2-Keto-3-deoxy-d-glycero-d-galacto-nononic Acid (KDN)- and N-Acetylneuraminic Acid-cleaving Sialidase (KDN-sialidase) and KDN-cleaving Hydrolase (KDNase) from the Hepatopancreas of Oyster, Crassostrea virginica
1999
Abstract KDN (2-keto-3-deoxy-d-glycero-d-galacto-nononic acid), a sialic acid analog, has been found to be widely distributed in nature. Despite the structural similarity between KDN and Neu5Ac, α-ketosides of KDN are refractory to conventional sialidases. We found that the hepatopancreas of the oyster, Crassostrea virginica, contains two KDN-cleaving sialidases but is devoid of conventional sialidase. The major sialidase, KDN-sialidase, effectively cleaves α-ketosidically linked KDN and also slowly cleaves the α-ketosides of Neu5Ac. The minor sialidase, KDNase, is specific for α-ketosides of KDN. We were able to separate these two KDN-cleaving enzymes using hydrophobic interaction and cation-exchange chromatographies. The rate of hydrolysis of 4-methylumbelliferyl-α-KDN (MU-KDN) by KDN-sialidase is 30 times faster than that of MU-Neu5Ac in the presence of 0.2 mNaCl, whereas in the absence of NaCl this ratio is only 8. KDNase hydrolyzes MU-KDN over 500 times faster than MU-Neu5Ac and is not affected by NaCl. KDN-sialidase purified to electrophoretically homogeneous form was found to have a molecular mass of 25 kDa and an isoelectric point of 8.4. One of the three tryptic peptides derived from KDN-sialidase contains the consensus motif, SXDXGXTW, that has been found in all conventional sialidases. Kinetic analysis of the inhibition of the hydrolysis of MU-KDN and MU-Neu5Ac by 2,3-dehydro-2-deoxy-KDN (KDN2-en) and 2,3-dehydro-2-deoxy-(Neu5Ac2-en) suggests that KDN-sialidase contains two separate active sites for the hydrolysis of KDN and Neu5Ac. Both KDN-sialidase and KDNase effectively hydrolyze KDN-GM3, KDNα2→3Gal β1→4Glc, KDNα2→6Galβ1→4Glc, KDNα2→6-N-acetylgalactosaminitol, KDNα2→6(KDNα2→3)N-acetylgalactosaminitol, and KDNα2→6(GlcNAcβ1→3)N-acetylgalactosaminitol. However, only KDN-sialidase also slowly hydrolyzes GM3, Neu5Acα2→3Galβ1→4Glc, and Neu5Acα2→6Galβ1→4Glc. These two KDN-cleaving sialidases should be useful for studying the structure and function of KDN-containing glycoconjugates.
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