TRANSPORT OF L-TYROSINE BY B16/F10 MALIGNANT MELANOCYTES - CHARACTERIZATION OF THE PROCESS

The main characteristics of L-tyrosine (L-Tyr) uptake by B16/F10 malignant melanocytes are reported. This amino acid can be taken up by two systems, both of them being saturable. The first one would be system L. This system can be studied in cells preloaded with amino acids that are a good substrate for system L, such as L-methionine or L-tryptophan. The kinetic parameters for L-Tyr uptake by this transport system are V(m) = 6.5 pmol L-Tyr/10(3) cells.min and K(m) around 130-mu-M. The second system, probably the system ASC, shows lower capacity but higher affinity than the former. This system can be detected only in cells previously depleted of amino acids, showing approximate kinetic values of V(m) 0.05 pmol L-Tyr/10(3) cells.min and K(m) around 5-mu-M. It is shown that the increase in cell density yields a decrease in the rate of L-Tyr uptake by system L, but this increase does not affect the high affinity system. alpha-MSH does not affect significantly the L-Tyr uptake by both systems. 2-Amino bicyclo-(2,2,1)-hepatane-2-carboxylic acid produces a remarkable inhibition of the rate of L-Tyr uptake, but alpha-methylaminoisobutyric acid does not affect the rate of transport of this amino acid. The absence of sodium produces a slight but reliable decrease in the rate of L-Tyr uptake, supporting the involvement of two different transport systems. The ionophores monensin and nigericin enhance the transport by system L, but this effect is suppressed by the presence of ouabain. This finding indicates that the (Na + -K+)-ATPase is essential for the stimulating action of ionophores. Finally, it is shown that gamma-glutamyl cycle is not involved in L-Tyr uptake, since the inhibition of gamma-glutamyl transpeptidase by periodate treatment does not affect the rate of transport.