Evidence for Calcitonin Receptor Heterogeneity: Binding Studies with Nonhelical Analogs*

Abstract

Binding of the nonhelical salmon calcitonin (sCT) analog, [Gly8,Ala16]-des-Leu19-sCT to membrane preparations from rat brain could be analyzed in terms of two independent binding sites. The high and low affinity binding sites for this analog were named CT-L (L, linear) and CT-H (H, helix), respectively. Although the CT-H type receptor has a low affinity for the nonhelical analogs, it binds the helical sCT with high affinity and therefore represents a CT binding site. The physiological significance for the existence of subtypes of specific CT receptors is not clear. The [Gly8,Ala16]-des-Leu19-sCT suppressed the osteoclastic bone resorption in tissue culture at low concentration (0.1 nM). The dose of [Gly8,Ala16]-des-Leu19-sCT required for this hypocalcemic activity was highly correlated with the binding affinity of this analog to the CT-L receptor subtype. In addition, human CT interacted with the CT-L type receptor at about 100th the concentration of that required for the displacement of sCT. We conclude that binding to the CT-L type receptor is required for hypocalcemic activity in mammals.

Correlations between biological activities and conformational properties for human, salmon, eel, porcine calcitonins and Elcatonin elucidated by CD spectroscopy

“…This is consistent with the hypothesis of 'linear' and 'helical' calcitonin receutors INakanuta, H., Orlowski, R. C. & Epand, R. M. (1990) Endocrinology 127, 163-1691 Human, salmon, eel and porcine calcitonins are peptide hormones of 32 amino acid residues with an N-terminal disulphide bridge (between residues 1 and 7) and a C-terminal proline amide residue (Azria, 1988). Elcatonin (ECT) is a synthetic analogue of eel calcitonin that differs from the natural peptide hormone by replacement of the disulphide bridge with an ethylene bridge and deletion of the N-terminal amino group (Morikawa et al, 1976).…”

“…The proposal has been made by Nakamuta et al (1990) that an extended conformation of the thyroidal calcitonins may be important in explaining biological activity. In terms of recognised secondary structure this can only mean a /Isheet-like or 3, -helix-like conformation.…”

“…Prompted by brain membrane binding studies, Nakamuta et al (1990) have proposed the existence of two distinct calcitonin receptors that can discriminate different conformational features of CT molecules. In particular, it is suggested that a non-a-helical structure recognizes and binds selectively to the linear receptors whilst an a-helical structure binds specifically to the helical receptors.…”

Structural Determinants of Salmon Calcitonin Bioactivity

“…Kaiser and coworkers (13,14) provided evidence that the model of an amphipathic helix in the region 8 -22 is a useful guide to designing potent sCT activity, although the primacy of conforma-tional flexibility over amphipathicity has been invoked as an important parameter for activity (15). These apparently conflicting results could be explained by the existence of subtypes of CT receptor (CTR) showing variable affinity for helical and non-helical peptides (1,16). Alternative RNA splicing yields multiple CTR mRNA isoforms.…”

“…The ␣-helical central region of CT peptides has been reported to interact directly with the receptor N terminus (20), and a short segment of the hCTRa close to the transmembrane domain 1 is proximal to amino acid 19 of helix (21). Furthermore, a strict relationship between the binding receptor affinity and the helicity of the hormone has been observed, with a correlation between amphipathicity and potency (16,22).…”