Studies on parainfluenza-3 virus neuraminidase.

Various parainfluenza-3 virus (PIV-3) strains differ in neuraminidase (NA) activity, some being neuraminidase strong (NAS) and some neuraminidase weak (NAW). No difference in the ability of these strains to elicit antibody activity to hemagglutinin (HA) or NA of PIV-3 was observed. Samples of formalin-treated PIV-3 released more N-acteylneuraminic acid (NANA) than samples of Piv-3 not treated with formalin when these samples were incubated with the substrate sialolactose for 16 h at 37 degrees C. The initial activity of PIV-3 NA, and the Km-value, were similar for a NAS strain whether it was treated with formalin or not. The results suggest that formalin stabilized the NA structure of PIV-3. A NAS strain of PIV-3 dialyzed against distilled water showed an increased initial neuraminidase activity as compared to virus dialyzed against isotonic NaCL or to non-dialyzed virus. The increased enzyme activity indicates an increased affinity of NA tothe substrate sialolactose as an increased Km-value was also obtained. It is suggested that PIV-3 neuraminidase facilitates the penetration of the virion through the mucociliary barrier of the respiratory tract by releasing the virus from inhibitors in the gel phase. It has been found that antibody in the gel phase of nasal secretion inhibits the NA activity of PIV-3 Following exposure of a gel phase of bovine tracheal secretion to a NAS strain of PIV-3 an increased initial viscosityof this phase was observed. The gel phase of bovine nasal secretion showed a structural change in scanning electronmicroscopy following exposure to a NAS strain. Stimultaneously an increased amount of free N-acetylneuraminic acid was found in the gel.