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Background: Quercetin, a flavonol contained in various vegetables and fruits, has various biological activities including anticancer, antiviral, anti-diabetic, and anti-oxidative. However, it has low oral bioavailability due to 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 3CL pro 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 contained 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. Results: With 10% rubusoside treatment, quercetin showed solubility of 7.7 mg/ml in water, and its solubility increased as the concentration of rubusoside increased; the quercetin solubility in water increased to 0.83 mg/mlas rubusoside concentration increased to 1 mg/ml. Quercetin solubilized in rubusoside solution showed DPPH radical-scavenging activity and mushroom tyrosinase inhibition activity, similar to that of quercetin solubilized in dimethyl-sulfoxide. Quercetin-rubusoside also showed 1.2 and 1.9 folds higher inhibition activity against 3CL pro of SARS and human intestinal maltase, respectively, than those of quercetin in DMSO. Conclusions: Quercetin can be solubilized in water with rebaudioside or rubusoside treatment. As Ru concentration increases, the solubility of quercetin in water increases. The solubilization of quercetin in Ru solution did not reduce its biological functions such as the DPPH radical-scavenging and mushroom tyrosinase activity; also, quercetin-rubusoside increased the inhibition activity against the 3CL pro of SARS and human intestinal maltase, when compared with the activity of quercetin in DMSO. Thus, rubusoside and rebaudioside are promising compounds which enhance the solubility of poorly water soluble compounds. Keywords:  rubusoside, rebaudioside, flavonol, quercetin, human maltase, 3CL pro

Enhancement of quercetin water solubility with steviol glucosides and the studies of biological properties