Substrate specificity of iodothyronine 5′-deiodinase in rat liver homogenates and its requirements of divalent cations [formula omitted

Abstract

Studies were carried out to compare the 5′-deiodination reactions of thyroxine (T 4) and 3,3′ −5′-triiodothyronine (rT 3) in 2.5% rat liver homogenates. The 5′-deiodinase activity was assayed by the 3,5,3′-triiodothyronine (T 3) produced from T 4 or by 125I-rT 3. Under our experimental conditions, the two 5′-monodeiodination reactions resulted in similar apparent K Ms: 1.5 μM for T 4 and 1.1 μM for rT 3. However, the apparent V max values of T 4 and rT 3 deiodination reactions were, respectively, 0.91 and 222 pmol/mg protein/min. Both reactions were stimulated by thiol reagents but only rT 3 deiodination showed complete thiol dependence. The inhibitory effect of 6-propyl-2-thiouracil on the 5′-deiodination of rT 3 was at least 50 fold greater than that of T 4. The divalent ion requirement of the deiodination system was tested with CaCl 2, MgCl 2, and ZnCl 2 at a range of concentrations. Zinc ion appeared to be a potent inhibitor in both T 4 and rT 3 deiodination systems. Only the 5′-deiodination of rT 3 was inhibited slightly by low concentrations of calcium and magnesium ions. Our results suggest that based on their apparently distinct regulation mechanisms, the 5′-monodiodination of T 4 and rT 3 in rat liver homogenates is likely mediated by more than one enzyme, despite the similarity of observed K Ms.