The time course of digestion of porcine thyroglobulin with subtilopeptidase at an enzyme to substrate ratio of 1:200 was analyzed by sodium dodecylsulfate/polyacrylamide gel electrophoresis after reduction with dithiothreitol. Two fragments of Mr 155000and 135000 were formed which showed a precursor‐product relationship, the 135000‐Mr species accumulating between 1 and 4 h of digestion. After purification by elution from electrophoretic gels, these fragments were treeated by CNBr and analyzed by dodecylsulfat/polyacrylamide gel electrophoresis. Both mixtures of CNBr peptides showed a closely similar electrophoretic band pattern. These results suggest the presence in the thyroglobulin chain (Mr about 140000 associated throught the continuity of the chain to a less structured fragment of Mr about 20000. A 15000‐Mr peptide was purified by Sephadex G‐200 gel filtration from CNBr‐treated thyroglobulin. Its uniqueness was demonstrated by N‐terminal sequence analysis and by tryptic fingerprinting. Using densitometer recording of stained electrophoretic gels in sodium dodecylsulfate, the molar ratio of this peptide was estimated to be 2 mol/mol of 12‐S thyroglobulin chain. These results are consistent with a protein structure in which two domains of similar size and structure are linked to form a single chain. Thus, the possibility exists that the structural gene for thyroglobulin might have evolved from duplication and fusion of an half‐sized ancestral gene.

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