Thyroglobuline

BeschreibungThyroglobulin is of special importance, because it serves as the matrix for the synthesis of T4 and T3 and as the storage form of the hormones and iodide. Thyroglobulin, a large glycoprotein with a molecular weight of 660,000 Da, accounts for about one-third of the weight of the thyroid gland. Thyroglobulin carries an average of 6 tyrosyl residues as monoiodo-L-tyrosine, 5 residues as diiodo-L-tyrosine, 0.3 residues as T3, and 1 residue as T4. From these values, it can be estimated that a 20 g thyroid stores roughly 10 μmol (7.8 mg) of T4 and 3 μmol (2.0 mg) of T3 and that the normal human thyroid gland contains enough potential T4 to maintain a euthyroid state for 2 months without new synthesis. The synthesis of the thyroid hormones, T3 and T4, is regulated by thyrotropin (thyroid-stimulating hormone [TSH]), which stimulates the synthesis of thyroglobulin, thyroperoxidase (TPO), and hydrogen peroxide. The formation of the thyroid hormones depends on an exogenous supply of iodide. The thyroid gland is unique in that it is the only tissue of the body able to accumulate iodine in large quantities and incorporate it into hormones. Approximately 25% of the body's supply of iodide is located in the thyroid gland. The iodine atoms play a unique role in the conformational preferences for T3 and T4 because of their large steric bulkiness. The metabolism of iodine is so closely related to thyroid function that the two must be considered together. The formation of thyroid hormones involves the following complex sequence of events: 1) active uptake of iodide by the follicular cells, 2) oxidation of iodide and formation of iodotyrosyl residues of thyroglobulin, 3) formation of iodothyronines from iodotyrosines, 4) proteolysis of thyroglobulin and release of T4 and T3 into blood, and 5) conversion of T4 to T3.

VerwendenProloid (Parke-Davis).