Abstract

Native bovine neurophysin II has one binding site per monomer for lysine-vasopressin at physiological pH (pH 7.38), as revealed by equilibrium dialysis. Oxytocin is bound to neurophysin II with the same or slightly greater affinity than vasopressin, but binding of the two hormones is competitive. The affinities of vasopressin and oxytocin for neurophysins I and II are essentially the same. Substitution of the asparagine (position 5) and glycinamide (position 9) residues in oxytocin by valine and glycine, respectively, has no substantial effect on binding of the resulting analogues to neurophysin; substitution of the glutamine residue by ornithine in position 4 of oxytocin slightly increases the affinity of the analogue for neurophysin II. The results are in agreement with the contention that the specific association of neurophysin I with oxytocin and of neurophysin II with vasopressin in neurosecretory granules is a result of compartmentalization during the biosynthesis of the neurohypophysial hormones and neurophysin proteins. The relatively low binding constants of neurohypophysial hormones to neurophysin proteins at physiological pH assure complete dissociation of the hormone-protein complexes after their release into the blood. Our observations also show that the specificity of the hormone-binding site of neurophysin proteins for the neurohypophysial hormones differs from that of the hormone receptors.