L-cysteine influx and efflux in human erythrocytes: The role of red blood cells in redox and metabolite homeostasis in the plasma

Abstract

The objective of this study was to investigate L-cysteine influx and efflux in human erythrocytes. L-cysteine is an amino acid required for glutathione synthesis in erythrocytes. In addition to being incorporated into glutathione, the soluble antioxidant L-cysteine plays a role in the maintenance of a proper intracellular or extracellular redox status. Recent investigations have pointed out that L-cysteine may contribute to redox homeostasis in the plasma and in the periplasm of some bacteria. Thus L-cysteine availability in the plasma may influence the oxidized/reduced state of several other metabolites normally found in the plasma. Our L-cysteine uptake studies demonstrated that erythrocytes can respond to an increase in the L-cysteine concentration in the extracellular media and influx L-cysteine in a concentration dependent-manner. The L-cysteine efflux is also time and concentration dependent. Erythrocytes pretreated with higher concentration of L-cysteine displayed higher efflux rates. Erythrocytes pretreated with L-cysteine 1 mM displayed efflux and increased the free-SH concentrations up to 0.184 ± 0.010 mM in the incubation media in 1 hr. While this concentration reached 0.843 ± 0.012 mM in 10 mM-L-cysteine pretreated erythrocytes. Our results also showed that the L-cysteine efflux is partly mediated by the Alanine-Serine-Cysteine (ASC) system. The presence of alanine or serine in the incubation media decreased the rate of efflux by about 16%. Our results also showed that the L-cysteine efflux process is not a simple diffusion but a carrier-mediated process. When compared with N-acetyl-L-cysteine (NAC), which is known to diffuse through the membranes, L-cysteine displayed a higher efflux rate under the same conditions. Pretreatment of erythrocytes with L-cysteine 4 mM increased the free-SH concentration to 0.48 ± 0.005 mM whereas the same concentration of NAC brought the free-SH concentration to 0.36 ± 0.01 mM in the incubation media. Our results suggest that erythrocytes may contribute to redox and metabolite homeostasis of the plasma.