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Results for “DIIODOTYROSINE”

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PubMed1974

A multiple ligand-binding radioimmunoassay of diiodotyrosine.

A radioimmunoassay has been developed for the measurement of 3,5-diiodo-L-tyrosine (DIT) in serum. DIT was coupled to porcine thyroglobulin (PTg) with a molar ratio of 205:1. Rabbits were immunized with 1 mg of immunogen emulsified in complete Freund's adjuvant. Sera were screened for their ability to bind trace amounts of [125I]DIT. A serum that bound 40% of the tracer at a final dilution of 1:1,750 was used in the assay. Assay specificity was improved by the use of thyroxine (T4)-binding globulin as a second ligand-binding protein to decrease T4 and triiodothyronine (T3) cross-reactivity with the antibody. Double antibody and polyethylene glycol radioimmunoassays were compared. DIT present in the second antiserum shifted the double antibody assay standard curve and altered estimates of assay specificity and assay sensitivity. By using the polyethylene glycol system and butanol:ethanol extracts of serum, DIT was measured in human serum. In 35 apparently healthy young adult controls DIT levels averaged 156 ng/100 ml. Random DIT levels averaged 158 ng/100 ml in 11 untreated hyperthyroid patients and 84 ng/100 ml in 15 untreated primary hypothyroid patients. No diurnal pattern in DIT levels could be demonstrated. Thyroid-stimulating hormone administration led to a variable but small rise in DIT levels, but short term T3 suppression was not associated with a measurable fall in DIT concentrations. Paired serum samples from the carotid artery and thyroid vein of 10 euthyroid goiter patients and one patient with a toxic solitary adenoma all showed a positive transthyroidal gradient indicating the thyroidal release of DIT in each patient. Measurable DIT levels of 45, 47, 68, and 80 ng/100 ml, respectively, were found in four fasting athyrotic patients indicating that the thyroid is not the only source of serum DIT.

Adult↗
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↗
PubMed1986

In vivo formation of diiodotyrosine by extrathyroidal thyroxine ether-link cleavage and effects of mononitrotyrosine on this pathway in the rat.

The formation of DIT from T4 was quantitatively studied in thyroidectomized (T) rats given 16 micrograms synthetic T4 daily. Measurement of the elimination of radioiodinated DIT and T4 tracers from serum yielded MCRs of 19.0 ml/h X 100 g body weight for DIT and 0.65 ml/h X 100 g body weight for T4. Mean serum concentrations +/- SD (nanomoles per liter; n = 8) measured by RIA 24 and 48 h after the last T4 administration were as follows: DIT, 0.243 +/- 0.130 and 0.150 +/- 0.070, respectively; T4, 173 +/- 34 and 97 +/- 20, respectively. In T rats which had not received T4, DIT and iodothyronines in serum were undetectable. From the results of kinetic studies and RIA measurements, the fraction of circulating T4 converted to DIT was calculated to be 3.9-4.3%. After administration of the iodotyrosine inhibitor 3-nitro-L-tyrosine (MNT) to T4-treated T rats at a dosage of 50 mumol/day for 1 week or longer, it was possible to observe, on the one hand, the expected delay of DIT tracer elimination from serum resulting in a decreased MCR of 9.9 ml/h X 100 g body weight. On the other hand, MNT treatment led to a strong decline of DIT serum levels below the detection limit in all animal groups. This effect of MNT on the peripheral T4-to-DIT conversion requires further studies using other experimental systems for confirmation and elucidation of its mechanism. It is concluded that peripheral DIT formation in the animal model used occurs via ether-link cleavage of administered T4 and/or some of its iodothyronine metabolites. On the basis of data from recent studies, the peripheral DIT turnover resulting from iodothyronine degradation can be estimated to be about 35% in intact rats. Our data confirm the in vivo generation of extrathyroidal DIT from T4 in the rat. Although the experiments were performed at unphysiologically high T4 serum levels and our quantitative data, therefore, cannot be applied to euthyroid conditions with absolute certainty, the results suggest that ether-link cleavage of T4 yielding DIT is not an insignificant pathway of peripheral T4 metabolism in the rat.

Animals↗