Purification of cytosolic beta-glucosidase from pig liver and its reactivity towards flavonoid glycosides

Flavonoid glycosides are common dietary components which may have health-pr omoting activities. The metabolism of these compounds is thought to influen ce their bioactivity and uptake from the small intestine. It has been sugge sted that the enzyme cytosolic beta-glucosidase could deglycosylate certain flavonoid glycosides. To test this hypothesis, the enzyme was purified to homogeneity from pig liver for the first time. It was found to have a molec ular weight (55 kDa) and specific activity (with p-nitrophenol glucoside) c onsistent with other mammalian cytosolic beta-glucosidases. The pure enzyme was indeed found to deglycosylate various flavonoid glycosides. Genistein 7-glucoside, daidzein 7-glucoside, apigenin 7-glucoside and naringenin 7-gl ucoside all acted as substrates, but we were unable to detect activity with naringenin 7-rhamnoglucoside. Quercetin 4'-glucoside was a substrate, but neither quercetin 3,4'-diglucoside, quercetin 3-glucoside nor quercetin 3-r hamnoglucoside were deglycosylated. Estimates of K-m ranged from 25 to 90 m u M while those for V-max were about 10% of that found with the standard ar tificial substrate p-nitrophenol glucoside. The non-substrate quercetin 3-g lucoside was found to partially inhibit deglycosylation of quercetin 4'-glu coside, but it had no effect upon activity with p-nitrophenol glucoside. Th is study confirms that mammalian cytosolic beta-glucosidase can deglycosyla te some, but not all, common dietary flavonoid glycosides. This enzyme may, therefore, be important in the metabolism of these compounds. (C) 1999 Els evier Science B.V. All rights reserved.