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E Kaptein

Publications and source records attributed to E Kaptein.

At least 37 records · Page 2Linked to original sources
PubMed1994

Rapid glucuronidation of tri- and tetraiodothyroacetic acid to ester glucuronides in human liver and to ether glucuronides in rat liver.

T3 is the principal bioactive thyroid hormone, although its metabolite 3,3',5-triiodothyroacetic acid (TA3) shows higher affinity for the nuclear T3 receptor. However, TA3 has a low in vivo potency because of its short half-life in both humans and rats. We have compared the glucuronidation of TA3, 3,3',5,5'-tetraiodothyroacetic (TA4), T3, and T4 by human and rat liver microsomes. In rat liver, TA3 and TA4 were glucuronidated about 20 times faster than T3 and T4. Both TA3 and TA4 glucuronides were stable during treatment with dilute base or methanol, suggesting that they represent ether glucuronides with the phenolic hydroxyl group. In human liver, TA3 and TA4 were glucuronidated about 1500 and 200 times faster than T3 and T4, respectively. Both TA3 and TA4 glucuronides were hydrolyzed by treatment for 30 min at 37 C with 0.1 M NaOH and showed transesterification to the methyl esters by treatment with methanol, suggesting that they represent ester glucuronides with the carboxyl group. Therefore, both TA3 and TA4 undergo very rapid, but different, types of glucuronidation in human and rat liver. The high glucuronidation rates may explain their short half-lives and the low bioactivity of TA3 in vivo.

Animals↗
PubMed1994

Different effects of continuous infusion of interleukin-1 and interleukin-6 on the hypothalamic-hypophysial-thyroid axis.

The cytokines interleukin-1 (IL-1) and IL-6 are thought to be important mediators in the suppression of thyroid function during nonthyroidal illness. In this study we compared the effects of IL-1 and IL-6 infusion on the hypothalamus-pituitary-thyroid axis in rats. Cytokines were administered by continuous ip infusion of 4 micrograms IL-1 alpha/day for 1, 2, or 7 days or of 15 micrograms IL-6/day for 7 days. Body weight and temperature, food and water intake, and plasma TSH, T4, free T4 (FT4), T3, and corticosterone levels were measured daily, and hypothalamic pro-TRH messenger RNA (mRNA) and hypophysial TSH beta mRNA were determined after termination of the experiments. Compared with saline-treated controls, infusion of IL-1, but not of IL-6, produced a transient decrease in food and water intake, a transient increase in body temperature, and a prolonged decrease in body weight. Both cytokines caused transient decreases in plasma TSH and T4, which were greater and more prolonged with IL-1 than with IL-6, whereas they effected similar transient increases in the plasma FT4 fraction. Infusion with IL-1, but not IL-6, also induced transient decreases in plasma FT4 and T3 and a transient increase in plasma corticosterone. Hypothalamic pro-TRH mRNA was significantly decreased (-73%) after 7 days, but not after 1 or 2 days, of IL-1 infusion and was unaffected by IL-6 infusion. Hypophysial TSH beta mRNA was significantly decreased after 2 (-62%) and 7 (-62%) days, but not after 1 day, of IL-1 infusion and was unaffected by IL-6 infusion. These results are in agreement with previous findings that IL-1, more so than IL-6, directly inhibits thyroid hormone production. They also indicate that IL-1 and IL-6 both decrease plasma T4 binding. Furthermore, both cytokines induce an acute and dramatic decrease in plasma TSH before (IL-1) or even without (IL-6) a decrease in hypothalamic pro-TRH mRNA or hypophysial TSH beta mRNA, suggesting that the acute decrease in TSH secretion is not caused by decreased pro-TRH and TSH beta gene expression. The TSH-suppressive effect of IL-6, either administered as such or induced by IL-1 infusion, may be due to a direct effect on the thyrotroph, whereas additional effects of IL-1 may involve changes in the hypothalamic release of somatostatin or TRH.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

