RESUMO
A simple assay for the determination of sialic acid (N-acetylneuraminic acid) in human tear fluid was evaluated. Sialic acid, terminally bound on carbohydrate side-chains of glycoproteins, was released after treatment with neuraminidase and measured by an enzymatic colorimetric test. Tear fluid samples were collected from ten healthy adults, using glass capillaries and cellulose sponges. Sialic acid levels in tears collected with sponges (0.8-1.8 mmol l-1) did not differ significantly from those found in capillary tears (0.9-1.8 mmol l-1). Sialic acid, expressed as mmol g-1 protein, was significantly lower in tears collected with sponges (0.18-0.32 mmol g-1) than with capillaries (0.19-0.42 mmol g-1). Recovery of sialic acid and protein after incubation of cellulose sponges with tears was more than 99%. Sialic acid levels in human tears, which had been centrifuged to remove insoluble material, remained unchanged. Furthermore, tear sialic acid activity did not pass a filter with a molecular weight cut-off point of 10,000. Our data indicate that with the assay used in this report, sialic acid in tears is not due to secretory immunoglobulin A (sIgA), lactoferrin and lysozyme. The fact that the major tear proteins do not contribute to the sialic acid levels detected in tears suggests that other as yet unknown soluble glycoproteins are involved.
