Cooperative binding of ocytocin to bovine neurophysin. A comparison of two theoretical models.

The in vitro formation of specific complexes between the 10 000 dalton brain protein, neurophysin, and the neurohypophyseal peptide hormones (ocytocin and vasopressin), constitutes an interesting system for the understanding at the molecular level of both the mechanism by which a peptide can bind a protein molecule and of the biological function of this class of proteins [ 1-12 1. It is now well established that the bovine neurophysins I [8] and II [2,8] tend to self-associate in aqueous solution. From sedimentation equilibrium studies at pH 5 60, 10.1, values of apparent weight average molecular weight as a function of total protein concentration (in the range 0.05-4 mg/ml) were shown to be consistent with a model specifying that protein solutions comprised reversible equilibrium mixtures of monomer and dimer governed by a single equilibrium association constant X0 = 5.8 X 10’ M-r [8]. The addition of peptide hormones imposes a constraint on this equilibrium toward the dimeric form of the protein [8]. The Scatchard analysis of the binding isotherms indicated a number of,features: (i) The apparent existence of 1 ocytocin site/IO 000 daltons. (ii) The curvilinearity of the isotherm showing evidence for a low degree of positive cooperativity. (iii) The clear dependence upon protein concentration of the average slope of the binding isotherms [8].