Two hypotheses on absorption mechanisms of flavonoid glucosides across the small intestine have been proposed: active uptake of the quercetin glucoside by the sodium-dependent glucose transporter (SGLT1) with subsequent deglycosylation within the enterocyte by cytosolic β-glucosidase, or luminal hydrolysis of the glucoside by lactase phlorizin hydrolase (LPH) and absorption by passive diffusion of the released aglycone. To test the above hypotheses we employed phlorizin (as an inhibitor of SGLT1) and N-(n-butyl)-deoxygalactonojirimycin (as an inhibitor of the lactase domain of LPH) in a rat everted-jejunal sac model. Quercetin-4′-glucoside mucosal hydrolysis was 10 times greater than quercetin-3-glucoside hydrolysis in the absence of inhibitors (449 and 47 nmol g-1 tissue, respectively), despite the similar amounts (13 ± 4 and 9 ± 1 nmol g-1, respectively) being transferred to the serosal compartment during the 15 min incubation. Apical hydrolysis of both quercetin glucosides was significantly reduced in the presence of NB-DGJ (80%), and transfer of quercetin (measured as quercetin metabolites) to the serosal solution was also significantly reduced (40-50%). In the presence of phlorizin, transfer of metabolites to the serosal solution was only reduced in the case of quercetin-4′-glucoside. Evidently the mechanism of absorption of quercetin-4′-glucoside involves both an interaction with SGLT1 and luminal hydrolysis by LPH, whereas quercetin-3-glucoside appears to be absorbed only following hydrolysis by LPH.
- Intestinal absorption
- Lactase phlorizin hydrolase (LPH)
- Quercetin glucoside
- Sodium-dependent glucose transporter 1 (SGLT1)