Hormonal Interactions at the Molecular Level

Abstract

Bovine neurophysins I and II have been obtained in a highly purified state using isoelectric focusing. The interactions of oxytocin and [8‐lysine]vasopressin with these proteins have been investigated by thin‐film equilibrium dialysis using highly radioactive hormones retaining their full biological activities. Binding of both oxytocin and [8‐lysine]vasopressin to neurophysins I and II as a function of pH leads to typical “bell‐shaped” curves. The patterns obtained indicate functional differences in the binding of oxytocin to neurophysin I compared with neurophysin II. No significant change was detected in the case of [8‐lysine]vasopressin. Scatchard plots indicate that both hormones bind neurophysins I and II with similar affinities. They demonstrate that at pH 5.7 one oxytocin molecule and two [8‐lysine]vasopressin molecules can be bound per neurophysin I or II monomer. Ca 2+ and Mg 2+ had no effect on the binding process of either [8‐lysine]vasopressin or oxytocin. Iodination on the tyrosyl ring of [8‐lysine]vasopressin and oxytocin resulted in the suppression of their binding ability. Competition experiments between oxytocin and vasopressin show that each compound inhibits the binding of the other peptide in a non‐competitive fashion.