Deletion sequences of salmon calcitonin that retain the essential biological and conformational features of the intact molecule
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Salmon calcitonin has an amino acid sequences that would allow it to form an amphipathic helix from approximately residue 9 to residue 22. We have synthesized a number of analogues of this peptide hormone with deletions in the carboxyl terminus of this putative amphipathic helix. These analogues include deletions of single amino acid residues at positions 19, 20, 21, or 22 as well as deletions of progressively larger segments starting with residue 19 and including deletions of residues 19 and 20; 19, 20, and 21; or 19, 20, 21, and 22. There is a small decrease in the helical content of these analogues compared with the native hormone, both in the presence and absence of amphiphiles. However, the extent of formation of secondary structure, as measured by circular dichroism, is similar for these deletion sequences as it is for the native hormone. In all cases, there is a large increase in the helical content of the peptide in the presence of dimyristoylphosphatidylglycerol, lysolecithin, or sodium dodecyl sulfate. All of the analogues have hypocalcemic activity in vivo in rats, comparable to the native hormone, except for des-Leu19-salmon calcitonin, which is about twice as active as the unmodified hormone. With use of an in vitro assay of adenylate cyclase activation in purified rat kidney membranes, des-Tyr22-salmon calcitonin, des-Leu19,Gln20,Thr21-salmon calcitonin, and des-Leu19Gln20,Thr21,Thr22-salmon calcitonin exhibited about one-tenth the stimulatory activity of the native hormone. Des-Tyr22-sCT and des-Leu19,Gln20,Thr21,Tyr22-sCT were also tested for their activity in inhibiting prolactin release from isolated rat pituitary cells. Both of these analogues exhibited inhibitory activity. Thus, the region of residues 19-22 does not greatly affect either the conformational or the biological properties of salmon calcitonin.
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