INTERACTION OF OXYTOCIN WITH CA2+ .1. 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 cation s. 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 trifluoroethanlo, a lipid-mimetic solven t, using CD and florescence spectroscopy. Binding Ca2+ (K-d similar to 50 mu M) caused drastic CD and fluorescence changes leading to a heli cal conformation. Mg2+ caused CD changes smaller than and opposite to Ca2+. However, the helical conformation was enhanced when both Ca2+ an d Mg2+ were present together. CD changes in the tail peptide of oxytoc in showed its ability to bind Ca2+ and Mg2+ whereas the vasopressin ta il 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 segm ent of oxytocin potentiates Ca2+ binding by the ring. While vasopressi n displayed a CD spectrum similar to that of oxytocin, CD spectra of i ts cation-bound forms were markedly different from those of oxytocin; the Ca2+ induced CD changes in vasopressin were very much smaller and of opposite sign, and Mg2+ -induced ones significantly larger than in oxytocin. Taken together, our observations bring out the structural di fferences between oxytocin and vasopressin in the context of their int eraction with Ca2+ and Mg2+. This may be relevant to understanding the differences in the bioactive conformations and receptor inter-actions of the two hormones. (C) 1997 John Wiley & Sons, Inc.