Substrate specificity for the 2α-hydroxylation of ursolic acid by Alternaria alternata and the antitumor activities of those metabolites

Ursolic acid (UA, 1), a major bioactive constituent of apple peels, exhibits a wide spectrum of biological activities, including antioxidant, antitumor, anti-inflammatory, and hypoglycemic properties. The bioconversion of 1 by Alternaria alternata was investigated, and 8 metabolites were isolated and identified. The chemical structures of these metabolites were found to be 2α,3β-dihydroxyurs-12-en-28-oic acid (corosolic acid, 2), urs-12-en-2α,3β,28-triol (3), 3β,23-dihydroxyurs-12-en-28-oic acid (4), 2α,3β,23-trihydroxyurs-12-en-28-oic acid (5), 2α,3β,23,24-tetrahydroxyurs-12-en-28-oic acid (6), 3β,28-dihydroxy-12-ursene (7), urs-12-en-3β-ol (8), and urs-12-en-2α,3β-diol (9), as determined through chemical and spectroscopic analyses. Among these metabolites, 2 was found to be the main bioconversion product, with a yield of 77.6%. The proposed biosynthetic pathways of UA by A. alternata were drawn and are presented in this paper. Metabolites 2, 5 and 6 showed potent antiproliferative activities against human breast cancer (MCF-7, MDA-MB-231), human colon cancer (Caco-2) and human liver cancer (HepG2) cell lines.