Thyroxine binding and deiodination of thyroxine by human lymphocytes

Binding and deiodination of T4 by lymphocytes from euthyroid, hypothyroid and hyperthyroid subjects were studied. Two sets of binding sites were found on normal lymphocytes. One set with equilibrium association constant of 6.9 X 10(9) l/mol and a maximal binding capacity of 3000 mol T4 per lymphocyte, and another set with equilibrium association constant of 4.1 X 10(6) l/mol and a maximal binding capacity of 7.8 X 10(6) mol T4 per lymphocyte. The major part of the binding sites were non specific and the binding reversible. Debinding assays revealed also two types of binding site and indicated that deiodination took place predominately from one type of binding site. Deiodination assay showed that the deiodination process had a maximal deiodination capacity of 6 X 10(-13) mol T4/10(6) lymphocytes. Paper chromatography revealed only I and T4 after incubation of lymphocytes with T4. In comparing the patient groups with the control group, lymphocytes from hyperthyroids and hypothyroids exerted a higher T4 turnover (deiodinated T4/bound T4) although the difference was not statistically significant. Lymphocytes exerted a high T4 turnover at low temperature (3 degrees C) and propylthiouracil inhibited deiodination of T4 but not binding.