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

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 plantaCO labeling experiments monitored by nuclear magnetic resonance spectroscopy (NMR) and high-resolution mass spectrometry (HRMS) unraveled the SGA biosynthetic pathways from CO photosynthates via early precursors to the SGAs. After a pulse of ~ 700 ppm CO for four hours, followed by a chase period for seven days, specific C-distributions were detected in SGAs from the leaves of the labeled plant. NMR analysis determined the positional C-enrichments in α-solanine and α-chaconine characterized by C-pairs in their aglycones. These patterns were in perfect agreement with a mevalonate-dependent biosynthesis of the isopentenyl diphosphate and dimethylallyl diphosphate precursors. The C-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.