THE MODIFIED DIPEPTIDE, ENALAPRIL, AN ANGIOTENSIN-CONVERTING ENZYME-INHIBITOR, IS TRANSPORTED BY THE RAT-LIVER ORGANIC ANION TRANSPORT PROTEIN

Oatp1, the organic anion transport polypeptide, is an integral membran e protein cloned from rat liver that mediates the uptake of various or ganic anions such as bromosulfophthalein (BSP) and taurocholate (TCA). Recent studies by others revealed that the thrombin inhibitor, CRC 22 0, a modified dipeptide, was transported by oatp1. The present study w as designed to examine whether another modified peptide, enalapril, an angiotensin-converting enzyme inhibitor, was also a substrate. Transp ort was studied with enalapril (1 to 800 mu mol/L, with [H-3]enalapril ) in a HeLa cell line stably transfected with oatp1-cDNA under the reg ulation of a Zn2+-inducible promoter. Noninduced transfected cells (wi thout zinc) that did not express oatp1 failed to take up enalapril. In contrast, cells expressing oatp1 transported enalapril, estrone sulfa te (E1S), taurolithocholic acid sulfate (TLCAS), and the glutathione c onjugate of BSP (BSPGSH). Uptake of enalapril by oatp1 at 37 degrees C was substantially higher than that at 4 degrees C. The rate at 37 deg rees C (uptake rates for induced - noninduced, transfected cells) was linear over 5 minutes and was concentration-dependent, characterized b y a K-m of 214 +/- 67 mu mol/L and a V-max of 0.51 +/- 0.15 nmol/min/m g protein Enalapril uptake was inhibited competitively by BSP (at 1, 5 , 10, and 50 mu mol/L) and TCA (at 5, 25, and 100 mu mol/L) with inhib ition constants (K-i) of 2 and 32 mu mol/L, respectively. The metaboli te enalaprilat was, however, not transported by oatp1. That oatp1 is n ot a general transporter of anionic compounds was further shown by the lack of transport of harmol sulfate, benzoate, and hippurate. These o bservations attest to the role of oatp1 as a specific transporter for at least two classes of pharmacologically important peptides.