Effects of temperature and urea concentration on the chemical modification and ionization behavior of tyrosyl and iodoamino acid residues of porcine thyroglobulin.

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Abstract
Acetylation of porcine thyroglobulin with N-acetyl imidazole was performed at various temperatures in the presence and absence of urea. In the medium without urea, the number of O-acetylated iodoamino acid residues formed was constantly 15 above 10.degree. C (originating from all diiodotyrosyl and a few of monoiodotyrosyl residues), while the number of O-acetyl tyrosyl residues increased gradually from 15 at 0.degree. C to a maximum of 70 at 70.degree. C. All (about 25) iodoamino acid residues were acetylated at 20.degree. C in 2 M urea, while the presence of 8 M urea was necessary to acetylate all tyrosyl residues at 20.degree. C. Experiments on the nitration of thyroglobulin with tetranitromethane showed that diiodotyrosyl and thyroxyl residues were easily nitrated even at low temperatures and the low ratio of the reagent to the protein, while tyrosyl residues consisted of 3 groups: the 1st group (about 14%) which was very easily nitrated even at 0.degree. C and the low ratio of the reagent to the protein, the 2nd group (about 45%) which was nitrated depending on temperature and the molar ratio of the reagent, the remaining which was definitely unreactive even under extreme conditions. The ionization of tyrosyl and iodoamino acid residues was studied by spectrophotometric titration at various temperatures in the presence and absence of 8 M urea. Diiodotyrosyl and thyroxyl residues did not change very much with respect to ionization behavior by the addition of urea, in contrast to clear dependency of tyrosyl residues on urea. Based on these observations, the location of these residues and the structural change of the protein with temperature are discussed.