Gut Bacterial 20-Hydroxysteroid Dehydrogenases Modify Endogenous Glucocorticoids and Corticosteroid Drugs

Bacterial 20-hydroxysteroid dehydrogenases (20-HSDHs) from the human gut are known to reduce the C-20 carbonyl group of cortisol, producing either 20α- or 20β-dihydrocortisol. Two 20-HSDHs from Bifidobacterium adolescentis L2-32 and Agathobaculum desmolans ATCC 43058, both members of the short-chain dehydrogenases/reductases superfamily, are known to produce 20β-dihydrocortisol, while the 20-HSDH from Clostridium scindens ATCC 35704, belonging to the zinc-containing alcohol dehydrogenase family, produces 20α-dihydrocortisol. These three enzymes were characterized for their activity toward cortisol and structurally related therapeutic corticosteroids. Kinetic analyses revealed narrow substrate specificity, with all enzymes preferring cortisol but maintaining significant activity for prednisone and prednisolone (k_cat/K_M values between 10³-10⁶ M^-1 s^-1 for the 20β-HSDH enzymes), and some detectable activity of the 20β-HSDHs to reduce triamcinolone. The 20β-HSDHs exhibited pH-dependent substrate inhibition, influencing their activity profile. Structural docking studies indicated that suitable substrates occupy a single productive binding mode within the 20β-HSDH enzyme active site. Our findings show that enzymes in the gut microbiome can metabolize corticosteroid drugs by reducing the 20-keto group, which could have implications for drug efficacy and side effects. This work highlights the importance of gut microbial enzymes in the biotransformation of both endogenous and therapeutic steroids, informing future research into drug-microbiome interactions and personalized medicine.