Site-selective biotransformation of ursane triterpenes by Streptomyces griseus ATCC 13273

Abstract

The oxidization of unactivated C-H bonds of pentacyclic triterpenes (PTs) is of great interest for the structural modification of PTs. Herein, we discovered the unique capability of Streptomyces griseus ATCC 13273 to catalyze the site-selective oxidation of the C-30 methyl group to the carboxyl group and hydroxylation of the C-24 methyl group over a range of ursane triterpenes, including ursolic acid (1), 3-oxo ursolic acid (2), and corosolic acid (3). It is noteworthy that while using asiatic acid (4) and madasiatic acid (5), which bear one hydroxyl group on C-23 as substrates, the hydroxylation on C-24 was blocked. As a result, eight new compounds (1a-3a, 5a, 1b-3b and 5b) of the metabolites were isolated and their structures were elucidated based on 1D and 2D NMR and HR-MS data. In addition, the cytotoxicity of substrates and transformed products was preliminarily evaluated by an MTT assay.