Comparative metabolism of four limonoids in human liver microsomes using ultra-high-performance liquid chromatography coupled with high-resolution LTQ-Orbitrap mass spectrometry.

Rationale: Limonoids, characterized by a triterpenoid skeleton with a furan ring, are unique secondary metabolites widely distributed in the families of Rutaceae, particularly in Citrus species and Meliaceae. Studies on health benefits have demonstrated that limonoids have a range of biological activities. Dietary intake of citrus limonoids may provide a protective effect against the onset of various cancers and other xenobiotic related diseases. However, few studies about the metabolic profiles of limonoids have been carried out.

Methods: The objectives of this study were to investigate the metabolic profiles of four limonoids (limonin, obacunone, nominin and gedunin) in human liver microsomes (HLMs) using ultra-high-performance liquid chromatography coupled with high-resolution mass spectrometry (UHPLC/HRMS) and to identify the cytochrome P450 (CYP) enzymes involved in the formation of their metabolites by recombinant human CYP enzymes.

Results: Based on the accurate HR-MS/MS spectra and the proposed MS/MS fragmentation pathways, four metabolites of limonin (M1-1, M1-2, M1-3 and M1-4), eight metabolites ofobacunone (M2-1, M2-2, M2-3, M2-4, M2-5, M2-6, M2-7 and M2-8), six metabolites of nominin (M3-1, M3-2, M3-3, M3-4, M3-5 and M3-6) and three metabolites of gedunin (M4-1, M4-2 and M4-3) in HLMs were tentatively identified and the involved CYPs were investigated.

Conclusions: The results demonstrated that reduction at C-7 and C-16, hydroxylation and reaction of glycine with reduction limonoids were the major metabolic pathways of limonoids in HLMs. Among them, glycination with reduction was the unique metabolic process of limonoids observed for the first time. CYP2D6 and CYP3A4 played an important role in the isomerization and glycination of limonoids in HLMs, whereas other CYP isoforms were considerably less active. The results might help to understand the metabolic process of limonoids in vitro such as the unidentified metabolites of limonin glucoside observed in the medium of microbes and the biotransformation of limonin in juices. Moreover, it would be beneficial for us to further study the pharmacokinetic behavior of limonoids in vivo systematically.