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Biomedical subjects

A Taurog

Publications and source records attributed to A Taurog.

At least 73 records · Page 4Linked to original sources
PubMed1978

Improved assay procedures for thyroid peroxidase; application to normal and adenomatous human thyroid tissue.

We have developed assays for thyroid peroxidase in crude thyroid tissue preparations, in which a linear relationship between activity and amount of tissue could be demonstrated. Linear assays were developed based on the following peroxidase catalyzed reactions in the presence of H2O2:(1) oxidation of I- to I(-3), (2) oxidation of guaiacol, and (3) iodination of human goiter thyroglobulin. To attain satisfactory linearity we found it necessary to solubilize the enzyme beforehand. This was accomplished by a brief treatment of the particulate fraction with trypsin and deoxycholate, followed by centrifugation at 40 000 X g and dialysis. Not only did this treatment facilitate the development of linear assays, but it also resulted in a substantial increase in enzyme activity compared with that in the untreated particulate fraction. The use of a Polytron homogenizer for the initial disruption of the tissue also proved helpful in developing these assay procedures. The three different assays were used to measure peroxidase activities in human thyroid adenomas and in normal tissue derived from adenomatous glands. T he adenomas generally displayed a higher level of peroxidase activity than normal tissue. The greatest difference was observed with the iodination assay and the smallest difference with the guaiacol assay.

Adenoma↗
PubMed1978

Plasma thyroxine and triiodothyronine levels in spontaneously metamorphosing Rana catesbeiana tadpoles and in adult anuran amphibia.

We have developed sensitive and reliable radioimmunoassays for T4 and T3 in amphibian plasma and have used these procedures to measure plasma T4 and T3 levels in spontaneously developing Rana catesbeiana tadpoles at various stages of metamorphosis. During premetamorphosis circulating levels of both T4 and T3 were below the limits of detection of the RIA procedures (T4 less than 50 ng/100 ml, T3 less than 5 ng/100 ml). A gradual rise in plasma T3 and T4 became apparent during prometamorphosis, and at the onset of metamorphic climax the levels of both T4 and T3 increased sharply. Peak levels for both hormones were observed in the middle of metamorphic climax (stage XXIII). The circulating T3 level reached a mean peak of 78 ng/100 mg, at least 15 times greater than the level during premetamorphosis. The peak T4 level was 0.5 microgram/100 ml, about a 10-fold increase over the premetamorphosis level. The surge in thyroid hormone secretion lasted only for several days, and during the latter half of metamorphic climax there was a fairly rapid decrease in plasma T4 and T3. By 2 days post-climax the levels had declined to about 20% of their peak values. Free T4 and T3 levels in plasma followed the same general pattern as the total hormone levels during the various stages of tadpole development. In adult R. catesbeiana, plasma T4 and T3 levels were surprisingly low. Similarly low values were observed in Bufo marinus and in Rana pipiens. The very low levels of circulating T4 and T3 both in premetamorphosis tadpoles and in adults suggest that thyroid hormones in anuran Amphibia may be of importance only during the period of metamorphosis.

Animals↗
PubMed1978

Mechanism of action of thioureylene antithyroid drugs: factors affecting intrathyroidal metabolism of propylthiouracil and methimazole in rats.

Experiments were performed with rats to test the physiological significance of a previously proposed mechanism of action of thioureylene antithyroid drugs, which had been derived from results obtained with a model system containing purified thyroid peroxidase. Two features of the previously proposed scheme were tested:1) the effects of drug dosage and 2) the effects of iodine deficiency. In the dosage experiments, rats were injected with graded doses of [35S]PTU (0.18-59 mumol or [35S]mmi (0.16-18 mumol). Thyroid glands were removed 1 and 6--8 h later and 35S distribution in the homogenates was determined by paper chromatography. Serum samples were also analyzed by the same procedure. From the measured 35S activity in the various components and from the known specific activity of the injected drugs, it was possible to calculate thyroidal concentrations of unchanged drug and drug metabolites. At low doses, thyroidal concentrations of unchanged 6-propyl-2-thiouracil (PTU) and 1-methyl-2-mercaptoimidazole (MM) significantly exceeded their concentrations in serum, as reported by previous investigators. A major new finding in the present investigation was the observation that intrathyroidal metabolism of PTU and MMI is greatly affected by dosage. Marked inhibition of intrathyroidal drug metabolism was observed at 6-8 when the dosage was increased from 5.9 to 18 mumol for [35S]PTU and from 0.88 to 2.2 mumol for [35S]MMI (per 200 g rat). These findings demonstrate that with increasing dosage, PTU and MMI inhibit their own intrathyroidal metabolism. These dosage effects are similar to results previously reported for in vitro oxidation of PTU and MMI by the thyroid peroxidase system, and they offer support for the physiological significance of the previously proposed scheme. Further evidence for the physiological validity of this scheme was obtained in the experiments with iodine-deficient rats. As predicted from the in vitro findings, intrathyroidal metabolism of [35S]PTU and [35S]MMI was markedly reduced in rats on low iodine diet.

