Fig. 6.

GAME25 enzymes play a key role in the formation of steroidal specialized metabolites in a proposed sequence of three reactions. A proposed three-step reaction sequence for the conversion of dehydrotomatidine to tomatidine in tomato, solanidine to demissidine in wild potatoes (e.g., S. chacoense), and solasodine to soladulcidine in certain Solanum species (e.g., S. dulcamara). Given our results, we propose a three-step reaction for the conversion of unsaturated steroidal saponin aglycone to saturated steroidal saponin aglycone. GAME25, a 3β-hydroxysteroid dehydrogenase/isomerase, performs the first step in this reaction sequence, producing 3-oxo-Δ^5,4 steroidal alkaloid/saponin aglycone derivatives from the respective unsaturated steroidal alkaloid/saponin aglycone substrates, which are further converted to saturated products by successive actions of putative 5-reductases and aldo-keto reductases, respectively. This multistep conversion partly resembles steroid metabolism in species such as Digitalis spp. that produce cardiac glycosides (cardenolides). Dashed arrows indicate multistep reactions.