Rat alpha 1 macroglobulin inhibits hemagglutination by influenza C virus

Purified alpha 1-macroglobulin (RMG) isolated from rat plasma was found to be a potent inhibitor of hemagglutination by influenza C virus. Neuraminidase treatment of purified RMG reduced its inhibitory activity by more than 80% indicating that sialic acid is required for maximal HI-activity. The inhibitory activity of RMG was shown to be sensitive to the receptor-destroying activity (RDA) of influenza C virus. Methylation analysis of the glycopeptides of RMG indicated the presence of only one major type of oligosaccharide which is a complex N-linked oligosaccharide with a biantennary structure. Comparison of the glycopeptides before and after neuraminidase treatment revealed that the oligosaccharides are terminated by sialic acid residues attached to galactose residues at position C-6. Methylation analysis was also performed on RMG which had lost its inhibitory activity upon incubation with RDA of influenza C virus. No difference between the glycopeptides of native and inactive RMG could be detected. Galactose was found to be substituted at position C-6 in both samples, indicating that also the oligosaccharides of inactive RMG are terminated by sialic acid. The implications of these results are discussed.