Alexine, a naturally occurring pyrrolizidine alkaloid, isolated from Alexa leiopetala, and four stereoisomers, isolated from Castanospermum australe, were investigated for inhibitory activity against the growth of HIV-1. Only treatment with the 7,7a-diepialexine restricted virus growth (IC50 0.38 mm) although it was less active than the indolizidine alkaloid castanospermine (IC50 0.02 mm). The antiviral effects of 7,7a-diepialexine, like castanospermine, correlated with the inhibitory activity against purified pig kidney α-glucosidase 1 of the glycoprotein processing enzymes and the reduced cleavage of the precursor HIV-1 glycoprotein gp160.
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