RESUMO
In mammalian cells, amino acids are taken up by different transport systems present in the plasma membrane. The transport systems were originally characterized by kinetic and competition studies. However, it was difficult to assign specific amino acids to specific transport systems. With recent advances in molecular biology, it has been possible to identify the specific transporter proteins for specific amino acids. In this review we describe the anionic and cationic amino acid transport systems reported at the molecular level. The anionic amino acids are movilized mainly by the XaG- and Xc- systems which are important in the inactivation of glutamatergic nervous transmission in the brain and for the synthesis of glutathione, respectively. Four isoforms of the XAG- system in the brain belong to the family of Na+ dependent amino acid transporters. Transport systems for cationic amino acids also recognize zwitterionic substrates, and the better characterized systems at the present time are y+, y+L, bo,+ and Bo,+. The regulation of the entrance of cationic amino acid such as arginine, lysine, and ornithine to the cell is important in the biosynthesis of nitric oxide, creatine, carnitine, and polyamines. An inherited defect associated to bo,+ system is cysteinuria.
Assuntos
Aminoácidos/metabolismo , Transportadores de Cassetes de Ligação de ATP/metabolismo , Erros Inatos do Metabolismo dos Aminoácidos/metabolismo , Sistema X-AG de Transporte de Aminoácidos , Sistemas de Transporte de Aminoácidos Básicos , Aminoácidos/química , Aminoácidos/farmacocinética , Animais , Ânions/metabolismo , Transporte Biológico , Encéfalo/metabolismo , Proteínas de Transporte/metabolismo , Cátions/metabolismo , Cistinúria/metabolismo , Ácido Glutâmico/metabolismo , Humanos , Mucosa Intestinal/metabolismo , Substâncias Macromoleculares , Glicoproteínas de Membrana/metabolismo , Proteínas de Membrana/metabolismo , Camundongos , Modelos Moleculares , RatosRESUMO
Previous studies have proposed the possibility that erythrocytes (RBC) are involved in the interorgan transport of amino acids; however, this role has not been confirmed. In order to study the likelihood that erythrocytes are involved in the interorgan transport, rates of influx and efflux of glycine, threonine, lysine, histidine and leucine were measured in rat red blood cells. Time course of influx of leucine, a large neutral amino acid, was very rapid (319 mumoles/L RBC. min), and a steady state was reached between 5 to 10 min of incubation, whereas glycine influx was very slow (5.04 mumoles/L RBC. min). Threonine influx was similar to leucine although the rate was slower (41.4 mumoles/L RBC. min); however, the steady state was reached in 30 minutes and its uptake showed less capacity. Histidine and lysine showed a continuous influx, and did not reach a steady state after 60 min of incubation. Efflux of leucine was extremely rapid indicating a rapid equilibration between the incubation medium and the intracellular space of the erythrocytes. Threonine efflux had a half life (t1/2) of between two to three min, independently of the medium used. Histidine showed a t1/2 of around six min, whereas for the small neutral amino acid glycine it was of 14 to 17 min. These results indicate that some large neutral amino acids are not involved in the potential interorgan transport by red blood cells due to the rapid equilibration of the concentration of amino acid between cells and the medium.