SUBSTRATE SPECIFICITY OF Β-GLUCOSIDASE FROM YUCCA GLORIOSA LEAVES

Yucca gloriosa leaves contain a considerable number of steroid glycosides. In the plant's intact leaves, the biosynthesis of furostanol glycosides occurs, which are then converted into spirostanol glycosides by the action of β-glucosidase. Two forms of β-glucosidase are found in Yucca gloriosa leaves. Form I (molecular weight 32,000) hydrolyzes both oligofurostanosides, converting them into the corresponding oligospirostanosides, as well as the synthetic substrate 4-nitrophenyl-β-D-glucopyranoside. Form II (molecular weight 68,000) hydrolyzes only 4-nitrophenyl-β-D-glucopyranoside and does not cleave oligofurostanosides. Both enzymes have an optimum temperature of 37°C and an optimum pH of 6.3-6.5. Glucono-1,5-lactone inhibited the activity of both enzymes. The β-glucosidase of Form I shows higher affinity for its natural substrates than for the synthetic ones. The Km value for the β-glucosidase of Form I is 7.7 mM in relation to the total oligofurostanosides of the leaves of Yucca gloriosa, and 18.3 mM in relation to the synthetic substrate. The affinity for the natural substrates is higher than for the synthetic ones. The data received allow us to conclude that the affinity of Form I β-glucosidase from Yucca gloriosa leaves does not depend on either the structure of the oligosaccharide fragment linked to the nucleus or the structure of the aglycone (of steroid origin).