Metabolites of the Ellagitannin Geraniin and Their Antioxidant Activities

Different types of ellagitannins are reported to have various biological activities, such as antioxidant, antiviral, and antitumor activities. However, there are few definitive studies on the absorption and metabolism of ellagitannins. This review compares the absorption and metabolism of ellagitannins, and the antioxidant properties of their metabolites in rats, with those of intact ellagitannins by means of in vitro and in vivo assays. We isolated 7 urinary and intestinal microbial metabolites in rats after the ingestion of geraniin, which is a typical ellagitannin isolated from Geranium thunbergii, an antidiarrheic remedy in Japan. The structures of these metabolites were determined to be dibenzopyran derivatives (1-7), using NMR and mass spectroscopic data. Four major metabolites (1-4) prepared by chemical synthesis were evaluated for their antioxidant activities by using 2,2-diphenyl-1-picrylhydrazyl radical scavenging and oxygen radical absorbance capacity (ORAC) methods. The metabolites exhibited more potent antioxidant activities in the ORAC assay than intact ellagitannins, such as geraniin and corilagin. Furthermore, plasma ORAC scores increased with increases in the plasma concentration of the metabolites after the oral administration of geraniin to rats. These findings suggest that these metabolites may contribute to the health benefits of ellagitannins as antioxidants in the body.