Fluorometric measurement of neuraminidase activity of influenza viruses.

A rapid and simple assay method for determining neuraminidase activity was investigated fluorometrically with 4-methylumbelliferyl-N-Ac-a-D-neuraminide as a substrate. The time required for performance was less than 3 hr and the activity could be simply measured with a fluorescent spectrophotometer. Neuraminidase activity in culture supernatant of MDCK cell infected with a small quantity of influenza A/ Aichi/2/68 virus was demonstrated by the fluorometric assay 24 hr after infection, when no viral cytopathogenic effect could be observed and no viral multiplication could be demonstrated by the hemagglutination test and the colorimetric neuraminidase assay method using fetuin as a substrate. The result was that the minimum number of virion detectable by the fluorometric neuraminidase assay method was approximately 103 order. 287