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Biomedical subjects

A Taurog

Publications and source records attributed to A Taurog.

At least 55 records · Page 3Linked to original sources
PubMed1984

Mechanism of iodide-dependent catalatic activity of thyroid peroxidase and lactoperoxidase.

Mechanisms that have been proposed for peroxidase-catalyzed iodination require the utilization of 1 mol of H2O2 for organic binding of 1 mol of iodide. When we measured the stoichiometry of this reaction using thyroid peroxidase or lactoperoxidase at pH 7.0, we consistently obtained a ratio less than 1.0. This was shown to be attributable to catalase-like activity of these enzymes, resulting in unproductive cleavage of H2O2. This catalatic activity was completely iodide-dependent. To elucidate the mechanism of the iodide-dependent catalatic activity, the effects of various agents were investigated. The major observations may be summarized as follows: 1) The catalatic activity was inhibited in the presence of an iodine acceptor such as tyrosine. 2) The pseudohalide, SCN-, could not replace I- as a promoter of catalatic activity. 3) The inhibitory effects of the thioureylene drugs, methimazole and carbimazole, on the iodide-dependent catalatic activity were very similar to those reported previously for thyroid peroxidase-catalyzed iodination. 4) High concentrations of I- inhibited the catalatic activity of thyroid peroxidase and lactoperoxidase in a manner similar to that described previously for peroxidase-catalyzed iodination. On the basis of these observations and other findings, we have proposed a scheme which offers a possible explanation for iodide-dependent catalatic activity of thyroid peroxidase and lactoperoxidase. Compound I of the peroxidases is represented as EO, and oxidation of I- by EO is postulated to form enzyme-bound hypoiodite, represented in our scheme as [EOI]-. We suggest that the latter can react with H2O2 in a catalase-like reaction, with evolution of O2. We postulate further that the same form of oxidized iodine is also involved in iodination of tyrosine, oxidation of thioureylene drugs, and oxidation of I-, and that inhibition of catalatic activity by these agents occurs through competition with H2O2 for oxidized iodine.

Animals↗
PubMed1983

Iodide-dependent catalatic activity of thyroid peroxidase and lactoperoxidase.

Thyroid peroxidase (TPO) and lactoperoxidase (LPO) display significant catalatic activity at pH 7.0 in the presence of low concentrations of iodide, based both on measurements of H2O2 disappearance and O2 evolution. In the absence of iodide only minor catalatic activity was detected. The stimulatory effect of iodide could not be explained by protection of the enzymes against inactivation by H2O2. A mechanism is suggested involving an enzyme-hypoiodite complex as an intermediate.

Catalysis↗
PubMed1983

Differences in iodinated peptides and thyroid hormone formation after chemical and thyroid peroxidase-catalyzed iodination of human thyroglobulin.

The distribution of iodine among the polypeptides of human goiter thyroglobulin (Tg) was examined. Tg was iodinated in vitro with 131I to levels of 2 to 84 gram atoms (g.a.)/mol using thyroid peroxidase (TPO) or a chemical iodination system. The samples were reduced, alkylated, and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two low-molecular-weight peptides appeared preferentially in radioautograms of the sodium dodecyl sulfate (SDS) gels of TPO-iodinated samples. Iodination of these peptides increased sharply in the TPO-treated Tg as the level of total iodine/molecule rose. Radioiodine was incorporated into these same gel regions in the chemically treated Tg, but only after much higher levels of total iodination were reached. Differences in iodoamino acid distribution were also noted between the chemically and enzymatically iodinated thyroglobulins. In the chemically iodinated samples, little thyroxine (T4) was synthesized, even at high iodine levels. In the TPO-treated samples only small amounts of T4 were seen below 14 g.a. total I/mol, while at or above that level of iodination T4 formation increased sharply. To examine the coupling process, Tg was chemically iodinated, excess I- removed, and the samples treated with TPO and a H2O2-generating system in the absence of iodide. Radioautograms obtained from SDS-polyacrylamide gels of reduced and alkylated protein from such coupling assays showed an increase in the level of iodine in the low-molecular-weight peptides after TPO treatment. Thyroxine production also increased with TPO treatment. The addition of free DIT (a known coupling enhancer) to the [131I]Tg/TPO incubation increased both the production of T4 and the amount of iodine in the smaller polypeptides. Two-dimensional maps prepared from CNBr-digested TG showed differences between the coupled and uncoupled samples. Our observations confirm the importance of the low-molecular-weight peptides derived from Tg in thyroid hormone synthesis. At total iodine levels above 14 g.a./mol Tg in enzymatically treated samples there is selective incorporation of iodine into both the low-molecular-weight polypeptides and into thyroid hormone.

