CORRELATIONS BETWEEN BIOLOGICAL-ACTIVITIES AND CONFORMATIONAL PROPERTIES FOR HUMAN, SALMON, EEL, PORCINE CALCITONINS AND ELCATONIN ELUCIDATED BY CD SPECTROSCOPY

Calcitonin (CT) inhibits osteoclastic bone resorption and induces calc ium uptake from body fluids. A comparative study of the conformational behaviours of therapeutic calcitonins [salmon (s), eel (e), a synthet ic eel calcitonin analogue (Elcatonin), porcine (p) and human (h) calc itonins] as a function of solvent polarity and temperature have been p erformed by circular dichroism spectroscopy. Elements of secondary str ucture were lacking in H2O but could be observed in 2,2,2-trifluoroeth anol and sodium dodecyl sulphate. In particular, similar amounts of al pha-helical content (four alpha-helical turns) were estimated in trifl uoroethanol despite the considerable differences in amino acid sequenc es. The relative ability to form an alpha helix, assessed by trifluoro ethanol/H2O titration, was found to be Elcatonin > sCT > pCT > eCT > h CT. In Elcatonin, sCT, pCT and eCT the four alpha-helical turns were p romoted almost completely in a single step, between 0 and 35% trifluor oethanol, unlike hCT where helical structure formation has been report ed to involve two steps over the whole trifluoroethanol/H2O range [Arv inte, T and Drake, A. E (1993) J. Biol. Chem. 268, 6408-6414]. In SDS, which mimics the membrane environment, conformational differences (3- 4 helical turns in Elcatonin, sCT, eCT versus one helical turn in pCT, hCT) were observed and correlate well with biological activity (Elcat onin = sCT = eCT > pCT = hCT). Low-temperature studies in a cryogenic solvent mixture showed the formation of high alpha-helix content (simi lar to that in trifluoroethanol) in Elcatonin, sCT, eCT and pCT, whils t a left-handed extended helix (3(1) helix) was formed in hCT. This is consistent with the hypothesis of 'linear' and 'helical' calcitonin r eceptors [Nakanuta, H., Orlowski, R. C. and Epand, R. M. (1990) Endocr inology 127, 163-169].