Purpose: It was previously thought that the surface tension of tears was due to dissolved mucin, but it has recently been shown that very little mucin is present. The surface tensions of solutions of commercial mucin, lysozyme, lactoferrin or secretory IgA are all higher than that of tears. The influence of tear lipocalin and lipids remained to be tested.
Methods: Surface tension was determined by a micro-method on pooled intact stimulated human tears, and following extraction with lipid solvents. The extracted material was also added back, as was a variety of lipid standards (phospholipids, glycolipids, sterols, etc.). TLC and GLC were used in partial identification of the extract. Another lipocalin, bovine beta-lactoglobulin, was also tested alone and mixed with tear lipids, model lipids, or model tear proteins.
Results: Intact tears had a surface tension of 42-46 mN/m, but after lipid extraction this rose to 53-55.5 mN/m. Addition of lipids to the delipidised tear fluid gave a range of tensions from 42 to 49 mN/m, with the greatest effects shown by phospholipids (phosphatidylcholine, sphingomyelin), but full recovery was only achieved by using the extracted lipid material. Human meibomian oil was less effective. The GLC peak profile of the extract was markedly different from meibomian oil, and the TLC pattern was consistent with the presence of glycolipids.
Conclusions: The surface tension of tears is due to a complex of tear lipocalin with a polar lipid fraction extractable from tears by lipid solvents and different from meibomian lipid. Lipocalin and this lipid fraction may be secreted together by the lacrimal gland.