Conversion of furostanol glycoside to spirostanol glycoside by beta-glucosidase in Costus speciosus

A diosgenin-rich plant, Costus speciosus (Koenig) Smith was found to contain beta-glucosidase which converts a furostanol glycoside, protogracillin, to a spirostanol glycoside, gracillin. It was demonstrated that the enzyme was present in rhizomes of plants grown in a greenhouse and whole parts of plantlets cultured in vitro. The soluble enzyme from the cultivated rhizomes showed pH optima of 5.0 and the apparent K-m for protogracillin of 0.10 mM. Activity of the enzyme was inhibited by typical beta-glucosidase inhibitors, such as D-glucono-1,5-lactone and conduritol-beta-epoxide.