The Interaction of Lysine Vasopressin with Free and Agarose‐Bound Bovine Neurophysin II

The interactions of tritiated lysine vasopressin with free and agarose‐coupled bovine neurophysin II were studied. Neurophysin II obtained by extraction of acetone‐dried hypophyses with strong acid contains at least two discrete populations of binding sites for lysine vasopressin, with association constants of 106 to 107 M−1 and 3×104 mM−1, respectively. The first population has a Hill coefficient higher than unity and accounts for 7–30% of the total binding sites in different batches of agarose‐bound neurophysin. With free neurophysin in solution, the size of this population was strongly dependent on the neurophysin concentration and decreased from about 17% at 0.05 mg/ml to less than 0.05% at 1.4 mg/ml. A somewhat lower Hill coefficient was found for the second class of binding sites. It is suggested that these two classes of binding sites are associated with polymerization equilibria of neurophysin in solution, the high‐affinity species being the monomeric, the low affinity species a polymeric form. For the agarose‐bound neurophysin, the pH optimum for binding was found to be 5.6–5.9 (vasopressin concentrations in the range of the first association constant). This binding was not affected by calcium chloride up to a concentration of 10 mM.