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

The type III iodothyronine deiodinase (ID-III) catalyzes the inner ring deiodination and, thus, the inactivation of the thyroid hormones T4 and T3. ID-III activity in rat brain, rat placenta and embryonic chicken liver is inhibited by the affinity label N-bromoacetyl-T3 (BrAcT3) with an affinity similar to that of T3. Reaction of rat brain and placenta microsomes with BrAc[125I]T3 resulted in the extensive labeling of a 32 kDa protein (p32). However, p32 was also prominently labeled in fetal rat liver microsomes which have no ID-III activity. Labeling of p32 was not influenced by 100 microM substrate analogs or inhibitors of ID-III, some of which completely inhibit ID-III activity at 1 microM. BrAc[125I]T3 labeling of embryonic chicken liver microsomes did not reveal p32 or another protein possibly related to ID-III. In contrast to previous suggestions, it is unlikely that p32 represents ID-III or a subunit thereof.

Affinity Labels↗
PubMed1993

Glucuronidation of thyroid hormone by human bilirubin and phenol UDP-glucuronyltransferase isoenzymes.

The glucuronidation of thyroid hormone by UDP-glucuronyltransferases (UGTs) stably transfected in Chinese hamster V79 lung fibroblasts was investigated. Human bilirubin UGT (HP3) and phenol UGT (HP4) both catalysed the glucuronidation of T4 and rT3, whereas glucuronidation of T3 was not significant, rT3 was the preferred substrate for both isoenzymes, glucuronidation rates being 1.6- and 6.4-times higher than conjugation of T4 by HP3 and HP4 clones, respectively. This is the first identification of thyroid hormone as potential alternative endogenous substrate for bilirubin UGT.

Animals↗
PubMed1993

Different thyroid hormone-deiodinating enzymes in tilapia (Oreochromis niloticus) liver and kidney.

Enzymes catalyzing the outer ring deiodination (ORD) of iodothyronines are important for the regulation of thyroid hormone bioactivity. We have studied ORD of thyroxine (T4) and 3,3',5'-triiodothyronine (rT3) in liver and kidney microsomes of fish, i.e. tilapia (Oreochromis niloticus). Tilapia kidney contains an enzyme which resembles the mammalian selenoenzyme type I iodothyronine deiodinase (ID-I) with respect to substrate preference (rT3 > T4) and high (approximately microM) Km values, but is much less sensitive to selenocysteine (Sec)-targeted inhibitors, including 6-propyl-2-thiouracil (PTU). In contrast, tilapia liver contains an enzyme very similar to mammalian type II deiodinase (ID-II) with respect to substrate preference (T4 > rT3), low (approximately nM) Km values, and lack of sensitivity to Sec inhibitors.

Animals↗
PubMed1993

Increased glucuronidation of thyroid hormone in hexachlorobenzene-treated rats.

Metabolism of thyroid hormones was investigated in WAG/MBL rats that had been exposed to hexachlorobenzene (HCB). Serum thyroxine (T4) levels were lowered by 35.5%, whereas triiodothyronine (T3) levels were not changed. Bile flow, as well as T4 excretion in bile were increased by HCB treatment. Analysis of bile by HPLC revealed a more than 3-fold increase of T4 glucuronide (T4G) and a concomitant reduction of non-conjugated T4. T4 UDP-glucuronyltransferase activity (T4 UDPGT) activity in hepatic microsomes was increased more than 4.5-fold in animals exposed to HCB. p-Nitrophenol (PNP) UDPGT showed a comparable increase by HCB. Both T3 and androsterone UDPGT activities were low in WAG/MBL rats compared with normal Wistar rats. T3 UDPGT activity was increased 2.5-fold by HCB, but androsterone UDPGT activity was unchanged. These results suggest that T4 is a substrate for HCB-inducible PNP UDPGT and T3 for androsterone UDPGT. In the absence of the latter, T3 is also glucuronidated to some extent by PNP UDPGT. Type 1 iodothyronine deiodinase activity was decreased by HCB treatment. It is concluded that decreased T4 levels in serum of animals after exposure to HCB may be due to a combined effect of displacement of T4 from carriers, an increased glucuronidation of T4 and enhanced bile flow.

Androsterone↗
PubMed1993

Multiple UDP-glucuronyltransferases for the glucuronidation of thyroid hormone with preference for 3,3',5'-triiodothyronine (reverse T3).

