Microbial metabolism of biologically active secondary metabolites from Nerium oleander L.

Ursolic acid (1) and kaempferol (3) are two major constituents of the Mediterranean plant Nerium oleander L. Microbial metabolism of (1) with Aspergillus flavus (ATCC 9170) resulted in the formation of 3-oxo-ursolic acid derivative, ursonic acid (2). On the other hand, Cunninghamella blakesleeana (ATCC 8688A) was able to convert (3) into kaempferol 3-O-beta-D-glucopyranoside (4) as well as the new natural product kaempferol 4'-sulfate (5). Incubation of kaempferol with Mucor ramannianus (ATCC 9628) led to the isolation of one metabolite identified as kaempferol 4'-O-alpha-L-rhamnopyranoside (6). Transformation of kaempferol to the new compound kaempferol 7-O-beta-D-4-O-methylglucopyranoside (7) and herbacetin 8-O-beta-D-glucopyranoside (8) was observed after fermentation with Beauveria bassiana (ATCC 13144). Cytotoxic as well as antioxidant activities of the isolated metabolites were determined.