Adult↗
PubMed1983

Reversible and irreversible inhibition of thyroid peroxidase-catalyzed iodination by thioureylene drugs.

The mechanism of reversible and irreversible inhibition of thyroid peroxidase (TPO)-catalyzed iodination by thioureylene drugs was investigated using a model incubation system. The major observations may be summarized as follows. 1) TPO is inactivated by 1-methyl-2-mercaptoimidazole and propylthiouracil even in the presence of a relatively high concentration of iodide. The extent of this inactivation depends on the ratio of iodide to drug. 2) Spectral changes observed on oxidation of the drugs with the peroxidase-iodide system were very similar to those observed when the drugs were oxidized nonenzymatically with I3-. These findings support the view that oxidized iodine is an intermediate in TPO-catalyzed oxidation of the drugs. 3) Under conditions where TPO is largely inactivated, inhibition of iodination is complete and irreversible. Drug metabolism, on the other hand, occurs to a limited extent. 4) Under conditions where TPO is only partially inactivated, inhibition of iodination is transient (reversible). In this case, drug metabolism is extensive, and higher oxidation products (sulfate and sulfinic acid) are observed. Inhibition of iodination occurs only during the interval required to reduce the drug concentration to a low level. Thereafter, iodination may occur at a rate close to that observed in the absence of drug. Based on these and other observations, a scheme is presented to explain the mechanism of reversible and irreversible inhibition of iodination. In essence, the type of inhibition depends on the relative rates and extent of TPO inactivation and drug oxidation. These rates, in turn, depend primarily on the iodide to drug concentration ratio. A high ratio favors extensive drug oxidation and reversible inhibition. A low ratio favors TPO inactivation and irreversible inhibition.

Animals↗
PubMed1983

Mechanism of action of thioureylene antithyroid drugs in the rat: possible inactivation of thyroid peroxidase by propylthiouracil.

We have previously shown that the thioureylene antithyroid drugs 6-propyl-2-thiouracil (PTU) and 1-methyl-2-mercaptoimidazole (MMI) can inactivate thyroid peroxidase (TPO) in a model iodination system containing relatively high concentrations of iodide. The purpose of the present study was to determine whether these drugs may also inactivate TPO in vivo in rats. Assays for total TPO activity after injection of PTU or MMI did not prove to be a valid approach. As TPO inactivation might be expected to result in a relatively prolonged inhibition of enzyme activity, most of our experiments involved measurement of the duration of the inhibitory effect of a single injection of drug. Young rats were injected with low doses of PTU or MMI, and the effect on thyroidal organic iodine formation was determined at intervals after injection, either by 1-h pulse labeling with 131I- in vivo or by incubation of excised thyroid lobes in a medium containing 131I-. Results of both types of experiment demonstrated that the inhibitory effect of a small dose of PTU (1 mumol/100 g BW) was still very marked 17-18 h after injection. Moreover, an inhibitory effect of this small dose of PTU on the metabolism of [35S]MMI could also be demonstrated. Administration of MMI to rats, on the other hand, did not show the prolonged inhibitory effect observed with PTU. This is most likely attributable to the much lower thyroidal uptake of MMI than of PTU in rats. Intrathyroidal metabolism of [35S]PTU and [35S]MMI was also investigated. In contrast to the rapid disappearance of 35S from plasma, both drugs showed accumulation and retention of 35S in the thyroid. However, we obtained no evidence that thyroidal accumulation of PTU or one of its metabolites could explain the prolonged inhibitory effect of this drug. It seemed more likely that this was attributable to TPO inactivation. The clinical implications of our findings are discussed with relation to the dosage schedule commonly employed in the treatment of Graves' disease with antithyroid drugs.

Animals↗
PubMed1982

Mechanism of inactivation of thyroid peroxidase by thioureylene drugs.

