Splitting of human thyroglobulin. III. Comparison of fragments obtained during enzymatic digestion and by reduction and alkylation

Purified thyroglobulin was digested with trypsin and papain and was also reduced with dithiothreitol and alkylated with iodoacetamide. The resulting fragments were separated and characterized by immunological techniques. Following enzymatic degradation a small fragment, termed fraction 2, was isolated. It had a sedimentation coefficient of approximately 3S and a mol. wt determined by SDS polyacrylamide gel electrophoresis of approximately 37,000. In the Ouchterlony test, it was antigenically deficient as compared with intact thyroglobulin, when rabbit antisera to human thyroglobulin were used. With human autoantisera, fraction 2 did not show any precipitin reaction in the Ouchterlony test. However, it produced weak inhibition in the tanned cell haemagglutination test. The fractions obtained from trypsin and papain digestion appeared to be immunologically identical. However, when these fractions were compared with the similar product obtained from reduced and alkylated thyroglobulin, the latter fraction showed a reaction of non identity in the Ouchterlony test, and had a weaker inhibiting capacity in tanned cell haemagglutination test. The fraction produced by reduction and alkylation had a sedimentation coefficient of 8S and an approximate mol. wt of 38,500. It can be concluded that the small mol. wt fractions derived from enzymatic breakdown and by reduction and alkylation are different although both possess a number of antigenic determinants recognized by the rabbit antiserum and lack most of the autoantigenic determinants.