MOLECULAR-CLONING AND CHARACTERIZATION OF A NITROBENZYLTHIOINOSINE-INSENSITIVE (EI) EQUILIBRATIVE NUCLEOSIDE TRANSPORTER FROM HUMAN PLACENTA

Mammalian equilibrative nucleoside transporters are typically divided into two classes, es and ei, based on their sensitivity or resistance respectively to inhibition by nitrobenzylthioinosine (NBMPR). Previous ly, we have reported the isolation of a cDNA clone encoding a prototyp ic es-type transporter, hENT1 (human equilibrative nucleoside transpor ter 1), from human placenta. We now report the molecular cloning and f unctional expression in Xenopus oocytes of a cDNA from the same tissue encoding a homologous ei-type transporter, which we designate hENT2. This 456-residue protein is 46 % identical in amino acid sequence with hENT1 and corresponds to a full-length form of the delayed-early prol iferative response gene product HNP36, a protein of unknown function p reviously cloned in a form bearing a sequence deletion. In addition to placenta, hENT2 is found in brain, heart and ovarian tissue. Like hEN T1, hENT2 mediates saturable transport of the pyrimidine nucleoside ur idine (K-m 0.2 +/- 0.03 mM) and also transports the purine nucleoside adenosine. However, in contrast with hENT1, which is potently inhibite d by NBMPR (K-i 2 nM), hENT2 is NBMPR-insensitive (IC50 < 1 mu M). It is also much less sensitive to inhibition by the coronary vasoactive d rugs dipyridamole and dilazep and to the lidoflazine analogue draflazi ne, properties that closely resemble those reported for classical ei-t ype transport in studies with intact cells.