Nucleobase and nucleoside transport in mammalian cells

Membrane transport of nucleobases and nucleosides has been an actively pursued research field for the past 25 years. Not only are these substances of physiological interest; derivatives are in clinical use or under investigation for their pharmacological activity against viral and neoplastic disease. An understanding of the molecular pharmacology of these substances includes a detailed knowledge of how they reach their intracellular targets. Membrane transport systems which have so far been found in all cells examined play an important role in this process. Since the transporters are minor membrane components, little is known about them on a molecular basis. This review discusses methodological approaches used to measure initial rates of membrane transport and summarizes current knowledge of the various transport systems which have been characterized with these kinetic methods.