Fig. 1

Schematic profiles of the catabolism of sterol side-chain. Sterols share a conserved degradation pathway. Thus, β-sitosterol was used as a model substrate to depict the route of sterols to target product 9-OHAD (in green color). This process involved multi-step NAD⁺-dependent (in blue color) and FAD-dependent enzymatic reactions (in red color). The re-oxidation of FAD depended on oxygen. Oxygen acted as an electron acceptor and was reduced to H₂O₂ during the reaction. Further, the metabolism of acetyl-CoA and propionyl-CoA (in orange color) also involved the utilization of FAD. Calculations of the NAD⁺ and FAD requirement and the generation of NADH and FADH₂ during the conversion of sterols into 9-OHAD are listed at the bottom. 9-OHAD, 9a-hydroxy-androst-4-ene-3,17-dione; AD, androst-4-ene-3, 17-dione; ADD, androst-1,4-diene-3,17-dione