ABSTRACT
beta-D-Glucopyranosyl-(1S and 1R)-epoxyethanes (I and II), 1-(beta-D-glucopyranosyl)-(2R and 2S)-2,3-epoxypropanes (III and IV), beta-D-glucopyranosyl isothiocyanate (V) and beta-D-galactopyranosylepoxyethane (VI) are active-site-directed irreversible inhibitors of sweet-almond beta-glucosidase B (beta-D-Glucoside glucohydrolase, EC 3.2.1.21). Formation of the covalent bond is preceded by the binding of these inhibitors in the active site of the enzyme. This is testitified by the competitive character of inhibition of beta-glucosidase component B by compounds I-VI at the early period and by the protection of the enzyme from inactivation by its competitive inhibitors D-glucose and 1,5-D-gluconolactone. Epoxides I-IV are bound covalently with componet B at a molar ratio 1 : 1 as shown with the aid of 14C-labelled inhibitors. The release of the label from modified enzyme (E-I covalent) by treatment with hydroxylamine suggests the formation of an ester bond between inhibitors I-IV and the carboxyl group of the enzyme active site. The pH dependence curve of the inactivation rate of beta-glucosidase B is of a bell-shaped form for V and of a sigmoid character for I-IV and points to the involvement of the active site groups with pKa 5.6-5.9 and 4.2-4.4.
