Gastric and salivary mucins inhibit angiotensin-converting enzyme. Inhibition is partly due to oligosaccharides.

Pig gastric mucin, a highly glycosylated glycoprotein, inhibits angiotensin-converting enzyme (ACE) with an IC50 of 2 mM-neutral hexose content. Pig submaxillary mucin at 2.3 mM inhibits by 73%. To determine whether the oligosaccharide moieties of the mucins contribute to this inhibition, oligosaccharides were prepared from each mucin by reductive beta-elimination and their effects on enzyme activity determined. Total oligosaccharides from gastric mucin inhibited enzyme activity with an IC50 of 0.3 mM based on the neutral hexose content of the oligosaccharide solution. Fractions isolated from gastric mucin by chromatography on DEAE-cellulose and Bio-Gel P-2 inhibited ACE with IC50 values ranging from 2 to 16 mM-oligosaccharide. Larger oligosaccharides inhibited with lower IC50 values than did smaller oligosaccharides. Fractions of average molecular mass 1100 and 740 Da prepared from submaxillary mucin inhibited with IC50 values of 40 and 80 mM-oligosaccharide respectively. Monosaccharides commonly present in serum and membrane glycoproteins were also tested for their effect on ACE. Galactose, N-acetylglucosamine, N-acetylgalactosamine and glucosamine were inhibitory. N-Acetylneuraminic acid stimulated the activity of ACE. Fucose, ethylene glycol and sucrose had no effect on the activity of the enzyme. The influences of different buffers, ion concentrations, pH and substrate structure on the effect of carbohydrate on enzyme activity were also evaluated. The extent of inhibition by the monosaccharide galactose was strongly influenced by buffer ion and substrate concentration. The effects of the oligosaccharide moieties and intact mucins were less sensitive to assay conditions.