Inhibition of rat hepatic thyroxine 5'-monodeiodinase by propylthiouracil: relation to site of interaction of thyroxine and glutathione

Kinetics and thiol requirements of iodothyronine 5'-deiodination are tissue-specific in common carp (Cyprinus carpio L.)

Iodothyronine deiodinases determine the biological activity of thyroid hormones. Despite the homology of the catalytic sites of mammalian and teleostean deiodinases, in-vitro requirements for the putative thiol co-substrate dithiothreitol (DTT) vary considerably between vertebrate species. To further our insights in the interactions between the deiodinase protein and its substrates: thyroid hormone and DTT, we measured enzymatic iodothyronine 5'-deiodination, Dio1 and Dio2 mRNA expression, and Dio1 affinity probe binding in liver and kidney preparations from a freshwater teleost, the common carp (Cyprinus carpio L.). Deiodination rates, using reverse T3 (rT3, 3,3',5'-triiodothyronine) as the substrate, were analysed as a function of the iodothyronine and DTT concentrations. In kidney rT3 5'-deiodinase activity measured at rT3 concentrations up to 10 μM and in the absence of DTT does not saturate appreciably. In the presence of 1mM DTT, renal rT3 deiodination rates are 20-fold lower. In contrast, rT3 5'-deiodination in liver is potently stimulated by 1mM DTT. The marked biochemical differences between 5'-deiodination in liver and kidney are not associated with the expression of either Dio1 or Dio2 mRNA since both organs express both deiodinase types. In liver and kidney, DTT stimulates the incorporation of N-bromoacetylated affinity labels in proteins with estimated molecular masses of 57 and 55, and 31 and 28 kDa, respectively. Although primary structures are highly homologous, the biochemistry of carp deiodinases differs markedly from their mammalian counterparts.

Mode of Action: Developmental Thyroid Hormone Insufficiency—Neurological Abnormalities Resulting From Exposure to Propylthiouracil

Because thyroid hormone is essential for normal brain development before and after birth, environmental chemicals that interfere with thyroid hormone signaling can adversely affect brain development. Adverse consequences of thyroid hormone insufficiency depend both on severity and developmental timing, indicating that environmental antithyroid factors may produce different effects at different developmental windows of exposure. Mechanistic studies can provide important insight into the potential impact of chemicals on human thyroid function, but relevance to humans must be systematically evaluated. This kind of analysis depends on data sets that include information about animals and humans. The drug 6-n-propyl-2-thiouracil (PTU) is used in animals to experimentally manipulate serum thyroid hormone levels, and in humans to treat patients, including pregnant women, with Graves' disease. A systematic analysis of the mode of action (MOA) of PTU in rats and in humans discloses similar modes of action. While the analysis predicts that PTU doses that produce thyroid hormone insufficiency in humans would adversely affect the developing brain, careful monitoring of PTU administration in pregnant and lactating humans keeps infant serum thyroid hormone levels within the normal range.

Thyroid hormone regulates the hypotriglyceridemic gene APOA5

The apolipoprotein AV gene (APOA5) is a key determinant of plasma triglyceride levels, a major risk factor for coronary artery disease and a biomarker for the metabolic syndrome. Since thyroid hormones influence very low density lipoprotein triglyceride metabolism and clinical studies have demonstrated an inverse correlation between thyroid status and plasma triglyceride levels, we examined whether APOA5 is regulated by thyroid hormone. Here we report that 3,5,3'-triiodo-L-thyronine (T3) and a synthetic thyroid receptor beta (TRbeta) ligand increase APOA5 mRNA and protein levels in hepatocytes. Our data revealed that T3-activated TR directly regulates APOA5 promoter through a functional direct repeat separated by four nucleotides (DR4). Interestingly, we show that upstream stimulatory factor 1, a transcription factor associated with familial combined hyperlipidemia and elevated triglyceride levels in humans, and upstream stimulatory factor 2 cooperate with TR, resulting in a synergistic activation of APOA5 promoter in a ligand-dependent manner via an adjacent E-box motif. In rats, we observed that apoAV levels declines with thyroid hormone depletion but returned to normal levels upon T3 administration. In addition, treatments with a TRbeta-selective agonist increased apoAV and diminished triglyceride levels. The identification of APOA5 as a T3 target gene provides a new potential mechanism whereby thyroid hormones can influence triglyceride homeostasis. Additionally, these data suggest that TRbeta may be a potential pharmacological target for the treatment of hypertriglyceridemia.

Mechanistic basis of enzyme-targeted drugs

Enzymes offer unique opportunities for drug design that are not available to cell surface receptors, nuclear hormone receptors, ion channels, transporters, and DNA. Here, we review the variety of inhibition mechanisms for enzyme-targeted drugs, and establish an enzyme target database for drugs currently marketed in the United States. From an analysis of the FDA Orange Book, there are 317 marketed drugs that work by inhibiting an enzyme. These drugs inhibit 71 enzymes, including 48 human, 13 bacterial, five viral, four fungal, and one protozoal enzyme. Among the 317 drugs, 65% either undergo reactive chemistry in the active site of the target enzyme or contain a structural motif related to the substrate. Among the 71 enzyme targets, 25 are irreversibly inhibited by drugs, and 19 of the 25 irreversibly inhibited enzymes are covalently modified by the drug. In two additional cases, the drug forms a covalent complex with the substrate, and in three more cases, the drug traps a covalent enzyme-substrate intermediate. Four of the 71 enzymes are inhibited by transition-state analogues. Moreover, advanced methods for determining transition-state structure now offer the opportunity for direct drug design without resorting to expensive random testing campaigns. A full appreciation of enzyme mechanisms sets enzymes apart as a specialized class of targets for highly directed drug design.

