Anaerobic and aerobic cleavage of the steroid core ring structure by Steroidobacter denitrifi cans

The aerobic degradation of steroids by bacteria has been studied in some detail. In contrast, only little is known about the anaerobic steroid catabolism. Steroidobacter denitrifi cans can utilize testosterone under both oxic and anoxic conditions. By conducting metabolomic investigations, we demonstrated that S. denitrifi cans adopts the 9,10- seco-pathway to degrade testosterone under oxic conditions. This pathway depends on the use of oxygenases for oxygenolytic ring fi ssion. Conversely, the detected degradation intermediates under anoxic conditions suggest a novel, oxygenase- independent testosterone catabolic pathway, the 2,3- seco-pathway, which differs signifi cantly from the aerobic route. In this anaerobic pathway, testosterone is fi rst transformed to 1-dehydrotestosterone, which is then reduced to produce 1-testosterone followed by water addition to the C-1/C-2 double bond of 1-testosterone. Subsequently, the C-1 hydroxyl group is oxidized to produce 17-hydroxy-androstan- 1,3-dione. The A-ring of this compound is cleaved by hydrolysis as evidenced by H 2 18 O-incorporation experiments. Regardless of the growth conditions, testosterone is initially transformed to 1-dehydrotestosterone. This intermediate is a divergence point at which the downstream degradation pathway is governed by oxygen availability. Our results shed light into the previously unknown cleavage of the sterane ring structure without oxygen. We show that, under anoxic conditions, the microbial cleavage of steroidal core ring system begins at the A-ring.