Species-related difference between limonin and obacunone among five liver microsomes and zebrafish using ultra-high-performance liquid chromatography coupled with a LTQ-Orbitrap mass spectrometer

RATIONALE Limonin and obacunone are two major limonoids distributed in the Rutaceae and Meliaceae families. Their defined anti‐tumor activity is closely connected with the furan ring and the multi‐carbonyls in their structures. In vivo and in vitro biotransformations may influence their structures and further change their effects. The metabolic profiles of limonin and obacunone have not been studied previously. In order to clarify their in vivo and in vitro metabolism, a comparative investigation of their metabolic pathways in five different species of liver microsomes and zebrafish was carried out. METHODS In the present study, ultra‐high‐performance liquid chromatography coupled with high‐resolution mass spectrometry (UHPLC/HRMS) and related electrospray ionization (ESI) tandem mass spectrometric (MS/MS) dissociation of limonin and obacunone were applied for the analysis. Each metabolite was identified by its accurate mass data. Human liver microsomes (HLMs), monkey liver microsomes (MLMs), dog liver microsomes (DLMs), rat liver microsomes (RLMs), mice liver microsomes (XLMs) and zebrafish were included in the biotransformations. RESULTS One phase I metabolite of limonin (M1‐1) and two phase I metabolites of obacunone (M2‐1, M2‐2) were identified by accurate mass measurement and MS/MS fragmentation behaviors. A reduction reaction was regarded as the major metabolic pathway of limonoids in liver microsomes. The reduction reaction site of M1‐1 and M2‐1 was at the C‐16 carbonyl, while for M2‐2 it was at C‐7. M1‐1 was the major and unique metabolite of limonin and the metabolic rate of limonin varied from 11.5% to 17.8% in liver microsomes (LMs). M2‐2 was the main metabolite of obacunone in LMs and zebrafish. M1‐1 and M2‐1were only detected in LMs while M2‐2 was found in both LMs and zebrafish incubation systems. The metabolic rate of obacunone varied from 2.5% to 19.1% and the content of M2‐2 was about five times higher than that of M2‐1. CONCLUSIONS The ESI‐HR‐MS/MS fragmentation behaviors of limonin and obacunone were investigated for the first time. A qualitative and semi‐quantitative method was developed for the in vivo and in vitro metabolic analysis of limonin and obacunone. The results demonstrated that the metabolic processes of limonin and obacunone were different between LMs and zebrafish. However, both of these two parent compounds presented similar metabolic processes in five species of LMs. This was caused by the metabolic difference between ma