Enzymatic Deacetylation as a Plausible Final Step in Camptothecin Biosynthesis in Ophiorrhiza pumila

Camptothecin (CPT) is a monoterpenoid indole alkaloid produced in several plants and is a clinically important natural resource that serves as the precursor for the semi-synthetic anticancer drugs such as topotecan and irinotecan. CPT possesses a characteristic quinoline-containing scaffold; however, the molecular mechanisms underlying the downstream steps of its biosynthetic pathway remain unclear. In this study, we identified 17-O-acetylcamptothecinic acid (1) in hairy roots of Ophiorrhiza pumila. The abundance of compound 1 was markedly reduced following methyl jasmonate treatment, concomitant with increased accumulation of CPT. Metabolite profiling of CPT-producing plants revealed that compound 1 predominantly accumulates in the roots of O. pumila, O. kuroiwai, and Nothapodytes nimmoniana, but was not detected in Camptotheca acuminata. Clear conversion of compound 1 into CPT was observed upon incubation with cell lysates prepared from O. pumila hairy roots as well as under chemical deacetylation conditions. These findings suggest that 17-O-acetylcamptothecinic acid is a plausible final biosynthetic intermediate of CPT in O. pumila and that a protection-deprotection strategy may operate in CPT biosynthesis, similar to those reported for other alkaloid pathways. Furthermore, the absence of this acetylated intermediate in C. acuminata suggests possible convergent evolution of CPT biosynthetic mechanisms among distinct plant lineages.