Pathway of 17β-estradiol degradation by Nitrosomonas europaea and reduction in 17β-estradiol-derived estrogenic activity

In recent years, natural and synthetic estrogens have been recognized as endocrine disruptors in aquatic organisms. Although natural and synthetic estrogens are known to be degraded by microbes, only limited information about their degradation pathways is available. Here, we studied the degradation pathways of a natural estrogen, 17β-estradiol, by the nitrifying microorganism Nitrosomonas europaea, and we determined whether the degradation products of 17β-estradiol had estrogenic activity. To identify the degradation products, we subjected the culture solution to solid-phase extraction, and the extract was analyzed by gas chromatography–mass spectrometry. The potential estrogenic activity of the degradation products was investigated by means of a yeast two-hybrid assay. 1,3,5(10),16-Estratetraen-3-ol (estratetraenol) was newly identified as a degradation intermediate produced by dehydration of 17β-estradiol. Estratetraenol was also degraded by N. europaea, and its degradation rate was faster than that of 17β-estradiol. The two-hybrid assay confirmed that estratetraenol acted as a ligand for the estrogen receptor; estratetraenol thus has potential estrogenic activity. N. europaea eliminated the estrogenic activity derived from 17β-estradiol. This paper is the first to report dehydration as a mechanism of microbial estrogen degradation.