Effect of theophylline on binding of triiodothyronine, thyroxine, thyroxamine, tetraiodothyroacetic acid and cortisol in the cytosol of human leukocytes.

Inhibition constants describing competitive potency of thyroactive analogues and cortisol against labelled triiodothyronine (T3) in cytosol in human leukocytes were determined in the presence and the absence of theophylline by means of charcoal-dextran assay. L-triiodothyronine, L-thyroxine,L-thyroxamine,L-tetraiodothyroacetic acid, D-thyroxine and cortisol competed with T3 in binding sites of cytosolic proteins without theophylline addition. While theophylline addition increased the competitive potency (decreased the values of inhibition constants) of all thyroactive analogues, no such equivocal effect has been found in the case of cortisol.