Animals↗
PubMed1977

Acute and chronic responses to iodine deficiency in rats.

Various indices of thyroid function were measured in rats at early (2 to 26 days) and at late (up to 55 weeks) intervals after the onset of an iodine deficient diet (LID). Control groups received the same diet with iodine-supplemented drinking water. The measurements included: thyroid total 127I concentration, thyroid weight, [131i] mit/[131i]dit and [131i]t3/[131i]t4 after a labeling period of 18--24 h, serum T4, serum T3, and serum TSH. In the acute experiments serum T4 was significantly decreased at 6--7 days. Thereafter, the decrease was rapid and progressive, and by 26 days serum T4 was reduced to undetectable levels (less than .05 mug per 100 ml). Thyroid T4 decreased more rapidly than serum T4, suggesting that the turnover rate of thyroid T4 increased soon after the onset of LID. Serum T3, in contrast to serum T4 showed no significant change during the 26 day period. Thyroid T3 decreased less rapidly than thyroid T4, but was greatly reduced by 26 days. Presumably the turnover rate of thyroih was slightly but significantly elevated at 4 days, before there was any detectable decrease in serum T4. Thereafter, the results were somewhat variable, but there appeared to be no significant further rise in serum TSH up to about day 15, despite the observation that serum T4 fell rapidly during this period. Only after 15 days did serum TSH display a sharp increase. It is apparent from these results that there is no simple inverse relationship between plasma T4 and plasma TSH. In the chronic experiments serum T4 remained undetectable. Serum T3 was decreased to about 50% of the normal level at 7 weeks but there appeared to be no further decrease even at 55 weeks. Serum TSH rose to very high levels at 12 weeks but showed no further increase thereafter. Thyroid 127I concentration reached its lowest value at 15 weeks and showed no further decrease. Thyroid weight, on the other hand, appeared to increase progressively. To evaluate the effect of severe iodine deficiency on thyroid status, rats that had been on LID for 3-4 months were exposed to a cold environment (4--5 C). Body temperature and survival rates were compared with those of matched controls receiving LID + KI drinking water. The body temperatures of the latter group increased significantly on exposure to cold, and all animals survived. On the other hand, the body temperature of rats on LID alone began to decrease about 5 days after the onset of cold exposure, and when the diet was sufficiently low in iodine the majority of the rats died within 15 days. These results suggest that thyroid function in severely iodine deficient rats is not adequate to meet the challenge of acute cold stress. In this sense, therefore, these animals may be daid to display signs of hypothyroidism.

Acute Disease↗
PubMed1977

The importance of thyroglobulin structure in thyroid peroxidase-catalyzed conversion of diiodotyrosine to thyroxine.

We have previously demonstrated that thyroid peroxidase (TPO) not only catalyzes the iodination of thyroglobulin and other proteins, but that it also catalyzes the intramolecular conversion of DIT residues to T4 (coupling reaction). The present study was designed to determine whether the native structure of thyroglobulin contributes to the efficiency of TPO-catalyzed coupling. Two lines of evidence are presented in support of the view that the conformation of thyroglobulin is important for TPO-catalyzed coupling. The first was based on comparison of T4 yields in thyroglobulin and other proteins. The second involved the effect of guanidine pretreatment on T4 yields in thyroglobulin. Both types of experiment provided evidence that the native structure of thyroglobulin contributes to the efficiency of the coupling reaction. Specificity of thyroid peroxidase activity, on the other hand, does not appear to be of importance in the coupling reaction.

Caseins↗
PubMed1976

The mechanism of action of the thioureylene antithyroid drugs.

