R. Ci. méd. biol., Salvador, v. 3, n. 1, p. 13-19, jan./jun. 2004 13 Receptor-binding variants of H3N2 influenza A viruses: characterization of their sialidase activity towards different substrates Alessanda Abdo do Couto* José Júnior França de Barros** José Nelson dos Santos Silva Couceiro*** Abstract Influenza virus sialidase develops an essential activity on cellular glycoproteins, then permitting the dissemination of the virus infections by preventing virus-virus self aggregation and virus-cell rebinding. Two purified variant samples of influenza A/Memphis/102/72 (H3N2) viruses, which are recognized for their receptor-binding activity to a-2,6 or a- 2,3-sialyllactose structures, were analysed for their sialidase activity on different natural and artificial substrates. T he M1/ 5 sample exhibited higher sialidase activity on fetuin (O.D.=0.226), MPN (O.D.=0.110) and human erythrocytes (10,240 HAU/ml), while the activity of the M1/5HS8 sample on these substrates was expressed by O.D.=0.129, O .D .=0.065 and 2,560 HAU/ml when using fetuin, MPN and human erythrocytes as substrates, respectively. Howe- ver, the M1/5HS8 sample showed more significative sialidase activity on mucin when compared to the M1/5 sample: the enzyme activity of first sample was responsible for liberation of 3.5 nmol of free sialic acids while the last one produced 16.5 nmol of free sialic acids. Keywords: Influenza virus. Receptor-binding variants. Neuraminidase (NA). Natural and artificial substrates. * Bacharel em Microbiologia e Imunologia. Instituto de Microbiologia Prof. Paulo de Góes, CCS. UFRJ. ** Mestre em Ciências (Microbiologia). Instituto de Microbiologia Prof. Paulo de Góes, CCS. UFRJ. *** Professor de Virologia. Doutor em Ciências (Microbiologia). Instituto de Microbiologia Prof. Paulo de Góes, Laboratório de Virologia Molecular I. Departamento de Virologia. CCS, Bloco I. UFRJ. 21.941-590 - Rio de Janeiro RJ Brasil Tel.: (21)2260-9311; Fax: (21)2560-8344 E-mail: jncouceiro@micro.ufrj.br INTRODUCTION Influenza A virus is one of the most important re-emergent pathogens; it is responsible for flu outbreaks, epidemics and pandemics, conducing to significant morbity and mortality levels, and economic losses (LAMB; KRUG, 2001; POTTER, 1998). The RNA of these enveloped RNA viruses codifies for different glycoproteins, hemagglutinin (HA) and neuraminidase (NA, sialidase EC 3.12.1.18) both recognizing sialic acid structures. The 15 subtypes of HA spike mediates the binding of the virus particle to sialylglycoproteins and gangliosides of the host cell, while the 9 subtypes of NA act as recep- tor-destroying enzymes on sialic acids exposed on the same receptor carbohydrated structures (OKASAKI et al., 2000). T his virus sialidase is involved in the breakdown of the mucosal lining of the upper respiratory tract and cleavage of sialic acids from the respiratory tract mucus during the infection. It contributes to the release of the virion by the budding process and to prevent its entrapment in this substance (COUCEIRO et al., 1993;