Neuraminidase (NA) activity of 10 strains of influenza A and 10 strains of influenza B virus was determined by fluorometric and colorimetric assays using 4-methylumbelliferyl (4-MU)-N-Ac-α-D-neuraminide and fetuin as the substrate, respectively.
The ratio of NA activity measured by colorimetric assay to that measured by fluorometric assay was significantly higher with influenza B virus than with influenza A virus. When measured by fluorometric assay, NA activity of influenza A virus was strongly inhibited by etylenediaminetetraacetate, while that of influenza B virus was not. Influenza B virus NA was also significantly less susceptible to competitive inhibition by N-acetylneuraminic acid than influenza A virus NA, suggesting the different substrate specificity of the two NAs.
It is worth to note that such an enzymological heterogeneity between influenza A and B virus NAs is demonstrated only by the use of a low molecular weight synthetic substrate, 4-MU-N-Ac-α-D-neuraminide and fluorometric assay.