Human calcitonin. Structure of calcitonin M and D

Human calcitonin M and its dimer calcitonin D, two highly active peptides isolated from C cell tumours, were subjected to sequence studies using chemical and enzymatic methods. For calcitonin M, containing 32 amino acid residues, the following structure was derived: Though the disulphide bridge between position 1 and 7, and the C-terminal proline amide of human calcitonin M are the same as in porcine α-thyrocalcitonin, many amino acid residues - 18 in all - are different throughout the molecule. Arginine and tryptophan are absent; on the other hand, lysine and isoleucine are to be found at position 18 and 27 respectively. Methionine changes its place from position 25 to 8 adjacent to the disulphide bridge. Experimental evidence indicates that calcitonin D represents the antiparallel dimeer of calcitonin M.