Cholesterol, an essential lipid for mammalian cells, can be used as a carbon source by bacteria and as a precursor for steroid hormones in animal cells. The cholesterol C17 side-chain-cleavage pathways are the committed and rate-limiting steps in the biosynthesis of steroid drugs. Three cholesterol C17 side-chain degradation pathways have been identified in nature: the β-oxidation pathways in actinobacteria, the oxygen-independent degradation pathway in Sterolibacterium denitrificans, and the cholesterol side-chain degradation pathway in mammals. An in-depth understanding of the structures and molecular mechanisms of enzymes in these degradation processes will facilitate the creation of enzyme mutants with better catalytic capabilities. The introduction of the engineered enzymes into the microbial cell factories may contribute to the industrial production of steroid drugs.

Diosgenin (DSG), a steroidal sapogenin derived from the tuberous roots of yam, possesses multiple biological properties. DSG has been widely used as a starting material for the industrial production of steroid drugs. Despite its significant pharmacological activities, moderate potency and low solubility hinder the medicinal application of DSG. Biotransformation is an efficient method to produce valuable derivatives of natural products. In this work, we performed the biotransformation of DSG using five Rhodococcus strains. Compounds 1–4 were isolated and identified from Rhodococcus erythropolis. Compounds 1 and 2 showed potent cytotoxicity against the A549, MCF-7, and HepG2 cell lines. Compounds 3 and 4 are novel entities, and each possesses a terminal carboxyl group attached to the spiroacetal ring. Remarkably, 4 exhibited significant cell protective effects for kidney, liver, and vascular endothelial cells, suggesting the therapeutic potential of this compound in chronic renal diseases, atherosclerosis, and hypertension. We further optimized the fermentation conditions aiming to increase the titer of compound 4. Finally, the yield of compound 4 was improved by 2.9-fold and reached 32.4 mg/L in the optimized conditions. Our study lays the foundation for further developing compound 4 as a cell protective agent.