Microbiological transformation of a cardiac aglycone

New Cardenolides from Biotransformation of Gitoxigenin by the Endophytic Fungus Alternaria eureka 1E1BL1: Characterization and Cytotoxic Activities

Microbial biotransformation is an important tool in drug discovery and for metabolism studies. To expand our bioactive natural product library via modification and to identify possible mammalian metabolites, a cytotoxic cardenolide (gitoxigenin) was biotransformed using the endophytic fungus Alternaria eureka 1E1BL1. Initially, oleandrin was isolated from the dried leaves of Nerium oleander L. and subjected to an acid-catalysed hydrolysis to obtain the substrate gitoxigenin (yield; ~25%). After 21 days of incubation, five new cardenolides 1, 3, 4, 6, and 8 and three previously- identified compounds 2, 5 and 7 were isolated using chromatographic methods. Structural elucidations were accomplished through 1D/2D NMR, HR-ESI-MS and FT-IR analysis. A. eureka catalyzed oxygenation, oxidation, epimerization and dimethyl acetal formation reactions on the substrate. Cytotoxicity of the metabolites were evaluated using MTT cell viability method, whereas doxorubicin and oleandrin were used as positive controls. Biotransformation products displayed less cytotoxicity than the substrate. The new metabolite 8 exhibited the highest activity with IC₅₀ values of 8.25, 1.95 and 3.4 µM against A549, PANC-1 and MIA PaCa-2 cells, respectively, without causing toxicity on healthy cell lines (MRC-5 and HEK-293) up to concentration of 10 µM. Our results suggest that A. eureka is an effective biocatalyst for modifying cardenolide-type secondary metabolites.

Vegetables as chemical reagents

Biocatalysis is an important adjunct to the chemical armamentarium that organic chemists may bring to bear for the synthesis of important intermediates and finished pharmaceutical and other commercial products. For most of the world however, such catalytic reagents are not an option due to their high cost and import limitations. Recent studies indicate that the use of locally available vegetables may offer an alternative opportunity for countries to investigate their local resources for the effective conduct of key synthetic transformations with significant economic and ecological implications. This review offers a brief overview of the field of microbial and plant biocatalysts, discusses the studies thus far on the use of intact plant materials for conducting synthetic chemical reactions, and considers some opportunities for future development.