We have studied the glucuronidation of the thyroid hormones T4, T3 and rT3 by liver microsomes of Wistar, Gunn and WAG rats. Gunn rats have a defect in the gene coding for bilirubin and phenol UDP-glucuronyltransferase (UGT) isoenzymes; WAG rats have a genetic defect in androsterone UGT. In normal Wistar rats UGT activity was approximately 5-fold higher for rT3 than for T4 or T3. UGT activities for T4 and rT3, but not for T3, were impaired in Gunn rats. Conversely, UGT activity for T3, but not for T4 or rT3, was impaired in WAG rats. Thus, in rat liver rT3 is glucuronidated much more rapidly than T4 and T3. Our results support the view that T4 and rT3 are glucuronidated by bilirubin and phenol UGTs and T3 by androsterone UGT.

Androsterone↗
PubMed1993

Glucuronidation of thyroid hormone in rat liver: effects of in vivo treatment with microsomal enzyme inducers and in vitro assay conditions.

We investigated the effects of in vivo treatment with different microsomal enzyme inducers, including clofibrate (CLOF), hexachlorobenzene (HCB), 3-methylcholanthrene (MC), 3,3',4,4'-tetrachlorobiphenyl (TCB), and 2,3,7,8-tetrachloro-p-dioxin, as well as of in vitro addition of the detergent Brij 56 on the glucuronidation of T4, T3, and rT3 by UDP-glucuronyltransferase (UGT) activities of rat liver microsomes. The results were compared with measurements of UGT activities for bilirubin, p-nitrophenol (PNP), and androsterone. In general, glucuronidation rates were 5-fold or more higher with rT3 than with T4 or T3 as substrate. In liver microsomes from untreated rats, T4 UGT activity was stimulated by Brij 56 to a maximum of about 2-fold at 0.025% detergent. Treatment of Wistar rats for 4 days with CLOF (200 mg/kg BW.day) resulted in significant increases in UGT activities for T4 (to 154%), rT3 (to 155%), and bilirubin (to 194%), in particular if assayed in the presence of 0.025% Brij 56, but had little effect on the UGT activities for T3, PNP, and androsterone. The CLOF-induced increases in T4 and rT3 UGT activities were not observed in Gunn rats, which have a complete lack of bilirubin UGT activity and greatly impaired PNP UGT activity. Treatment of Wistar rats with a single injection of MC (50 mg/kg BW), TCB (50 mg/kg BW), or 2,3,7,8-tetrachloro-p-dioxin (6.25 micrograms/kg BW) resulted, after 4 days, in 6.3- to 7.3-fold increases in T4 UGT activity and 15.1- to 16.7-fold increases in rT3 UGT activity if determined in the absence of Brij 56, whereas T4 UGT activity was only increased by 33-68% when assayed in the presence of Brij 56. T3 glucuronidation was not affected (with Brij 56) or was increased by only 33-68% (without Brij 56) after treatment with these MC-type inducers. PNP UGT activity was induced 3.6- to 4.3-fold, whereas bilirubin and androsterone UGT activities were changed little by these treatments. Similar findings regarding T4, rT3, PNP, and bilirubin UGT activities were obtained after chronic treatment of WAG rats with HCB, another MC-type inducer. However, WAG rats lack androsterone UGT and show low T3 UGT activity, which was increased about 2.3-fold by HCB treatment. On the basis of these and previous findings it is concluded that at least three UGT isoenzymes are involved in the glucuronidation of thyroid hormone.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Myocardial texture and cardiac calcification in uremia.

Seventy patients with advanced chronic renal failure were examined by 2D echocardiography in an effort to determine the prevalence of changes in myocardial texture and of valvular calcification. Changes in myocardial texture may be due to calcium deposition in the myocardium, the relationship between changes in myocardial texture and factors that may enhance myocardial calcification (blood levels of parathyroid hormone, PTH, and calcium-phosphorus product) were also evaluated. Myocardial texture was abnormal in 36 patients. In 68 patients, structural calcification could be assessed; a high degree of calcification was found. No correlation was found between either the echocardiographic parameter and blood levels of PTH or calcium-phosphorus product. We conclude that myocardial textural changes and intracardiac calcification are common in chronic renal failure and that these abnormalities do not correlate with blood PTH levels or calcium-phosphorus product.

