Interaction of bovine neurophysin with oxytocin and vasopressin measured by temperature-jump relaxation

Abstract

The interaction between bovine neurophysins I and II and oxytocin and vasopressin was measured using temperature-jump relaxation. A single relaxation time in the 10-90 ms range was noted for each solution. This time depended on the concentration of both neurophysin and hormone, and increased with increasing pH. The formation rate constants (.+-. SE) for the interaction of neurophysin I dimer with the protonated form of oxytocin and vasopressin at pH 7.4 in 0.1 M KNO3 and 25.degree. C were 2.8 (.+-. 0.4) .times. 106 and 2.3 (.+-. 0.5) .times. 106 M-1 s-1, respectively; the Kd were 11 and 15 s-1, respectively. For neurophysin II dimer, formation rate constants were 6.0 (.+-. 0.2) .times. 106 and 2.4 (.+-. 0.3) .times. 106 M-1 s-1; Kd were 24 and 16 s-1 for oxytocin and vasopressin, respectively. Formation rate constants for the interaction of neurophysin monomer with protonated hormone were approximately an order of magnitude lower than those for the dimer. The results are in general agreement with circular dichroism and pH titration studies which indicate that this interaction occurs between a negatively charged carboxyl group on the neurophysin and a protonated .alpha.-amino group on the hormone. Oxytocin and vasopressin apparently bind with greater affinity to the neurophysin dimer than the monomer, and the binding of oxytocin and vasopressin in neurophysin dimer at pH 7.4 and concentrations between 10-4 and 10-5 M is apparently nearly identical for each hormone.