Abstract

Rat organic anion transporting polypeptide 2 (rOatp2) is a member of the OATP family. It exhibits broad substrate specificity and accepts amphipathic organic anions, cardiac glycosides (digoxin and ouabain; a neutral compound), and organic cations (rocuronium andN-(4,4-azo-n-pentyl)-21-deoxyajamalinium). In the present study, kinetic analyses were carried out to investigate whether taurocholate (TCA), digoxin, and 17β-estradiol-d-17β−glucuronide (E₂17βG) share the same recognition site on rOatp2 for their transport. The transport of TCA and digoxin was mutually inhibited, and theK_i values of digoxin and TCA for the transport of TCA and digoxin were 0.58 and 160 μM, respectively. TheK_m and V_maxvalues of TCA and digoxin were 190 μM and 140 pmol/min/mg of protein and 1.1 μM and 6.6 pmol/min/mg of protein, respectively. TheK_m and K_i values were consistent. In addition, digoxin (1 μM) and TCA (100 μM) increased the K_m values of TCA and digoxin, respectively, but they did not affect theV_max values, suggesting that their inhibition is competitive. The transport of digoxin via rOatp2 was inhibited slightly by E₂17βG, whereas the uptake of TCA was stimulated by E₂17βG in a concentration-dependent manner. These results suggest that rOatp2 has at least two substrate recognition sites, one for TCA and digoxin and the other for E₂17βG.