Polyspecific substrate uptake by the hepatic organic anion transporter Oatp1 in stably transfected CHO cells

The rat liver organic anion transporting polypeptide (Oatp1) has been exten sively characterized mainly in the Xenopus laevis expression system as a po lyspecific carrier transporting organic anions (bile salts), neutral compou nds, and even organic cations. In this study, we extended this characteriza tion using a mammalian expression system and confirm the basolateral hepati c expression of Oatp1 with a new antibody. Besides sulfobromophthalein [Mic haelis-Menten constant (K-m) of similar to 3 mu M], taurocholate (K-m of si milar to 32 mu M), and estradiol-17 beta-glucuronide (K-m of similar to 4 m u M), substrates previously shown to be transported by Oatp1 in transfected HeLa cells, we determined the kinetic parameters for cholate (K-m of simil ar to 54 mu M), glycocholate (K-m of similar to 54 mu M), estrone-3-sulfate (K-m of similar to 11 mu M), CRC-220 (K-m of similar to 57 mu M), ouabain (K-m of similar to 3,000 mu M), and ochratoxin A (K-m of similar to 29 mu M ) in stably transfected Chinese hamster ovary (CHO) cells. In addition, thr ee new substrates, taurochenodeoxycholate (K-m of similar to 7 mu M), tauro ursodeoxycholate (K-m of similar to 13 mu M), and dehydroepiandrosterone su lfate (K-m of similar to 5 mu M), were also investigated. The results estab lish the polyspecific nature of Oatp1 in a mammalian expression system and definitely identify conjugated dihydroxy bile salts and steroid conjugates as high-affinity endogenous substrates of Oatp1.