enkephalin--leucine-2-alanine and naltrexonazine

Biphalin is a dimeric opioid peptide, composed of two tetrapeptides connected 'tail-to-tail', that exhibits a high affinity for all three opioid receptor types (i.e. mu, delta and kappa). This study presents the X-ray crystal structure of biphalin sulfate and compares it to other opioids that interact with the same biological targets. Both halves of the molecule have a folded backbone conformation but differ significantly from one another. Residues 1-4 in biphalin, which compare well with the delta selective opioid peptide DADLE, fold into a random coil. Residues 5-8, which can be fit to the mu selective peptide D-TIPP-NH2, exhibit a fairly normal type III' beta bend. Biphalin also exhibits structural similarities with two naltrexone analogs, naltrexonazine and norbinaltorphamine, that are specific to mu and kappa receptor sites.

Topics: Binding Sites; Crystallography, X-Ray; Enkephalin, Leucine-2-Alanine; Enkephalins; Hydrogen Bonding; Naltrexone; Narcotics; Oligopeptides; Pliability; Protein Conformation; Protein Folding; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu; Structure-Activity Relationship; Substrate Specificity; Tetrahydroisoquinolines

A series of opiate azines, including naloxonazine, naltrexonazine and oxymorphonazine, produce both a wash-resistant inhibition of 3H-opioid binding and prolonged actions in vivo. Opiate diacylhydrazones synthesized from succinic, adipyl and suberic dihydrazides possess similar actions against 3H-opioid binding. Competition studies measuring inhibition of binding in the presence of the compounds revealed little difference between standard, reversible opiates such as naloxone, oxymorphone and naltrexone and our two series of compounds, the diacylhydrazones and the azines. In these assays, the diacylhydrazones, the azines, oxymorphone, naloxone and naltrexone all inhibited 3H-opioid binding with very similar IC50 values, typically under 5 nM. At concentrations under 5 nM, the inhibition of all the compounds was reversible. At higher concentrations, however, much of the inhibition of the diacylhydrazones and azines was not freely reversible, in distinction to oxymorphone, naloxone and naltrexone. Washing after the incubation of membranes with the naloxone, naltrexone or oxymorphone (50 nM) returned binding to control levels. Despite the extensive washing, the diacylhydrazones, on the other hand, lowered binding by as much as 90%. Mu binding was most sensitive to wash-resistant binding. In general, the longer dihydrazide derivatives produced wash-resistant inhibition more effectively than either the shorter dihydrazide derivatives or the corresponding azines. The ability of these compounds to produce wash-resistant inhibition of binding probably did not result from a bivalent attachment of the ligand to two binding sites at once. Additional assymetric azines and diacylhydrazones unable to bind simultaneously to two sites still produced wash-resistant inhibition of binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Binding, Competitive; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; Hydromorphone; In Vitro Techniques; Male; Naloxone; Naltrexone; Narcotic Antagonists; Narcotics; Oxymorphone; Rats; Rats, Inbred Strains; Receptors, Opioid; Structure-Activity Relationship