Isolation of a multispecific organic anion and cardiac glycoside transporter from rat brain

A novel multispecific organic anion trans porting polypeptide (oatp2) has been isolated from rat brain, The cloned cDNA contains 3,640 bp, The coding region extends over 1,983 nucleotides, thus encoding a polypeptide of 661 amino acids, Oatp2 is homologous to other members of the oatp gene family of membrane transporters with 12 predicted transmembrane domains, five potential glycosylation, and six potential protein kinase C phosphorylation sites, In functional expression studies in Xenopus laevis oocytes, oatp2 mediated uptake of the bile acids taurocholate (K-m approximate to 35 mu M) and cholate (K-m approximate to 46 mu M), the estrogen conjugates 17 beta-estradiol-glucuronide (K-m approximate to 3 mu M) and estrone-3-sulfate (K-m approximate to 11 mu M), and the cardiac gylcosides ouabain (K-m approximate to 470 mu M) and digoxin (K-m approximate to 0.24 mu M). Although most of the tested compounds are common substrates of several oatp-related transporters, high-affinity uptake of digoxin is a unique feature of the newly cloned oatp2, On the basis of Northern blot analysis under high stringency conditions, oatp2 is highly expressed in brain, liver, and kidney but not in heart, spleen, lung, skeletal muscle, and testes, These results provide further support for the overall significance of oatps as a new family of multispecific organic anion transporters, They indicate that oatp2 may play an especially important role in the brain accumulation and toxicity of digoxin and in the hepatobiliary and renal excretion of cardiac glycosides from the body.