[Studies on sialidase and esterase in influenza viruses]

The main contributions of the author and collaborators about sialidase (EC 3.2.1.18) of influenza virus types A and B and O-acetylesterase (EC 3.1.1.53) of type C are summarized. After a short introduction on the topic, the negative results obtained by the author on inhibitors are commented. Then, the peculiarities of the three procedures assayed, based on the NADH determination as a measurement for the sialidase activity, are discussed. The spectrofluorimetric measurement of NADH concentration is a more sensitive and convenient procedure than that by spectrophotometry, although it is less sensitive than that based on bioluminiscence. Sialidase activity is generally higher in influenza virus type A than in type B; however, some differences have been found between the three sub-types A analysed. Furthermore, thermal stability and stability against changes in the pH values are higher for influenza virus from ducks, followed by those from humans and, finally, by those from pigs. O-acetylesterase of influenza virus type C shows a broad specificity; it acts on O-acetyl-containing compounds which may not be sialic acids. It seems that this enzyme might contribute to facilitate the action of sialidase of influenza virus types A and B. The peculiarities of influenza virus type C suggest to include this type as a new genus in the future classification of viruses.