Changes in the Particulate Subcellular Component of Hepatic Thyroxine-5&-Monodeiodinase in Hyperthyroid and Hypothyroid Rats*

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Abstract
T4-5'-monodeiodination was studied in fractions of liver homogenates from rats made hyperthyroid by injections of T4 (10 μg/100 g BW-day, for 5 days), rats made hypothyroid by thyroidectomy, and euthyroid rats. In the presence of 5 mM dithiothreitol (DTT), reaction rates in the whole homogenates (H), 2,000 × g supernatants (Si), and 2,000–160,000 × g pellets (P2) were significantly greater than those in preparations without DTT. In H, Si, and P2 from hyperthyroid rats, reaction rates were approximately 3.5 times the rates in the corresponding fractions from euthyroid rats (P < 0.005 or less). DTT-stimulated reaction rates were reduced in H, Si, and P2 from hypothyroid rats to about 30% of the rates in these fractions from euthyroid rats (P < 0.005 or less). Increasing the T4 concentration from 1.3 to 9.1 μM in incubations of Si without DTT resulted in a progressive increase in the absolute rate of T4-5'-monodeiodination in all three treatment groups, but there was no change in the relative rates in Si from the hyperthyroid and hypothyroid groups compared to normal. Reaction rates in the presence of 5 mM DTT were consistently higher in Si from the hyperthyroid group and lower in Si from the hypothyroid group compared to normal in the T4 concentration range 1.3–26 JUM. In these incubations with DTT, 26 μM T4 appeared to be far below a saturating concentration. Incubation of P2 from the three groups with T4 concentrations between 0.13–26 μM in the presence of 8 mM reduced glutathione did show substrate saturation. The apparent Km for T4 was 2.9, 3.6, and 1.3 μM and the Vmax was 2362, 438, and 83 fmol T3/mg protein min in the hyperthyroid, euthyroid, and hypothyroid groups, respectively. A reconstitution experiment was performed by mixing P2 pools from each of the three treatment groups with cytosol (S2) from all three groups. The reconstitution procedure caused little or no enzyme inactivation. The S2 pools from the hyperthyroid, euthyroid, and hypothyroid groups had equal capacities to stimulate T4-5'-monodeiodination in P2 from the hyperthyroid group and in P2 from the euthyroid group. The reaction rates in the reconstituted mixtures containing P2 from hypothyroid rats were too low for useful comparisons. These data suggest that hyperthyroidism causes an increase in the hepatic T4-5'-monodeiodination rate by increasing the tissue content of the T4-5'-monodeiodinase enzyme and that hypothyroidism causes a decrease in the hepatic T4-5'-monodeiodination rate by decreasing the tissue content of this enzyme. Neither hyperthyroidism nor hypothyroidism substantially changes the cofactor activity of hepatic cytosol.