We have investigated the mechanism by which the thioureylene drugs, 1-methyl-2-mercaptoimidazole (MMI) and 6-n-propylthiouracil (PTU), inactivate thyroid peroxidase (TPO). Our results indicate that inactivation of TPO by MMI and PTU involves a reaction between the drugs and the oxidized heme group produced by interaction between TPO and H2O2. This conclusion is supported by the following observations. First, addition of a low concentration of H2O2 to a solution of TPO shifted lambda max of the Soret band from 411 to 420 nm, reflecting the formation of an oxidized form of TPO (TPOox). Addition of MMI or PTU to TPOox produced a Soret spectrum that was significantly different from the spectrum of native TPO or TPOox, whereas addition of MMI or PTU to native TPO produced no significant change in the heme spectrum. Second, studies with radiolabeled MMI and PTU combined with simultaneous assays of enzyme activity (guaiacol assay) showed that firm binding of the drugs to TPO and inactivation of the enzyme occurred on addition of the drugs to TPOox. However, neither binding nor inactivation occurred on addition of the drugs to native TPO. Third, the presence of a low concentration of iodide prevented the shift in the Soret spectrum, the binding of labeled drug, and the loss of enzyme activity associated with the addition of thioureylene drugs to TPO + H2O2. Under these conditions we assume that the enzyme was present as TPO X Iox, a form in which the heme is present in the same reduced state as in native TPO. This would explain the protective action of iodide on the inactivation of TPOox by MMI and PTU.

Antithyroid Agents↗
PubMed1982

Preferential inhibition of thyroxine and 3,5,3'-triiodothyronine formation by propylthiouracil and methylmercaptoimidazole in thyroid peroxidase-catalyzed iodination of thyroglobulin.

The present study was undertaken to determine whether the thioureylene antithyroid drugs propylthiouracil [6-propyl-2-thiouracil (PTU)] and methylmercaptoimidazole [1-methyl-2-mercaptoimidazole (MMI)] have a specific inhibitory effect on the thyroid peroxidase (TPO)-catalyzed conversion of diiodotyrosine to T4 (coupling reaction) independent of their well known inhibitory effect on peroxidase-catalyzed iodination. We have employed model incubation systems containing highly purified TPO to examine this question. Most experiments were performed with a model iodination system containing TPO, low iodine thyroglobulin, [131]iodide, and glucose-glucose oxidase. Both PTU and MMI are effective inhibitors of iodination of this system at physiological concentrations, and the system is well suited for studying the simultaneous action of these drugs on iodination and coupling. The addition of graded doses of the drugs to the iodination system demonstrated a relatively greater inhibitory effect on iodothyronine than on iodotyrosine formation. However, this observation in itself does not establish a specific inhibitory effect on coupling, since the formation of T4 involves a reaction between two molecules of 3,5-diiodotyrosine (DIT). The rate of this reaction, therefore, is second order with respect to DIT concentration, and the inhibition of DIT formation by thioureylene drugs would be expected to result in a disproportionately greater reduction in T4 formation even id there were no selective inhibitory effect of the drugs on the coupling reaction. Under certain conditions of incubation, however, it was possible to demonstrate a significant inhibitory effect on T4 and T3 formation without any decrease (in fact, a slight increase) in diiodotyrosine formation. These observations indicate that, at least under some conditions, PTU and MMI can exert a specific inhibitory effect on the coupling reaction. In the case of PTU, a specific inhibitory effect on coupling was also demonstrated with an incubation system in which TPO-catalyzed coupling was measured in the absence of iodination.

Animals↗
PubMed1982

[Effects of exogenous TSH on the thyroid activity of adult or neotenic amphibians].

In adult Anuran and neotenic Urodela, bred in laboratory conditions, the levels of plasma thyroid hormones are undetectable (T3 less than 50 ng/100 ml, T4 less than 5 ng/100 ml). Thyroid function can be reactivated after ovine TSH treatment. Under those conditions, metamorphosis is induced in the axolotl and T4 plasmatic levels reaches 0,53 +/- 0,13 micrograms/100 ml and those of T3 9 +/- 2,64 ng/100 ml. In adult Anuran, thyroid reactivation under thyrotropic treatment determines an increased secretion of T4 whereas T3 remains below the limits of detection of the assay procedure. This aptitude of adult Anuran thyroid to answer thyrotropic stimulation suggests a cyclic function of the gland after metamorphosis.

