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). Previously, we have reported the isolation of a cDNA clone encoding a prototypic es-type transporter, hENT1 (human equilibrative nucleoside transporter 1), from human placenta. We now report the molecular cloning and functional 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 proliferative response gene product HNP36, a protein of unknown function previously cloned in a form bearing a sequence deletion. In addition to placenta, hENT2 is found in brain, heart and ovarian tissue. Like hENT1, hENT2 mediates saturable transport of the pyrimidine nucleoside uridine (Km 0.2±0.03 mM) and also transports the purine nucleoside adenosine. However, in contrast with hENT1, which is potently inhibited by NBMPR (Ki 2 nM), hENT2 is NBMPR-insensitive (IC50 < 1 μM). It is also much less sensitive to inhibition by the coronary vasoactive drugs dipyridamole and dilazep and to the lidoflazine analogue draflazine, properties that closely resemble those reported for classical ei-type transport in studies with intact cells.
Molecular Cloning, Functional Expression and Characterization of d-Limonene Synthase from Agastache rugosa
