Studies on mucus biosynthesis in the gastrointestinal tract

Mucin biosynthesis in the rat gastrointestinal tract was measured by following the incorporation of radioactive precursors, [3H] glucosamine, [3H] galactose and [3H] serine. The radioactive mucin was isolated by a combination of equilibrium centrifugation in a CsCl density gradient and gel-filtration or, by papain digestion and removal of digested peptides by dialysis. Radioactive precursors were incorporated into mucin by stomach, duodenum and colon in vivo and by stomach in organ culture in vitro. In the latter, incorporation was approximately 20-fold greater than in vivo, over 6 h. In all cases lower molecular weight non-mucin glycoprotein, of the same buoyant density as mucin, became radioactively labelled (non-dialysable) and usually accounted for the majority of the incorporated radioactivity. Acetylcholine, dibutyryl cAMP and secretin stimulated radioactive incorporation into both gastric mucin and non-mucin components with acetylcholine and dcAMP stimulating specifically [3H] glucosamine into gastric mucin. Using these conditions no stimulation of incorporation into gastric mucin of all three radioactive precursors could be obtained with topical PGE2. In some biosynthesis studies by others, methods used to precipitate radioactive mucins do not distinguish between mucin and lower sized material. This study points to the need for a reappraisal of existing methods using radioactive isotopes to study mucin biosynthesis.