The effects of iodination on the polypeptide heterogeneity of thyroglobulin.

19 S thyroglobulins from several mammals were prepared under conditions designed to minimize endogenous proteolytic activities. The purified iodoproteins were then fractionated by RbCl isopycnic gradient centrifugation and fractions of varying iodine content analyzed by SDS-polyacrylamide gel electrophoresis. The dependence on the iodination level of molecular dissociability, previously reported in preparations "markedly' differing in their iodination degree, has been now ultimately assessed in all species investigated and demonstrated even for fraction differing from each other only by a few iodine atoms. Upon reduction fractions across the isopycnic gradients showed the occurrence of discrete electrophoretic bands faster than the 300,000 Da subunit: the relative amount of these peptides has been found to be linearly related to the iodine content of the molecules analyzed. This behaviour has been observed in all animal species investigated. In conclusion, our data suggest that iodination, beside the well-documented effects on thyroglobulin conformational properties, may also affect its "apparent' polypeptide chain composition. The origin of such heterogeneity is discussed.