were applied to the aqueous subphase in a Langmuir trough, and changes in surface pressure were measured. Changes in the contact angle of tear components were observed on Teflon and ferric-ste arate-treated surfaces. A nitroxide-labeled derivative of lauric acid and a fluorescence-labeled derivative of palmitic acid were used to monitor the dynamic interaction of Lipid removed from a hydrophobic surface by the majo r tear components in solution. RESULTS. TLs increase the surface pressure at the aqueous-air interface by penetrating, spreading, and rearranging on the surface. Apo-TLs show a long er diffusion-dependent induction time than holo-TLs due to more extensive o ligomerization of the apoprotein. Kinetic analysis of relaxation time sugge sts that apo-TLs have more rapid surface penetration and rearrangement than holo-TLs, indicative of a more flexible structure in apo-TLs. TLs reduce t he contact angle of solutions on lipid films, a property that is greater wi th TLs than other tear proteins. TLs, unlike lysozyme and lactoferrin, remo ve labeled lipids from hydrophobic surfaces and deliver them into solution. CONCLUSIONS. TLs are potent lipid-binding proteins that increase the surfac e pressure of aqueous solutions while scavenging lipids from hydrophobic su rfaces and delivering them to the aqueous phase of tears. These data sugges t important functional roles for TLs in maintaining the integrity of the te ar film.