Distinct metabolic handling of 3β-hydroxy-17a-oxa-D-homo-5α-androstan-17-one by the filamentous fungus Aspergillus tamarii KITA: Evidence in support of steroid/hydroxylase binding hypothesis

Aspergillus tamarii KITA transforms progesterone in to testololactone in high yield through a sequential four-step enzymatic pathway which also has the flexibility to transform a range of steroidal substrates. This study has investigated the further metabolism of testololactone and a range of fully saturated steroidal lactone analogues. In contrast to testololactone, which even after 120 h incubation did not undergo further metabolism, the lactone analogues entered the minor hydroxylation pathway. Uniquely, after forming 3β-hydroxy-17a-oxa-D-homo-5α-androstan-17-one (48 h) 4 distinct positions on the steroid skeleton were monohydroxylated (11β, 6β, 7β, 11α) which geometrically relate to the four binding positions (normal, reverse, inverted normal and inverted reverse) possible within the steroidal hydroxylase(s). This is the first evidence demonstrating the four possible steroid/hydroxylase(s) binding interactions with a single molecule that has previously been hypothesized with a single organism. In addition a rare 1β-monohydroxylation was observed, this may be indicative of dehydration generating 1-ene functionality in A. tamarii rather than dehydrogenation as reported in man and microorganisms. The importance of these findings in relation to steroid/hydroxylase binding interactions is discussed.