UDP-glucuronic acid:soyasapogenol glucuronosyltransferase involved in saponin biosynthesis in germinating soybean seeds

Abstract.

We detected UDP-glucuronic acid:soyasapogenol glucuronosyltransferase (UGASGT) activity in the microsomal fraction from germinating soybean (Glycine max [L.] Merr.) seed. A microsomal fraction was isolated from germinating soybean seed and treated with various detergents to solubilize the enzyme. UGASGT activity was monitored throughout purification using UDP-[U-¹⁴C]glucuronic acid and soyasapogenol B as substrates. Purification of UGASGT was achieved by HiTrap Q, Superdex 200, and HiTrap Blue chromatography procedures. This resulted in >205-fold enrichment relative to the starting homogenate. UGASGT was found to require divalent cations for activity. Studies on the substrate specificity of UGASGT demonstrated that the specificity for the sugar residue transferred was very high, as activity was scarcely found when UDP-glucuronic acid was replaced by other UDP sugars: UDP-glucose and UDP-galactose. Soyasapogenols, which are the aglycons of soybean saponin, are usable acceptors, but glycyrrhetinic acid, sophoradiol, β-amyrin, and flavonoids are not. These findings suggest that this UGASGT was a specific enzyme for UDP-glucuronic acid as a donor and soyasapogenols as acceptors, and that it was related to the biosynthesis of the sugar chain in soybean saponin. This study provides a basis for the molecular characterization of a key enzyme in saponin biosynthesis in soybean. The isolation of the gene may enable its use in the elucidation of the biosynthesis and physiological role of saponins in soybean.