CYSTEINE AS A SYSTEM-SPECIFIC SUBSTRATE FOR TRANSPORT-SYSTEM ASC IN RAT HEPATOCYTES

The rapid transport of L-cysteine into isolated rat hepatocytes escapes detectable inhibition by 2-(methylamino)-isobutyric acid at levels up to 50 mM. The system transporting cysteine instead is convincingly similar to the ASC system described for the Ehrlich cell in structural and steric specificity and in pH sensitivity. The Na+-dependent uptake of 2-aminoisobutyric acid is almost evenly divided between Systems A and ASC, showing better accommodation of its two α-methyl groups by ASC than in the Ehrlich cell. The hepatocyte ASC system tolerates Li+-for-Na+ substitution better than does System A, although the tolerance depends on amino acid structure. Adaptive regulation and insulin and glucagon stimulation were not seen under conditions producing these effects for System A. © 1979.