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Studies on the protein portion of thyroglobulin. Amino acid compositions and terminal amino acids of several thyroglobulins.

Abstract Amino acid analyses have been performed on the thyroglobulins from calf, sheep, pig, and man. These proteins contain approximately one-fifth of their amino acid residues in the form of dicarboxylic amino acids, almost one-half of these being in the amide form. The basic amino acids account for about 10% of the total amino acids. Other prominent amino acids in the thyroglobulins are serine, leucine, alanine, and glycine, and there is a substantial amount of half-cystine present, representing approximately 250 moles per mole of protein. The amino acids, which number close to 5500 per molecule, account for 87 to 91% of the weight of the proteins, the remainder being made up of 8 to 10% of carbohydrate (approximately 300 monosac-charide residues per molecule) and iodine. The terminal amino acids of the four thyroglobulins have been investigated. Amino-terminal amino acid analysis by the fluorodinitrobenzene method has indicated the presence of 2 residues of dinitrophenyl-aspartic acid per molecule. A third terminal position appears to be occupied by leucine in the sheep and calf proteins, by glycine in the pig, and by serine in the human. Each protein, in addition, has partial residues of several dinitrophenyl-amino acids which account for another mole of terminal residues per mole of protein. These findings would be consistent with the presence of four peptide chains in the thyroglobulin molecule. Carboxyl-terminal amino acid analyses, performed by hydrazinolysis, resulted in the release of 2 to 3 moles of amino acids per mole of protein after 8 hours of treatment. These amino acids included serine, glutamic acid, glycine, alanine, and leucine. Longer exposure to hydrazine was not possible because of the considerable hydrolytic cleavage of amino acid hydrazides or of peptide bonds which occurred, as evaluated by hydrazine treatment of the methylated protein. Carboxypeptidase digestion led to the release of a limited number of amino acids, most prominent among them being leucine and serine.