DESETHYLAMIODARONE IS A NONCOMPETITIVE INHIBITOR OF THE BINDING OF THYROID-HORMONE TO THE THYROID-HORMONE BETA(1)-RECEPTOR PROTEIN

It has been hypothesized that amiodarone (A), a potent antiarrythmic a nd antianginal drug, induces a local hypothyroid-like condition in ext rathyroidal tissues. This might be related to competitive antagonism o f A for the thyroid hormone receptor reported in some studies but deni ed in others. These conflicting results are presumably due to the poor solubility of A in a hydrophilic environment. We, therefore, studied the effect of the drug and its major metabolite, desethylamiodarone (D EA), on the in vitro binding of thyroid hormone (T3) to its receptor p rotein using the rat beta1-thyroid hormone receptor expressed in Esche richia coli. A and DEA stayed in solution up to 10(-4) M when 0.05% Tr iton X-100 was added to the incubation buffer, as evidenced by a recov ery of 80-90% for both chemicals, as measured by HPLC. DEA, but not A, had a clear inhibitory effect on the binding of T3 to its receptor (I C50, 1-3 x 10(-5) M). Scatchard analysis in the presence of DEA demons trated a dose-dependent decrease in the K(a) as well as the maximum bi nding capacity. Lineweaver-Burke analysis indicated noncompetitive inh ibition. Plots of the intercepts of Lineweaver-Burke plots vs. DEA con centration were linear (y = 0.334 + 0.098x), giving a K(i) of 30 muM f or the binding of DEA to the occupied receptor. Plots of the slopes vs . inhibitor concentration were parabolic (y = 3.01 + 0.06x + 0.16x2), indicating a progressively stronger effect of DEA on the unoccupied re ceptor as concentrations rise. This preference for the unoccupied rece ptor is reflected in experiments that show a progressive loss of T3 bi nding when the receptor was incubated for increasing periods with DEA before adding T3. We conclude that DEA is a noncompetitive inhibitor o f the binding of T3 to the beta1-thyroid hormone receptor protein, int eracting preferably with the unoccupied T3 receptor.