Isolation and Chemical Properties of Porcine Thyrocalcitonin

Thyrocalcitonin has been isolated from porcine thyroid tissue following a 40,000-fold purification. Specific biological activity is 200 MRC units per mg. The purified peptide was shown to be homogenous by multiple criteria and to consist of 32 amino acids: Arg2, His1, Thr2, Ser4, Glu1, Pro2, Gly3, Ala1, Val1, Met1, Asn4, Leu3, Phe3, Tyr1, Trp1, half-Cys2. The purified hormone is resolved by chromatography on carboxymethyl cellulose or cellulose thin layer plates into two components. The only difference between these components is that one form of the hormone contains methionine as the sulfoxide; the other component, methionine in the reduced form. Both forms have equal specific activity. The peptide contains no covalently bound carbohydrate, iodine, or other substituted amino acids. The 2 half-cystine residues are present in the hormone as an intrachain disulfide bond. Since 1 half-cystine residue is the NH2-terminal amino acid, the amino-terminal portion of the molecule is in the form of a 23-membered ring. The lack of a high degree of ordered structure in the molecule is reflected in the complete solvent accessibility of the methionine, tryptophan, and tyrosine residues without prior denaturation of the polypeptide.