L-cysteine influx in erythrocytes as a function of human age.

In erythrocytes, although three amino acids are required for the synthesis of reduced glutathione (GSH), the rate of GSH synthesis is determined only by the availability of L-cysteine. Cysteine supplementation has been shown to ameliorate several parameters that are known to degenerate during human aging; this has led to an interesting hypothesis that aging could be a cysteine deficiency syndrome. In the present study, we measured L-cysteine influx in human erythrocytes by suspending cells in solution containing 10 mM L-cysteine. We show a significant decline in the influx of L-cysteine in erythrocytes during aging in humans. The decrease in cysteine influx correlates with the decrease in antioxidant potential of plasma measured in terms of FRAP (ferric-reducing ability of plasma) during aging. We conclude that a decreased influx of L-cysteine may be an important factor contributing to the development of oxidative stress in human erythrocytes during aging.