KDN, a member of the sialic acid family, includes N-acetylneuramininc acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). The stereochemistry of KDN is completely identical to that of Neu5Ac or Neu5Gc, except that the N-acylamino group at C-5 position of N-acylneuraminic acid is replaced by a hydroxyl group (Figure 1). KDN is 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid or 3-deoxy-D-glycero-D-galacto-nonulsonic acid, and also designated as deaminoneuraminic acid. In 1986, KDN was discovered as a minor sugar component of polysialoglycoprotein from rainbow trout eggs 1.

Figure 1
KDN-ganglioside is a class of glycosphingolipids having KDN residue(s), and was first found in 1991 in rainbow trout sperm a (KDN)GM3, where KDN is alpha 2,3-ketosidically linked to lactosyl ceramide instead of Neu5Ac 2. KDN is a ubiquitous sugar component of glycoconjugates found in life form from bacteria to mammals. KDN-gangliosides have been found not only in fish egg, ovarian fluid, sperm, and testis 3, but also as minor gangliosides in various cells and tissues of mammals. KDN-gangliosides thus far identified are: (KDN)GM3, II3KDN alpha-LacCer; (KDN)GD1a, IV3KDN alpha, II3KDN alpha-Gg4Cer; (KDN,Neu5Ac)GD1a, IV3KDN alpha, II3Neu5Ac alpha-Gg4Cer and IV3Neu5Ac alpha, II3KDN alpha-Gg4Cer; (KDN)GD1 alpha, IV3KDN alpha, III6KDN alpha-Gg4Cer; (KDN,Neu5Ac)GD1 alpha, IV3KDN alpha, III6Neu5Ac alpha-Gg4Cer; (O-acetylated KDN)GD1a, IV39-O-acetyl-KDN alpha, II3KDN alpha-Gg4Cer.

KDN residues are known to be resistant or of low sensitivity to the action of various known bacterial, viral, and animal-derived sialidases. Recent studies have shown the presence of two classes of sialidase that can cleave the ketosidic linkages of KDN: One is designated as KDN-sialidases, which can cleave ketosidic linkages of Neu5Ac and Neu5Gc as well as KDN. This class of enzyme is known to occur in ovary of rainbow trout, liver of loach, hepatopancreas of oyster and starfish; the other is KDNase, which can only cleave the ketosidic linkages of KDN, but not that of Neu5Ac or Neu5Gc. KDNase Sm from Sphingobacterium multivorum(4)and KDNase from the hepatopancreas of oyster(5) are known.
Ken Kitajima (Nagoya University Bioscience Center)
References(1) Nadano, D, Iwasaki, M, Endo, S, Kitajima, K, Inoue, S, Inoue, Y, J. Biol. Chem. 261, 11550-11557, 1986
(2) Yu, S, Kitajima, K, Inoue, S, Inoue, Y, J. Biol. Chem. 266, 21929-21935, 1991
(3) Yu, S, Kitajima, K, Inoue, S, Khoo, K-H, Morris, H R, Dell, A, Inoue, Y, Glycobiology 5, 207-218, 1995
(4) Kitajima, K, Kuroyanagi, H, Inoue, S, Ye, J, Troy, FA II, Inoue, Y, J. Biol. Chem. 269, 21415-21419, 1994
(5) Pavlova, NV, Yuziuk, JA, Nakagawa, H, Kiso, M, Li, SC, Li YT, J. Biol. Chem. 274, 31974-31980, 1999
Revised; Apr. 15, 2001

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