Thyroid hormone regulation and cholesterol metabolism are connected through Sterol Regulatory Element-Binding Protein-2 (SREBP-2)

High affinity uptake of serum-derived low density lipoprotein (LDL) cholesterol is accomplished through the LDL receptor in the liver. In mammals, thyroid hormone depletion leads to decreased LDL receptor expression and elevated serum cholesterol. The clinical association in humans has been known since the 1920s; however, a molecular explanation has been lacking. LDL receptor levels are subject to negative feedback regulation by cellular cholesterol through sterol regulatory element-binding protein-2 (SREBP-2). Here we demonstrate that the SREBP-2 gene is regulated by thyroid hormone and that increased SREBP-2 nuclear protein levels in hypothyroid animals results in thyroid hormone-independent activation of LDL receptor gene expression and reversal of the associated hypercholesterolemia. This occurs without effects on other thyroid hormone-regulated genes. Thus, we propose that the decreased LDL receptor and increased serum cholesterol associated with hypothyroidism are secondary to the thyroid hormone effects on SREBP-2. These results suggest that hypercholesterolemia associated with hypothyroidism can be reversed by agents that directly increase SREBP-2. Additionally, these results indicate that mutations or drugs that lower nuclear SREBP-2 would cause hypercholesterolemia.

How do eating disorders affect thyroid function?

Abnormal thyroid function in patients with eating disorders can result from malnutrition. A low serum triiodothyronine (T3) level is commonly noted in starvation states and is caused by reduced peripheral conversion of thyroxine (T4) to T3. Diminished T4 concentrations are also observed. Adequate nutrition normalizes this type of aberrant laboratory profile. Thyroid function tests that give results below the normal range are best repeated initially for verification of results and again after adequate nutrition is reestablished. If no primary endocrinopathy is present, spontaneous correction of these laboratory values can be expected with conventional dietary habits.

Mode of death effect on rat liver iodothyronine 5' deiodinase activity: role of adenosine 3',5' monophosphate

Liver thyronine 5'-deiodinase activity assayed in crude homogenates in the absence of dithiothreitol (DTT) is increased in rats killed by asphyxia when compared to that of animals killed by phenobarbital injection or decapitation. The addition of cyclic adenosine monophosphate leads to a consistent decrease in observed deiodinase activity, suggesting the possible involvement of this nucleotide in the regulation of this enzyme. The addition of DTT eliminates this effect and suggest a dual regulation of the enzyme by cAMP and physiological sulfhydryl compounds.

Lysosomal thyroid hormone 5'-deiodinase

Lysosomes prepared from rat liver and kidney after loading with the detergent Triton WR-1339 show membrane-bound 5'-deiodinase activity with marked specificity for 3,3',5'-triiodothyronine (reverse T3), lesser activity with respect to thyroxine (T4) and almost none towards 3,3',5'-triiodothyronine (T3). The enzyme is thiol dependent and shows maximal catalysis at pH 7.2. As many of the states known to alter thyroid hormone levels also affect lysosomal function, inhibition of the lysosomal 5'-deiodinase leading to an increase in intracellular reverse T3 may be an initiating mechanism for thyroid hormone change.

Selective modification of the active center of renal iodothyronine 5'-deiodinase by iodoacetate

Pretreatment of renal iodothyronine 5'-deiodinase with sulfhydryl reagents, iodoacetate, iodoacetamide and N-alkylmaleimides, results in irreversible loss of catalytic activity. Iodoacetate and iodoacetamide were the most potent inhibitors, being 100- to 1000-times more potent than N-alkylmaleimides. Iodoacetate and iodoacetamide inactivation followed pseudo-first-order kinetics with maximum inactivation rate constants of 1.56 min-1 and 0.87 min-1, respectively. Thyroxine and 3,3',5'-triiodothyronine and the competitive inhibitor iopanoate, protected the enzyme against iodoacetate inhibition. Protection by 3,3',5'-triiodothyronine was competitive with iodoacetate with a dissociation constant (Kd) of 113 nM; in close agreement with the Km for rT3 of 190 nM determined under similar reaction conditions. [3H]Carboxymethylation of renal membranes in the absence and presence of 3,3',5'-triiodothyronine showed specific incorporation of iodo[3H]acetate into substrate-protected sites of 35-40% of total when non-essential residues were first blocked with excess unlabeled iodoacetate. ' Protectable ' [3H]acetate incorporation followed pseudo-first-order kinetics and the rate constant for incorporation was identical to the rate constant for inactivation. These results indicate that iodoacetate fulfills the minimum criteria for an active-site-directed reagent for renal 5'-deiodinase and that a sulfhydryl group is in close proximity to the iodothyronine-binding site.