Characterization of the major polypeptide chains of reduced bovine thyroglobulin

The finding that reduced 19S bovine thyroglobulin showed two major, closely migrating polypeptide bands during electrophoresis in SDS-polyacrylamide gels was presented as evidence that thyroglobulin consists of two nonidentical subunits, referred to as S and F, of approximately the same size (J. Biol. Chem. 253, 1853-1858 (1978)). It was, however, not clear whether the difference in migration rates of the two subunits was due to differences in amino acid or carbohydrate composition or a small difference in size. In this study, several physical and chemical properties of purified S and F polypeptides were investigated. Three methods revealed important differences between them. It was found by cyanogen bromide degradation that S contained one fragment (Mr approximately equal to 20 000) more than F. S eluted ahead of F by column chromatography in SDS. The sedimentation coefficient of S was found to be greater than that of F. However, the two subunits were shown to have similar amino acid and carbohydrate compositions and to be indistinguishable in their interaction with SDS. The different migration rates of S and F can therefore be explained by a small difference in size which agrees with the finding (Endocrinology 108, 1285-1292 (1981)) that a proteolytic enzyme present in commercial horseradish peroxidase can convert S into F.