Biosynthesis of α-solanine and α-chaconine in potato leaves (Solanum tuberosum L.) - A 13CO2 study

Abstract

α-Solanine and α-chaconine are the major glycoalkaloids (SGAs) in potatoes, but up to now the biosynthesis of these saponins is not fully understood. In planta¹³CO₂ labeling experiments monitored by nuclear magnetic resonance spectroscopy (NMR) and high-resolution mass spectrometry (HRMS) unraveled the SGA biosynthetic pathways from CO₂ photosynthates via early precursors to the SGAs. After a pulse of ~ 700 ppm ¹³CO₂ for four hours, followed by a chase period for seven days, specific ¹³C-distributions were detected in SGAs from the leaves of the labeled plant. NMR analysis determined the positional ¹³C-enrichments in α-solanine and α-chaconine characterized by ¹³C₂-pairs in their aglycones. These patterns were in perfect agreement with a mevalonate-dependent biosynthesis of the isopentenyl diphosphate and dimethylallyl diphosphate precursors. The ¹³C-distributions also suggested cyclization of the 2,3-oxidosqualene precursor into the solanidine aglycone backbone involving a non-stereoselective hydroxylation step of the sterol a mixture of 25S-/25R-epimers of the SGAs.

Keywords: (13)CO(2); Isotopologue profiling; Potato; Solanum tuberosum L.; Steroidal glycoalkaloids, α-solanine; α-Chaconine.