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

We examined uptake of l -thyroxine sulphate (T(4)S) and possible interactions between T(4)S and thyroxine (T(4)) uptake in the choriocarcinoma cell line JAr. Cells were incubated with 50 p m(125)I-T(4)S in the absence (total uptake) and in the presence (non-specific uptake) of 10 microm T(4)S. Cells were also incubated at 37 degrees C for 2 min with 50 p m(125)I-T(4)in the presence of an increasing amount of unlabelled T(4)(0-10 microm) or T(4)S (0-30 microm). There was negligible total uptake of(125)I-T(4)S (1.14+/-0. 05 fmol/mg cellular protein, mean+/-sem) and no specific uptake after 120 min incubation. Minor inhibition of(125)I-T(4)uptake by T(4)S could be explained entirely by a low level of residual T(4)(0. 2 per cent) in the T(4)S preparation. These findings indicate that T(4)S does not share the T(4)membrane transporter.