Degradation of Thyroxine by a Thyroidal Peroxidase1

Dawber, Nancy A.; Galton, Valerie Anne; Ingbar, Sidney H.

doi:10.1210/endo-88-1-144

Cited by 12 publications

(2 citation statements)

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“…No clear evidence that an enzymic reaction is involved in the mechanism of this conversion is as yet at hand. In rats a role for peroxidase in thyroxine deiodination has been proposed (Galton & Ingbar, 1963;Dawber et al, 1971), as well as the involvement of reduced flavin nucleotide as a cofactor in the deiodination of diiodotyrosine (Rosenberg & Ahn, 1969). These two supposed enzymic processes, however, seem to be functionally separated (Haibach, 1971).…”

mentioning

confidence: 99%

Conversion of thyroxine into tri-iodothyronine by rat liver homogenate

Visser

Does-Tobé

Docter

et al. 1975

Biochemical Journal

126

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By using a highly specific radioimmunoassay the formation of tri-iodothyronine by the deiodination of thyroxine was studied in rat liver homogenate. Several observations suggest that the reaction observed is enzymic in nature. Pre-heating the homogenate for 30 min at 56 degrees C completely abolished conversion of thyroxine into tri-iodothyronine; the component of rat liver homogenate responsible could be saturated with substrate; iodotyrosines displayed competitive activity. Between 0 degrees and 37 degrees C, the tri-iodothyronine-production rate was positively correlated with incubation temperature. The addition of NAD+ enhanced conversion into tri-iodothyronine, which suggests that an oxidative mechanism is involved. 5-Propyl-2-thiouracil and 6-propyl-2-thiouracil, both known to prevent deiodination in vivo, greatly decreased the deiodiantion activity of rat liver homogenate.

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mentioning

confidence: 99%

Conversion of thyroxine into tri-iodothyronine by rat liver homogenate

Visser

Does-Tobé

Docter

et al. 1975

Biochemical Journal

126

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“…This observation suggests the presence of adsorption of T4 on the surface of the cells, together with the accumulation in the cells. Dawber et al (1971) demonstrated that degradation of T4 in thyroid homogenate is mediated by thyroidal peroxide-peroxidase system. The significance of the present tindings in the physiology of thyroidal secretion is not definitely clear.…”

Section: Discussionmentioning

confidence: 99%

Studies on the Accumulation of Thyroxine in Isolated Bovine Thyroid Cells

et al. 1971

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SynopsisBy the use of isolated bovine thyroid cells (ITC), accumulation of T4 in thyroid tissue has been studied. ITC were chosen to disclose this thyroid cell function per se by eliminating the effect of follicular structure and contaminated serum. Cell/medium ratio (C/M) of 125I-T4, used as an indicator of T4 accumulation, was 17.7 when ITC were incubated in Eagle's solution. On the other hand, the ratio in the thyroid slice and erythrocytes was 1.1 and 3.0 respectively. By equilibration chamber method, T4 accumulated more in ITC than in erythrocytes. The observed accumulation of T4 in ITC was reversible when re-incubated with normal human serum. Displacement of 1251-T4 by non-labelled T4 was not observed at the range of added T4 from 10 to 1000 ital. Essentially the same result were obtained when the liver, thyroid slice or erythrocytes were studied. Therefore, the number of T4-binding sites appears to be very large. Increased deiodination of 1-25I-T4 was observed when 1251-T4 was incubated with ITC, whereas conversion of 125I-T4 to 1251-T3 was not remarkable. It is concluded that ITC obviously concentrate T4 by an entirely different mechanism from that in serum TBP.

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