Filamentous fungi as a biotechnological tool for the biotransformation of betulin

Betulin, an inexpensive substrate extracted primarily from birch bark, exhibits wound-healing properties; yet its high hydrophobicity limits its bioavailability. Given that biotransformation can produce metabolites with modified chemical and biological properties, we sought to develop an efficient biocatalyst for the biotransformation of betulin. We isolated, characterized, and evaluated the ability of six fungi to biotransform betulin. The isolates were identified as Penicillium citreonigrum, Fusarium oxysporum, Fusarium redolens, Blastobotrys sp., Aspergillus niger, and Penicillium pimiteouiense and were cultured in biotransformation medium supplemented with betulin (10 mg/mL) for 4-7 days. Under the biotransformation conditions, Aspergillus niger exhibited the highest metabolic activity, producing biomass up to 1.29 g/L, acidifying the medium by increasing redox potential (404 mV), and lowering pH (3.29), while Blastobotrys sp. and Penicillium pimiteouiense had the lowest metabolic activity. All six isolates converted betulin to betulonic acid with Fusarium oxysporum exhibiting the highest bioconversion (0.74 g/L), followed by Fusarium redolens (0.72 g/L). Furthermore, at the adopted concentration, betulin biotransformation exerts no toxicity on fungal biomass. Therefore, the obtained results imply that the application of fungal cells as biological systems for betulin biotransformation can produce betulonic acid and for the first time, fungal-mediated biotransformation of betulin to betulonic acid was demonstrated.