Lack of membrane transport of L-thyroxine sulphate in the human choriocarcinoma cell line, JAr

We examined uptake of L-thyroxine sulphate (T4S) and possible interactions between T4S and thyroxine (T-4) uptake in the choriocarcinoma cell line JAr . Cells were incubated with 50 pM I-125-T4S in the absence (total uptake) a nd in the presence (non-specific uptake) of 10 mu M T4S. Cells were also in cubated at 37 degrees C for 2 min with 50 pM I-125-T-4 in the presence of a n increasing amount of unlabelled T-4 (0-10 mu M) or T4S (0-30 mu M). There was negligible total uptake of I-125-T4S (1.14 +/- 0.05 fmol/mg cellular p rotein, mean +/- SEM) and no specific uptake after 120 min incubation. Mino r inhibition of I-125-T-4 uptake by T4S could be explained entirely by a lo w level of residual T-4 (0.2 per cent) in the T4S preparation. These findin gs indicate that T4S does not share the T-4 membrane transporter. (C) 2000 Harcourt Publishers Ltd.