Interaction of oxytocin with Ca2+: I. CD and fluorescence spectral characterization and comparison with vasopressin.

Extracellular Ca2+ is required for the action of oxytocin and both the hormone and its receptor have binding sites for divalent metal cations. To characterize the cation-bound form of oxytocin, we monitored the binding of Ca2+ and Mg2+ to oxytocin as well as peptides representing its ring and tail regions in trifluoroethanol, a lipid-mimetic solvent, using CD and fluorescence spectroscopy. Binding Ca2+ (Kd approximately 50 microM) caused drastic CD and fluorescence changes leading to a helical conformation. Mg2+ caused CD changes smaller than and opposite to Ca2+. However, the helical structure was enhanced when both Ca2+ and Mg2+ were present together. CD changes in the tail peptide of oxytocin showed its ability to bind Ca2+ and Mg2+ whereas the vasopressin tail peptide did not bind either cation. CD spectral changes on Ca2+ and Mg2+ binding to tocinoic acid (the ring moiety of oxytocin) were much smaller than those of oxytocin. These data suggest that the tail segment of oxytocin potentiates Ca2+ binding by the ring. While vasopressin displayed a CD spectrum similar to that of oxytocin, CD spectra of its cation-bound forms were markedly different from those of oxytocin; the Ca(2+)-induced CD changes in vasopressin were very much smaller and of opposite sign, and Mg(2+)-induced ones significantly larger than in oxytocin. Taken together, our observations bring out the structural differences between oxytocin and vasopressin in the context of their interaction with Ca2+ and Mg2+. This may be relevant to understanding the differences in the bioactive conformations and receptor interactions of the two hormones.