Tunable Toxicity of Bufadienolides is Regulated through a Configuration Inversion Catalyzed by a Short‐Chain Dehydrogenase/Reductase

Bufadienolides are toxic components widely found in amphibious toads that exhibit a wide range of biological activities. Guided by UPLC‐QTOF‐MS analysis, several 3‐epi‐bufadienolides with unique structures were isolated from the bile of the Asiatic toad, Bufo gargarizans. However, the enzymatic machinery of this epimerization in toads and its significance in chemical ecology remains poorly understood. Herein, we firstly compared the toxicities of two typical bufadienolides, bufalin (featuring a 14β‐hydroxyl) and resibufogenin (containing a 14, 15‐epoxy group), with their corresponding 3‐epi isomers in a zebrafish model. The results of the toxicology assays showed that the ratio of maximum non‐toxic concentrations of these two pairs of compounds are 256 and 96 times, respectively, thereby indicating that 3‐hydroxyl epimerization leads to a significant decrease in toxicity. Aiming to investigate the biotransformation of 3‐epi bufadienolides in toads, we applied liver lysate to transform bufalin and found that it could stereoselectively catalyze the conversion of bufalin into its 3α‐hydroxyl epimer. Following this, we cloned and characterized a short‐chain dehydrogenase/reductase, HSE‐1, from the toad liver cDNA library and verified its 3(β→α)‐hydroxysteroid epimerization activity. To the best of our knowledge, this is the first hydroxyl epimerase identified from amphibians that regulates the toxicity of animal‐derived natural products.