Regioselective formation of quercetin 5- O-glucoside from orally administered quercetin in the silkworm, Bombyx mori

Abstract

The cocoons of some races of the silkworm, Bombyx mori, have been shown to contain 5- O-glucosylated flavonoids, which do not occur naturally in the leaves of their host plant, mulberry ( Morus alba). Thus, dietary flavonoids could be biotransformed in this insect. In this study, we found that after feeding silkworms a diet rich in the flavonol quercetin, quercetin 5- O-glucoside was the predominant metabolite in the midgut tissue, while quercetin 5,4′-di- O-glucoside was the major constituent in the hemolymph and silk glands. UDP-glucosyltransferase (UGT) in the midgut could transfer glucose to each of the hydroxyl groups of quercetin, with a preference for formation of 5- O-glucoside, while quercetin 5,4′-di- O-glucoside was predominantly produced if the enzyme extracts of either the fat body or silk glands were incubated with quercetin 5- O-glucoside and UDP-glucose. These results suggest that dietary quercetin was glucosylated at the 5- O position in the midgut as the first-pass metabolite of quercetin after oral absorption, then glucosylated at the 4′- O position in the fat body or silk glands. The 5- O-glucosylated flavonoids retained biological activity in the insect, since the total free radical scavenging capacity of several tissues increased after oral administration of quercetin.