Thyroxine 5′-Monodeiodination in Rat Anterior Pituitary Homogenates*

Abstract

The conversion of T₄ to T₃ by 5′-monodeiodination was studied in anterior pituitary homogenates. No T₄ 5′- monodeiodination was seen unless dithiothreitol (DTT) or reduced glutathione was added. DTT (5 mM) caused measurable conversion, and maximum rates occurred between 50–100 mM DTT. Glutathione was a less potent stimulant. T₄ 5′-monodeiodination was pH dependent, with a pH maximum of 7.5, and temperature dependent between 4–37 C. Preheating the homogenate to 75 C for 30 min destroyed more than 95% of the activity. The reaction rate was proportional to the tissue content in the homogenate. At submaximal but not at saturating DTT concentrations, incubations under nitrogen increased the reaction rate to 161% of the rates in incubations under air. There was no significant effect of 5 mM EDTA, 5′ M α-tocopherol, 100 mM KC1, 100 –M Nal, 140 μM TRH, 300 μM propylthiouracil, or 300 HM methimazole. Catalase (2000 U/ml) had a variable inhibitory effect depending on the DTT concentration. The T₄ 5′-monodeiodination rate was dependent on T₄ concentration, and the Km for T₄, was 8.8 nM. With 1.3 JUM T₄, and 100 mM DTT, both saturating concentrations, the mean T₃, 5′-monodeiodination rate in nine experiments was 78 fmol T₄/mg protein-min, with a range of 33–149 fmol Ta/mg protein-min. There was little or no activity in homogenates of posterior pituitary, hypothalamus, or frontal cortex. The T₄ 5′-monodeiodination rates in homogenates of anterior pituitaries from hyperthyroid rats were 15–35% of the rates in homogenates of simultaneous euthyroid controls (P < 0.05 or less), and the rates in homogenates from hypothyroid rats were 3.1–12.1 times greater than those in homogenates from simultaneous euthyroid controls (P < 0.001). T₄ 5′-monodeiodinase activity in anterior pituitary homogenates from fasted rats was reduced to 76 ± 5% (SE) of control values when the results of five experiments were combined (P < 0.005). Iopanoic acid added in vitro inhibited T₄ 5′-monodeiodination to 48% of the control rate at 10 μM and to 33% of control at 50 JUM (both P < 0.01). Iopanoic acid, administered in vivo in dosage of 5 mg/100 g BW twice on 1 day, caused the T.(5′-monodeiodination rate in anterior pituitary homogenate to fall to 13% of control (P < 0.001). In summary, T₄ 5′-monodeiodination in anterior pituitary homogenates is enzymatic and tissue specific. It requires thiolreducing agents and is inhibited by iopanoic acid, as does the same reaction in liver and kidney homogenates. Anterior pituitary T₄ 5′-monodeiodination differs from the hepatic and renal reactions quantitatively in requiring 20 to 50–fold greater DTT concentrations for maximum rates and in having a markedly lower Km for T₄. The anterior pituitary process responds qualitatively differently from those in liver and kidney during hyperthyroidism and hypothyroidism. The responses of the pituitary to hyper- and hypothyroidism suggest that of the different cell types, the thyrotrophs have the greatest T₄ 5′-monodeiodinase activity. (Endocrinology106: 567, 1980)