Type I deiodinase

A recently recognized property of the type I deiodinase is its particular activity towards sulfated iodothyronine substrates [19,20,34], This was first discovered in... 

The type I deiodinase of liver and kidney is inactivated by different SH-selective reagents. In particular, it shows an extremely high susceptibility to carboxymethy-lation by iodoacetate and a somewhat lesser sensitivity for iodoacetamide and bro-moacetate [42]. In comparison, A-alkylmaleimides are only inhibitory at high concentrations (> 0.1 mM) [42]. Enzyme inactivation by iodoacetate follows pseudo... 

Thiourea derivatives are known for their anti-thyroid effects due to inhibition of thyroid peroxidase [1], Two thiourea compounds especially, have found wide application in the treatment of patients with hyperthyroidism, i.e., PTU and 2-mer-capto-l-methylimidazole (methimazole). It was soon recognized, however, that while methimazole only blocks thyroid hormone synthesis PTU has an additional effect on thyroid hormone metabolism [13]. These clinical findings have been confirmed in vitro showing that PTU, but not methimazole, is a potent inhibitor of the type I deiodinase [5-8]. Structure-activity studies of thiourea analogues [44,45] have... 

Inhibition of the type I deiodinase by PTU is uncompetitive with substrate and competitive with cofactor. This is the case for the ORD of T4 and rT3 as well as for the IRD of T3 and T3S [7,8]. Persistent inactivation of enzyme by PTU and covalent labelling with radioactive inhibitor requires the presence of substrate and is only reversed with high DTT [42,47], All available evidence indicates that PTU reacts with a substrate-induced enzyme intermediate. As thiourea derivatives are particularly reactive towards sulfenyl iodide (SI) groups, generation of an enzyme-SI intermediate is thought to precede thiouracil inhibition through mixed disulfide formation [7,8]. 

Significant stimulation of type I deiodinase activity is observed with GSH only if the rate of E-SI generation is limited, i.e., at low rT3 concentrations or with T4 as the substrate which is slowly deiodinated. The low reactivity of GSH in the absence of glutaredoxin may be due to the formation of a stable enzyme-glutathione mixed... 

In contrast to the type I deiodinase which shows a high preference for rT4 over T4 as the substrate (Table II), the type II enzyme is somewhat more effective in the deiodination of T4 than of rT3 (Table III). Under the conditions tested, the Km value of T4 for the type II enzyme is three orders of magnitude lower than the Km of T4 for the type I deiodinase. The Km of rT3 for the type II deiodinase is somewhat greater than that of T4 and differs less from the Km of rT3 for the type I enzyme. The Umax of the conversion of T4 to T3 by the type II enzyme depends on the tissue and the thyroid status of the animal (see below). In cerebral cortex of hypothyroid rats [82] it is roughly one-thousandth of the maximum T3 production by the hepatic type I deiodinase of euthyroid animals determined under similar conditions [32]. The VmiJKm ratio of this reaction is, therefore, similar for the type II deiodinase of hypothyroid rat brain and the type I deiodinase of euthyroid rat liver and much greater than that for the hepatic enzyme of hypothyroid rats [86], In view of the reaction kinetics of the type II deiodinase (see below), it is questionable if the Vm,JKm ratios estimated in vitro also apply to physiological conditions with unknown cofactor availability. 

Type II deiodinase activity is low in unsupplemented tissue homogenates but is stimulated by DTT [71-74,82,83] and to a lesser extent also by GSH [72]. The DTT concentrations required for maximal enzyme stimulation in the CNS and pituitary seem higher than in BAT and also than those necessary for the type I deiodinase in liver and kidney. Kinetic analysis of the deiodination of varying substrate (T4, rT3) concentrations at different cofactor (DTT) levels have indicated a sequential reaction mechanism for the type II deiodinase [73,82,83]. This is very suggestive of the formation of a ternary enzyme-substrate-cofactor complex in the catalytic process [82], The physiological cofactor of the type II deiodinase has not been identified but it has been observed that GSH depletion with diamide or diethylmaleate impairs T4 to T3 conversion in GH3 pituitary tumor cells [93]. 

The insensitivity of the type II enzyme to PTU seems to exclude the generation of an enzyme SI intermediate as is the case with the type I deiodinase (see Section 2.4). The lack of involvement of a catalytic enzyme SH group in type II deiodination is also suggested by the weak effects of iodoacetate [82], a potent inhibitor of the type I deiodinase. It may be speculated that the type II enzyme catalyses the transfer of I+ from the substrate directly to the SH group of the cofactor [82], In contrast to PTU, iopanoic acid has similar inhibitory effects on the type I and II deiodinases [71-73,84,89]. 

Thus, it has been shown that sensitivity to PTU is not an absolute indicator for the type I deiodinase. At very high (mM) concentrations it also inhibits the type II enzyme especially with limited DTT [101,102]. Moreover, since PTU is an uncompetitive inhibitor of the type I deiodinase (Section 2.3), it is relatively less potent at low substrate levels. It is, therefore, important to realize that under assay conditions for the low-/Cm type II enzyme PTU may not always completely inhibit the type I deiodinase. This is also a potential pitfall of the use PTU to investigate the origin of T3 in plasma and tissues of rats, especially at the low iodothyronine levels in hypothyroid animals. Additional information is then required to assess the contribution of the different enzymes. Previous findings [103] suggesting that peripheral production of T3 in hypothyroid rats is provided primarily by PTU-insensitive (type II) deiodination of T4 have been confirmed in subsequent experiments [90],... 