Adult↗
PubMed1992

Selenouracil derivatives are potent inhibitors of the selenoenzyme type I iodothyronine deiodinase.

Type I iodothyronine deiodinase (ID-I) is a selenoenzyme, which is important for the conversion of the prohormone thyroxine (T4) to the bioactive thyroid hormone 3,3',5-triiodothyronine (T3). 2-Thiouracil derivatives inhibit ID-I by interaction with an enzyme form generated during catalysis. We have now tested the potential inhibitory effects of the selenocompounds 6-methyl- (MSU) and 6-propyl-2-selenouracil (PSU) in comparison with their thioanalogs 6-methyl- (MTU) and 6-propyl-2-thiouracil (PTU) on rat liver ID-I activity using 3,3',5-triiodothyronine (reverse T3, rT3) as substrate and dithiothreitol (DTT) as cofactor. All compounds showed dose-dependent inhibition of ID-I with IC50 values of 1, 0.5, 0.4 and 0.2 microM for MTU, MSU, PTU and PSU, respectively. Our results further suggest that these inhibitions are uncompetitive with substrate and competitive with cofactor. The high potency of selenouracils may be due to reaction with a substrate-induced enzyme selenenyl iodide intermediate under formation of a stable enzyme-selenouracil diselenide.

Animals↗
PubMed1989

Alterations in 3,3'5'-triiodothyronine metabolism in response to propylthiouracil, dexamethasone, and thyroxine administration in man.

To elucidate the mechanisms involved in altering serum 3,3',5'-triiodothyronine (rT3) levels with absolute or relative low 3,5,3'-triiodothyronine (T3) states in man, agents capable of lowering circulating T3 levels were sequentially administered to six euthyroid subjects. These agents included propylthiouracil (PTU) (300 mg/6 h X 5 d), dexamethasone (DEX) (2 mg/6 h X 5 d), and thyroxine (T4) (3.0 mg load and 0.3 mg/d X 5 d). [125I] rT3 clearance rates and rT3 production rates were then determined. Increased serum rT3 levels and rT3/T4 values occurred with both PTU and DEX as compared with control, while T4 increased serum rT3 but did so without changing rT3/T4 values. The rT3 clearance rate was significantly decreased by PTU without altering production rate, while DEX increased the rT3 production rate without altering the rT3 clearance rate. T4 administration did not change rT3 clearance but proportionately increased rT3 production. These responses indicate that circulating rT3 predominantly originates from a non-PTU inhibitable deiodinase enzyme system located in extrahepatic tissues. This enzyme system appears to have a high capacity and low affinity for T4 and can be stimulated by DEX administration.

Adult↗
PubMed1989

Increased plasma 3,5,3'-triiodothyronine sulfate in rats with inhibited type I iodothyronine deiodinase activity, as measured by radioimmunoassay.

