Some properties of neurophysins isolated from bovine neurosecretory granules

Our experiments with the neurophysin-related proteins from bovine NSG have demonstrated that these species differ in several important respects from the materials conventionally prepared. (1) In structural terms, the NSG proteins are essentially identical with the conventional neurophysins in amino acid composition and closely similar in immunoreactivity; however, the presence of carbodydrate and lipid moieties in the NSG material, no matter how they are attached, constitutes a structural difference apparently sufficient to cause considerable changes in properties. (2) The affinities for the neurohypophyseal hormones of the NSG proteins are very much higher than those of the "conventional" neurophysins and, moreover, the binding properties of the NSG material are much more stable with time under the conditions of the binding experiments. (3) The low binding capacities of the NSG materials, even when they are purified to apparent molecular homogeneity, indicate a functional heterogeneity perhaps related to supramolecular structure. (4) The conversion of the NSG proteins by acid or alkali treatment to materials resembling the "conventional" neurophysins in their binding properties suggests that the latter may be isolation artifacts. Although we cannot as yet consistently explain the properties of our neurophysins from NSG, we offer the hypothesis that the low binding capacity, as also the Hill coefficient greater than 1 (cf. Reference 25) are indicative of molecular aggregation, perhaps mediated or facilitated by the nonprotein components. It is conceivable that such aggregation, proceeding in the more "natural" environment of the NSG in a more precisely organized manner, might constitute the truly "native," fully functional state of the neurophysins. In this context it is of interest to record our preliminary observations, which suggest the presence of a protein of 10,000 mol. wt. in the NSG membrane fraction (SDS gel) as well as electron-microscopic indications30 of a highly organized ("crystalline") structure within these membranes. Although, therefore, the materials we have described may not merit the description of "native" neurophysins, we believe that they are certainly closer to the native state than the proteins conventionally isolated; and we would suggest that any discussion of the biological role of the neurophysins based on the properties of the conventional preparations may be at best speculative, and at worst misleading.