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At least 811 records · Page 45Linked to original sources
PubMed1976

Iodothyronine content of human thyroid albumin.

Stable thyroid hormones (T4 and T3)1) have been demonstrated in pure albumin isolated from normal human thyroid tissue iodinated in vivo. Five samples of albumin were separated from other thyroid proteins by acrylamide gel electrophoresis. After pronase hydrolysis, the content of Thyroid hormones was measured chemically (T4 + T3) as well as by competitive radioactive measurement (T4) and radioimmunoassay (T3). The purity of the albumin and validity of these measurements were confirmed by different techniques. The synthesis of thyroid hormones is not therefore a property unique to Tg and may occur in albumin. However the amount of iodothyronines in the albumin (average 0.004 residue per molecule) is much less than that found in Tg (0.5 residue per molecule). In the albumin as in Tg the number of hormone residues per molecule is proportional to the number of atoms of iodine. At an equivalent iodine concentration, the albumin seems capable of forming the thyroid hormones as well as Tg. The difference between these two proteins, in their capacity to synthesize thyroid hormones, seems to depend on their capacity for iodination. This difference of iodination does not seem to be linked with the number of tyrosyl residues, but might be related to the position of these residues.

Albumins↗
PubMed1977

Transplantable thyroid tumour in rats: iodoamino acid distribution in successive tumour generations.

All first generation thyroid tumours in rats, produced by 3 micronc 131I followed by LID, were uniformly functional containing MIT, DIT, T4, T3, and in some cases, T'3. All fourth generation tumours were nonfunctional as tested. Second and third generation tumours were biochemically unpredictable, some containing all IAA including T'3, others only MIT, I-and a trace DIT. Degree of dedifferentiation and of autonomy did not necessarily correlate with loss of function. Results could be explained by postulating nodules with differing biochemical properties within individual tumours.

Amino Acids↗
PubMed1981

Effect of cycloheximide on iodothyronine formation in vitro.

The unique inhibitory effect of cycloheximide (CH) on the coupling of iodotyrosines was examined in vitro. Rat thyroid lobes were incubated for 8 h under our improved condition. In the presence of 10(-4) - 10(-3) M CH, the per cent uptake of 131I decreased, proportionate synthesis of [131I]MIT increased slightly, and that of [131I]T4 or [131I]T3 decreased markedly. The incorporation of medium 127I into T4 or T3 during the 8 h incubation period decreased markedly, but was fairly constant into MIT and only slightly decreased into DIT. Thus the inhibitory effect of CH seemed more prominent on iodothyronine formation than on iodotyrosine formation in this in vitro system. Inhibition of formation of newly labelled iodothyronines seemed to occur almost in parallel with the inhibition of [3H]amino acid incorporation into the thyroidal soluble protein. However, the coupling of iodotyrosines prelabelled in the absence of CH did not seem to be affected by CH. The presence of 10(-4) M CH induced the Wolff-Chaikoff effect at a lower iodide concentration than that which occurred in the absence of CH, suggesting that CH sensitized the Wolff-Chaikoff effect. However, the organification of 127I and T4 synthesis were markedly reduced in the presence of CH even before the apparent Wolff-Chaikoff effect was initiated. These results give further support to out contention that prethyroglobulin is more important for organification of iodide than pre-existing thyroglobulin. We conclude that CH reduces coupling efficiency indirectly, probably by inhibiting the formation of prethyroglobulin with a favourable structure for coupling.

Amino Acids↗
PubMed1990

Inhibition by immunoglobulin G of synthesis of thyroid hormone in thyroid cultures from hypothyroid patients with goitrous Hashimoto's thyroiditis.

Recently, thyroid microsomal antigen was identified as thyroid peroxidase, and thyroid microsomal antibody was found to inhibit thyroid peroxidase activity in vitro. We investigated the possibility that anti-microsomal antibody inhibits the iodination of tyrosine, in vivo. Immunoglobulin G with or without anti-microsomal antibody from hypothyroid patients with goitrous Hashimoto's thyroiditis inhibited thyroid hormone synthesis in cultured slices of normal human thyroid tissue. IgGs with anti-microsomal antibody inhibited 125I thyroidal uptake and thyroid hormone synthesis stimulated by TSH more than normal IgG did. However, the same results were obtained with IgGs without anti-microsomal antibody. This effect did not involve anti-microsomal antibody, anti-thyroglobulin antibody, TSH-binding inhibitor immunoglobulin, thyroid stimulation-blocking immunoglobulin, or the cAMP level of the thyroid tissue. The ratio of organic I to inorganic I with stimulation by TSH in slices incubated with IgG from hypothyroid patients with goitrous Hashimoto's thyroiditis or normal IgG was not significantly different, but was significantly higher in slices incubated with methylmercaptoimidazole. Therefore, IgG from hypothyroid patients with goitrous Hashimoto's thyroiditis mainly suppressed 125I thyroidal uptake, rather than inhibiting thyroid peroxidase activity. In addition, this IgG was present in the serum of 11 of the 12 hypothyroid patients with Hashimoto's thyroiditis studied. This IgG may be involved in the mechanism that causes hypothyroidism in some patients with goitrous Hashimoto's disease.

Adult↗
PubMed1993

Thyroid peroxidase: kinetics, pH optima and substrate dependency.

The oxidation of iodide, guaiacol and 2,2'-azino-di[3-ethyl-benzthiazoline-(6)-sulphonic acid] and the iodination of tyrosyl residues in bovine serum albumin, catalysed by partly purified thyroid peroxidase, were studied. The enzyme showed pH optima with all electron donors. With the exception of guaiacol, the position of the pH optima depended upon both the electron donor and hydrogen peroxide concentrations. With increased hydrogen peroxide concentrations the optima shifted to lower pH, and with increased iodide concentration to higher pH. For monoiodotyrosine (MIT) formation in bovine serum albumin the position of the pH optimum was also dependent on the hydrogen peroxide concentrations. The position of the pH optimum of the oxidation of guaiacol was pH 9 and independent of substrate and hydrogen peroxide concentrations. It is obvious from these findings that iodination reactions must be studied under well-defined conditions.

Animals↗