In contrast to the glucuronide conjugate, T3 sulfate (T3S) undergoes rapid deiodinative degradation in the liver and accumulates in rats and rat hepatocyte cultures if type I iodothyronine deiodinase activity is inhibited. We here report the RIA of plasma T3S in rats treated with the antithyroid drugs propylthiouracil (PTU) or methimazole (MMI), of which only PTU inhibits type I deiodinase. Male Wistar rats were treated acutely by ip injection with 1 mg PTU or MMI/100 g BW and subsequently for 4 days by twice daily injections with these drugs together with 0.5 microgram T4 or 0.25 microgram T3/100 g BW. Blood was obtained 4 h after the last injection, and plasma T4, rT3, T3, and T3S were determined by RIA and compared with pretreatment values. Serum concentrations (mean +/- SEM; nanomoles per liter) in untreated rats were: T4, 51 +/- 1; T3, 1.37 +/- 0.03; T3S, 0.09 +/- 0.01; and rT3, 0.03 +/- 0.002. Serum T3 was decreased, and T3S and rT3 were increased by acute PTU treatment [T3, 1.16 +/- 0.05 (P less than 0.01); T3S, 0.33 +/- 0.04 (P less than 0.001); rT3, 0.27 +/- 0.02 (P less than 0.001)], but unaffected by acute MMI treatment (T3, 1.37 +/- 0.05; T3S, 0.09 +/- 0.01; rT3, 0.02 +/- 0.003). In T4-treated rats, serum T3 was decreased and T4, T3S, and rT3 were increased by PTU vs. MMI [T4, 86 +/- 5 vs. 58 +/- 4 (P less than 0.001); T3, 0.51 +/- 0.07 vs. 0.88 +/- 0.06 (P less than 0.001); T3S, 0.38 +/- 0.03 vs. 0.12 +/- 0.01 (P less than 0.001); rT3, 0.86 +/- 0.19 vs. 0.08 +/- 0.01 (P less than 0.005)]. In T3-substituted rats T3S was increased by PTU vs. MMI (1.09 +/- 0.13 vs. 0.25 +/- 0.03; P less than 0.001). The T3S/T3 ratio in the PTU-treated T3 -replaced rats (0.60 +/- 0.09) was in agreement with that determined by HPLC of serum radioactivity in animals that in addition to this treatment also received about 10 microCi [125I]T3 with the last two injections (0.92 +/- 0.13). In conclusion, this investigation demonstrates the feasibility of the measurement of serum T3S by RIA. Our findings confirm previous observations with radioactive isotopes, suggesting that sulfation is an important pathway for the metabolism of T3 in rats. Analogous to rT3, the accumulation of T3S in PTU-treated rats indicates that this conjugate is metabolized predominantly by type I deiodination.

Animals↗
PubMed1989

Metabolism of triiodothyronine in rat hepatocytes.

The metabolism of T3 by isolated rat hepatocytes was analyzed by Sephadex LH-20 chromatography, HPLC, and RIA for T3 sulfate (T3S) and 3,3'-diiodothyronine (3,3'-T2). Type I iodothyronine deiodinase activity was inhibited with propylthiouracil (PTU), and phenol sulfotransferase activity by SO4(2-) depletion or with competitive substrates or inhibitors. Under normal conditions, labeled T3 glucuronide and I- were the main products of [3'-125I]T3 metabolism. Iodide production was decreased by inhibition (PTU) or saturation (greater than 100 nM T3) of type I deiodinase, which was accompanied by the accumulation of T3S and 3,3'-T2S. Inhibition of phenol sulfotransferase resulted in decreased iodide production, which was associated with an accumulation of 3,3'-T2 and 3,3'-T2 glucuronide, independent of PTU. Formation of 3,3'-T2 and its conjugates was only observed at T3 substrate concentrations below 10 nM. Thus, T3 is metabolized in rat liver cells by three quantitatively important pathways: glucuronidation, sulfation, and direct inner ring deiodination. Whereas T3 glucuronide is not further metabolized in the cultures, T3S is rapidly deiodinated by the type I enzyme. As confirmed by incubations with isolated rat liver microsomes, direct inner ring deiodination of T3 is largely mediated by a low Km, PTU-insensitive, type III-like iodothyronine deiodinase, and production of 3,3'-T2 is only observed if its rapid sulfation is prevented.

Animals↗
PubMed1989

Serum triiodothyronine sulfate in man measured by radioimmunoassay.

