STUDIES OF BOVINE NEUROPHYSIN‐NEUROHYPOPHYSEAL HORMONE INTERACTIONS

The complex molecular organization of cells and their constituent organelles is based on the association of different classes of small and macromolecules. In the regulation of many metabolic processes in living cells, macromolecular association reactions are known to play a central role.' In the last few years, a considerable effort has been made to understand these interactions at the molecular level, applying to the study of different s y s t e m s ' ~ ~ the most refined analytical, chemical, and physical methods. The neurosecretory granules produced in the magnocellular nuclei of the hypothalamus in different vertebrates contain the nonapeptide hormones, oxytocin and vasopressin, in association with their carrier proteins, the neur~physins.~" In vitro and probably in vivo these components form stereospecific complexes. The in vitro interactions of neurohypophyseal hormones with neurophysins provide a relatively simple and unique model system for studying in detail the structural features of hormone binding to a protein. In addition, studies of this model system may give information about the conformation of the hormone molecules bound to a protein compared to their conformation free in solution. From the point of view of biochemical and biophysical methodology, the hormone-neurophysin system is a particularly suitable one for a number of reasons, including the fact that both compounds are chemically well-defined,' stable molecules and are relatively small (MW 1 O3 and lo4, respectively). Although x-ray crystallographic data on the three-dimensional structures of neurophysins and of the hormones are still lacking, a considerable number of nuclear magnetic