On the Inhibition Mechanism of the Sialidase Activity from Newcastle Disease Virus

N-Acetylneuraminic, 2-deoxy-2,3-didehydro-N-acetylneuraminic acid and the beta anomer of methoxyneuraminic acid (Neu5Ac, Neu5Ac2en, MeONeu) have been used as probes for the catalytic mechanism of the activities of the outer membrane-bound haemagglutinin-neuraminidase (HN) from newcastle disease virus (NDV). Neu5Ac and Neu5Ac2en produced a competitive inhibition of the sialidase (= neuraminidase) activity, whereas MeONeu had no effect on this activity. This lack of inhibition can be explained by the free amino-acid group lacking the acetyl substituent in the MeONeu. Neu5Ac2en produced the highest inhibition. Based on the effect of the inhibitors, a reaction mechanism is suggested. On the other hand, the above mentioned inhibitors of the sialidase activity had no effect on haemagglutinating activity, suggesting different active sites for the both activities.