Ginsenosides, ingredients of the root of Panax ginseng, are not substrates but inhibitors of sodium-glucose transporter 1

Recent pharmacokinetic studies have revealed that ginsenosides, the major ingredients of ginseng (the roots of Panax ginseng), are present in the plasma collected from subjects receiving ginseng, and speculated that ginsenosides might be actively transported via glucose transporters. We evaluated whether ginsenosides Rb-1 and Rg(1), and their metabolites from enteric bacteria act as substrates of sodium-glucose cotransporter (SGLT) 1, the major glucose transporter expressed on the apical side of intestinal epithelial cells. First, we evaluated the competing effects of ginseng extract and ginsenosides on the uptake of [C-14]methyl-glucose, a substrate of SGLT1, by SGLT1-overexpressing HEK293 cells. A boiling water extract of ginseng inhibited SGLT1 in a concentration-dependent manner with an IC50 value of 0.85 mg/ml. By activity-guided fractionation, we determined that the fraction containing ginsenosides displayed an inhibitory effect on SGLT1. Of the ginsenosides evaluated, protopanaxatriol-type ginsenosides were not found to inhibit SGLT1, whereas protopanaxadiol-type ginsenosides, including ginsenosides Rd, Rg(3), Rh-2, F-2 and compound K, exhibited significant inhibitory effects on SGLT1, with ginsenoside F-2 having the highest activity with an IC50 value of 23.0 A mu M. Next, we measured the uptake of ginsenoside F-2 and compound K into Caco-2 cells, a cell line frequently used to evaluate the intestinal absorption of drugs. The uptake of ginsenoside F-2 and compound K into Caco-2 cells was not competitively inhibited by glucose. Furthermore, the uptake of ginsenoside F-2 and compound K into SGLT1-overexpressing HEK293 cells was not significantly higher than into mock cells. Ginsenoside F-2 and compound K did not appear to be substrates of SGLT1, although these compounds could inhibit SGLT1. Ginsenosides might be absorbed by passive diffusion through the intestinal membrane or actively transported via unknown transporters other than SGLT1.