Neutral amino acid transport systems in Chinese hamster ovary cells

The neutral amino acid transport systems A, ASC, and L have been characterized in the Chinese hamster ovary cell. System A, defined by its sodium ion dependency and inhibition by 2-methylaminoisobutyric acid, was found to be extremely sensitive to the pH of the external medium and to increase in response to starvation for amino acids. System ASC, identified by its sodium ion dependency and intolerance of N-methylation of substrates, was found to be relatively insensitive to external pH and nutrient limitation. System ASC in Chinese hamster ovary cells has been shown to be the major mode of entry of neutral amino acids. A much broader substrate specificity was observed for System ASC than has been reported for other mammalian cell types, with nearly every amino acid tested showing significant uptake by this system. In addition, the highest observed velocities of uptake were for System ASC. System L, defined by its sodium ion independency and inhibition by 2-aminobicyclo-[2,2,2]-heptane-2-carboxylic acid, was enhanced in activity by lowered pH. The starvation-induced enhancement in System A activity was prevented by the presence of a single substrate of this system, the nonmetabolizable analog 2-methylaminoisobutyric, or by the presence of the protein synthesis inhibitor cycloheximide.