Immunochemical studies on human calcitonin M leading to information on the shape of the molecule.

1. Two antisera were obtained from a single rabbit. Both are highly specific for human calcitonin M but react with different parts of the amino acid sequence. 2. The different sequences that react with the antibodies of the two antisera were located. The first antiserum reacts at two sites in the molecule, one in the sequence residues 11-18, probably with residue 17 as the immunodominant group, and another on either side of the 28-29 peptide bond. The second antiserum, harvested 9 months later, reacts principally at one site bridging the 28-29 peptide bond. 3. A consideration of the properties of the hormone's binding sites and of data relating biological activity to structure enables some conclusions to be drawn with regard to the shape of the molecule. It appears that the peptide chain is folded to bring N- and C-termini closer together and that there is non-covalent interaction between regions in the chain near both termini. One of these is located near residue 8.