Sequential deiodination of thyroxine to 3,3'-diiodothyronine via 3,5,3'-triiodothyronine and 3,3',5'-triiodothyronine in rat liver homogenate. The effects of fasting versus glucose feeding.

The characteristics of thyroxine (T4) deiodination to 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) and of each of the latter to 3,3'-diiodothyronine (3,3'T2) were examined in rat liver homogenate. Each of the four reactions was enzymatic in nature, demonstrating pH and temperature optima, and tissue and time dependence. All reactions were considerably augmented (greater than 10-fold) by the presence of a thiol agent. At pH 7.2 with 2 muM T4 as substrate, rT3 generation was 3.3 +/- 0.44 (S.E.) and T3 formation was 4.8 +/- 0.57 pmol/min/100 mg of homogenate protein. Fasting for 72 h resulted in a significant inhibition of T4 deiodination, compared to that in the glucose-fed animals, in a 2% homogenate preparation. Enzyme activity for T4 to T3 was reduced by 54% (p less than 0.05) in the homogenate from the fasted rats. Fasting lowered the enzyme activity of T4 to rT3 by 56% (p less than 0.05). Although the monodeiodination of T3 to 3,3'-T2 was also significantly depressed (p less than 0.01) by fasting, rT3 deiodination to 3,3'-T2 was not. The in vitro additon of 5 mM dithioerythritol did not reverse the effect of fasting on any reaction. These results demonstrate that a 72-h fast significantly impairs the sequential deiodination of T4 in liver homogenate. The effect of fasting appears to be mediated mainly through a reduction in enzyme concentration rather than co-factor availability.