Biosynthesis and partial characterization of tear film glycoproteins. Incorporation of radioactive precursors by human lacrimal gland explants in vitro

A system for studying the biosynthesis of tear glycoproteins by human lacrimal gland is described. Sufficient quantities of the newly-synthesized glycoproteins were obtained to permit some analyses of their physical and chemical properties. Separation of glycoproteins by gel filtration on Sephadex G-200 showed three major heterogeneous glycoprotein fractions with molecular weights of 580 000, 100 000 and 39 000 daltons. The sulfate, carbohydrate, and amino acid composition of each fraction was determined. The protein moieties were found to be acidic in nature. The smaller molecular weight fractions were moderately richer in carbohydrate [26% (w/w) than the higher molecular weight fractions (8% w/w)]. The low carbohydrate contents were comparable to those of plasma-type proteins. DEAE ion exchange chromatography of the major fraction also showed an elution pattern characteristic of plasma-type proteins. Using immunodiffusion techniques, at least five serum-type proteins, including IgA, IgG, albumin, transferrin and cerulo-plasmin were identified from the different fractions of the cultured lacrimal gland secretions. It appeared that human lacrimal gland secreted similar plasma proteins to those found in the tears. The significance of these identified tear proteins to the physiology of tears is discussed.