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Thousands of specialized, steroidal metabolites are found in a wide spectrum of plants. These include the steroidal glycoalkaloids (SGAs), produced primarily by most species of the genus Solanum , and metabolites belonging to the steroidal saponins class that are widespread throughout the plant kingdom. SGAs play a protective role in plants and have potent activity in mammals, including antinutritional effects in humans. The presence or absence of the double bond at the C-5,6 position (unsaturated and saturated, respectively) creates vast structural diversity within this metabolite class and determines the degree of SGA toxicity. For many years, the elimination of the double bond from unsaturated SGAs was presumed to occur through a single hydrogenation step. In contrast to this prior assumption, here, we show that the tomato GLYCOALKALOID METABOLISM25 (GAME25), a short-chain dehydrogenase/reductase, catalyzes the first of three prospective reactions required to reduce the C-5,6 double bond in dehydrotomatidine to form tomatidine. The recombinant GAME25 enzyme displayed 3β-hydroxysteroid dehydrogenase/Δ 5,4 isomerase activity not only on diverse steroidal alkaloid aglycone substrates but also on steroidal saponin aglycones. Notably, GAME25 down-regulation rerouted the entire tomato SGA repertoire toward the dehydro-SGAs branch rather than forming the typically abundant saturated α-tomatine derivatives. Overexpressing the tomato GAME25 in the tomato plant resulted in significant accumulation of α-tomatine in ripe fruit, while heterologous expression in cultivated eggplant generated saturated SGAs and atypical saturated steroidal saponin glycosides. This study demonstrates how a single scaffold modification of steroidal metabolites in plants results in extensive structural diversity and modulation of product toxicity., Competing Interests: The authors declare no conflict of interest.

The Colorado potato beetle Leptinotarsa decemlineata (Say) (CPB) is a coleopteran herbivore that feeds on the foliage on Solanum species, in particular, potato. Six resistant wild Solanum species were identified, and two of these species had low levels of glycoalkaloids. Comparative analysis of the untargeted metabolite profiles of the foliage using UPLC-qTOF-MS was done to find metabolites shared between the wild species but not with Solanum tuberosum (L.) to identify resistance-related metabolites. It was found that only S. tuberosum produced the triose glycoalkaloids solanine and chaconine. Instead, the six wild species produced glycoalkaloids that shared in common tetrose sugar side chains. Additionally, there were non-glycoalkaloid metabolites associated with resistance including hydroxycoumarin and a phenylpropanoid, which were produced in all wild species but not in S. tuberosum.

The C-26 amino group of tomatine, a representative Solanaceae steroidal alkaloid, is introduced in an early step of its biosynthesis from cholesterol. We recently proposed a transamination mechanism for the C-26 amination as opposed to the previously proposed mechanism involving a nitrogen nucleophilic displacement. In the present study, a deuterium labeled C-26 aldehyde, (24,24,27,27,27-(2)H5)-3β-hydroxycholest-5-en-26-al, was synthesized and fed to a tomato (Solanum lycopersicum) seedling. LC-MS analysis of the biosynthesized tomatine indicated that the labeled aldehyde was incorporated into tomatine. The finding strongly supports the intermediacy of the aldehyde and the transamination mechanism during C-26 amination., (Copyright © 2014. Published by Elsevier Ltd.)

The degradation fo steroid glycoalkaloids (SGAs) has been studied in model solutions. The number of colony forming units (CFU) was determined using a nondirect (cultivation) method during all stages of fermentation. The changes in SGAs content were observed by HPLC on the Supelcosil LC-NH2 column. The changes in alpha-tomatine concentration added to fermented Lactobacillus MRS broth have been studied. A mathematical model of steroid glycoalkaloids degradation during lactic fermentation was proposed. The mathematical model was based on the experimental data of SGA and glucose concentration and should be used for study and prediction of SGA concentration changes of fermented samples. The ratio of SGA degradation rate by fermentation and by lactic acid hydrolysis was calculated. The experimental data evaluated by proposed mathematical model for the selected strain of Lactobacillus plantarum 976H show real feasibility for SGA degradation by lactic acid fermentation.

The microbial dehydrogenation of steroid alkaloids follows the dehydrogenation pattern of steroids until the 3-keto-1,4-diene stage. No side-chain cleavage or degradation of the steroid nucleus is observed. Side-chain cleavage of tomatidine is achieved only by previous induction of side-chain splitting enzymes.

The bioconversion of steroid alkaloid tomatine by Nocardia restricta yields the conjugate with lactic acid. We studied the bioconversion of some steroid glycosides without a nitrogen atom in the molecule to determine the effect of the nitrogen atom. The glycosides were of three different types: sterol glycosides, bufadienolide rhamnoside and steroid saponine. The results of bioconversions showed that Nocardia restricta converts steroid glycosides differently according to the sugar bound to the steroid aglycone. It can be concluded that in the absence of a nitrogen atom in the steroid molecule no conjugation with lactic acid by Nocardia restricta occurs.

In the degradation pathway of the steroid alkaloid tomatidine by Gymnoascus reesii the A-ring of tomatidine is opened with the formation of the 4-hydroxy-3,4-secotomatidine-3-oic acid, which was identified in the form of N-acetyl-3,4-tomatidine-carbolactone by mass, IR and 1H NMR spectra. Cleavage of the A-ring in the starting reaction indicates that an alternative pathway must be operating, instead of the general oxidative one.

(22S,25S)-5alpha-tomatanin-3beta-ol, N-acetyl-(22S,25S)-5alpha-tomatanin-3beta-ol, (22R,25R)-5alpha-tomatanin-3beta-ol and (22R, 25S)-22, 26-epimino-5alpha-cholestane-3beta, 16beta, 16beta-diol are transformed by Nocardia restrictus into corresponding 3alpha-ol compounds with yields from 7o to 5%.