Microbial Transformation of ent-Kaurenoic Acid and Its 15-Hydroxy Derivatives by the SG138 Mutant of Gibberella fujikuroi

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Abstract
Feeding experiments with ent-kaurenoic acid (4), 15α-hydroxy-ent-kaurenoic acid (5), 15β-hydroxy-ent-kaurenoic acid (6), and mixtures of 4 plus 5 and 4 plus 6 were conducted using the SG138 mutant of Gibberellafujikuroi, to gain information about the phenotype of this unique strain. The biotransformation of 5 gave 7β,15α-dihydroxykaurenolide (9) and 7β,15α-dihydroxy-ent-kaurenoic acid (13). The incubation of 6 produced 7β,15β-dihydroxy-ent-kaurenoic acid (7) and 7β,15β-dihydroxykaurenolide (10). No 15-hydroxylated gibberellins were detected in any of these experiments. The results indicated that a hydroxy group at C-15 does not inhibit 7β-hydroxylase activity but in the SG138 strain obstructs the enzymatic ring-B contraction of ent-kaurenoids to gibberellins. Exogenous 4 stimulated both the excretion of ent-kaurene and the fungal metabolism of 5 and 6.