INTERACTIONS BETWEEN TRANSPORT OF TRIIODOTHYRONINE AND TRYPTOPHAN IN JAR CELLS

[摘要] We studied the effect of a number of amino acids on uptake of L-triiodothyronine (T-3) in the human choriocarcinoma cell line, JAR. Tryptophan inhibited saturable T-3 uptake by about 57% without any significant effect on the non-saturable uptake. Michaelis constant (K-m) for T-3 uptake was 1.06 +/- 0.15 mu M (n = 15) with the corresponding maximum velocity (V-max) of 24.2 +/- 3.1 pmol/min/mg cellular protein. For tryptophan uptake the K-m was 1.31 +/- 0.26 mu M (n = 7) and V-max was 166.4 +/- 35.7 pmol/min/mg protein. The kinetic parameters for both uptake processes were similar to those reported in normal placenta. Uptake of T-3 was inhibited by tryptophan but not phenylalanine, but tryptophan uptake was inhibited both by T-3 and phenylalanine. Inhibition of T-3 uptake by tryptophan was dose dependent, with an inhibition constant (K-i) of 2.9 +/- 0.5 mM. Similarly, tryptophan uptake was inhibited by T-3 and phenylalanine in a dose dependent way with K-i values of 4.9 +/- 0.5 mu M and 15.6 +/- 4.8 mu M respectively. K-m for T-3 uptake was significantly increased to 1.86 +/- 0.42 mu M (n = 4) in the presence of 3 mM unlabelled tryptophan and, similarly, K-m for tryptophan uptake was significantly increased to 9.91 +/- 2.57 mu M (n = 3) in the presence of 5 mu M unlabelled T-3. Efflux of T-3 was progressively inhibited by increasing concentrations of both ligands, i.e. was saturable. We conclude that there is mutual competitive inhibition between uptake systems for T-3 and tryptophan in JAR cells, but the kinetic parameters of cross-inhibition of uptake by the substrates suggest that the carriers are distinct. T-3 may be transported in JAR cells by at least two transport systems with differing substrate specificities. We also demonstrated the presence of a saturable membrane carrier mediating the efflux of T-3 from the cells which was subject to trans-inhibition by T-3 and tryptophan.

[发布日期] 1994-05-01

[关键词] MEMBRANE TRANSPORT;TRIIODOTHYRONINE;TRYPTOPHAN;CHORIOCARCINOMA CELL LINE (JAR);PLACENTA