8-Affinity with deltorphins

When the efficacy of biphalin-stimulated G protein activation was examined (Table 3) in delta opioid receptor-transfected CHO cells, an efficacy ratio of 0.42 was determined as compared with deltorphin-II and DPDPE, the latter a reference delta-selective agonist. Such low efficacy values suggest that biphalin does not efficiently stimulate the G protein through the delta receptor [9]. Relative affinities of biphalin and morphine for mu, delta, and kappa binding sites in guinea pig brain membranes are shown in Table 4. 

Furthermore, the substitution of methyl groups at the T and 6 position of Tyr (Dmt) yielded Dmt-Tic- peptides (3) endowed with dramatically increased 8-affinity, comparable to the 8-affinity observed with deltorphins [59-63]. On the other hand, p-affinity of 3 rose 20-fold relative to 1 and 2, while k-affinity increased over 40-fold in guinea pig brain membranes, 20-fold in rat brain synaptosomes compared to 2 and approximately 40-fold for both membrane preparation relative to 1. Inclusion of the Tyr-D-Ala-Phe message domain (4) essentially eliminated opioid interaction with k-receptors, whereas 8- and p-affinities were only moderately altered in comparison to DYN (1-11) -NH2 (Table... 

Erspamer Y, Melchiorri P, Falconieri-Erspamer G, et al. Deltorphins a family of naturally occurring peptides with high affinity and selectivity for opioid binding sites. Proc Natl Acad Sci USA 1989 86 5188-5192. 

Erspamer, V., Melchiorri, P., Falconieri-Erspamer, G., Negri, L., Corsi, R., Severin , C., Barra, D., Simmaco, M., Kreil, G. Deltorphins a family of naturally occurring peptides with high affinity and selectivity for Sopioid binding sites, Proc. Natl. Acad. Sci. USA 1989, 86, 5188-5192. 

Subsequently, two additional peptides with even higher affinity for the delta opiate receptor were isolated from the skin of Ph. bicolor [12], Like dermorphin, these peptides contain D-alanine as the second amino acid. They have been termed D-Ala-deltorphin-I and D-Ala-deltorphin-II. 

Despite a common N-terminal tripeptide (Tyr-D-Xaa-Phe), the two groups of opioid peptides, dermorphins and deltorphins, differ enormously in receptor selectivities but bind to their own receptors with similar affinities. The N-terminal domain contains the minimum sequence essential for binding to opioid receptors whereas the C-terminal domain contains the address requisites for receptor selectivity. 

Deltorphins are endogenous linear heptapeptides, isolated from skin extracts of frogs belonging to the genus Phyllomedusa, that have a higher affinity and selectivity for delta opioid-binding sites than any other natural compound known [36]. Two deltorphins with the sequence Tyr-Ala-Phe-Asp (or Glu)-Val-Val-Gly-NH2 have been identified and well characterized [36]. 

The high-affinity GTPase activity in the mouse spinal cord is increased in a concentration-dependent manner by [D-Ala2]deltorphin II [82]. This increase of GTPase activity induced by [D-Ala2]deltorphin II is completely blocked by coincubation with a selective delta opioid receptor antagonist NTB [82]. 

Based on their finding amphibian skin peptides, which were counterparts to other mammalian bioactive peptides, Erspamer and coworkers examined amphibian skin for opioid peptides (see Ref 663 for a review). This led first to the isolation and characterization of dermorphin (212, Fig. 7.41), which is a ja-se-lective peptide (see Table 7.13), from the skin of South American Phyllemedusinae hylid frogs in the early 1980s (664). Inspection of the sequence of one of the cloned cDNAs for the precursor of dermorphin suggested the existence of another heptapeptide with a similar iV-terminal sequence (665). This then led to the isolation of deltorphin (alsocalled dermenkephalin or deltorphin A, 213, Fig. 7.41), the first S-selective amphibian opioid peptide, from these frogs (666, 667). Synthesis confirmed that the amino acid in position 2 of deltorphin was o-methionine rather than L-methionine (666,668, 669). Two additional peptides [o-Ala ldeltorphin I (also referred to as deltorphin C, 214, Fig. 7.41) and [o-Ala ]deltorphin II (also referred to as deltorphin B, 25, Fig. 7.5) were subsequently discovered (106) which exhibited even greater 8-receptor affinity and exceptional selectivity... 

In positions 5 and 6 of [D-Ala ]deltorphin II replacement of Vaf and Val individually by Ala suggested that VaP is more important than Val for 8-receptor selectivity (920). Replacement of valine in both positions by norleucine, He, or y-methylleucine enhances both 6-receptor affinity and selectivity four- to eightfold (920). Replacing Gly in [n-Ala ]-deltorphin I with Asp, which is found in this position in deltorphin, decreases affinity for both 6 and ix receptors approximately 10-fold (919). 

The four isomers of j3-MePhe were incorporated into both [D-Ala ]deltorphin I and deltorphin (930). The peptides containing the 2jS isomers exhibit higher Breceptor affinity and agonist activity in the MVD than the analogs with the 2R isomers for both [D-Ala ]deltorphin I and deltorphin, consistent with the preference for l- over D-Phe in this position. The stereochemistry of the methyl group at the j3 position, however, had different effects on the 6 affinity and agonist... 

The peptides subtly modified in the side chain at N-terminus residue yielded an opioid with high dual affinities to both 8- and g-opioid receptor types. At present, it appears that the replacement of the Tyr residue by the innatural Dmt analogue in the 5- selective opioid agonist, deltorphin B, produced a peptide with high affinity for both g- and 8-opioid receptors, thereby yielding a 5-opioid receptor ligand which, to all intents and purposes, lacks 5-opioid receptor selectivity. 

While the high affinity for the g-opioid receptor was confirmed in the in vitro pharmacological assay (guinea- pig ileum), in vivo antinociception apparently failed to correlate with high g-opioid receptor activity however, the activity profile of peptide 1 [DMT1] deltorphin B was more closely related to that of the 8- selective opioid peptides 2 (deltorphin B) and 3 (deltorphin C). 

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