Noncovalent interactions of a 26,000-dalton peptide with 19 S human thyroglobulin.

Extensive succinylation of 19 S normal human thyroglobulin having a high iodine content results in the formation of a 26,000-Da peptide. One-half mole of the peptide is obtained from 1 mol of the high molecular weight glycoprotein. The dissociation of the peptide is accompanied by the appearance of an intense absorption band which has a maximum at 264 nm. The absorption band is associated exclusively with the 26,000-Da peptide. The amino acid composition of the peptide differs from 19 S thyroglobulin by having no cysteine and higher contents of serine, alanine, tyrosine, phenylalanine, lysine, glycine, isoleucine, and histidine. The peptide also has a high thyroxine content. There were no detectable carbohydrates in the peptide. The fluorescence spectrum of the 26,000-Da peptide shows an emission maximum at 405 nm which we have recently assigned to iodotyrosine-iodotyrosine interactions (Shifrin, S., Consiglio, E., and Kohn, L. D. (1983) J. Biol. Chem. 258, 3780-3786). A 26,000-Da peptide with the same physicochemical properties is found in extracts of normal human thyroid glands.