In humans deiodination and perhaps glucuronidation are important pathways of thyroid hormone metabolism. In animals, sulfation plays an important role in T4 and especially in T3 metabolism, but little is known about sulfate conjugation of thyroid hormone in humans. In this study we used a specific T3 sulfate (T3S) RIA to address this question. Eight normal subjects were given oral T3 (1 microgram/day.kg BW) for 7 weeks. During the fifth week they also received propylthiouracil (PTU; four doses of 250 mg/day) for 2 days and during the seventh week iopanoic acid (IOP; 1 g/day) for 3 days. The mean pre-T3 serum iodothyronine values were: T4, 92 +/- 6 (+/- SE) nmol/L; rT3, 0.24 +/- 0.02 nmol/L; T3, 2.30 +/- 0.10 nmol/L; and T3S, less than 0.1 nmol/L (at or below the detection limit of the RIA). After 4 weeks of T3 administration the mean serum values were: T4, 39 +/- 6; rT3, 0.11 +/- 0.01; T3, 5.31 +/- 0.39; and T3S, 0.10 +/- 0.01 nmol/L. After 2 days of PTU administration, mean serum T4 increased to 48 +/- 7 (P less than 0.005), rT3 to 0.20 +/- 0.03 (P less than 0.025), and T3S to 0.13 +/- 0.01 nmol/L (P = NS), but serum T3 did not change (4.91 +/- 0.35 nmol/L). The effect of IOP was more pronounced; after its administration for 3 days the mean serum T4 was 49 +/- 8 (P less than 0.001), rT3 was 0.48 +/- 0.09 (P less than 0.005), and T3S was 0.29 +/- 0.04 nmol/L (P less than 0.005), and serum T3 decreased to 3.95 +/- 0.25 nmol/L (P less than 0.005). The T3S/T3 ratio was increased by PTU from 0.018 +/- 0.003 to 0.024 +/- 0.004 (P less than = NS) and by IOP to 0.055 +/- 0.007 (P less than 0.005). In conclusion, 1) serum T3S is virtually undetectable (less than 0.1 nmol/L) in normal subjects; 2) low serum T3S concentrations are detected in humans given T3; 3) serum T3S in T3-treated subjects is increased by inhibition of type I deiodinase activity with PTU and especially IOP; and 4) in comparison with previous estimates of the serum T3S/T3 ratio in rats, the low ratio in humans may indicate that sulfation is not an important mechanism of T3 metabolism in humans and/or the kinetics of plasma T3 and T3S differ in humans and rats.

Adult↗
PubMed1988

Development of a radioimmunoassay for triiodothyronine sulfate.

This paper is the first description of a radioimmunoassay (RIA) for triiodothyronine sulfate (T3S). Rabbits were immunized against T3S coupled to bovine serum albumin using carbodiimide. All animals produced antibodies to T3S but also even higher titers of T3 antibodies. Ka values for binding of T3 and T3S to these antisera varied between 2 x 10(10) and 8 x 10(10) M-1. One of the antisera (#8193) was selected for use in the T3S RIA because of a high titer of T3S antibodies (final dilution 1:50,000), a high sensitivity to T3S (less than 2.5 fmol/tube), and a low crossreactivity by T3 (0.4%). This RIA provides a tool for the study of the importance of sulfation as a metabolic pathway for T3.

Antibody Affinity↗
PubMed1988

Deiodination of thyroid hormone by human liver.

Liver is an important site for the peripheral production of T3 by outer ring deiodination (ORD) of T4 as well as for the clearance of plasma rT3, which is produced by inner ring deiodination (IRD) of T4 in other tissues. However, little is known about the underlying enzymatic reactions, and current concepts about thyroid hormone deiodination are largely based on studies in rat tissue. Here we describe the results of detailed studies of the catalytic properties of the iodothyronine deiodinase activity of human liver. The results demonstrated a high degree of similarity with the type I deiodinase of rat liver. The enzyme activity was found in the microsomal fraction. rT3 was the preferred substrate, since its ORD was catalyzed roughly 400 times more efficiently than the ORD or IRD of T4 or the IRD of T3. The deiodination of sulfated substrates was more rapid, as demonstrated by the roughly 30-fold increase in the IRD of T3 sulfate (T3S) compared with T3. The deiodinations exhibited ping-pong-type kinetics with dithiothreitol as the cofactor. Inhibition by propylthiouracil was uncompetitive with substrate and competitive with dithiothreitol, and PTU was an equally effective inhibitor of the ORD of rT3 and the IRD of T3S (Ki, 0.10-0.16 mumol/L). Various compounds with widely different inhibitory potencies had similar effects on ORD (rT3) and IRD (T3S). These results suggest that in human liver microsomes a single enzyme catalyzes the deiodination of the outer as well as the inner ring of iodothyronines by the same catalytic mechanism and with the same substrate specificity as the type I deiodinase of rat liver.

Binding, Competitive↗