A model incubation system containing purified thyroid peroxidase (TPO) was used to study the mechanism of action of the thioureylene anti-thyroid drugs--propylthiouracil (PTU), methylmercapto imidazole (MMI) and carbimazole. Two general types of experiments were performed: a) measurement of the inhibitory effects of the drugs on TPO-catalyzed iodination and on TPO-catalyzed oxidation of guaiacol, and b) studies of the metabolism of PTU and MMI by the TPO model system. The major observations can be summarized as follows: 1) The thioureylene drugs are potent inhibitors of TPO-catalyzed iodination of protein and tyrosine. Their potency increases greatly as the concentration of I- decreases. 2) The thioureylene drugs are also potent inhibitors of TPO-catalyzed oxidation of guaiacol, a reaction that does not involve iodide. 3) MMI and PTU are readily oxidized in the model incubation system when iodide is present but not in the absence of iodide. The rate of oxidation increased as the iodide concentration was increased from 10 to 100 muM. 4) Oxidation of PTU and MMI by the model incubation system is inhibited by relatively slight increases in the concentration of PTU and MMI. These drugs are capable of inhibiting their own and each other's metabolism. 5) Inhibition of iodination is competitively antagonized by iodide at low drug concentrations, but not at higher drug concentrations. 6) Inhibition of iodination by MMI and PTU may be either reversible (low ratio of drug to iodide), or irreversible (higher ratio of drug to iodide). In reversible inhibition the iodination is inhibited for a period which may be as brief as 2 min or as long as 20 min, but thereafter, iodination begins, and there is escape from inhibition. During the lag-period there is extensive metabolism of the drug. In the case of irreversible inhibition of iodination is inhibited completely or almost completely for 60 min, and drug oxidation during this period is relatively low. 7) Irreversible inhibition may be transformed into reversible inhibition by increasing the concentration of TPO or the concentration of iodide. However, increasing the concentration of H2O2 or of tyrosine does not overcome irreversible inhibition. On the basis of these findings and of current views concerning the mechanism of enzymatic iodination, a scheme is proposed for the mechanism of inhibition by thioureylene drugs of TPO-catalyzed iodination of protein and tyrosine.

In Vitro Techniques↗
PubMed1976

Variations in the response of the thyroid gland of the rat to different low-iodine diets: correlation with iodine content of diet.

Large variations are frequently encountered in the thyroidal responses of rats to commercially available low-iodine diets. The major aim of this investigation was to attempt to correlate these variations with differences in the iodine content of the diets. A method was developed for measuring the iodine content of low-iodine diets which was sufficiently sensitive to discriminate between a diet containing 15-20 ng of iodine per g and one containing 30-40 ng of iodine per g. Large differences were observed between various commercial low-iodine diets in their ability to induce goiter and to affect other indices of thyroid function, and these differences could be correlated with differences in the iodine content. The most severe iodine deficiency occurred in rats that were fed a Remington diet containing 15-20 ng of iodine per g. After 3 months on this diet, thyroid weight increased about 7-fold, thyroid 127I concentration was reduced to about 0.5% of control values, serum thyroxine (T4) was reduced to less than 0.25 mug/100 ml, the ratio of labeled triiodothyronine to labeled T4 was increased to 5.2, and that of labeled monoiodotyrosine to labeled diiodotyrosine was increased to 4.2. Much smaller changes were observed in the rats on the other low-iodine diets, which ranged from 30-40 to about 100 ng of iodine per g. A daily supplement of only 0.2 mug of iodide per day administered to rats on a Remington low-iodine diet produced a significant changes in thyroidal responses. These results suggest that iodine deficiency alone is sufficient to explain the effects of the Remington low-iodine diet and that it is not necessary to postulate the presence of a goitrogen.

Animals↗
PubMed1976

Formation of 3,3'-diiodothyronine and 3',5',3-triiodothyronine (reverse T3) in thyroid glands of rats and in enzymatically iodinated thyroglobulin.

It was observed in the present investigation that labeled thyroxine (T4) comprised less than 2% of the total 131I in the thyroids of severely iodine-deficient rats labeled with 131I for 18-24 h, a much lower value than had previously been reported for iodine-deficient rats. This low value was attributable to two factors: 1) the use of a diet low enough in iodine content to produce extreme iodine deficiency, and 2) the use of a paper chromatography system that successfully separates T4 from the minor iodothyronines, 3,3'-diiodothyronine (T2) and 3',5',3-triiodothyronine (reverse T3; T3'). Formation of the minor iodothyronines, while low, becomes appreciable in relation to T4 formation in severe iodine deficiency. In the present study, the formation of labeled T2 was significant only in iodine deficiency, and the highest values were observed in the most severely iodine-deficient rats. In the latter, labeled monoiodotyrosine (MIT) comprised approximately 60% of the total 131I in the thyroid, and the increased formation of T2 could be attributed to the increased probability of coupling between two molecules of MIT. The formation of labeled T3', on the other hand, was significant in thyroids from both iodine-deficient and iodine-sufficient rats. Similarly, in thyroglobulin iodinated in vitro with thyroid peroxidase to varying levels of iodination, the formation of T2 was evident only at lower levels of iodination, whereas the formation of T3' was significant at all levels of iodination. The comparison of relative T3' and T4 formation in enzymatically iodinated thyroglobulin with corresponding values reported for the intermolecular (DIHPPA) model for T4 formation, indicates that the peroxidase model system simulates much more closely the relative formation of T3' and T4 seen in vivo.

Animals↗