Inner ring deiodination

T3 (rT3), that is metabolically inert. Thus outerring deiodination is the step-up process to increase metabolic activity and the inner-ring deiodination is the step-down inactivation process. Further deiodination of the molecule abolishes hormonal activity. Drugs such as ipodate, beta-blockers, and corticosteroids influence 5 -deiodinase, resulting in low T3 and high rT3 levels in serum. 

Abbreviations BAT, brown adipose tissue BrAc, /V-bromoacetyl- BSA, bovine serum albumin CNS, central nervous system DEP, diethylpyrocarbonatc DIT, diiodotyrosine DTT, dithiothreitol G, glu-curonide Grx, glutaredoxin GSH, reduced glutathione GSSG, oxidized glutathione IRD. inner ring deiodination MIT, monoiodotyrosine ORD, outer ring deiodination PTU, propylthiouracil S, sulfate rT3, 3,3, 5 -triiodothyronine (reverse T,) Trx, thioredoxin T2, diiodothyronine T3, 3,3, 5-triiodothyronine T4, 3,3, 5,5 -tetraiodothyronine (thyroxine). 

Fig. 3. The main pathways for TH monodeiodination. ORD, outer-ring deiodination IRD, inner-ring deiodination.

Deiodination In order to initiate TH action, T4 originating from the thyroid gland a must be activated in tissues by outer ring deiodination (ORD) to form T3. To balance the activation pathway, both T4 and T3 are irreversibly inactivated by monodeiodination of the tyrosyl ring of the iodothyronines, called inner ring deiodination (IRD). Mammals and birds have three types of deiodinases type I deiodinase (Dl) with ORD and IRD activity, type II (D2) with only ORD activity, and type 111 with only IRD activity VisserT. J. (1990). 

As discussed earlier, T4 is considered to be a prohormone, and its peripheral metabolism occurs in two ways outer ring deiodination by the enzyme 5 -D, which yields T3, and inner ring deiodination by the enzyme 5-D, which yields rT3, for which there is no known biological function (Fig. 34.4). In humans, deiodination is the most important metabolic pathway of the hormone, not only because of its dual role in... 

Inner ring deiodination by the enzyme 5-D, which yields rTs. 

The most important pathway for the metabolism of T4 is monodeiodination. The removal of an iodide from the outer ring of T4 yields T3. Since the affinity of nuclear TRs is much higher for T3 than T4, outer ring monodeiodination of T4 to yield T3 produces a more active metabolite. Conversely, removal of an iodide from the inner ring of T4 yields an inactive metabolite, rTj. Both T3 and rT3 may undergo subsequent deiodinations to yield totally deiodinated thyronine (To). 

Up to 80% of the circulating T3 originates from deiodination of T4. This is due mainly to a deiodinase (Dl) activity in the Uver, where most of the T3 formed is exported into the circulation. Monodeiodination of T4 to yield Tj is catalyzed by another deiodinase (D2). It appears that D2 catalyzes Tj from T4 for local cellular demands independent of circulating Tj. The third enzyme involved in the reductive deiodination of T4, Tj, and other iodothyronines is D3. The sole action of this enzyme is the removal of iodide from the inner ring of iodothyronines. 

Evidence has accumulated for the existence of a specific deiodinase for the inner ring of iodothyronines which is further distinguished from the type I enzyme because of its insensitivity to sub-mM PTU concentrations. Thus, type III iodothyronine deiodinase converts T4 to rT3 but not to T3 and produces 3,3 -T2 from T3 but not from rT, (Table I). It has been detected in chick embryo heart [94] and liver [95] cells, monkey hepatocarcinoma cells [96], rat CNS [71,75,97], human [98], rat [98] and guinea pig [99] placenta, and rat skin [100], With higher enzyme activities in cerebral cortex than in cerebellum, the distribution of the type III deiodinase is different from that of the type II enzyme [75], In brain cell cultures type III deiodination appears associated with the presence of glial cells [76,78,79],... 

