Evidence for the mechanism of action of the antifungal phytolaccoside B isolated from Phytolacca tetramera Hauman.

Phytolaccoside B (1), an antifungal monodesmoside triterpenoid glycoside isolated from berries of Phytolacca tetramera Hauman (Phytolaccaceae), alters the morphology of yeasts and molds. The malformations were similar to those produced by enfumafungin, a known inhibitor of (1-->3)-beta-D-glucan synthase, an enzyme that catalyzes the synthesis of (1-->3)-beta-D-glucan, one of the major polymers of the fungal cell wall. However, enzymatic assays revealed that 1 did not inhibit (1-->3)-beta-D-glucan synthase, but it did produce a notable enhancement of the chitin synthase 1 activity and, concomitantly, a rise in chitin, another important polymer of the fungal cell walls. This finding was corroborated by fluorescence microscopy and also by quantification of the chitin. In addition, a 2-fold increase in the thickness of the fungal cell wall was observed with transmission electronic microscopy. On the other hand, 1 neither bound to ergosterol nor caused hemolysis of red blood cells, although some fungal membrane damage was observed at the MIC of 1.