The nature of inhibition of thyroid monoamine oxidase by thyroxine in rat thyroid.

The effect of thyroxine on rat thyroid monoamine oxidase (MAO) activity in vivo and in vitro was studied in order to evaluate the nature of the inhibition of the enzyme. Monoamine oxidase activity was measured in rat thyroid homogenate. Thyroxine (200 micrograms kg-1) or triiodothyronine (40 micrograms kg-1) injected in vivo inhibited the activity of monoamine oxidase (MAO) significantly (P less than 0.05). Thyroxine (T4), 10 micrograms added to the incubation mixture during 20 min also inhibited MAO activity in thyroid homogenate. A Lineweaver-Burk plot of the data obtained shows that the inhibition is of the uncompetitive type.