Binding studies of polypeptide hormones to bovine neurophysins

Experimental binding isotherms of [9 glycinamide 1 ¹⁴C]oxytocin and [9 glycinamide 1 ¹⁴C]arginine vasopressin to purified neurophysins I and II at pH = 4.4, 5.4, 6.5, 7.4, and 8.5 and 6°, 22°, and 37° in aqueous buffers are reported. For purposes of comparison, binding isotherms for [4 glycine 1 ¹⁴C]oxytocin to neurophysin II and I in aqueous buffer, and [9 glycinamide 1 ¹⁴C]oxytocin to neurophysin II in dimethylsulfoxide under selected conditions are also reported. A brief discussion of the interpretation of binding isotherms is entered into and apparent binding constants are derived. The results indicate that the interpretations presented in the literature up to now are much too simple. There are, in contrast, multiple binding sites of oxytocin and vasopressin to the neurophysins and large temperature dependences of the number of sites and their binding constants. We find, in fact, that at 37° the binding of neurohypophysial hormones to the supposed storage proteins is rather weak even at the pH of maximum binding.