Functional characterization of 3β-hydroxysteroid dehydrogenases in Asiatic toad reveals an efficient broad substrate specificity of Bg-3βHSD2.

3β-Hydroxysteroid dehydrogenases (3βHSDs) are key enzymes in steroid metabolism, catalyzing C3 oxidation-reduction and Δ5→Δ4 isomerization reactions that govern metabolic flux across multiple steroidogenic pathways. However, the functional diversity of 3βHSDs involved in bufadienolide metabolism in amphibians remains poorly explored. Here, we systematically characterized the 3βHSD gene family in the Asian toad (Bufo bufo gargarizans) using integrated transcriptomic, biochemical, and metabolomic analyses. Seven Bg-3βHSD genes were identified from multi-tissue transcriptomes generated under control and Pb2+ exposure conditions, and six were heterologously expressed for functional evaluation. In vitro assays revealed pronounced functional divergence among Bg-3βHSD isoforms. Bg-3βHSD1 primarily catalyzed bidirectional C3 redox reactions of C21 steroids and bile acid-related substrates, consistent with canonical steroidogenic roles. In contrast, Bg-3βHSD2 enzyme exhibited broad substrate specificity and high catalytic efficiency toward hormones, bile acids, and bufadienolides. In addition to canonical C3 redox reactions and Δ5→Δ4 isomerization, Bg-3βHSD2 also displayed additional oxidation activity at the C17 position for several steroid substrates. A third homolog, Bg-HSD3B7 (GenBank accession no. XM 044303756.1), selectively converted 7α-hydroxylated sterols, suggesting a potential role in classical bile acid metabolism. Integration of tissue-specific expression profiles with bufadienolide distribution patterns suggests that Bg-3βHSD2 may contribute to connecting classical steroid metabolism with bufadienolide biosynthesis in adrenal tissue. Together, the present study identifies Bg-3βHSD2 as an efficient and versatile steroid-transforming enzyme, expands our knowledge of functional diversity within the amphibian 3βHSD family, and provides insights into the enzymatic basis of steroid and bufadienolide metabolism in B. bufo gargarizans.