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S.M. Gouveia, J.M. Tiffany; Low-shear Aggregate Formation in Human Tears . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2526.
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Purpose: Human tears show non-Newtonian shear thinning, similar to solutions of linear polyanions such as hyaluronan, yet tear proteins are relatively small and globular, and their ability to self-aggregate is unknown. Shear-thinning is also found to depend on lipocalin/lipid interactions, and is enhanced in model solutions of two or more tear proteins compared to the near-Newtonian viscosities of individual proteins. Methods: Viscosity/shear rate relationships in human tears and model solutions were measured with the Contraves Low-Shear 30 Rheometer. Compositions of tears and protein solutions were monitored by size-exclusion HPLC and by SDS-PAGE. To stabilize aggregates at near-zero shear rates, the cross-linking agent BS3 was diffused into tears and model solutions in Slide-A-Lyzer dialysis cassettes. Results: The viscosities of the individual model tear proteins sIgA, lactoferrin, and lysozyme were Newtonian (approx. 1.0 mPa.s) at normal tear concentrations. However, the viscosity of model mixtures became shear-thinning when lysozyme was present (up to 120 mPa.s at very low shear). Recombinant tear lipocalin also showed shear-dependent viscosity, but binding of tear lipids reduced the effect. In whole tears, high molecular weight aggregates (up to 600 kDa) and a new peak of about 30 kDa, heavier than lipocalin and lysozyme, were detected both by SE-HPLC and SDS-PAGE, relative to controls. This was unaffected by removal of tear lipids. Conclusions: Human tears appear to form loose aggregates involving major tear proteins, especially lysozyme, at near-zero shear rates, but these are dispersed as shear rate rises. It is likely that these interactions contribute to the physical properties of tears, and may have consequences for the stability of the precorneal tear film, including under the high shear rates in saccades or blinking.
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