Goswami and Rosenberg have suggested that liver and kidney microsomes contain in addition to the type I deiodinase multiple low-Mm enzymes for the ORD of T4 and rT3 that differ from the type II enzyme [60,64,65]. This was mainly based on different susceptibilities to iopanoic acid and PTU if reactions were carried out at low substrate concentrations in the presence of various cofactors, i.e., DTT, GSH, glutaredoxin and thioredoxin [60,64,65]. It was even reported that the deiodinase activity stimulated by the thioredoxin system accepted rT3 but not T4 as substrate [64]. The uncertainty in the estimation of the low conversion rates in the nM substrate range which are not accounted for by residual activity of the type I deiodinase, however, questions the validity of the above conclusions. The possible exist-... 

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. 

Figure 5 is a model of the peripheral metabolism of thyroid hormone in normal humans which places the production of plasma T3 and the clearance of plasma rT3 predominantly in tissues with PTU-sensitive, type I deiodinase activity. Although the role of the liver is emphasized, contribution of the kidneys is not excluded. Clearance of plasma T3 and production of plasma rT3 is located mainly in tissues such as brain and perhaps skin with PTU-insensitive, type III deiodinase activity. 

The low T3 syndrome is induced by a decrease in the production of plasma T3 as well as the clearance of plasma rT3 and is observed in several clinical situations such as starvation, systemic illness and the use of certain drugs [115]. In fasting [70] and illness [108] the abnormal thyroid hormone metabolism appears to result from a defective liver uptake and, therefore, a decreased supply of T4 and rT3 for intracellular deiodination. In other conditions such as treatment with PTU or propranolol [116], the defect appears localized in the type I deiodinase itself leading to a decline in T3 formation and rT3 breakdown. 

Type I deiodinase appears to be responsible for most of the T3 in the bloodstream. The type I enTyme cataly.zes the conversion of T4 to T3 within the thyroid gland. This is a 5 -deiodination reaction. The dominant role of this enzyme is... 

Some evidence for adverse effects on the endocrine system has also been found following intermediate and chronic oral exposure to elevated levels of dietary selenium in humans and animals. Human studies have demonstrated a decrease in triiodothyronine levels in response to increased dietary selenium, although the hormone levels remained within the normal range. Intermediate-duration studies of rats have shown reductions in type-I-deiodinase activity in response to selenium. However, the levels of thyroid hormones in these animals did not show a consistent pattern. 

Wistar) ad lib type I deiodinase activity) sodium selenite... 

Selenium supplementation has been shown to affect type-I-deiodinase activity in male rats (Behne et al. 1992 Eder et al. 1995 Hotz et al. 1997). Exposure to 0.055 or 0.27 mg selenium/kg/day as sodium selenite in food for 40 days produced a significant decrease (approximately 50%) in serum levels of T3 and a nonsignificant reduction in type-I-deiodinase activity compared with rats receiving 0.009 or 0.026 mg selenium/kg/day (Eder et al. 1995). Exposure to 0.27 mg selenium/kg/day did not produce any other adverse signs, such as weight loss or decreased food consumption, and serum T4 levels were similar in all groups. 

Selenium is a component of all three members of the deiodinase enzyme family, the enzymes responsible for deiodination of the thyroid hormones (Kohrle 1994 St. Germain and Galton 1997). The deiodinases contain a selenocysteine at the active site, which is required for catalytic activity. There are three types of deiodinases and they differ in terms of tissue distribution, reaction kinetics, efficiency of substrate utilization, and sensitivity to inhibitors. The first to be recognized as a selenoprotein was type I iodothyronine 5 -deiodinase which converts the prohormone thyroxine (T4) to the active form, triiodothyronine (T3) and to date, studies of the effects of excess selenium have focused on this protein. Under normal circumstances the human thyroid produces only 20-30% of its hormone as T3 the remainder is T4 (a minute amount of reverse T3 (rT3) is also produced), which is largely converted to active T3 by type I deiodinase located within the liver, euthyroid pituitary, kidney, thyroid, and brain. Type I deiodinase is a membrane bound protein and, thus, its activity has not been directly measured in studies of humans supplemented with selenium. Human studies have instead measured serum levels of T3, rT3, T4, and TSH. 

Berry MJ, Banu L, Chen Y, et al. 1991. Recognition of UGA as a selenocysteine codon in Type I deiodinase requires sequences in the 3 untranslated region. Nature 353 273-276. 

In extrathyroidal tissues, the thyroid hormones are catabolized in a series of deiodination, oxidative, or conjugation reactions. Deiodination of circulating T4 occurs mainly in the liver and kidney by type I deiodinases with either 5 -deiodination yielding T3 or 5-deiodination yielding inactive rTj. The production of reverse Tj is thought to be another extrathyroidal control mechanism regulating the delivery of free T3 to the tissues. Type II deiodinases carry out 5 -deiodination in the cerebral cortex and brown adipose tissues. 

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). 

A single dose of TCDD was reported to decrease type I deiodinase activity in a dose-dependent manner in adult rats (Raasmaja et al, 1996). Hydroxylated PCB metaboHtes also interfere with T4 metabolism by inhibition of deiodinase activity, which prevents the formation of T3 (Adams etal, 1990). 

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). 

Type I deiodinase activity is primarily associated with the microsomal fraction of liver tissue (14-15). The deiodinase activity was assayed using conditions adapted from the literature... 

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