The specificity of viral sialidases. The use of oligosaccharide substrates to probe enzymic characteristics and strain-specific differences.

1. The action of sialidases from Newcastle disease virus (NDV), influenza A2 virus (IA2V) and fowl plague virus (FPV) on sialyloligosaccharide substrates containing alpha 2-3, alpha 2-6 or alpha 2-8 linkages was studied. 2. In all cases 2-3-linked sialic acids were preferentially released. Compared with II6Neu5AcLac, all 2-6-linked substrates, including sialyl-N-acetyllactosamine and its asparaginyl derivative, a urinary hexasaccharide and Neu5Ac(2-6)GalNAc were cleaved at improved rates by NDV and less by FPV sialidases. In the case of IA2V sialidase the asparaginyl oligosaccharide was very poorly cleaved, illustrating a variation in viral strain specificity. 3. A decrease in relative rates was observed in the order NDV greater than IA2V greater than FPV for substrates with 2-3 linkages relative to II6Neu5AcLac. The greatest relative rate was 470-fold higher. The 2-3-linked sialyl-N-acetyllactosaminylasparagine and IV3Neu5AcLcOse4 were poor substrates for the IA2V sialidase, but the rates were greater than with the 2-6 linked substrates. 4. The ganglioside substrate II3Neu5AcLacCer showed lower activity than its oligosaccharide analogue, but neither II3Neu5AcGgOse4Cer nor its oligosaccharide were substrates. 5. The Km values for 2-6-linked substrates were generally of the order 10 mM while those for the 2-3-linked substrates were approximately 1 mM. The V values were consistently higher for the 2-3-linked substrates. IV3Neu5AcLcOse4 showed high Km and very high V values, while the 2-8-linked disialyllactose showed this trend only with NDV enzyme, the IA2V and FPV sialidases exhibiting high Km and low V values. 6. The results are discussed in the light of the current knowledge of viral sialidase specificity and relative to the binding of virus particles to cell surfaces.