Sialic acids comprise a large family of derivatives of neuraminic
acid containing methyl, acetyl,
sulfate, and
phosphate among other groups, which confer specific physicochemical properties (e.g., hydrophobicity and resistance to
hydrolases) to the molecules carrying them. Several years ago, a
monoclonal antibody, designated H185, was developed, which binds to cell membranes of human corneal, conjunctival, laryngeal, and vaginal epithelia and whose distribution is altered on the ocular surface of patients with keratinizing disease. Recent findings using immunoprecipitation and immunodepletion techniques have demonstrated that, in human corneal epithelial cells, the H185
antigen is carried by the membrane-associated
mucin MUC16. In this study, we show that the H185
epitope on human corneal cells and in tear fluid is an O-acetylated
sialic acid epitope that can be selectively hydrolyzed in an
enzyme-concentration-dependent manner by
sialidase from Arthrobacter ureafaciens and to a lesser extent by sialidases from Newcastle disease virus, Clostridium perfringens, and Streptococcus pneumoniae. Binding of the H185 antibody was impaired by treatment of tear fluid with a recombinant 9-O-acetylesterase from influenza C virus. Two O-acetyl derivatives, Neu5,7Ac(2) and Neu5,9Ac(2), were identified in human tear fluid by fluorometric high-performance liquid chromatography (HPLC) and electrospray mass spectrometry (MS). Immunoprecipitation of the H185
epitope from human corneal epithelial cells revealed that Neu5,9Ac(2) was the major derivative on the
mucin isolate. These results indicate that exposed wet-surfaced epithelia are decorated with
O-acetyl sialic acid derivatives on membrane-associated
mucins and suggest that O-acetylation on cell surfaces may protect against pathogen
infection by preventing degradation of membrane-associated
mucins.