Sialic acid in thyroglobulin. II. Influence of sialic acid on ionization of tyrosine residues.

During electrophoresis in polyacrylamide gel at pH 9-5, it was noticed that sialic acid-poor thyroglobulin contains fewer low molecular weight fragments than sialic acid-rich thyroglobulin. Splitting off of sialic acid diminishes thenumber of phenolic groups of tyrosine, MIT, DIT and T4, which dissociate below pH 12-5. This phenomenon was interpreted as a result of lowered hydrophilia of these regions of the thyroglobulin molecule which contains tyrosine and its iodinated derivatives.