Mucin is a glycoprotein present in the mucous membrane of digestive organs and respiratory organs, and plays a role in protecting their epithelial tissues. It has so far been determined that the structure of mucins consists of many O -linked chains, and recently, the glycoprotein is also noted in relation to metastasis and specific antigens of several tumor cells secreting the glycoprotein. It has been shown that Olinked chains of mucin are closely involved in the viscosity, the tertiary

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... re, and tissue-specific antigens, and moreover, in this paper, the effect of those on the activity of angiotensinconverting enzyme(ACE) is reported. ACE, a metal protease, is a dipeptidyl carboxypeptidase stimulating the contraction of blood vessels and the secretion of aldosterone from the adrenal cortex. ACE has been found in endothelial cells of pulmonary blood vessels and the brush border of the small intestine, in which the enzyme resides as a membrane-bound ectoenzyme. In this paper, the effect of mucin, its oligosaccharides, and its constitutive sugars on the activity of ACE is investigated. First of all, the enzyme was inhibited at 50% by pig gastric mucin at 2 mM neutral hexose, which is estimated by phenol-sulfuric acid method, and is near the content of Olinked chains. 50% inhibitory concentration (IC50) is 0.3 mM with oligosaccharides from the gastric mucin by alkalifraction obtained by Bio-gel P-2 gel filtration of the oligosaccharides, and 8.5-16 mM with neutral sugar chains by DEAF-cellulose chromatography of the oligosaccharides. Second, ACE was inhibited at 73 % by pig submaxillary mucin at 2.3 mM neutral hexose, and at 50 % with oligosaccharides by alkali-reductive reaction at 62-110 mM. The different inhibitory effect of oligosaccharides from gastric and submaxillary mucins could be partially explained by carbohydrate compositions; oligosaccharides from submaxillary mucin contains nearly equi-molar Sia, GalNAC, GalNACol, Gal, and Fuc while one from gastric mucin contains less Fuc and GlcNAc substituted for Sia. However, in both cases, it couldn't be excluded that the inhibition of ACE is caused by the contamination of peptides at 2-3% in