Comparison of substrate specificities of sialidase activity between purified enzymes from influenza virus A (H1N1 and H3N2 subtypes) and B strains and their original viruses.

Sialidases possessing enzyme activity were solubilized from mouse-adapted influenza viruses A/PR/8/34 (A/PR8, H1N1), A/Guizhou/54/89 (A/Guizhou, H3N2) and B/Ibaraki/2/85 (B/Ibaraki) by proteolytic digestion and purified by affinity chromatography and/or sucrose density gradient centrifugation. The purified sialidases were observed as a single protein band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The optimum pH of purified sialidases from A/PR8, A/Guizhou and B/Ibaraki against sodium p-nitrophenyl-N-acetyl-alpha-D-neuraminate were 6.5, 7.5 and 5.5, respectively. The purified sialidase (N1) from A/PR8 and its original virus showed enzyme activity with similar substrate specificity, and preferentially hydrolyzed alpha (2-->3)sialyllactose and bovine submaxillary mucin (BSM). Purified sialidase from B/Ibaraki hydrolyzed alpha (2-->3)sialyllactose, alpha (2-->6)sialyllactose and most glycoproteins, especially BSM, but the intact virus showed higher sialidase activity against sialyllactoses than against glycoproteins and gangliosides. These results indicate that the purified enzyme and the original virus of B/Ibaraki have different substrate specificities of sialidase activity. Purified A/Guizhou sialidase (N2) hydrolyzed alpha (2-->3)sialyllactose and porcine stomach mucin but not alpha (2-->6)sialyllactose and BSM. The original virus of A/Guizhou showed substrate specificity similar to its purified enzyme, except that the virus was active against BSM.