Studies on thyroid proteins. VII. Immunochemical properties of some enzymatic fragments of rabbit thyroglobulin.

Rabbit thyroglobulin was enzymatically digested with trypsin, and fragments were isolated through gel filtration. A 10-hour enzyme digest was studied in detail. Upon examination in a synthetic-boundary cell in the ultracentrifuge it revealed a predominantly slow-sedimenting component with a 1 S value. The various fragments of this digest were tested against three different antisera, i.e. heteroantisera to whole thyroglobulin, and to fragments of thyroglobulin, and an isoantiserum, containing autoantibodies, to whole thyroglobulin. The thyroglobulin fragments reacted strongly with the heteroantisera, especially with the antiserum to thyroglobulin fragments, but the same fragments reacted very poorly with the autoantibodies. The former antiserum revealed six lines of precipitation whereas the latter antiserum showed only two lines. These observations supported the hypothesis that a lesser number of antigenic determinants on the thyroglobulin molecule were active against autoantibodies as compared to the heteroantibodies.