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 organic anion transporting polypeptide. Methods: A human liver complementary DNA library was screened with a specific rat liver complementary DNA probe. The human liver transporter was cloned by homology with the rat protein and functionally characterized in Xenopus laevis oocytes. Results: The cloned human liver organic anion transporting polypeptide consists of 670 amino acids and shows a 67% amino acid identity with the corresponding rat liver protein. Injection of in vitro transcribed complementary RNA into frog oocytes resulted in the expression of sodium-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 mol/L), [H-3]- taurocholate (K-m, similar to 60 mu mol/L), [C-14]glycocholate, [H-3]taurochenodeoxycholate, and [H-3]tauroursodeoxycholate (K-m similar to 19 mu mol/L). Northern blot analysis showed cross-reactivity with messenger RNA species from human liver, brain, lung, kidney, and testes. Polymerase chain reaction analysis of genomic DNA from a panel of human-rodent somatic cell hybrids mapped the cloned human organic anion transporter to chromosome 12. Conclusions: These studies show that the cloned human liver organic anion transporter is closely related to, but probably not identical to, the previously cloned rat liver transporter. Furthermore, its additional localization in a variety of extrahepatic tissues suggests that it plays a fundamental role in overall transepithelial organic anion transport of the human body.