Abstract
Adenosine is a hydrophilic molecule that requires specialized transport proteins for permeation of cell membranes. Functional studies have identified two types of nucleoside transport processes: equilibrative bidirectional processes driven by chemical gradients and inwardly directed concentrative processes driven by the sodium electrochemical gradient. The equilibrative nucleoside transport processes (es, ei) are widely distributed among various cell types, whereas the concentrative nucleoside transport processes (cit, cif, cib, csg, cs) are present primarily in specialized epithelial Using molecular cloning techniques and functional expression of cDNAs in oocytes of Xenopus laevis, we have isolated and characterized cDNAs encoding integral membrane proteins of the four major nucleoside transport processes of rat and human cells (es, ei, cit, cif). The equilibrative and concentrative nucleoside transport processes are mediated by members of two previously unrecognized groups of integral membrane proteins, which we have designated the Equilibrative Nucleoside Transporter (ENT) and the Concentrative Nucleoside Transporter (CNT) protein families. This review summarizes the current state of knowledge of the molecular biology of the ENT and CNT protein families, focusing on the role of these proteins (r/hENT1, r/hENT2, r/hCNT1, r/ hCNT2) in the transport of adenosine.
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