Use of [(13)C-methyl]Tropinone to follow the fate of the methyl group during calystegine formation in root cultures of Solanum dulcamara (Solanaceae)
Résumé
Tropane alkaloids are a class of naturally-occurring compounds, found mainly in the Solanaceae. They display anticholinergic, anti-emetic and parasympatholytic activities and are used in a variety of medicinal applications. Examples of the medical exploitation of tropane alkaloids include the use of hyoscyamine, to treat gastrointestinal tract disorders and bladder spasms, and of scopolamine to treat motion sickness. Roots are the major organ of tropane alkaloid biosynthesis. The pathway has been elucidated in a number of Solanaceae [1, 2]. Tropinone, produced from putrescine and acetate, is reduced by two different enzymes, tropinone reductase I and tropinone reductase II, which catalyse the formation of two stereo-isomeric alcohols, tropine and pseudotropine, respectively. Tropine is esterified to yield hyoscyamine and scopolamine, while pseudotropine undergoes N-demethylation and conversion to calystegines, a class of polyhydroxy-nortropane alkaloids. This study reports the metabolism of [13C-methyl]tropinone fed to root cultures of Solanum dulcamara, in order to shed light on the fate of the methyl group. The metabolic outcome of 13C-tropinone was analysed by NMR and MS over time, exploiting an approach that has already proved to be a powerful tool to investigate the N-demethylation of nicotine in Nicotiana cell cultures [3, 4]. It was revealed that 13C-tropinone was metabolised to 13C-pseudotropine very rapidly. After 6 days of feeding, asparagine and glutamate were found to be labelled, showing clearly that the N-methyl group can be recycled into primary metabolism. Intriguingly, label is not redistributed in the same manner as found for nicotine demethylation [3].