Formation of series reactive dialdehyde intermediates by metabolism of monotropein

Monotropein, an iridoid glycoside, is the principal bioactive constituent with the highest amount isolated from Morindae officinalis radix, which has been reported to have potent pharmacological properties. Monotropein contains a hemiacetal structure. The hemiacetal may be converted into a reactive dialdehyde intermediate and covalently bind to the primary amine. Therefore, we speculated that the metabolites of monotropein with hemiacetal can generate dialdehyde intermediates, a class of highly reactive metabolites that enable covalent modification of proteins. This study characterized the metabolic profiling of monotropein, especially the unstable dialdehyde intermediates, using UPLC-Q/Orbitrap MS. A total of eighteen stable metabolites were characterized in monotropein-treated rats. The main metabolic pathways were dehydration, deglycosylation, and decarboxylation. It is noteworthy that six unstable dialdehyde intermediates of monotropein were characterized for the first time. They were the hemiacetal ring-opening products of M4 (monotropein aglycone), M5 (decarboxylated aglycone), M6 (methylated aglycone), M7 (dehydrated aglycone), M8 (hydrogenated aglycone), and M9 (dehydroxymethyl aglycone). The dialdehyde intermediates were identified by their stable dihydropyridine conjugates with amino acids. Five dialdehyde intermediates enabled covalent modification of proteins in the intestine. In conclusion, eighteen stable metabolites and six reactive dialdehyde intermediates were characterized in this study, which contributes to understanding the pharmacological effects of monotropein.