Enzymatic bioconversion of rutin to quercetin-3-O-glucoside (Q-3-G) by Penicillium decumbens naringinase was increased with reaction pH increased approximately to pH 6.0. It resulted in greater than 92% production of Q-3-G due to the removal of the terminal rhamnose at the controlled pH 6.0. The enzymatic bioconversion of rutin to Q-3-G was repetitively performed, yielding 84% after 5 batches with little quercetin formation. Interestingly, the water solubility of Q-3-G was enhanced 69- and 328-fold over those of rutin and quercetin, which may make Q-3-G more bioavailable in food. Q-3-G was approximately 6- and 1.4-fold more potent than rutin as an inhibitor of human intestinal maltase and human DL-3-hydroxy-3-methylglutalyl coenzyme A reductase. Q-3-G was less potent (16- and 1.3-fold, respectively) than quercetin as an inhibitor of these enzymes. However, the results suggest that Q-3-G may be confirmed more effective and bioavailable food component than rutin and even quercetin because of its enhanced solubility and inhibitory properties.