Membrane cooperative enzymes. High molecular specificity for blocking action of thyroxine on triiodothyronine effect in rat erythrocyte and Escherichia coli systems.

The molecular specificity for the blocking action of thyroxine on the triiodothyronine effect in the cooperativity of membrane-bound rat erythrocyte acetylcholinesterase and Escherichia coli Ca2+-ATPase was analyzed. Changes in the values of n (Hill coefficient) were obtained at strict physiological levels of these hormones. The structural requirements of the thyroid hormones to modify the membrane-bound systems were studied using various analogues of these hormones. In the erythrocyte system, a very high molecular specificity for triiodothyronine and thyroxine actions was found. The L-alanine side is essential to carry out both the allosteric desensitization and the blocking effects. The blocking ability of thyroxine is characterized by the presence of iodine in the 5' position. The bacterial system presented only specificity for the triiodothyronine allosteric desensitization. A system of membrane-bound enzymes for the study of the actions of thyroid hormones, is presented here.