PROPERTIES OF THYROGLOBULIN .18. ISOLATION OF THYROGLOBULIN SUBUNITS

After mild reduction of bovine thyroglobulin with dithiothreitol at alkaline pH, followed by alkylation, two new, slower sedimenting species are observed by centrifugation. These species have been isolated in purified form by sucrose gradient centrifugation and found to have one-fourth to one-half the molecular weight of the native protein. Cleavage with cyanogen bromide of the isolated subunits, after extensive reduction and alkylation, revealed no differences by disc electrophoresis between these two species and native thyro-globulin. The number of disc electrophoretic bands ob served was close to half the number of methionine res idues present in thyroglobulin and implies that the dif ferent chains of thyroglobulin must exist in identica pairs. The slowest sedimenting component therefore appears to be a mixture of two different subunits with very similar molecular properties. The faster sedimenting component(s) appears to be an unfolded form of the 12S dissociation product of thyroglobulin. © 1969, American Chemical Society. All rights reserved.