Reaction of the type III iodothyronine deiodinase with the affinity label N-bromoacetyl-triiodothyronine

Leptin stimulates hepatic activation of thyroid hormones and promotes early posthatch growth in the chicken

Hepatic iodothyronine deiodinases (Ds) are involved in the conversion of thyroid hormones (THs) which interacts with growth hormone (GH) to regulate posthatch growth in the chicken. Previous studies suggest that leptin-like immunoreactive substance deposited in the egg may serve as a maternal signal to program posthatch growth. To test the hypothesis that maternal leptin may affect early posthatch growth through modifying hepatic activation of THs, we injected 5.0μg of recombinant murine leptin into the albumen of breeder eggs before incubation. Furthermore, chicken embryo hepatocytes (CEHs) were treated with leptin in vitro to reveal the direct effect of leptin on expression and activity of Ds. In ovo leptin administration markedly accelerated early posthatch growth, elevated serum levels of total and free triiodothyronine (tT3 and fT3), while that of total thyroxin (tT4) remained unchanged. Hepatic mRNA expression and activity of D1 which converts T4 to T3 or rT3 to T2, were significantly increased in leptin-treated chickens, while those of D3 which converts T3 to T2 or T4 to rT3, were significantly decreased. Moreover, hepatic expression of GHR and IGF-I mRNA was all up-regulated in leptin-treated chickens. Males demonstrated more pronounced responses. A direct effect of leptin on Ds was shown in CEHs cultured in vitro. Expression and activity of D1 were increased, whereas those of D3 were decreased, in leptin-treated cells. These data suggest that in ovo leptin administration improves early posthatch growth, in a gender-specific fashion, probably through improving hepatic activation of THs and up-regulating hepatic expression of GHR and IGF-I.

Effect of methyl iodide on deiodinase activity

Methyl iodide (MeI) has been proposed as an alternative for methyl bromide in pre-plant soil fumigation applications that does not affect stratospheric ozone. Preliminary studies in rabbits noted fetal resorptions if the pregnant does were exposed to MeI during a critical period during gestation. In addition, abnormalities in thyroid hormone parameters were also observed in animals exposed to MeI. Since monodeiodination is the major metabolic pathway of the thyroid hormones, we examined the effect of MeI on deiodinase activity as a possible etiology for the alteration in thyroid hormone parameters and ultimate fetal demise. In vitro studies using tissue microsomes and cell culture showed that MeI has no effect on type I 5'-deiodinase (D1) or type II 5'-deiodinase (D2) at physiologically relevant concentrations. At high concentrations (>10 mM,>10,000 ppm), MeI caused a nonspecific inactivation of D1 and D2. Analysis of D1 and D2 activity in rats exposed by inhalation to increasing concentrations of MeI showed a significant decrease in enzyme activity at 100 ppm, while brain type III 5'-deiodinase (D3) was unaffected by MeI at the exposures studied. While the drop in D1 can be explained by the induction of a hypothyroid state in the exposed rats, there is no clear explanation for the fall in D2 levels. In the rabbit studies, there was a significant decrease in kidney D1 in the adult rabbits exposed to 20 ppm MeI. However, there was no effect on liver D1, brain D2, or placental D3 in the MeI-exposed rabbits. Similarly, there was no effect of MeI on fetal D1 or D2 activity. The lack of a significant direct effect of MeI on deiodinase activity and the absence of a change in placental or fetal deiodinase activity make it unlikely that alterations in deiodinase activity plays a role in the fetal resorptions in the MeI-exposed rabbits.

