A novel function of glutamine in cell culture: utilization of glutamine for the uptake of cystine in human fibroblasts

Transport and metabolism of glutamine has been investigated in human diploid fibroblasts, IMR-90. Glutamine was taken up via System ASC (Na+-dependent amino acid transport system especially reactive with short or polar side chain amino acids). In the routine culture medium the cells contained a large quantity of glutamate; its major source was shown to be glutamine in the medium. Previously we described a transport system that mediates the entrance of cystine in exchange for the exit of glutamate (Bannai, 1986). Since the cystine taken up is reduced to cysteine and the cysteine readily exits to the medium where it is oxidized to cystine, a cystine-cysteine cycle across the plasma membrane has been postulated. When the cells were cultured in glutamate/glutamine-free medium, intracellular glutamate decreased, depending on the amount of cystine in the medium; in the absence of cystine, glutamate decreased very slowly. When the cells were cultured in ordinary medium, glutamine in the medium decreased, and glutamate in the medium increased. Both changes were well correlated with cystine concentration in the medium. These results are consistent with the view that the intracellular glutamate, of which the source is glutamine in the medium, is released from the cells into the medium in order to take up cystine and thereby to rotate the cystine-cysteine cycle. In the routine culture one-third to one-half of the total consumption of glutamine seems to be used for the uptake of cystine.