Glutathione (GSH) is an important pulmonary antioxidant. It has been established that the rate limiting step of GSH synthesis is the intracellular availability of cysteine. Type II cells isolated from rat lung were utilized to study the mechanism of cysteine and cystine uptake in alveolar epithelial cells.
Sodium-dependent uptake of cystine and glutamate (glu) was observed and found to be totally inhibited by micromolar concentrations of glu and aspartate (asp). Using inhibitors specific for different amino acid transporters only aspartate-β-hydroxamate (AβH), a specific inhibitor of system XAG, effectively inhibited sodium-dependent glu and cystine uptake. These results suggested cystine, glu and asp are transported via an XAG-like carrier protein.
Addition of physiological concentrations of glutathione (60μM) or cysteinylglycine (30 μM) as components of an in vivo reductive environment enhanced (2-fold) sodium-dependent cystine uptake by reducing this amino acid to cysteine. Cysteine uptake could be partially inhibited by AβH or serine and completely by both. Therefore, both an XAG-like carrier and system ASC (inhibited by serine) are involved in sodium-dependent cysteine transport. System XAG was further characterized and found to transport glutamate, cysteine and cystine(Km= 7.5 ± 0.1, 82.6 ± 7.9 and >500 μM respectively).
In a sodium-free environment, quisqualate-inhibitable uptake of cystine and glu was observed, demonstrating that Type II cells also possess the system xc carrier protein. In a reducing environment, sodium-independent cysteine uptake was also observed via a carrier yet to be characterized.
These results suggest that in vivo, the redox state surrounding Type II cells maintains significant amounts of cysteine in the reduced form. The subsequent uptake of cyst(e)ine by either sodium-dependent or -independent carriers is dependent on endogenous levels of amino acids competing for transport. Because of their role in GSH synthesis, these transporters contribute to the protection of Type II cells against oxidant stress.
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Knickelbein, R., Seres, T., Lam, G. et al. Demonstration of Multiple Transporters for Cyst(e)ine Uptake in Type II Pneumocytes. ▴ 2311. Pediatr Res 39, 388 (1996) doi:10.1203/00006450-199604001-02336