As discussed in previous sections, the stepwise deiodination of T4 is mediated by at least three different enzymes. Deiodination of the outer ring of T4 and reverse T3 is mediated by the type I and II enzymes while deiodination of the inner ring of T4 and T3 is catalysed by the type I and III enzymes. The contribution of the different enzymes to the peripheral production and clearance of T3 and rT3 can be estimated using PTU as a specific inhibitor of the type I deiodinase (for potential pitfalls of this approach, see Section 3.3). Thyroid hormone has a positive effect on the type I and type III enzymes but down-regulates the type II deiodinase. 

Although in principle the metabolic clearance of plasma T3 may occur via several pathways, direct deiodination of the inner ring of T3 by the type I enzyme seems of minor importance (Section 2.2). Also, glucuronidation in the liver does not represent an irreversible pathway of T3 elimination since enzymatic hydrolysis of the conjugate in the intestine allows for the reabsorption of free T3 (enterohepatic cycle). Further, the finding that plasma T3 clearance is not affected in patients with liver cirrhosis [115] suggests that hepatic metabolism of T3 by sulfation and subsequent deiodination is less important than in rats. It appears, therefore, that the type III deiodinase of extrahepatic tissues is a major site for the clearance of plasma T3 as it is also for the production of plasma rT3. 

Two reactions account for the metabolic fate of about 80% of the T4 in plasma about 40% is converted to T3 via 5 -deiodination (activation), and another 40% of the T4 is converted to rT3 by 5 -deiodination (inactivation). These two reactions are catalyzed by three enzymes designated types I, II, and III iodothyronine deiodinases (Figure 33-5 and Table 33-2). Types I and II both catalyze the 5 -deiodination reaction but differ with respect to substrate specificity, tissue distribution, and regulation. Type III is a 5-deiodinase, which catalyzes the removal of iodine from position 5 of the inner ring. Type I is deiodinase selenocysteine-containing microsomal enzyme present in the liver, kidney, and thyroid, with specificity for... 

Reaction catalyzed 5-deiodination (outer ring) 5-deiodination (outer ring) 5-deiodination (inner ring)... 

Three types of deiodinases are currently known, and these are distinguished from each other primarily based on their location, substrate preference, and susceptibility to inhibitors. Type I deiodinase is found in liver and kidney and catalyzes both inner ring and outer ring deiodination (i.e., T4 to T3 and rTs to 3,3 -T2). Type II deiodinase catalyzes mainly outer ring deiodination (i.e., T4 to T3 and T3 to 3,3 -T2) and is found in brain and the pituitary. Type III deiodinase is the principal source of rTs and is present in brain, skin, and placenta (14). 

DEIODINATION SITE OUTER AND INNER RING OUTER RING INNER RING... 

Having defined critical conditions for the examination of outer ring deiodination, similar techniques were applied to the Type III or inner ring deiodinase activity (17-19). The results were anticipated by the initial studies of the enzyme homogenates. The Type III enzyme prefers T3 as a substrate, follows sequential, rather than ping-pong, kinetics and is insensitive to PTU at 10 M (Table 1). Despite the fact that T3 is the preferred substrate for the Type III activity, it is this enzyme which produces rT3 from T4 in the central nervous system. 

Differiences in placentation among species may determine (diether the placenta is also a site for peripheral deiodination of maternal iodothyronines. In the hemochorial placenta, maternal and fetal blood is in direct contact with placental tissue. therefore, in these placentas, iodothyronines from both the maternal and fetal circulations are litely to provide substrates for inner ring deiodinase activity. Species with hemochorial placentation include the rat, guinea pig, and human. In the she, ihich has endotheliochorial placentation, only fetal blood is in contact with placental tissue. For this reason the placental deiodination of maternal iodothyronines in the sheep may be limited. 

Type I iodothyronine deiodinase is defined as the enzyme which catalyses the (mono)deiodination of the inner or the outer ring of different iodothyronines and which is inhibited by jrM concentrations of PTU [5-8]. In rats and humans, such enzyme activities are present at high levels in liver, kidneys and interestingly also in thyroid, and at low levels in many other tissues [5-8]. The deiodinase is associated with the microsomal fractions of these tissues but is only active in the presence of a cytoplasmic cofactor [5-8], Also, in the absence of cytosol, deiodinase activity is stimulated by simple thiols such as dithiothreitol (DTT) but the physiological cofactor has not yet been identified with certainty. The effects of synthetic and natural thiols will be discussed in Section 2.5. 

---