Site-dependent effect of O-glycosylation on the conformation and biological activity of calcitonin

We synthesized seven O-glycosylated calcitonin derivatives, each with a sin gle GalNAc residue attached to either Ser or Thr, and studied their three-d imensional structure and biological activity to examine site-dependent effe cts of O-glycosylation. The CD spectra in an aqueous trifluoroethanol solut ion showed that the GalNAc attachment at Thr6 or Thr21 reduced the helical content of calcitonin, indicating that the O-glycosylated residue functions as a stronger helix breaker than the original amino acid residue. Only the GalNAc attachment at Ser2 or Thr21 retained the hypocalcemic activity of c alcitonin. This result corresponded well to that of the calcitonin-receptor binding assay. The GalNAc attachment other than Ser2 or Thr21 perturbed th e interaction with the receptor, resulting in the loss of the hypocalcemic activity. The biodistribution did not change much among the seven derivativ es, but some site dependency could also be observed. Thus, we can conclude that the O-glycosylation affects both the conformation and biological activ ity in a site-dependent manner.