Transformation of structurally diverse steroidal analogues by the fungus Corynespora cassiicola CBS 161.60 results in generation of 8β-monohydroxylated metabolites with evidence in favour of 8β-hydroxylation through inverted binding in the 9α-hydroxylase

Corynespora cassiicola has a unique but unexplored ability amongst fungi, in that it can hydroxylate 17α-hydroxyprogesterone at the highly hindered C-8 position of the steroid nucleus. In order to gain greater understanding of the mechanistic basis and capability of the 8β-hydroxylase we have transformed a range of structurally diverse androgens and progestogens with this organism. This has revealed that both steroid types can be hydroxylated at the 8β-position. The collective data has demonstrated the first time that 8β-hydroxylation occurs through inverted binding within a 9α-hydroxylase of the fungus. In the case of the progestogens, for this to occur, the presence of 17α-oxygen functionality (alcohol or epoxide) was essential. Remarkably monohydroxylation of 17α-hydroxyprogesterone at carbons 8β and 15β has strongly indicated that the responsible hydroxylase has 2 different binding sites for the ring-A ketone. Unusually, with one exception, all hydroxylation occurred at axial protons and in the case of the progestogens, all above the plane of the ring system. In general all maximally oxidised metabolites contained four oxygen atoms. The importance of these findings in relation to 8β-hydroxylation of these steroids is discussed.