Identification, developmental expression and tissue distribution of deaminoneuraminate hydrolase (KDNase) activity in rainbow trout

A deaminoneuraminosyl-glycohydrolase (KDNase), which catalyses the hydrolysis of α-ketosidic 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (or naturally occurring deaminated neuraminic acid; KDN) linkages in KDN-glycoconjugates, is required for their structural and functional studies since KDN residues are usually resistant to the action of known sialidases. A search for KDNase was initiated by examining various cells and tissues of rainbow trout because KDN-glycoconjugates were first found in this animal species. Tissue localization studies of KDNase activity showed it to be present in kidney, spleen and ovary. The highest KDNase activity was found in ovarian postovulatory follicles obtained from female fish at the time when the reproductive organ was undergoing natural effacement. Little if any activity was found in brain, heart, liver, muscle, mature eggs and testis. Developmentally, higher levels of KDNase were usually expressed 3–4 months before ovulation or spermiation. An exception to this was in the ovary (or ovarian follicles) where the most striking increase in KDNase occurred 1–2 months after the maturation of gamete cells. Enzyme extracts containing KDNase activity also contained sialidase activity. From the data based on a kinetic study using mixed substrates, both KDNase and sialidase activities were indicated to reside on a single enzyme protein. The KDN-sialidase displayed broad specificity, which could possibly limit its usefulness as a probe for KDN-glycoconjugates. Nevertheless, unlike sialidases, KDNase can selectively remove KDN residues, thus making it an important new reagent to identify KDN-glycoconjugates in vivo. The present findings also indicate that KDNases are likely to be present in other species, which encourages the search for a specific KDNase to be extended to other organisms.