Anti-allergic effects of enzymatically modified isoquercitrin (α-oligoglucosyl quercetin 3-O-glucoside), quercetin 3-O-glucoside, α-oligoglucosyl rutin, and quercetin, when administered orally to mice.

Quercetin, a flavonol distributed widely in edible and medicinal plants, has beneficial biological activities in vitro. However, the low water solubility of quercetin limits its bioavailability to exhibit activity in vivo. We evaluated the anti-allergic effects of quercetin, quercetin 3-O-glucoside (isoquercitrin, IQC), α-oligoglucosyl rutin (αOR), and enzymatically modified isoquercitrin (α-oligoglucosyl isoquercitrin; EMIQ) in the murine ear passive cutaneous anaphylaxis (PCA) reaction using ovalbumin as an antigen. The substances to be tested were dissolved or suspended in water, and administered orally to mice (4 mmol 10 ml⁻¹ kg⁻¹). EMIQ exhibited a significant inhibitory effect on the PCA reaction. We detected 600 μM IQC and 95.1 μM Q3Glcn in the plasma of mice 30 min after EMIQ treatment, suggesting that IQC and Q3Glcn might be the genuine active compounds mediating the anti-allergic effects of EMIQ. Oral treatments of quercetin and αOR at this dosage exhibited no anti-allergic effect, and IQC showed less effect than EMIQ. Since IQC and quercetin cannot completely dissolve in water at this concentration, the water solubilities of these substances might affect their biological activities. αOR dissolved well in water at the concentration used but plasma concentrations of quercetin metabolites in mice orally treated with αOR were low, suggesting that αOR might not be converted to IQC or quercetin by the enzymes in small intestine and thus not exhibit any activity. Glycosyl conjugation of quercetin with specific sugar motifs is an effective strategy to improve the biological activity of quercetin in vivo.