Ambystoma↗
PubMed1981

Elevation of serum 3,5,3'-triiodothyronine and thyroxine levels in rats fed Remington diets; opposing effects of nutritional deficiency and iodine deficiency.

In the course of experiments on iodine deficiency induced by Remington diets in rats, we observed that the Remington diet supplied by ICN Nutritional Laboratories, though very deficient in iodine (less than 20 micrograms I/kg), did not lead to the rapid loss of thyroid iodine and the rapid decrease in serum T4 expected on the basis of previous studies with a similarly iodine-deficient Remington diet from another source. In searching for an explanation for this observation, we noted that the ICN Remington diet was nutritionally much more deficient than Remington diets from other suppliers. We also noted that when rats were placed on the iodide-supplemented ICN Remington diet there was a marked increase in serum T3 and T4. In one experiment, rats receiving the ICN Remington diet plus KI in the drinking water for 16 days showed a serum T3 level of 109 +/- 16 ng/dl and a serum T4 level of 6.6 micrograms/dl compared to 52 +/- 7.8 and 4.4 +/- 0.8, respectively, in control rats on a stock diet. These elevations were not simply the result of increased binding to serum proteins. Serum protein-binding studies by the method of equilibrium dialysis showed a very slight decrease in the percent dialyzable fraction for T3 and T4. However, calculated free T3 levels were significantly elevated (P less than 0.001), and increases in free T4, though less striking, were also significant. These elevations were not accompanied by evidence of hyperthyroidism, as judged by measurements of O2 consumption or serum TSH. Although the specific nutritional factors and mechanisms have not yet been defined, our studies demonstrate that nutritional deficiencies in a Remington diet may act to oppose the effects of the iodine deficiency itself. Our observation that the iodide-supplemented ICN Remington diet has a marked serum T3- and T4-elevating effect offers a possible explanation for the blunted thyroidal responses of rats to the same diet lacking added iodide. Our studies also suggest that alteration of peripheral T4 and T3 conversion may not be the only mechanism by which nutritional factors affect serum T3 and T4 levels.

Animals↗
PubMed1981

Strain differences among rats in response to Remington iodine-deficient diets.

Male rats of five different strains (Simonsen albino, Wistar, Long-Evans, Holtzman Sprague-Dawley, and Charles River Sprague-Dawley) were tested for their response to the U.S. Biochemical Corp. Remington low iodine diet containing 15-18 microgram I/kg. Measurements made after the diet had been fed for 28-30 days indicated that Simonsen albino and Wistar strains consistently showed the greatest response, based on degree of thyroid enlargement, depletion of thyroidal iodine, reduction in serum T4, and elevation of serum TSH. Long-Evans and Holtzman Sprague-Dawley rats responded relatively poorly to the low iodine diet. One experiment included female rats, and the limited data suggested that within a given strain there was no significant sex difference. With more prolonged feeding (84 days), the difference between a rapidly responding strain (Simonsen albino) and a more slowly responding strain (Holtzman Sprague-Dawley) was not so marked. Our results indicate that given sufficient time and a diet sufficiently low in iodine, even a more slowly responding strain will ultimately develop signs of extreme iodine deficiency. However, it is inconvenient and expensive to maintain rats on a Remington low iodine diet for 3 months, and studies on the effect of severe iodine deficiency are much more rapidly performed using a rapidly responding strain such as the Simonsen albino. Our observation that rats of different strains differ markedly in their responses to an iodine-deficient diet suggests that hereditary factors play an important role in this response.

Animals↗
PubMed1981

Hypothyroidism in severely iodine-deficient rats.