Substitution of cysteine for selenocysteine in the catalytic center of type III iodothyronine deiodinase reduces catalytic efficiency and alters substrate preference

Human type III iodothyronine deiodinase (D3) catalyzes the conversion of T(4) to rT(3) and of T(3) to 3, 3'-diiodothyronine (T2) by inner-ring deiodination. Like types I and II iodothyronine deiodinases, D3 protein contains selenocysteine (SeC) in the highly conserved core catalytic center at amino acid position 144. To evaluate the contribution of SeC144 to the catalytic properties of D3 enzyme, we generated mutants in which cysteine (D3Cys) or alanine (D3Ala) replaces SeC144 (D3wt). COS cells were transfected with expression vectors encoding D3wt, D3Cys, or D3Ala protein. Kinetic analysis was performed on homogenates with dithiothreitol as reducing cofactor. The Michaelis constant of T(3) was 5-fold higher for D3Cys than for D3wt protein. In contrast, the Michaelis constant of T(4) increased 100-fold. The D3Ala protein was enzymatically inactive. Semiquantitative immunoblotting of homogenates with a D3 antiserum revealed that about 50-fold higher amounts of D3Cys and D3Ala protein are expressed relative to D3wt protein. The relative substrate turnover number of D3Cys is 2-fold reduced for T(3) and 6-fold reduced for T(4) deiodination, compared with D3wt enzyme. Studies in intact COS cells expressing D3wt or D3Cys showed that the D3Cys enzyme is also active under in situ conditions. In conclusion, the SeC residue in the catalytic center of D3 is essential for efficient inner-ring deiodination of T(3) and in particular T(4) at physiological substrate concentrations.

The deiodinase family of selenoproteins

The realization some forty years ago that several iodothyronine compounds are present in the circulation suggested that deiodination occurs in various tissues. Subsequently, deiodination was indeed documented in in vivo studies. Later, using in vitro assay techniques, three deiodinase processes, termed types 1, 2 and 3, were defined that differed in terms of tissue distribution, reaction kinetics, efficiency of substrate utilization and sensitivity to inhibitors. Although purification of the deiodinase enzymes has continued to be problematic, recent molecular cloning studies have identified cDNAs for these three deiodinase isoforms from multiple species. These cDNAs have provided important insights into the structural characteristics of this family of enzymes. Foremost among the structural features has been the demonstration that all three deiodinase isoforms contain at their active site the uncommon amino acid selenocysteine which is of critical importance to their catalytic activity. The availability of cDNAs for these enzymes provides important reagents for pursuing additional studies aimed at defining their biochemical features and roles in thyroid hormone economy.

Investigation of type I and type III iodothyronine deiodinases in rat tissues using N-bromoacetyl-iodothyronine affinity labels

In the present study the hypothesis was tested that N-bromoacetyl-3,3',5-[125I]triiodothyronine (BrAc[125I]T3) is a useful affinity label for both type I and type III iodothyronine deiodinases (ID-I and ID-III). Therefore, the microsomal fractions of various rat tissues were tested for ID-I and ID-III activities, and microsomal proteins were labeled with BrAc[125I]T3 and analyzed by SDS-PAGE. In agreement with previous observations, high ID-I activities were found in liver, kidney and thyroid, and high ID-III activities in brain, in particular fetal brain, and placenta. SDS-PAGE of BrAc[125I]T3-labeled microsomes showed a prominent radioactive approximately 27 kDa protein (p27) in liver, kidney and thyroid, which was previously identified as ID-I, and a approximately 32 kDa protein (p32) in brain, in particular fetal brain, and placenta. A good correlation was found between the affinity labeling of p32 and the inactivation of ID-III by BrAcT3, suggesting that p32 represents ID-III or a subunit thereof. After treatment of microsomes with 0.05% deoxycholate or carbonate buffer (pH 11.5) p32 was still labeled by BrAc[125I]T3, indicating that p32 is a transmembrane protein. Although 3,3',5'-triiodothyronine (rT3) is not a substrate for ID-III, p32 was readily labeled with BrAc[125I]rT3. Labeling of p32 in rat brain microsomes by BrAc[125I]rT3 was not affected by addition of 100 microM unlabeled thyroxine (T4) or T3, whereas deiodination of [125I]T3 by ID-III was inhibited by 91 and 96% in the presence of 1 microM T4 and T3, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)