Specific irreversible inhibition of sweet-almond β-glucosidase by some β-glycopyranosylepoxyalkanes and β-d-glucopyranosyl isothiocyanate

Abstract β- d -Glucopyranosyl-(1S and 1R)-epoxyethanes (I and II), 1-(β- d -glucopyranosyl)-(2R and 2S)-2,3-epoxypropanes (III and IV), β- d -glucopyranosyl isothiocyanate (V) and β- d -galactopyranosylepoxyethane (VI) are active-site-directed irreversible inhibitors of sweet-almond β-glucosidase B (β- 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 β-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 component B at a molar ratio 1 : 1 as shown with the aid of 14 C-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 β-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 p K a 5.6–5.9 and 4.2–4.4.