Human thyroglobulin: studies on the native and S-carboxymethylated protein.

Abstract

Abstract 1. 1. The properties of human thyroglobulin prepared from both normal thyroid glands and from subjects with non-toxic colloid goiters have been studied. No significant differences in composition were found with respect to the non-iodinated amino acids, the total amount of carbohydrate, and the kinds of sugar residues found nor were differences observed in electrophoretic and centrifugal properties. The amino acid compositions are also compared to those of sheep and of hog thyroglobulin. 2. 2. The amino acid analyses showed the presence of approximately 180 half-cystine residues per molecular weight of 660 000. Within experimental error all the half-cystine residues were found to be in disulfide linkage as shown by amino acid analyses of preprations which had been treated at pH 8.6 in 8 M urea with sodium iodoacetate with and without prior reduction by mercaptoethanol. This conclusion was supported by results of amperometric titrations. 3. 3. Evidence concerning subunit structure has been obtained by physical studies of the S-carboxymethyl derivative of reduced thyroglobulin. In the presence of sodium dodecyl sulfate at pH 9.6 this derivative undergoes dissociation and sediments as a single component having an s° 20, w of 3.2–3.4 S. The molecular weight of the dissociated species was estimated to be in the range of 110 000–125 000, thus indicating at least 5 or 6 physically identical subunits or peptide chains in the native thyroglobulin molecule.