Metabolites involved in aerobic degradation of the A and B rings of estrogen

Various bacteria, mainly actinobacteria and proteobacteria, are capable of aerobic estrogen degradation. In a previous study, we used the obligate aerobic alphaproteobacterium Sphingomonas sp. strain KC8 as a model microorganism to identify the initial metabolites involved in the oxygenolytic cleavage of the estrogen A ring: 4-hydroxyestrone, a meta-cleavage product, and a dead-end product pyridinestrone acid. In this study, we identified the downstream metabolites of this aerobic degradation pathway using ultraperformance liquid chromatography-high-resolution mass spectrometry (UPLC-HRMS). 4-Norestrogen-5(10)-en-3-oyl-coenzyme A and its closely related deconjugated (non-coenzyme A [non-CoA]) structure, 4-norestrogenic acid, were detected in the estrone-grown strain KC8 cultures. The structure of 4-norestrogenic acid was elucidated using nuclear magnetic resonance (NMR) spectroscopy. The extracellular distribution and the accumulation of 4-norestrogenic acid in the bacterial cultures indicate that the estrogen-degrading bacteria cannot degrade this deconjugated product. We also observed temporal accumulation and subsequent consumption of a common steroid metabolite, 3aα-H-4α(3=-propanoate)- 7aβ-methylhexahydro-1,5-indanedione (HIP), in the bacterial cultures. The metabolite profile and genomic analyses shed light on the biochemical mechanisms involved in the degradation of the A and B rings of natural estrogens. In this proposed aerobic pathway, C-4 of the meta-cleavage product is removed by a 2-oxoacid oxidoreductase through oxidative decarboxylation to produce the 4-norestrogen-5(10)-en-3-oyl- CoA. Subsequently, the B ring is cleaved by hydrolysis. The resulting A/B-ringcleaved product is transformed into a common steroid metabolite HIP through β-oxidation reactions. Accordingly, the A and B rings of different steroids are degraded through at least three peripheral pathways, which converge at HIP, and HIP is then degraded through a common central pathway.