Polypeptides non-covalently associated in 19-S thyroglobulin

Abstract

The present studies were performed on several preparations of mammalian (sheep, ox, rat, horse, pig) and human normal or goiter 19-S thyroglobulin. Homogeneity of the preparations was controlled by analytical centrifugation and disc electrophoresis. Succinylated or native thyroglobulins dissolved in buffers containing urea, guanidine·HCl or sodium dodecyl sulfate release discrete polypeptides (5–8) non-covalently associated with the remainder of the molecule and whose molecular weights are below 330 000. The fastly migrating components have been partially isolated by filtration on Sephadex G-200 equilibrated in buffers containing urea, guanidine·HCl or sodium dodecyl sulfate and their apparent molecular weights determined by electrophoresis in sodium dodecyl sulfate-acrylamide gels. They represent about 20% by weight of the native protein when gel filtration is performed in the presence of a sodium dodecyl sulfate-containing buffer (pH 8.0) and their molecular weights are between 14 000 and 150 000. The amino acid composition of the mixture of fastly migrating components differs from that of native thyroglobulin by a much lower content in cystine (about 1 3 ) and a higher content in isoleucine, tyrosine, histidine, lysine, thyroxine and 3,5,3′-triiodothyronine. Components of identical or almost identical apparent molecular weights, but in larger amounts, are separated from fully reduced and S-carboxymethylated 19-S thyroglobulins by electrophoresis in sodium dodecyl sulfate-acrylamide gels. It is concluded that native 19-S thyroglobulin contains polypeptides non-covalently associated with the bulk of the protein. This observation is discussed in relation to the subunit structure of 19-S thyroglobulin.