UDP-glucose: solasodine glucosyltransferase from eggplant [Solanum melongena L.] leaves: Partial purification and characterization

Uridine 5 '-diphosphoglucose-dependent glucosyltransferase which catalyzes the glucosylation of solasodine, i.e. UDP-glucose:solasodine glucosyltrans­ferase, is present in leaves, roots, unripe fruits and unripe seeds of eggplant (Solanum melongenaL.). The glucosylation product is chromatographically identical with authentic solasodine 3β-nmonoglucoside, a putative intermedi­ate in the biosynthesis of solasodine-based glycoalkaloids characteristic of the eggplant. The enzyme was purified about 50-fold from crude cytosol fraction of eggplant leaves by ammonium sulphate precipitation and column chromatog­raphy on Q-Sepharose and Sephadex G-100. The native enzyme has a molecular mass of approx. 55 and pH optimum of 8.5. metal ions are not required for its activity but the presence of free -SH groups is essential. Besides solasodine (Km = 0.04 ), the enzyme effectively glucosylates tomatidine, another steroidal alkaloid of the spirosolane type, but it is virtually inactive towards the solanidane-type steroidal alkaloids such as solanidine or demissid- ine. The enzyme is specific for UDP-glucose (Km= 2.1 u) since unlabelled ADP-, GDP-, CDP- or TDP-glucose could not effectively compete with UDP-[14]glucose used as the sugar donor for solasodine glucosylation. Moreover, no synthe­sis of labelled solasodine galactoside was observed when UDP-l14]glucose was replaced with UDP-[l4]galactose.