Nucleosides: Metabolites and Drugs

movement of numerous nonlipophilic compounds across cell membranes and are likely to have evolved soon after the appearance of the lipid-based biological membrane. Any substance that must enter or leave a eukaryotic or prokaryotic cell and that is not soluble in lipid or small enough to diffuse between the constituent molecules that make up a membrane requires a specialized transport mechanism such as a transport protein. Thus transporters are essential components of cellular physiology, allowing access of a wide variety of compounds, such as nutrients and substrates, and facilitating release of compounds, such as signaling molecules. For nucleosides and nucleobases involved in myriad physiological processes, the specialized transport proteins responsible are currently accepted to be primarily the members of the gene families SLC28 and SLC29 (although the existence of additional or novel, previously unidentified transport proteins cannot be ruled out). Here, we discuss various aspects of the physiology of NTs. For more comprehensive discussions of all the relevant literature on NTs, we direct the reader to a number of excellent recent reviews (6, 8, 35, 56, 58, 61).