Substrate specificity of iodothyronine 5'-deiodinase in rat liver homogenates and its requirements of divalent cations in vitro.
Studies were carried out to compare the 5'-deiodination reactions of thyroxine (T4) and 3,3'-5'-triiodothyronine (rT3) in 2.5% rat liver homogenates. The 5'-deiodinase activity was assayed by the 3,5,3'-triiodothyronine (T3) produced from T4 or by 125I-rT3. Under our experimental conditions, the two 5'-monodeiodination reactions resulted in similar apparent KMs: 1.5 microM for T4 and 1.1 microM for rT3. However, the apparent Vmax values of T4 and rT3 deiodination reactions were, respectively, 0.91 and 222 pmol/mg protein/min. Both reactions were stimulated by thiol reagents but only rT3 deiodination showed complete thiol dependence. The inhibitory effect of 6-propyl-2-thiouracil on the 5'-deiodination of rT3 was at least 50 fold greater than that of T4. The divalent ion requirement of the deiodination system was tested with CaCl2, MgCl2, and ZnCl2 at a range of concentrations. Zinc ion appeared to be a potent inhibitor in both T4 and rT3 deiodination systems. Only the 5'-deiodination of rT3 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 T4 and rT3 in rat liver homogenates is likely mediated by more than one enzyme, despite the similarity of observed KMs.
S
S
Kobayshi
Y
Y
Gao
R L
RL
Ong
C S
CS
Pittman
D000818:Animals; D002122:Calcium Chloride / Q000494:pharmacology; D002413:Cations, Divalent; D002712:Chlorides; D004226:Dithioerythritol / Q000494:pharmacology; D007453:Iodide Peroxidase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008099:Liver / Q000201:enzymology*; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D008297:Male; D011441:Propylthiouracil / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013379:Substrate Specificity; D013974:Thyroxine / Q000378:metabolism; D014284:Triiodothyronine / Q000378:metabolism; D014285:Triiodothyronine, Reverse / Q000378:metabolism; D015032:Zinc / Q000494:pharmacology; D017967:Zinc Compounds
D002413:Cations, Divalent; D002712:Chlorides; D017967:Zinc Compounds; D015636:Magnesium Chloride; D014284:Triiodothyronine; D014285:Triiodothyronine, Reverse; D004226:Dithioerythritol; D011441:Propylthiouracil; C016837:zinc chloride; D007453:Iodide Peroxidase; D008274:Magnesium; D015032:Zinc; D002122:Calcium Chloride; D013974:Thyroxine