Structural requirements for opioid activity of analogues of the enkephalins

Abstract

Structure-activity relations of a series of analogues of the two endo­genous morphine-like peptides, leucine-enkephalin and methionine-enkephalin are examined on the basis of ( a ) effects on the mouse vas deferens and the guinea pig ileum and ( b ) affinities for the rat brain opiate receptor. In the mouse vas deferens, metabolism of the peptides by proteolysis is not a major influence on activity. In contrast, however, brain opiate re­ceptor preparations contain an abundance of proteolytic enzymes, the effects of which can be minimized by conducting opiate receptor binding assays at 0 °C and in the presence of bacitracin. The potentiation of biological activity and opiate receptor binding affinity by replacing the Gly 2 residue in the natural enkephalins by d-Ala, is discussed both in terms of increased stability of the Tyr-d-Ala bond to aminopeptidases and of the stabilization of the peptide conform­ation as present in the receptor-peptide complex. The substitution of the Leu 5 - or Met 5 -residue by the corresponding d-amino acid contributes little to proteolytic stability, which emphasizes that the predominating site at which metabolism occurs is the Tyr 1 -Gly 2 bond. Of the analogues described, [d-Ala 2 , d-Leu 5 ]-enkephalin is the most active peptide in the three assay systems, the mouse vas deferens, the guinea pig ileum and the rat brain opiate receptor preparations. Substitutions by the respective d-amino acids d-Tyr and d-Phe at positions 1 and 4 reduce both the potency and binding affinity and emphasize the importance of stereochemical acceptability at these positions. The promotion of receptor binding by d-amino acids is examined, particularly with respect to implied peptide conformations. The experi­mental data have been analysed for the relative influence of metabolic and conformational factors.

Exploring the Backbone of Enkephalins To Adjust Their Pharmacological Profile for the δ-Opioid Receptor

“…Compounds 3, 4, and 6 were found to bind DOPR with modest affinity, a finding consistent with our observation that these compounds were less efficient than Leu 5 -enkephalin and compound 1 at promoting DOPR internalization (Figures 6 and 7) and ERK1/2 phosphorylation (Figure 8 -enkephalin strongly reduces its affinity and activity for DOPR (17,18). In fact, it is known that the phenyl side chain of the L-Phe 4 residue of Leu 5 -enkephalin is critical for its biological activity (17) and that Leu -enkephalin were shown to bind and activate DOPR similarly (17,18). Still, the slightly better affinity of compound 6 led us to hypothesize that this compound contains a D-Leu at position 5 while compound 4 contains an L-Leu at the same position.…”

“…The Tyr-Gly-Gly-Phe//D-Leu stereochemistry was attributed to 6 because it is slightly more active than 4, similar to TyrGly-Gly-Phe-D-Leu, which is more active toward DOPR than Leu 5 -enkephalin (cf. Table 1) (17,18). Admittedly, this assumption is only based on comparative binding and activity with previous studies and remains to be confirmed by X-ray analysis.…”

“…Diastereomer 5, being virtually inactive, was assigned the Tyr-Gly-Gly//DPhe-Leu stereochemistry like its inactive parent (Tyr-GlyGly-D-Phe-Leu). In contrast, diastereomer 3 retains some affinity or activity for DOPR (17,18). Compounds 4 and 6 were prepared by a slightly different procedure because the first coupling to the resin would require too much of the dipeptide surrogate Phe//Leu.…”

“…Aside from small molecules, it has also been shown that several linear peptides similar to the enkephalins (i.e., DADLE (Tyr-D-Ala-Gly-Phe-D-Leu) and deltorphin II (Tyr-Ala-Phe-Glu-Val-Val-Gly-NH 2 )) and that some cyclic peptides topologically similar to enkephalins (DPDPE, JOM13) exhibit excellent affinity and selectivity for DOPR (Figure 2) (17)(18)(19)(20).…”

Structure−Activity Relationships of Bifunctional Peptides Based on Overlapping Pharmacophores at Opioid and Cholecystokinin Receptors

“…The high to moderate binding affinity and bioactivity for the opioid receptors are surprising since it was found that D-aromatic amino acid residues in this position resulted in inactive peptides. 19,30 Two strategies were combined to obtain the desired effect of increased binding at opioid receptors and at CCK receptors. Since substitution with D-Trp 4 successfully resulted in an antagonist effect at the CCK-1 receptor, and substitution with Nle 5 resulted in a balanced binding affinity between CCK-1 and CCK-2, a double substitution was attempted to obtain additive result.…”