Fungal products. Part XVI. Conversion of isosteviol and steviol acetate into gibberellin analogues by mutant B1–41a of Gibberella fujikuroi and the preparation of [3H]gibberellin A20
Isosteviol (ent-16-oxobeyeran-19-oic acid) and steviol acetate (ent-13-acetoxykaur-16-en-19-oic acid) are efficiently metabolised by cultures of resuspended mycelium of Gibberella fujikuroi, mutant B1–41a. Isosteviol is converted, inter alia, into ring-CD-rearranged derivatives of gibberellins A17, A19, A20, and 13-hydroxygibberellin A12. Steviol acetate is metabolised mainly to the 7β-hydroxy- and 6β, 7β-dihydroxy-derivatives and to the 13-acetates of gibberellins A17 and A20. The preparation of [3H]steviol acetate and its conversion into [3H]gibberellin A20 of high specific activity is described. These results provide further evidence that the fungal enzymes for gibberellin biosynthesis from ent-kaurenoic acid lack substrate specificity. They also reveal that structural changes in the CD ring system of ent-kaurenoid substrates suppress the 3-hydroxylating enzyme.