Phenolic and nonphenolic ring deiodinations of iodothyronines in cultured hepatocarcinoma cell homogenate from monkey.
Iodothyronine monodeiodinase activities in homogenates of cultured monkey hepatocarcinoma cells were measured by the deiodination of [3.5-(125)I]-diiodo-L-thyronine or 3-[3',5'-(125)I]triiodo-L-thyronine (phenolic ring-labeled 'reverse' triiodothyronine). The assay system utilized a small ion-exchange column (AG50W-X4, O.9 X approximately 1 cm) to measure 125I-. Both deiodinases were destroyed by boiling for 1 min. Maximal nonphenolic ring deiodination was observed at pH 7.9 whereas maximal phenolic ring deiodination was at pH 6.3. Both reactions were enhanced strongly by dithiothreitol (0.1-5mM), and slightly by 5 mM beta-mercaptoethanol. Phenolic ring deiodination was strongly inhibited by 0.1 mM propylthiouracil. Nonphenolic ring deiodination was accelerated by EDTA (1.2 MM) and inhibited by Mg(2+) (5mM). Methylmercaptoimidazol and Mg(2+), Ca(2+) and Mn(2+) (0.1-1.0 mM) had little or no effect on either reaction, but Zn(2+) (0.1 mM) strongly inhibited both. Both reactions were inhibited by excess iodothyronine analogues at 10 mM to 10 micron M, and thyroxine was shown to be a competitive inhibitor in both cases. On the basis of relative affinities and inhibitory effects, it appears that the order of affinity for the phenolic ring deiodinase is 3,3',5'-triiodo-L-thyronine(rT3) greater than L-thyroxine(T4) greater than 3,5,3'-triiodo-L-thyronine(T3), whereas for the nonphenolic ring deiodinase the order is T3 greater than T4 greater than rT3. Diiodotyrosine did not affect their deiodination.
D000818:Animals; D002413:Cations, Divalent / Q000494:pharmacology; D002478:Cells, Cultured; D004104:Diiodothyronines / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D000882:Haplorhini; D007453:Iodide Peroxidase / Q000378:metabolism*; D007455:Iodine / Q000378:metabolism; D008114:Liver Neoplasms, Experimental / Q000378:metabolism*; D008713:Methimazole / Q000494:pharmacology; D010544:Peroxidases / Q000378:metabolism*; D010636:Phenols / Q000378:metabolism; D011441:Propylthiouracil / Q000494:pharmacology; D013970:Thyronines / Q000378:metabolism* / Q000494:pharmacology; D013974:Thyroxine / Q000378:metabolism; D014284:Triiodothyronine / Q000378:metabolism
D002413:Cations, Divalent; D004104:Diiodothyronines; D010636:Phenols; D013970:Thyronines; D014284:Triiodothyronine; D008713:Methimazole; D011441:Propylthiouracil; D007455:Iodine; D010544:Peroxidases; D007453:Iodide Peroxidase; D013974:Thyroxine