Abstract

Polyspecific organic cation transporters in the liver mediate the elimination of a wide array of endogenous amines and xenobiotics. In contrast to our understanding of the mechanisms of organic cation transport in rat liver, little is known about the mechanisms of organic cation transport in the human liver. We report the cloning, sequencing, and functional characterization of the first human polyspecific organic cation transporter from liver (hOCT1). hOCT1 (554 amino acids) is 78% identical to the previously cloned organic cation transporter from rat, rOCT1 [Nature (Lond.) 372:549–552 (1994)]. InXenopus laevis oocytes injected with the cRNA of hOCT1, the specific uptake of the organic cation³H-1-methyl-4-phenylpyridinium (³H-MPP⁺) was significantly enhanced (8-fold) over that in water-injected oocytes. Uptake of³H-MPP⁺ was saturable (K _m = 14.6 ± 4.39 μm) and sensitive to membrane potential. Both small monovalent organic cations such as tetraethylammonium andN ¹-methylnicotinamide and bulkier organic cations (e.g., vecuronium and decynium-22) inhibited the uptake of³H-MPP⁺. In addition, the bile acid taurocholate inhibited the uptake of ³H-MPP⁺ in oocytes expressing hOCT1. Northern analysis demonstrated that the mRNA transcript of hOCT1 is expressed primarily in the human liver, whereas the mRNA transcript of rOCT1 is found in rat kidney, liver, intestine, and colon [Nature (Lond.) 372:549–552 (1994)]. In comparison to rOCT1, hOCT1 exhibits notable differences in its kinetic characteristics and tissue distribution. The functional expression of hOCT1 will provide a powerful tool for elucidation of the mechanisms of organic cation transport in the human liver and understanding of the mechanisms involved in the disposition and hepatotoxicity of drugs.