Purification and characterization of furostanol glycoside 26‐O‐β‐glucosidase from Costus speciosus rhizomes

In plants, spirostanol glycosides (steroid saponins) are known to be formed furostanol glycosides during postharvest treatment and storage. Furostanol glycoside 26-O-beta-glucosidase (F26G) involved in this conversion was purified to apparent homogeneity for the first time from Costus speciosus rhizomes which accumulate these glycosides. The enzyme was highly specific for cleavage of the C-26-boudn glucose moiety of furostanol glycosides showing Km for protogracillin of 50 microM. Glucono-1,5-lactone, a typical beta-glucosidase inhibitor, and diosgenin, an aglycone of spirostanol glycosides, strongly inhibited the enzyme activity. The purified F26G is dimeric with a native apparent molecular weight of 110,000 consisting of subunits of 54,000 and 58,000. The N-terminal sequence of the 54,000 protein has a high similarity to the sequences found in N-terminal regions of known plant beta-glucosidases.