Promotion of cystine uptake and its utilization for glutathione biosynthesis induced by cysteamine and <em>N</em>-acetylcysteine.

Chinese hamster ovary (CHO) cells obtain a high capacity to utilize cystine from the growth medium by exposure to cysteamine (2-mercaptoethylamine, MEA) or N-acetylcysteine (NAC). For uptake studies a modified McCoy&#39;s 5A medium supplemented with 0.1 mM [35S]cystine was used. The uptake of cystine was dependent on the time of exposure (0-60 min) and the concentrations of MEA or NAC (0-8 mM). At high concentrations of MEA or NAC, the uptake of cystine became saturated. Half-maximal uptake of cysteine was observed at concentrations of 0.12 mM MEA and 0.66 mM NAC, respectively. Increase in temperature (37-44&deg;) or pH (6.0-8.0) during MEA or NAC exposure further increased the cystine uptake. The increased uptake of cystine was not affected in the presence of glutamate or homocysteate which both inhibited the cystine uptake of control cells. Determination of both reduced (GSH) and oxidized (GSSG) cellular glutathione showed a twofold increase in MEA- or NAC-treated CHO cells. DL-buthionine-S,R-sulfoximine (BSO), an inhibitor of GSH biosynthesis completely blocked the promotion of cystine uptake by MEA and NAC. By further analysis using reversed-phase HPLC of cell extracts, more than 90% of the [35S] radioactive cystine taken up by the cells could be recovered within the pool of GSH. The results demonstrate that exposure of CHO cells with MEA and NAC leads to a promoted uptake of cystine from the culture medium and its rapid utilization for cellular GSH biosynthesis