Jacques Nunez

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.

Protéines iodées particulaires thyroïdiennes. I.

Abstract 1. 1. Particles of sheep thyroid slices incubated in vitro with 125 I contain several labeled iodoproteins extractable without denaturation by digitonin and deoxycholate. 2. 2. The digitonin solubilzees principally an iodoprotein with a sedimentation coefficient of 19 S, which behaves as thyroglobulin under different analytical conditions. The formation of 3–8-S, 13-S, 27-S and 32-S compounds is also observed. The lighter proteins predominate at short incubation times. 3. 3. The proteins extracted from the particles of rat thyroid labeled in vitro contain the same constituents, though the 19-S and 3–12-S iodoproteins predominate. 4. 4. Double-labeling experiments with 131 I and 125 I, with an intermediate chase by 127 I at various times, lead to the observation that the particle iodoproteins are younger than the thyroglobulin. The heavier the particulate iodoproteins and the longer the chase time, the more these proteins are labeled with the first isotope ( 131 I). 5. 5. The evolution of the isotope ratio obtained by double labeling and the correlation with the composition of the particle and soluble fraction iodoproteins deserve special emphasis. The results suggest: (1) that the lighter particulate iodoproteins are precursors of the heavier, (2) that particulate iodoproteins are precursors of thyroglobulin.

Nature des produits de la désiodation des hormones thyroidiennes marquées simultanément par le tritrium et l'iode radioactif

Abstract 1. 1. The deiodination of thyroid hormones has been studied with slices and in homogenates of rat liver, muscle and kidney, as substrates using 3,5,3′-triiodothyronine and thyroxine labeled either with tritium or with both tritium and radioactive iodine. 2. 2. The simultaneous evolution of the deiodination and degradation of the side chain of the hormones result in the formation of several metabolites. However, the formation of a tritiated and iodinated compound with an RF=o (PX) has been established as a general phenomenon in all the solvents used. At the same time small quantities of tritiated degradation products of the hormones have been identified as iodothyroacetic acid and presumably tyrosine and thyronine. 3. 3. The kinetics of the formation of the compound PX indicate that the latter is an intermediate in the deiodination of 3,5,3′-triiodothyronine and thyroxine. When isolated from a medium containing 3,5,3′-triiodo[3H]thyroxine, the compound PX appears to be associated with a protein. After enzymic hydrolysis, PX gives rise to tritium-labeled monoiodotyrosine, diiodotyrosine and tyrosine which suggests that their hormonal precursor might be bound to the tritium-labeled protein (PX). The formation of PX thus appears to tbe a normal step on the deiodination pathway of thyroid hormones.

Regulation of lipogenesis in adipocytes independent effects of thyroid hormones, cyclic AMP and insulin on the uptake of deoxy-d-glucose

Thyroidectomy is known to enhance fat cell phosphodiesterase activity; as a result, the response to lipolytic hormones is markedly reduced. Thyroidectomy also stimulates overall lipogenesis and the uptake of glucose: the present experiments investigated whether there was a correlation between cyclic AMP and glucose uptake. The parameter measured was the transport and phosphorylation (uptake) of deoxy-D-glucose in the presence of two modifiers of the cyclic AMP pool: phosphodiesterase inhibitors and the analogue, dibutyryl cyclic AMP. The inhibition by methylxanthines and dibutyryl cyclic AMP of deoxy-D-glucose uptake observed, was the same in fat cells from normal and thyroidectomized rats: the latter nonetheless still maintained their enhanced glucose uptake. It was therefore concluded that thyroid hormones and cyclic AMP control this step by different, separate pathways. Insulin, well known for its lipogenic effect, enhanced deoxy-D-glucose uptake in fat cells from both normal and thyroidectomized rats to the same extent (about 40%). An additive effect of thyroidectomy and insulin on glucose uptake was thus demonstrated. These results imply that glucose uptake in the adipocyte is controlled by at least three factors: thyroid hormones, cyclic AMP and insulin, each of which can act independently. Maximum glucose uptake is achieved in the presence of a combination of low concentrations of cyclic AMP, of insulin, and in the absence of thyroid hormones.

Conditions de formation d'une combinaison rotéique iodée et tritiée au cours de la désiodation des hormones thyroidiennes doublement marquées (131I et 3H)

Abstract 1. 1. Thyroid hormones doubly labeled in α-β position with 3H and in 3′,5′ or 3′ with 131I ([131I, 3H] dl -thyroxine and [131,3H]3,5,3′-triiodo- dl -thyronine) can be deiodinated by mitochondria and several acellular preparations: salt extracts of slices and mitochondria, purified preparations of thyroxine deiodinase. The only labelled compounds produced under the adopted experimental conditions are 131I- and a protein complex, PX, which contains both 3H and 131I. No trittium-labelled thyronine or tyrosine is formed. Since no 3H loss occurs during the incubation there is no exchange between the tritium atoms of the iodothyronine and the hydrogen atoms of the medium, neither do any metabolic modifications occur in the alanine side chain during the process. 2. 2. The ratio of the specific activities of the two isotopes in the chromatographic zones where residual dl -thyroxine or 3,5,3′-triiodo- dl -thyronine are located is identical to the blank after incubation with thyroxine deiodinase. However, this is not so in the other cases where a disagreement is observed in the rate of deiodination calculated from the measurements of 131I and 3H. PX does always, at the end of the reaction, contain a fraction of the 131I and the total 3H provided for by the disappeared iodothyronine. The 131I concentration of PX is not constant during the incubation period, but decreases when considered as a total and even in relationship to the concentration of 3H. The 131I deiodination of PX takes place in two steps which seems to indicate that the 131I is present in the protein in two different forms. 3. 3. All the used preparations lead to the same reaction products. Also, it seems that the mechanism of the deiodination is the same in the different cases. The deiodination of the thyroid hormones proceeds in two steps: (a) the fixation of the hormone to the protein, probably the enzyme, (b) the iodine is partially liberated from the complex which retains completely the organic skeleton of the iodothyronine. The formation of PX thus appears as a necessary step in the deiodination of the thyroid hormones.

Sur le métabolisme de la l-[3H]thyronine par les tissus du rat

Abstract Thyronine, one of the metabolites resulting from the diodination of thyroxine and 3,5,3′-triidothyronine, has been incubated with kidney and liver slices. The products of its metabolism are: tyrosine; 3′-hydroxythyronine and 3,4-dihydroxyphenyl-alanine; thyroacetic and p -hydroxyphenylacetic acids. These facts correspond to three mechanisms: the rupture of the diphenylether bridge, the o -hydrocylation of the rings and the degradation of the allanine chain via the general pathway for amino acids.