Thyroglobulin and related iodoproteins in the thyroid gland.

摘要

The object of this investigation was to study the metabolism of the thyroidal iodoproteins during goitrogonesis in the rat and to examine the physico-chemical properties of the iodoproteins in established goitrous human glands. Goitrogenesis was induced in rats, following 'equilibrium labelling' of thyroidal iodine, by daily administration of methyl-thiouracil (MTU). After a latent period of 5 days there was a rapid increase in gland weight to 3 times the control value after 10 days on the MTU regime. Throughout goitrogenesis, the total [125]I content of the gland decreased logarithmically but the [125]I was removed at a faster rate from the 125,000 g supernatant fraction than from the whole gland homogenate. These data suggest that the rate of colloid resorption is greater than the rate of proteolysis or the rate of acquisition of the hydrolytic enzymes by the colloid droplets. In the study of the metabolism of the soluble iodoproteins, the 125,000 g supernatants were fractionated by DEAE cellulose chromatography, sucrose gradient ultracentrifugation and polyacrylamide gel electrophoresis. Analysis of the decrease in [125]I content of the iodoprotein fractions, separated by these techniques, showed that the newly iodinated iodoprotein molecules are metabolised at a faster rate than the older more highly iodinated molecules and demonstrated that the 27S iodoprotein is metabolised at a slower rate than the 19S thyroglobulin. In another study, thyroidal protein synthesis during goitrogenesis was examined by 3H-leucine incorporation into thyroidal proteins 'in vivo'. A pronounced stimulation of labelled aminoacid incorporation occurred between the 3rd and 6th days of goitrogenesis and the 3H-leucine was incorporated into a protein with sedimentation constant lees than 19S. After 8 days, however, label could also be detected in a heavier protein component corresponding to 24 - 25S. It is suggested that this represents a precursor of the 27S iodo-protein which accumulates when iodide organification is inhibited by MTU and provides additional, evidence that iodination is not a prerequisite for subunit aggregation. In studying the physico-chemical properties of the iodoproteins in established goitrous human glands, the iodoproteins were purified by gel filtration from a number of goitres removed at operation from patients. In general, iodoproteins from non-toxic goitres contained considerably less iodine per mg protein than normal although they resembled thyroglobulin from normal tissue in carbohydrate content, electrophoretic mobility and sedimentation properties. An exception, however, occurred in the iodoproteins from cases of Hashimoto's thyroiditis where the hexose levels were elevated and the iodoprotein had a sedimentation constant of 20 - 21S. Estimation of the degree of heterogeneity of the iodoproteins from goitrous glands by DEAE cellulose revealed that in non-toxic goitres the iodoproteins compared with those from the normal gland in heterogeneity while in all other cases studies the iodoproteins were, in general, considerably more homogeneous. Peptide finger-prints of the S.carbosymethylated iodoproteins (SCM) did not reveal any gross dissimilarities in the primary structures of the proteins although a variability in the presence of certain groups of peptides was found between the native and SCM proteins. In some cases, these variations could be related to the absence of a 27S iodoprotein from these glands and a heterogeneity of the tyrosyl residues in the protein with regard to the extent of iodination. In general, in physical properties studied, the iodoproteins from goitrous glands were indistinguishable from normal thyroglobulin and the dissimilarities which occurred in the chemical properties could, for the most part, be related to the iodination level of the protein. In two goitrous glands studied, abnormalities were, however, detected in the iodoproteins isolated. In one case, in addition to a normal thyroglobulin component, an iodinated albumin and pre-albumin were isolated. However, a structural similarity between these iodoproteins and normal thyroglobulin was revealed by peptide fingerprinting and it is suggested that they represent iodinated subunits of thyroglobulin. In the other case, a high molecular weight iodoprotein was isolated which, although related to thyroglobulin, had only half the normal carbohydrate content and readily dissociated, under the conditions of isolation, to smaller subunits. Furthermore, an iodoalbumin was also isolated from this gland which produced a peptide fingerprint significantly different from normal thyroglobulin. It is suggested that in this gland an abnormal subunit is synthesised which prevents normal aggregation and stabilisation mechanisms taking place.