BIPHALIN, A HIGHLY POTENT DIMERIC ENKEPHALIN ANALOG. Judith. L. Flippen-Anderson, Jeffrey Deschamps and Clifford George, Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, D. C. 20375-5341; Victor J. Hruby, Guigen Li, Andrzej W. Lipkowski, and Aleksandra Misicka, Department of Chemistry, University of Arizona, Tucson, AZ 85721

One of the major achievements in the area of opioid research in the past twenty years has been the elucidation of multiple opioid receptors (u, [[kappa]], d) In the absence of direct structural information for these receptors current structure activity studies still center on understanding the effects of conformational changes on the opioid ligands. Structural modifications of endogenous ligands such as enkephalin have produced compounds with enhanced receptor selectivity, biological stability and antinociceptive potency. Most modifications of the opioid ligands have involved changes which reduce the flexibility of the molecule thereby significantly lowering the number of possible conformations to be studied. Combining X-ray results with modeling and NMR studies can provide a more reliable picture of the biological conformation of a highly potent ligand than one could get from any single analytical method.

In this paper we are reporting the solid state structure of biphalin [(Tyr-DAla-Gly-Phe-NH)₂], a highly potent site specific dimeric enkephalin analog. Biphalin is formed by replacing the Gly 2 residue in enkephalin with a D-Ala residue and replacing the C-terminal residue by a second identical enkephalin fragment connected by a hydrazine bridge. Biphalin displays an unusual activity profile in that it shows similar binding affinity at both u and d receptor sites. When administered i.c.v. it is more than 200 times as potent as morphine in antinociception. However, both u and d antagonists can block its antinociceptive effects. The X-ray structure revealed distinctly different conformations for the two halves of the dimer which is consistent with its activity profile. Comparison with the structures of known u and d agonists clearly reveals which half of the molecule binds at each receptor site.

This work was supported in part by ONR (NRL), the US Public Health Service (Arizona) and NIDA.