UDP-glucose:solasodine glucosyltransferase from eggplant (Solarium melongena L.) leaves: Partial purification and characterization

Abstract

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 melongena L.). The glucosylation product is chromatographically identical with authentic solasodine 3β-D-monoglucoside, 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 chromatog-raphy on Q-Sepharose and Sephadex G-100. The native enzyme has a molecular mass of approx. 55 kDa and pH optimum of 8.5. Divalent metal ions are not required for its activity but the presence of free -SH groups is essential. Besides solasodine (Km = 0.04 μM), 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 demissidine. The enzyme is specific for UDP-glucose (Km = 2.1 μM) since unlabelled ADP-, GDP-, CDP- or TDP-glucose could not effectively compete with UDP-[14C]glucose used as the sugar donor for solasodine glucosylation. Moreover, no synthesis of labelled solasodine galactoside was observed when UDP-[14C]glucose was replaced with UDP-[14C]galactose.