Distinct populations of high-Mr mucins secreted by different human salivary glands discriminated by density-gradient electrophoresis

Abstract

High-M(r) mucins [mucin glycoprotein 1 (MG1)] isolated from human saliva from the individual salivary glands were chemically characterized. The carbohydrate content of MG1 derived from palatal (PAL), submandibular (SM) and sublingual (SL) saliva was typical of mucins but showed heterogeneity, especially in the amount of sialic acid and sulphated sugar residues. The physicochemical properties of native MG1s make conventional SDS/PAGE and ion-exchange chromatography unsuitable for investigating differences between individual samples. Recently a density-gradient electrophoresis (DGE) device has been developed, primarily for separation based on the charge of entire cells or cell organelles [Tulp, Verwoerd and Pieters (1993) Electrophoresis 14, 1295-1301]. We have used this apparatus to study the high-M(r) salivary mucins. Using DGE, the MG1s of individual glands were seen to have clearly distinct electrophoretic mobilities, as monitored by ELISA using MG1-specific monoclonal antibodies. Even within a particular MG1 preparation, subpopulations could be distinguished. DGE analysis of a chemically and enzymically modified MG1 series, followed by ELISA and dot-blot detection using specific monoclonal antibodies, lectins and high-iron diamine staining, suggests that the high electrophoretic mobility of PAL-MG1 is mainly the result of a high sulphate content, whereas the SL subpopulations differ mainly in binding type and amount of sialic acid. SM-MG1 most resembles the low-mobility subpopulation of SL-MG1, except that it has a lower sulphate content. In conclusion, DGE appears to be a powerful method for analysis of native mucin; it has been used to demonstrate that MG1s from the various salivary glands are biochemically much more diverse than was previously assumed.