Jean Mauchamp

Biosynthesis of uniodinated pre-thyroglobulin: II. Biosynthèse d'une préthyroglobuline non iodée

Abstract 1. 1. Surviving sheep-thyroid slices synthesize a pre-thyroglobulin with a sedimentation coefficient of 17 S. Iodination is, however, found to take place on the preformed molecules already present in the tissue. 2. 2. Polymerisation of the precursors (3–8 S, 12 S) of thyroglobulin (19 S) does not depen on their iodination. 3. 3. Chemical iodination of the pre-thyroglobulin (17 S) and subsequent modification of its conformation leads to the formation of thyroglobulin (19 S). 4. 4. The formation of two types of thyroglobulin: 125I (18.4 S) and 14C (17 S), differing in their sedimentation coefficient, composition (absence of iodo [14C] tyrosine in the 17 S0 and behavior after chemical iodination, suggests a separate localisation and the existence of two iodination siets, the first being in the colloid and involving preformed molecules, the second being cellular and related to newly synthesized molecules. The second, cellular site, appears to be the most disturbed during the incubations in vitro.

Biosynthèse in vitro d'hormones marquées par 3H et 125I dans des coupes de corps thyroïde: I. Propriétés des coupes en survie

Abstract 1. 1. Sheep thyroid slices were incubated 24 h at 37° under appropriate conditions as described. They formed thyroid hormones labeled with 3 H and 125 I while histologically intact. 2. 2. The biological cycle of hormone formation was maintained in the gland slices studied in the presence of Na 125 I and of tritiated l -tyrosine. This was proven by several observations, among others the organification of 125 I, the incorporation of [ 3 H]tyrosine into thyroglobulin purified by Sephadex, and also the formation of labeled 3,5,3′-triiodothyronine and thyroxine.

Biogénèse de la thyroglobuline: Les transitions d'halogénation

Abstract A model for the changes in properties of thyroglobulin during he course of its iodination has been established. 1. 1. The initial component of the heterogeneous molecular population which constitutes thyroglobulin is a non-iodinated protein (prethyroglobulin) which can be partially separated by chromatography on DEAE-Sephadex. Prethyroglobulin appears to be homogeneous. 2. 2. The stable thyroglobulin preformed in vivo and the preparation labeled by 125 I inv vitro atre heterogeneous, as shown by chromatography on DEAE-Sephzdex. The 2 populations contains molecules with the same iodine content. 3. 3. The iodine content of the total thyroglobulin corresponds to an average value. This protein must contain molecules with more than 60–70 atoms of iodine. 4. 4. The sedimentation coefficientd for the stable thyroglobulin (19 S) and for the preparation labeled in vivo (18.4 S) are average values. The heterogeneous populations contain molecules with lower (17 S) and higher (20 S from model iodination experiments) sedimentation coefficients. The iodination heterogeneity introduces conformational heterogeneity. 5. 5. Introduction of halogen (up to 60–70 iodine atoms) produces discrete transitions of conformation (shape or density changes) manifested in higher sedimetation velocity. A more striking transition occurs when an iodination level of 60–70 atoms is reached.

Synthèse acellulaire de la thyroglobuline et site d'iodation

Abstract Cell-free synthesis of thyroglobulin and the iodination site 1. 1. Thyroid microsomes and polysomes synthesize thyroglobulin (approx. 19 S) in cell-free systems. If it is a true protein synthesis, membranes appear therefore to be unnecessary. 2. 2. The 12-S precursor, observed in cellular systems, is absent in cell-free systems. Thyroglobulin is totally free in the medium. The ribonucleoprotein particles contain only the light labelled fraction apparently not related to thyroglobulin. The processes of chain liberation and polymerization are therefore different in cellular and cell-free systems. In the latter case it seems that the newly synthesized chains are freed by hybridization with the preformed thyroglobulin which is present in the cell sap. 3. 3. Thyroid microsomes iodinate thyroglobulin but also other proteins present in the medium. The specificity observed in cellular systems is lost in cell-free systems. Polysomes are devoid of iodinating activity. The iodination enzyme seems to be on the membranes. These results agree with the fact, previously established, that biosynthesis and iodination are accomplished on two different sites in cellular systems; a specific transport mechanism thus allows the thyroglobulin molecule to reach the iodination site.

Dissociation de la thyroglobuline par l'ion tétraphénylborate

Abstract Purified bovine and ovine thyroglobulins (19 S) are partially dissociated into 12-S subunits after treatment with sodium tetraphenyl borate. The extent of dissociation obtained by sodium tetraphenyl borate or sodium dodecyl sulfate treatment is the same. The electrophoretic mobilities on acrylamide gels of sodium tetraphenyl borate-resistant molecules and of native thyroglobulin are identical. Sodium dodecyl sulfate-resistant molecules move more slowly than the native protein.