Transport and Metabolism of Equol by Caco-2 Human Intestinal Cells

Equol is a metabolite of daidzein with greater estrogenic activity and antioxidant capacity than its precursor. Although it is known that equol is produced by the gut microflora, information regarding its transport and metabolism in the intestine is lacking. This study investigated transepithelial transport, bioconversion, and efflux of equol using differentiated cultures of Caco-2 cells to characterize its bioavailability. Uptake was directly proportional to the initial concentration in the apical compartment with maximal intracellular concentrations being reached and 20% of the total added to the apical compartment present in the basolateral compartment as free equol after 1 h. By 4 h, 73% of equol was present as β-glucuronidase/sulfatase sensitive conjugates with approximately 47 and 26% of initial equol distributed in apical and basolateral compartments, respectively. Free equol in the basolateral compartment appeared to be retrotransported, largely conjugated, and effluxed across the apical membrane. These results suggest that differences in the synthesis and efflux of equol conjugates may contribute to the marked variance in the bioavailability of equol in “producer” phenotype.