with median IC50
of 1.8 mg/ml and 0.4 mg/ml, respectively; in Mu-NANA assay,
1G8 inhibited X1 NA activity at an IC50 of 25.4 mg/ml, while 1D1 did
not show significant inhibition even at the highest concentration
tested, 200 mg/ml. Fetuin, which has a molecular weight of 50 kd,
was used in ELLA as substrate, while Mu-NANA, which has a
molecular weight of 489 d, was used in Mu-NANA assay. It is likely
that although the binding of either antibody prevents the NA active
center from catalyzing the large substrate fetuin, 1D1 molecules
bound to NA do not block the access of the smaller substrate, Mu-
NANA, to the enzyme active center; by contrast, 1G8 molecules
bound to NA may prevent the enzyme from accessing Mu-NANA,
thus abolishes the cleavage of the substrate. Taken this into
consideration, it is plausible to postulate that the N2 epitope bound
by 1G8 is closer to that bound by 1D1.