Biotransformation of labdane and halimane diterpenoids by two filamentous fungi strains

Biotransformation of natural products by filamentous fungi is a powerful and effective approach to achieve derivatives with valuable new chemical and biological properties. Despite diterpenoid substrates exhibit good susceptibility towards fungi enzymes, there have been no studies concerning the microbiological transformation of halimane-type diterpenoids up to now. In this work, we investigated the capability of Fusarium oxysporum (a fungus isolated from the rhizosphere of Senna spectabilis) and Myrothecium verrucaria (an endophyte) to transform halimane (1) and labdane (2) acids isolated from Hymenaea stigonocarpa (Fabaceae). Feeding experiments resulted in the production of six derivatives, including hydroxy, oxo, formyl, and carboxy analogues. Incubation of 1 with F. oxysporum afforded 2-oxo-derivative (3), while bioconversion with M. verrucaria provided 18,19-dihydroxy (4), 18-formyl (5), and 18-carboxy (6) bioproducts. Transformation of substrate 2 mediated by F. oxysporum produced a 7-hydroxy (7) derivative, whilst M. verrucaria yielded 7- (7) and 3-hydroxy (8) metabolites. Unlike F. oxysporum, which showed a preference to transform ring B, M. verrucaria, exhibited the ability to hydroxylate both rings A and B from substrate 2. Additionally, compounds 1–8 were evaluated for inhibitory activity against Hr-AChE and Hu-BChE enzymes through ICER-IT-MS/MS assay.

利用丝状真菌对天然产物实施生物转化，是获取兼具新颖化学特性与生物活性价值衍生物的高效可靠途径。尽管二萜类底物对真菌酶类展现出良好的可转化性，但截至目前，尚未见关于halimane型二萜类化合物（halimane-type diterpenoids）微生物转化的研究报道。
本研究考察了尖孢镰刀菌（Fusarium oxysporum，分离自长穗决明Senna spectabilis根际）与疣孢漆斑菌（Myrothecium verrucaria，一种内生真菌），对分离自豆科（Fabaceae）植物Hymenaea stigonocarpa的halimane型二萜酸（1）以及labdane型二萜酸（2，labdane-type diterpenoid acids）的转化能力。
底物饲喂实验共获得6种衍生物，涵盖羟基、氧代、甲酰基与羧基类类似物。将底物1与尖孢镰刀菌共孵育，得到2-氧代衍生物（3）；而疣孢漆斑菌介导的生物转化则得到18,19-二羟基产物（4）、18-甲酰基产物（5）与18-羧基产物（6）。
尖孢镰刀菌介导的底物2转化得到7-羟基衍生物（7）；疣孢漆斑菌则得到7-羟基衍生物（7）与3-羟基代谢物（8）。相较于偏好修饰底物2的B环的尖孢镰刀菌，疣孢漆斑菌可对底物2的A环与B环均进行羟基化修饰。
此外，本研究通过ICER-IT-MS/MS实验法，对化合物1至8的Hr-AChE与Hu-BChE酶抑制活性开展了评价。