5-aminolevulinic acid, but not 5-aminolevulinic acid esters, is transported into adenocarcinoma cells by system BETA transporters

5-aminolevulinic acid (5-ALA) and its ester derivatives are used in photodynamic therapy as precursors for the formation of photosensitizers. This study relates to the mechanisms by which 5-ALA is transported into cells. The transport of 5-ALA has been studied in a human adenocarcinoma cell line (WiDr) by means of [C-14]-labeled 5-ALA. The rate of uptake was saturable following Michaelis-Menten kinetics (K-m = 8-10 mM and V-max = 18-20 nmol.(mg protein x h)(-1)), and Arrhenius plot of the temperature-dependent uptake of 5-ALA was characterized by a single discontinuity at 32 degrees C. The activation energy was 112 kJ.mol(-1) in the temperature range 15 degrees-32 degrees C and 26 kJ.mol(-1) above 32 degrees C. Transport of 5-ALA was Na+ and partly Cl--dependent. Stoichiometric analysis revealed a Na+:5-ALA coupling ratio of 3:1. With the exception of valine, methionine and threonine, zwitterionic and basic amino acids inhibited the transport of 5-ALA. 5-ALA methyl ester was not an inhibitor of 5-ALA uptake. The transport was most efficiently inhibited, ie. by 65-75%, by the beta-amino acids, beta-alanine and taurine and by gamma-aminobutyric acid (GABA). Accordingly, 5-ALA, but not 5-ALA methyl ester, was found to inhibit cellular uptake of [H-3]-GABA and [C-14]-beta-alanine. Protoporphyrin IX (PpIX) accumulation in the presence of 5-ALA (0.3 mM) was attenuated 85% in the presence of 10 mM beta-alanine, while PpIX formation in tells treated with 5-ALA methyl ester (0.3 mn) or 5-ALA hexyl ester (4 mu M) was not significantly influenced by beta-alanine. Thus, 5-ALA, but not 5-ALA esters, is transported by beta-amino acid and GABA carriers in this cell line.