[Transport of zwitterionic amino acids in mammalian cells]

Amino acid transport is an important metabolic process that regulates the amino acid flux between the extracellular and intracellular space of the cell. Amino acids enter to the cell through plasmatic membrane proteins that have been kinetically well characterized. System A is involved in the transport of zwitterionic amino acids with short lateral chains and plays a key role in the gluconeogenesis from amino acids, especially from alanine, and has been implicated in the process of cellular duplication. System N transports amino acids with a nitrogen side chain, specially glutamine, an important regulator of protein synthesis. System L allows the entrance of zwitterionic amino acids with large side chains: it is normally constitutive and it is important for the uptake of these amino acids into the brain where some of them are precursors of neurotransmitters. Amino acid transport has been studied at a molecular level since several cDNAs have been cloned opening the possibility to study their structure and regulation. Several isoforms of some zwitterionic amino acids have been cloned including systems ASC, Gly, beta and proline, which have been classified as a superfamily of carrier proteins containing 6 to 12 spanning membrane domains. This review shows the general aspects of amino acid transport and recent advances in the zwitterionic amino acid transport systems, emphasizing the molecular characteristics of cloned systems and their regulatory factors.