Background: Quercetin, a flavonol contained in various vegetables and fruits, has numerous biological activities which include anticancer, antiviral, anti-diabetic, and anti-oxidative properties. However, quercetin also has low oral bioavailability, due to its insolubility in water. Thus, the bioavailability of quercetin administered to human beings in a capsule form was reported to be less than 1%, with only a small percentage of ingested quercetin getting absorbed in the blood. This leads to certain difficulties in creating highly effective medicines.
Methods: Quercetin-rubusoside and quercetin-rebaudioside were prepared. The antioxidant activities of quercetin and Q-rubusoside were evaluated by DPPH radical scavenging method. Inhibition activities of quercetin and Quercetin-rubusoside were determined by measuring the remaining activity of 3CLpro with 200 μM inhibitor. The inhibition activity of quercetin, rubusoside and quercetin-rubusoside were determined by measuring the activity of human maltase, which remains at 100 μM rubusoside or quercetin-rubusoside. The mushroom tyrosinase inhibition was assayed with the reaction mixture containing 3.3 mM L-DOPA in 50 mM potassium phosphate buffer (pH 6.8), and 10 U mushroom tyrosinase/ml with or without quercetin or quercetin-rubusoside.