Contrasts in transport systems for anionic amino acids in hepatocytes and a hepatoma cell line HTC.

The uptake of anionic amino acids by the rat hepatoma cell line, HTC, occurs by two saturable routes, one Na+-dependent and one Na+-independent. Which of the two mediations will serve for transport of a given anionic amino acid depends on the chain length of that anionic substrate. The Na+-independent system, shared by glutamate, cystine, and anionic amino acids intermediate in length between the two, is stereoselective, pH-independent, and not inhibited by neutral amino acids. The pH-independent inhibition of this system by m-8-mercaptolactate-cysteine disulfide confirms that cystine reacts as an a ionic amino acid. Shorter anionic amino acids, including aspartate, are scarcely transported by this route. The accommodation of the cystine molecule calls for its stabilization as an anion even below pH 7. Inhibition triple that of cystine by the product of its hydrolytic monodeamination confirms this interpretation. A Na+-independent system similar to the one described here has been reported for fetal human lung fibroblasts (Bannai, S., and Kitamura, E. (1980) J BioL Chem. 255, 2372-2376). In contrast, the Na+-dependent route for anionic amino acid uptake by HTC cells is shared only by ‘the three short anionic amino acids, aspartate, cysteate, and cysteinesulfinate, among the analogs tested. Characteristics of the latter system include an unexplained type of pH dependence and inhibition by threonine and other amino acids known to be substrates for System ASC. Both systems for anionic amino acid uptake in the HTC cell have properties different from those of the two saturable, Na+-dependent routes of anionic amino acid uptake previously described for the cultured rat hepatocyte.