Primary Structure, Genomic Organization, and Functional and Electrogenic Characteristics of Human System N 1, a Na+- and H+-coupled Glutamine Transporter

Abstract

We have cloned the human Na ؉ -and H ؉ -coupled amino acid transport system N (hSN1) from HepG2 liver cells and investigated its functional characteristics. Human SN1 protein consists of 504 amino acids and shows high homology to rat SN1 and rat brain glutamine transporter (GlnT). When expressed in mammalian cells, the transport function of human SN1 could be demonstrated with glutamine as the substrate in the presence of LiCl (instead of NaCl) and cysteine. The transport activity was saturable, pH-sensitive, and specific for glutamine, histidine, asparagine, and alanine. Analysis of Li ؉ activation kinetics showed a Li ؉ :glutamine stoichiometry of 2:1. When expressed in Xenopus laevis oocytes, the transport of glutamine or asparagine via human SN1 was associated with inward currents under voltage-clamped conditions. The transport function, monitored as glutamine-or asparagine-induced currents, was saturable, Na ؉ -dependent, Li ؉ -tolerant, and pH-sensitive. The transport cycle was associated with the involvement of more than one Na ؉ ion. Uptake of asparagine was directly demonstrable in these oocytes by using radiolabeled substrate, and this uptake was inhibited by membrane depolarization. In addition, simultaneous measurement of asparagine influx and charge influx in the same oocyte yielded an asparagine:charge ratio of 1. These data suggest that SN1 mediates the influx of two Na ؉ and one amino acid substrate per transport cycle coupled to the efflux of one H ؉ , rendering the transport process electrogenic.

No effect of glutamine supplementation and hyperoxia on oxidative metabolism and performance during high-intensity exercise

“…Therefore, conclusions with regard to oxidative metabolism based on alterations in plasma lactate concentrations should be treated with caution. Indeed, the later peak plasma lactate concentration following glutamine ingestion in the present study demonstrates the many factors that may influence plasma lactate concentration (indeed it is possible that glutamine interferes with either the passive or active export of lactate from the muscle, via monocarboxylate transporter 1; Fei et al 2000). Furthermore, the reduction in lactate concentration in hyperoxia observed in previous studies during submaximal exercise is likely due to a reduction in muscle glycogenolysis (Stellingwerff et al 2005;Stellingwerff, LeBlanc, Hollidge, Heigenhauser & Spriet, 2006).…”

Heterologous Expression of the Glutamine Transporter SNAT3 in Xenopus Oocytes Is Associated with Four Modes of Uncoupled Transport

“…The carrier of these currents has been discussed controversially. Initially, the transporter was thought to be electrogenic (17), but this was refuted by two subsequent studies (14,15). Further analysis of the currents showed that the reversal potential of the substrate-induced currents varies with pH, and as a result a proton conductance was proposed by Chaudhry et al (14).…”

“…Uptake of glutamine is coupled to the cotransport of 1Na ϩ and the antiport of 1H ϩ and hence is unaffected by changes of the membrane potential (15,16). However, inward currents are observed during transport (14,15,17). The carrier of these currents has been discussed controversially.…”

Na+- and Cl–-coupled active transport of nitric oxide synthase inhibitors via amino acid transport system B0,+

“…Cloning of mouse ATB 0,+ . The SuperScript plasmid system (Life Technologies Inc., Rockville, Maryland, USA) was used to establish a unidirectional cDNA library with poly(A) + RNA isolated from mouse colon as described previously (23)(24)(25). The probe for library screening was prepared by RT-PCR using primers specific for mouse ATB 0,+ cDNA reported in the GenBank (accession no.…”