The molecular dimensions of porcine neurophysin and some thermodynamic parameters of the reaction with lysine vasopressin.

1. The molecular weight of porcine neurophysin was estimated by molecular sieve chromatography and by analytical ultracentrifugation and was found to be in the order of 13,000.2. Internal evidence for the homogeneity of the preparation of neurophysin with respect to molecular weight was obtained in the ultracentrifugation experiments.3. The frictional ratio of neurophysin was 1.1 which suggests that the molecular form of the protein approximates to a sphere.4. The molecular weight and frictional ratio were not affected by temperature change (10-34 degrees C) or by twofold change in protein concentration.5. The binding of [(14)C]lysine vasopressin to porcine neurophysin was studied at 0, 27 and at 45 degrees C, and double reciprocal plots of the binding were shown to be curvilinear at 27 and at 45 degrees C and rectilinear at 0 degrees C.6. Concordant estimates for maximum binding capacity were obtained by extrapolations from the data at 27 and 45 degrees C by applying two independent empirical methods of approximation; these agreed with the estimate obtained by extrapolation of the straight line, fitting data obtained at 0 degrees C, being approximately 1 mole lysine vasopressin per 13,000 g protein.7. The association constant and thermodynamic parameters of the reaction were estimated for near saturation conditions. The reaction is entropy driven.8. The binding of lysine vasopressin was found to be dependent on protein concentration. No dependence of oxytocin binding on protein concentration was apparent.