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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 extensively characterized mainly in the Xenopus laevis expression system as a polyspecific carrier transporting organic anions (bile salts), neutral compounds, and even organic cations. In this study, we extended this characterization using a mammalian expression system and confirm the basolateral hepatic expression of Oatp1 with a new antibody. Besides sulfobromophthalein [Michaelis-Menten constant ( K m) of ∼3 μM], taurocholate ( K m of ∼32 μM), and estradiol- 17β-glucuronide ( K m of ∼4 μM), substrates previously shown to be transported by Oatp1 in transfected HeLa cells, we determined the kinetic parameters for cholate ( K m of ∼54 μM), glycocholate ( K m of ∼54 μM), estrone-3-sulfate ( K m of ∼11 μM), CRC-220 ( K m of ∼57 μM), ouabain ( K m of ∼3,000 μM), and ochratoxin A ( K m of ∼29 μM) in stably transfected Chinese hamster ovary (CHO) cells. In addition, three new substrates, taurochenodeoxycholate ( K m of ∼7 μM), tauroursodeoxycholate ( K m of ∼13 μM), and dehydroepiandrosterone sulfate ( K m of ∼5 μM), were also investigated. The results establish 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.