The Effect of Azaserine on Cysteine Transport in Erythrocytes

PURPOSE: The objective of the present study was to investigate the effects of azaserine on cysteine influx and efflux in erythrocytes. Azaserine is a potent carcinogen and induces tumors especially in the pancreas. Azaserine structurally resembles glutamine and thus interferes with a variety biological processes including enzymatic activities and transmembrane transport of solutes. Glutamine and cysteine have been shown to share similar transport mechanisms. In this respect azaserine, structurally resembling glutamine, may effect the cysteine transport across the cell membrane or conversely cysteine availability may effect the uptake or release of azaserine. METHODS: In the present study, erythrocytes were treated with different concentrations of azaserine and cysteine and then cysteine influx and efflux processes in erythrocytes were measured through detection of nonprotein, soluble free –SH levels. The effect of glutamine on cysteine transport was also measured in a similar way. RESULTS: Our results indicate that azaserine has an effect on bi-directional transport of cysteine through the erythrocyte membranes. An induction of cysteine influx was observed in the presence of 2 mM of azaserine (2.4 ± 0.07 μmol/ml erythrocyte) compared to the absence of azaserine (1.9 ± 0.14 μmol/ml erythrocyte). However, the effect of azaserine on cysteine efflux from erythrocytes was more pronounced and started at 0.5 mM of azaserine and then increased in a concentration dependent manner. In the presence of 2 mM of azaserine, the cysteine efflux reached to 0.71± 0.05 μmol/ml erythrocyte and in the absence of azaserine the efflux reached to 0.51± 0.01 μmol/ml erythrocyte. Glutamine at 2 mM concentration also significantly increased the cysteine efflux from erythrocytes. CONCLUSION: Our results suggest that carcinogenic compound azaserine affects the flux of cysteine through erythrocyte membranes. This result suggest that azaserine may enter the erythrocytes by an azaserine/cysteine exchange mechanism. Glutamine which shares structural similarity with azaserine did not show any effect on cysteine influx. We suggest that in addition to its effect on induction of DNA damage and enzymatic activities, the described effects of azaserine in this study may contribute to its carcinogenity. Azaserine may limit the cysteine availability as it gains entry into the cells in exchange with cysteine. This process may represent a mechanism by which azaserine is accumulated in tissues in which it leads to carcinogenesis.