SHORT COMMUNICATION: 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 (T4) uptake in the choriocarcinoma cell line JAr. Cells were incubated with 50pm125 I-T4S in the absence (total uptake) and in the presence (non-specific uptake) of 10μm T4S. Cells were also incubated at 37°C for 2min with 50pm125 I-T4in the presence of an increasing amount of unlabelled T4(0–10μm) or T4S (0–30μm). There was negligible total uptake of125I-T4S (1.14±0.05fmol/mg cellular protein, mean±sem) and no specific uptake after 120min incubation. Minor inhibition of125I-T4uptake by T4S could be explained entirely by a low level of residual T4(0.2 per cent) in the T4S preparation. These findings indicate that T4S does not share the T4membrane transporter.