Interactions of bovine neurophysin II with oxytocin and [8 lysine] vasopressin. High resolution proton nuclear magnetic resonance spectroscopy studies

Interactions of bovine neurophysin II with oxytocin and [8 lysine]vasopressin were studied using proton Fourier transform nuclear magnetic resonance spectroscopy. The nuclear spin relaxation times, T₁ and T₂, of tyrosyl aromatic protons of oxytocin and of tyrosyl and phenylalanyl aromatic protons of [8 lysine]vasopressin were measured in the presence of varying amounts of neurophysin II. In each case, the relaxation time T₂ was markedly decreased in the presence of neurophysin II while the relaxation time T₁ was unaffected. The shortened transverse relaxation times for the protons of the bound hormones were independent of nuclear magnetic resonance frequency and were found to be dependent upon temperature in a manner that demonstrates that both hormones exchange rapidly between free solution and the hormone neurophysin II complex. The existence of fast exchange conditions permits the calculation of a rotational correlation time for the oxytocin neurophysin II complex of 5 x 10^-8 s. These results indicate that the motional freedom of residues 2 and 3 of the hormone molecules is markedly restricted upon complex formation with neurophysin II, thereby suggesting that residues 2 and 3 of oxytocin and [8 lysine]vasopressin are directly involved in the binding with neurophysin II.