Characterization and Quantitation of Steryl Glycosides in Solanum melongena

Glycosylated plant sterols or steryl glycosides (SGs) are a small group of glycolipids occurring ubiquitously in plants. In contrast to free sterols, they are insufficiently characterized concerning structural variety, quantity, and biological function. In particular, the type of sugar usually attached to the C-3 hydroxy function of the respective sterol is poorly studied. Eggplants ( Solanum melongena) are rich in phytochemicals including SGs. In the present work, the unique glycosylation pattern was investigated by a highly selective LC-MS/MS method that allowed quantitation of the glucosides and galactosides of the most common sterols: cholesterol, β-sitosterol, campesterol, and stigmasterol. The quantitatively most important structure was β-sitosteryl β-d-glucopyranoside, with 54.5 mg/kg fresh weight of total fruit (365.3 mg/kg dry weight) followed by stigmasteryl β-d-glucopyranoside and campesteryl β-d-glucopyranoside. Analyses were performed in different tissues of eggplants (i.e., exocarp and outer mesocarp vs the remaining inner part). Steryl galactosides were determined in eggplants for the first time at significantly lower concentrations by a factor of 100. Furthermore, the rare SG β-sitosteryl β-d-cellobioside (3-β-sitosteryl β-d-glucopyranosyl-(1→4)-β-d-glucopyranoside) was detected in eggplants for the first time. Finally, UV irradiation induced the formation of the vitamin D glucosides 7-dehydrocholesteryl β-d-glucopyranoside and cholecalciferyl β-d-glucopyranoside at very low levels.