Transport interaction of L-cystine and L-glutamate in human diploid fibroblasts in culture.

The uptake of L-cystine or L-glutamate into cultured human diploid fibroblasts, IMR-90, has been studied. These amino acids appeared to be taken up into the cells via sodium ion-independent uptake system. Inhibition studies using various amino acids provided information about the specificity of the uptake system. The L-cystine or L-glutamate uptake system was shown to exhibit very similar substrate specificity to each other. It was demonstrated that L-cystine uptake was potently inhibited by L-glutamate and vice versa. The inhibition was immediate and reversible, and the kinetic studies indicated that the inhibition was competitive. The K, values for uptake of L-cystine and L-glutamate were similar to their Kt values when acting as inhibitors. Among other 24 amino acids tested, D-glUtamate was a relatively effective inhibitor for either Lcystine or L-glutamate uptake, and the others showed only a limited or no inhibitory effect on the uptake of L-cystine or L-glutamate. The metabolic conversion of L-cystine or L-glutamate by the cells was examined. The L-glutamate was recovered unchanged from the cells, whereas the L-cystine had been converted very rapidly to L-cysteine, glutathione, and the mixed disulfide of glutathione and L-cysteine. However, it was found that L-glutamate inhibited the L-cystine uptake without disturbing the L-cystine metabolism. These results suggest that the bulk of L-cystine or L-glutamate is transported into the human fibroblasts via a common carrier system.