Quercetin (3,3Ј,4Ј,5,7-pentahydroxyflavone) is a widely distributed secondary metabolite in plants. Human beings regularly consume quercetin in various fruits, vegetables or herbal medicines; such as apple, onion, and sophora flower. Quercetin has been found to have various biological activities including antiproliferative effects on several cancer cells, [1][2][3] anti-inflammatory and anti-allergic effects, 4) anti-oxidative activity, 5) and atherosclerosis-preventing effect. 6) In clinical study, supplementation of quercetin in hypertensive patients for 28 d significantly reduced blood pressure. 7) However, orally administered quercetin is poorly absorbed, and the bioavailability of quercetin administered in capsule form to human beings was reported to be less than 1%. 8) Therefore, several trials have been conducted to increase the bioavailability of quercetin by optimizing its formulation. For example, a pharmacokinetic study in rats revealed that bioavailability of quercetin was increased 5.7-fold by administration using a solid lipid nanoparticle as an oral delivery carrier compared with that administered as a quercetin suspension.
9)Quercetin is generally accumulated in plants as glycosides such as glucosides, rutinosides and xylosides. These quercetin glycosides show higher solubility in water than quercetin due to the hydrophilicity of the sugar moieties. Indeed, quercetin-4Ј-O-b-D-glucopyranoside has higher bioavailability than its aglycone in human beings, suggesting that conjugation with glucose would enhance quercetin absorption in the small intestine. 10,11) Glucosyl conjugation of lipophilic small molecules leads to an increase in their water solubility.12) Several glucosides of quercetin have been prepared by enzymatic synthesis to enhance the water solubility, as exemplified by a-monoglucosyl rutin (aMR), 13) a-oligoglucosyl rutin (aOR), 14) and "enzymatically modified isoquercitrin" (EMIQ).15) In Japan, EMIQ has been approved as a food additive, 16) and in U.S.A., FDA declared EMIQ generally regarded as safe (GRAS) for use in multiple food categories. However, how glucosyl conjugation affects the pharmacokinetic behavior of quercetin remains unknown except that aMR exhibited 4.5-fold higher area under the plasma concentration-time curve (AUC ) value than quercetin after oral administration as a suspension in carboxymethylcellulose (CMC) solution in rats. 13) In the present investigation, we examined the intestinal absorption and bioavailability of quercetin after oral administration of various quercetin glucosides, and revealed that glucosyl conjugation of isoquercitrin (IQC) with a-glucosidic linkages is quite effective for improving the bioavailability. We also showed that hydrolysis of the glycosidic linkage prior to the absorption through small intestinal epithelium is essential. We further suggest that the collaborative action of two glucosidases, mucosal maltose-glucoamylase (MGAM) and lactase-phlorozin hydrolase (LPH), both anchoring in the mucosal membrane of the epithelial tissue, pla...