Characteristics of D- and L-Alanine Transport into Mouse Ehrlich Ascites Tumor Cells

Abstract

The present investigation was undertaken to examine the temperature dependence of the kinetic parameters for the uptake and exit of D- and L-alanine in Ehrlich ascites tumor cells, as one of a series of studies to account for the high in vivo uptake of radio-activity of ¹⁴C-labeled D-amino acids in various tumor cells. The V_max values for D- and L-alanine uptake decreased with a fall in temperature. The activation energies for both isomers obtained from plots of In V_max against 1/T suggested that these isomers were transported through a mediated process. On the other hand, K_m for D-alanine uptake was only slightly dependent on temperature, whereas K_m for L-alanine decreased with a fall in temperature. These temperature dependences of K_m are discussed in comparison with those of K_m for D- and L-leucine uptakes. Furthermore, it was observed that the exit rate of D-alanine was considerably slower than that of the L-isomer. The activation energies of the isomers obtained from plots of ln k_e against 1/T (k_e : the first-order rate constant for exit) suggested that free diffusion is predominant in the exit process of D-alanine, in contrast to that of the L-form. This slow exit of D-alanine may account in part for the high in vivo uptake of radioactivity of D-alanine-¹⁴C into Ehrlich tumor cells.