The thyroid status of severely iodine-deficient rats was assessed by measurement of the resting metabolic rate (RMR) and liver mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD). Rats maintained on the iodine-deficient diet for 2 or 3 months showed significantly reduced RMR and alpha-GPD, compared to rats on the same diet supplemented with KI in the drinking water. They also displayed markedly reduced serum T4 levels, slightly reduced serum T3 levels, and highly elevated serum TSH levels. A significant decrease in liver alpha-GPD was observed 29 days after the rats were placed in iodine-deficient diet. However, the decrease in RMR in the same animals was not statistically significant. These results suggest that measurement of liver alpha-GPD may be a more sensitive index of impending hypothyroidism than measurement of O2 consumption. The present study demonstrates that a hypothyroid state can be induced in rats exposed to a severely iodine-deficient diet. In severe iodine deficiency, the compensatory mechanisms of increased TSH stimulation and preferential T3 secretion from the thyroid are insufficient to prevent a fall in serum T3. The hypothyroid state results from the inability to maintain a normal serum T3 level and possibly also from the very low levels of serum T4.

Animals↗
PubMed1981

Elevated serum levels of T3 without metabolic effect in nutritionally deficient rats, attributable to reduced cellular uptake of T3.

Rats receiving a nutritionally deficient diet displayed markedly elevated serum free T3 levels but showed no increase in oxygen consumption. This was associated with greatly reduced ratios of hepatic cellular and nuclear 125I-T3 to serum 125I-T3. Kinetic data supported the conclusion that cellular uptake of T3 was decreased in the nutritionally deficient rats. The lack of metabolic effect, despite the elevated serum T3 levels, is attributable to reduced availability of serum T3 to tissue nuclear receptor sites.

Animals↗
PubMed1979

Rapid conversion of carbimazole to methimazole in serum; evidence for an enzymatic mechanism.

Carbimazole (CBZ) is one of the major drugs currently used for the treatment of Graves' disease. It is a carbethoxy derivative of methimazole (MMI), originally developed in the hope of obtaining a longer acting drug than methimazole. In the present study we have demonstrated that carbimazole is rapidly converted to methimazole in vitro by serum from rats and humans, and we have obtained evidence that this conversion is enzymatic. Experiments with [35S] CBZ in rats showed that the drug is so rapidly transformed to MMI after i.v. injection (within 3 min) that very little of the unchanged drug would be expected to reach the thyroid gland. The antithyroid action of CBZ in rats, therefore, can be ascribed entirely to the MMI to which it is rapidly converted. Although no experiments were performed with human subjects in vivo, the very rapid conversion of CBZ to MMI by human serum in vitro suggests that the antithyroid action of CBZ in humans can also be attributed to MMI. The original expectation of a longer acting drug has, therefore, not been met by CBZ. On the basis of the studies reported here there appears to be no advantage in using CBZ in preference to MMI for the treatment of Graves' disease. Although the in vivo action of CBZ must be attributed to its rapid conversion to MMI, the drug does possess inherent antithyroid activity. This was shown in the present study by the finding that CBZ is as potent as MMI in blocking thyroid peroxidase-catalysed iodination of thyroglobulin.

Animals↗
PubMed1979

Congenital goitre and hypothyroidism with impaired iodide organification and high thyroid peroxidase concentration.

A sibship of thirteen subjects whose parents were first cousins was studied for a defect in thyroid hormone synthesis. Five sibs were goitrous and had congenital hypothyroidism. All but one showed a positive perchlorate discharge test (PDT). Three other subjects were goitrous and euthyroid (one with a positive PDT), and the remaining five sibs were euthyroid with a presumably normal thyroid. However, an abnormally exaggerated TSH response to TRH was observed not only in the hypothyroid patients but also in six of the other subjects, indicating a decreased thyroid feedback at the pituitary level in the presence of a normal serum concentration of thyroid hormones. In two hypothyroid patients a normal serum T3, low serum T4 and a low reverse T3 were observed. Microscopic studies of thyroid tissue from three of the sibs disclosed marked cellular hyperplasia with no lymphocytic infiltration anywhere in the tissue. Peroxidase activity was determined on tissue from three sibs by three different assay procedures. It was within the normal range in one patient and was significantly elevated in the other two. There was no evidence for a qualitatively defective peroxidase. The defect in thyroid function in this family does not appear to involve a peroxidase deficiency. Thyroglobulin isolated from the thyroid glands of two of the goitrous, hypothyroid subjects was poorly iodinated but was judged to be normal by immunoreactive and ultracentrifugation procedures. Although the nature of the thyroid metabolic defect in this family was not elucidated, the evidence suggests a genetic defect, probably involving a recessive gene.

Adolescent↗