MOLECULAR AND FUNCTIONAL-CHARACTERIZATION OF AN ORGANIC ANION TRANSPORTING POLYPEPTIDE CLONED FROM HUMAN LIVER

Background & Aims: Based on a recently cloned rat liver organic anion transporter, we attempted to clone the corresponding human liver organ ic anion transporting polypeptide. Methods: A human liver complementar y DNA library was screened with a specific rat liver complementary DNA probe. The human liver transporter was cloned by homology with the ra t protein and functionally characterized in Xenopus laevis oocytes. Re sults: The cloned human liver organic anion transporting polypeptide c onsists of 670 amino acids and shows a 67% amino acid identity with th e corresponding rat liver protein. Injection of in vitro transcribed c omplementary RNA into frog oocytes resulted in the expression of sodiu m-independent uptake of [S-35]bromosulfophthalein (Michaelis constant [K-m], similar to 20 mu mol/L), [H-3]cholate (K-m, similar to 93 mu mo l/L), [H-3]- taurocholate (K-m, similar to 60 mu mol/L), [C-14]glycoch olate, [H-3]taurochenodeoxycholate, and [H-3]tauroursodeoxycholate (K- m similar to 19 mu mol/L). Northern blot analysis showed cross-reactiv ity with messenger RNA species from human liver, brain, lung, kidney, and testes. Polymerase chain reaction analysis of genomic DNA from a p anel of human-rodent somatic cell hybrids mapped the cloned human orga nic anion transporter to chromosome 12. Conclusions: These studies sho w that the cloned human liver organic anion transporter is closely rel ated to, but probably not identical to, the previously cloned rat live r transporter. Furthermore, its additional localization in a variety o f extrahepatic tissues suggests that it plays a fundamental role in ov erall transepithelial organic anion transport of the human body.