Haemagglutinin of influenza A virus is a target for the antiviral effect of Norakin.

The anticholinergic anti-parkinsonism drug Norakin is an inhibitor of influenza virus multiplication. By crossing a Norakin-resistant variant of fowl plague virus (FPV) strain Weybridge with the sensitive FPV/Rostock/34 wild-type virus, Norakin-resistant recombinants were obtained. Analyses of the gene composition showed that all Norakin-resistant recombinants had inherited their haemagglutinin gene from the Norakin-resistant parent strain. The majority of the recombinants had received all the other gene segments from the sensitive parent strain. Norakin was shown to inhibit red blood cell lysis induced either by purified virions or by the haemagglutinin of a sensitive FPV strain at low pH, but failed to affect the Norakin-resistant FPV variant. No aggregation of autoliposomes containing the haemagglutinin of a sensitive FPV strain or digestion of the HA1 subunit of haemagglutinin by trypsin occurred in the presence of Norakin at acid pH. The data suggest that the haemagglutinin of FPV is the target for the antiviral activity of Norakin, which acts by inhibiting the conformational change in the haemagglutinin at acid pH important for lysis.