Studies on the transport of tyrosine, leucine, and methionine in cultured B-16 mouse melanoma cells.

1. Linear uptake of tyrosine lasted for 1 min in cultured B-16 mouse melanoma cells preloaded with an amino acid such as tyrosine. 2. The tyrosine uptake increased markedly on preincubating the cells with 0.1 mM methionine, tyrosine, histidine or tryptophan and moderately with 1 mM phenylalanine, valine, isoleucine, or leucine. The effects of the preincubation on leucine uptake were similar to those on tyrosine uptake. 3. The tyrosine uptake was Na-independent under the experimental conditions used (0.05-1.0 mM); Km 75 micrometers and Vmax 15 nmol/min/mg protein. The methionine uptake (0.1-0.5 mM) was also Na-independent (Km 150 micrometers and Vmax 25 nmol/min/mg protein), whereas Na-dependent uptake could contribute at 2 mM methionine. 4. Inhibitions of tyrosine uptake by tryptophan, phenylalanine, leucine, isoleucine, methionine, valine, and histidine were competitive, giving K1 values of 70, 80, 100, 130, 160, 350, and 900 micrometers, respectively. 5. Exchange between intracellular methionine and extracellular tyrosine and vice versa was equimolar. Potencies of amino acids in stimulating tyrosine efflux were in the following order: methionine greater than tyrosine greater than histidine greater than tryptophan greater than phenylalanine greater than leucine greater than isoleucine much greater than valine. 6. The amino acid affinity of the system in the intracellular surface was suggested to be different from that in the extracellular surface. 7. The theophylline-treated cells showed a marked increase in tyrosine-uptake rate with elevated Vmax and unchanged Km.