Castanospermine inhibits the processing of the oligosaccharide portion of the influenza viral hemagglutinin.

Castanospermine (1,6,7,8-tetrahydroxyoctahydroindolizine) is a plant alkaloid that inhibits alpha- and beta-glucosidase in fibroblast extracts [Saul, R., Chambers, J. P., Molyneux, R. J., & Elbein, A. D. (1983) Arch. Biochem. Biophys. 221, 593-597]. In the present study, castanospermine also proved to be a potent inhibitor of glycoprotein processing by virtue of the fact that it inhibits glucosidase I. Thus, when influenza virus was raised in the presence of castanospermine, at 10 micrograms/mL or higher, 80-90% of the viral glycopeptides were susceptible to the action of endoglucosaminidase H, whereas in the normal virus 70% of the glycopeptides are resistant to this enzyme. The major oligosaccharide released by endoglucosaminidase H from castanospermine-grown virus migrated like a hexose10GlcNac on a calibrated Bio-Gel P-4 column. This oligosaccharide was characterized as a Glc 3 Man 7 GlcNAc on the basis of various enzymatic treatments, as well as by methylation analysis of the [2-3H]-mannose-labeled or [6-3H]galactose-labeled oligosaccharide. The presence of three glucose residues in the oligosaccharide was also confirmed by periodate oxidation studies of the [6-3H]galactose-labeled hexose10GlcNAc. Castanospermine did not inhibit the incorporation of [3H]leucine or [14C]alanine into protein in MDCK cells at levels as high as 50 micrograms/mL. In addition, influenza virus produced in the presence of this alkaloid were fully infective and apparently produced in similar amounts to that of control cells, as determined by plaque counts. Castanospermine did, however, cause considerable changes in cell surface properties, since MDCK cells grown in 10 micrograms/mL castanospermine were able to bind twice as much [3H]concanavalin A as were control cells.