Beta-glucosidase involved in saponin metabolism. Highly specific for the cleavage of C-26-bound glucose moiety of furostanol glycosides such as protogracillin and protodioscin. No activity with nuatigenin glycoside. Convers furostanol glycosides to spirostanol glycosides. H2O + protodioscin = 26-deglucoprotodioscin + D-glucose Partially inhibited by glucono-1,5-lactone, conduritol beta-epoxide and diosgenin, but not by beta-sitosterol or cholesterol. Optimum pH is 5.0-5.5. Heterodimer. The N-terminus of the larger subunit is blocked and the smaller subunit might be derived from the larger one. Enzymatic conversion of furostanol glycosides to spirostanol glycosides proceeds only after plants are harvested. Under normal physiological conditions, the enzyme and its substrate might be compartmented spatially and/or chemically. Belongs to the glycosyl hydrolase 1 family.