Activities of thyroid hormones and related compounds in an in vitro thymocyte assay

Prior studies have demonstrated that the thyroid hormones L-triiodothyronine and L-thyroxine stimulate the rapid uptake of 1-amino-cyclopentane-1-carboxylic acid into isolated rat thymocytes. In the present study the effects of several groups of thyroid hormones and structurally related compounds were investigated to determine the structure-function relations required for stimulation of this membrane process. Particular attention was given to (a) analogues with modifications at the oxygen bridge, the phenolic hydroxyl group, and the group at the 3' position of the outer ring, and (b) the steric orientation of the thyroid hormones. The following were found to be important for maximal activity: (a) the L-isomer configuration, (b) the presence of a 4'-hydroxyl group, (c) the presence of one substituent in both the inner and outer rings (3 and 3' positions), (d) the distal orientation of the 3' substituent in the outer ring of L-triiodothyronine, and (e) the lipophilic character of the 3' substituent. Of lesser importance was the presence of halogen atoms, or an oxygen atom in the ether position. Since these structure-function relations seen in thymocytes parallel in many respects those relations seen in whole-animal studies, it is believed that thymocytes will be a useful tool for further studies of thyroid hormones and their analogues.