The cDNA for the type I iodothyronine 5‘-deiodinase encodes an enzyme manifesting both high Km and low Km activity. Evidence that rat liver and kidney contain a single enzyme which converts thyroxine to 3,5,3‘-triiodothyronine.
1992; Elsevier BV; Volume: 267; Issue: 18 Linguagem: Inglês
10.1016/s0021-9258(18)42310-1
ISSN1083-351X
AutoresJill Sharifi, Donald L. St. Germain,
Tópico(s)Biotin and Related Studies
ResumoThe enzymatic conversion of thyroxine (T4) to 3,6,3'-triiodothyronine (Ts) by iodothyronine 6'-deiodinase(s) is an obligate step in the physiologic action of thyroid hormones in most extrathyroidal tissues.In the rat liver and kidney, S'-deiodinase processes having either high K , (micromolar range) or low K , (nanomokr range) values for thyroid hormone substrates have been described.The number of enzymes mediating these reactions, however, remains uncertain and controversial.To examine this question we have compared the 6'-deiodinase activity expressed in membrane preparations of Xenopus laevis oocytes after the injection of either rat liver poly(A)+ RNA or in vitro prepared RNA transcribed using the G21 full-length type I 6"deiodinase cDNA.In oocytes injected with rat liver poly(A)+ RNA, high K , (i.e.type I) activity was observed when 20 mM dithiothreitol was used as the thiol cofactor, whereas K,,, values in the nanomolar range were noted with 0.6 m M dithiothreitol, glutathione, or a reconstituted thioredoxin cofactor system.This complex pattern of 6'-deiodinase activity, which mimics that found in homogenates and subcellular fractions of rat liver and kidney, was reproduced exactly in oocytes by the microinjection of G21-derived RNA transcripts.Furthermore, hybrid arrest of translation in oocytes using a partial type I 6'-deiodinase cDNA completely inhibited the expression of both high and low K , activity after the injection of rat liver poly(A)+ RNA.These findings demonstrate that rat liver and kidney contain only a single 6"deiodinase which manifests either high or low K , activity depending on the reduced thiol cofactor utilized in the reaction.Thyroxine (TJ,' the principal secretory product of the thyroid gland, functions primarily as a prohormone and undergoes a single deiodination of the phenolic ring in extrathyroidal tissues to form the metabolically more active hormone 3,5,3'-triiodothyronine (T3) (1).The biochemical characterization of the iodothyronine 5"deiodinases which catalyze this reaction has been hampered by the inability to purify
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