Control of extracellular cysteine/cystine redox state by HT-29 cells is independent of cellular glutathione

Human cell lines regulate the redox state (Eh) of the cysteine/cystine (Cys/CySS) couple in culture medium to approximately −80 mV, a value similar to the average Eh for Cys/CySS in human plasma. The mechanisms involved in regulation of extracellular Eh of Cys/CySS are not known, but GSH is released from tissues at rates proportional to tissue GSH concentration, and this released GSH could react with CySS to contribute to maintenance of this balance. The present study was undertaken to determine whether depletion of cellular GSH alters regulation of extracellular Cys/CySS Eh. Decrease of GSH in HT-29 cells by inhibiting synthesis with l-buthionine-[ S, R]-sulfoximine showed no effect on the rate of reduction of extracellular CySS to achieve a stable Eh for Cys/CySS in the culture medium. Limiting Cys and CySS in the culture medium also substantially decreased cellular GSH but resulted in no significant effect on extracellular Cys/CySS Eh. Addition of CySS to these cells showed that extracellular Cys/CySS Eh approached −80 mV at 4 h while cellular GSH and extracellular GSH/GSSG Eh recovered more slowly. Together, these results show that HT-29 cells have the capacity to regulate the extracellular Cys/CySS Eh by mechanisms that are independent of cellular GSH. The results suggest that transport systems for Cys and CySS and/or membranal oxidoreductases could be more important than cellular GSH in regulation of extracellular Cys/CySS Eh.