To study the biotransformation of 16-O-demethylaconitine in human intestine, aconitine was incubated with human intestinal bacteria in vitro. The metabolites of 16-O-demethylaconitine were investigated using ion trap and Fourier transform ion cyclotron resonance electrospray ionization tandem mass spectrometry directly. Aconitum alkaloids formed protonated molecule [M＋H]^＋ in the positive ion mode. It was found that 16-O-demethylaconitine from biotransformation of aconitine could be further transformed by human intestinal bacteria. There are new metabolites such as mono-ester alkaloids, diester alkaloids and lipo-alkaloids produced by deacetylation, debenzoylation, dehydroxylation, demethylation and more than twenty kinds of esterification in the further transformation process of the 16-O-demethylaconitine. The 16-O- lipodemethylaconitines have been first reported as a metabolite of aconitine by human intestinal bacteria.

Key words: 16-O-demethylaconitine, human intestinal bacteria, biotransformation, electrospray ionization mass spectrometry