Analogues of enkephalins

SAR studies on the enkephalins have shown the importance of the tyrosine phenol ring and the tyrosine amino group. Without either, activity is lost. If tyrosine is replaced with another amino acid, then activity is also lost (the only exception being D-serine). It has also been found that the enkephalins are easily inactivated by 

Much work has been done therefore, to try and stabilize this bond towards hydrolysis. It is possible to replace the amino acid glycine with an unnatural D-amino acid such as D-alanine. Since D-amino acids are not naturally occurring, peptidases do not recognize the structure and the peptide bond is not attacked. The alternative tactic of replacing L-tyrosine with D-tyrosine is not possible, since this completely alters the relative orientation of the tyrosine aromatic ring with respect to the rest of the molecule. As a result, the analogue is unable to bind to the analgesic receptor and is inactive. 

Putting a methyl group on to the amide nitrogen can also block hydrolysis by peptidases. Another tactic is to use unusual amino acids which are either not recognized by peptidases or prevent the molecule from fitting the peptidase active site. Examples of these tactics at work are demonstrated in Fig. 12.40. 

Unfortunately, the enkephalins also have some activity at the mu receptor and so the search for selective agents continues. 

Longer enkaphahns/endorphins Slight increase in mu activity 

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Mitchell SA, Pratt MR, Hraby VJ, Polt R (2001) Solid-phase synthesis of O-linked giycopeptide analogues of enkephalin. J Org Chem 66 2327-2342... 

LY 127623 BCW59-A) is a synthetic pentapeptide analogue of enkephalin, and is an opioid RECEPTOR agonist with OPIOID analgesic activity. 

Sinisi R, Ghilardi A, Ruiu S, Lazzari P, Malpezzi L, Sani M, Pani L, Zanda M (2009) Synthesis and in vitro evaluation of triflu-oroethylamine analogues of enkephalins. ChemMedChem 4(9) 1416-1420... 

This synthetic strategy has been used to prepare some interesting dehydropeptides such as chromophoric dehydro analogues of leucine enkephalin/ potential angiotensin-converting enzyme inhibitors/ and dehydropeptides substituted with a p-lactam moiety. " ... 

The methods presented in the sections above are all suitable for the synthesis of lanthionines as amino acid derivatives. However, a very important synthetic goal remains the synthesis of lanthionine-containing peptides, for example the synthesis of lantibiotics and other peptide analogues where lanthionine has replaced cystine. In the following Sections some syntheses of lanthionine-containing peptides comprising analogues of cystine peptides, such as enkephalin, somatostatin, and oxytocin are described. 

Pless, J., Bauer, W., Cardinaux, F., Closse, A., Hauser, D., Hugenin, R., Roemer, D., Buescher, H.-H., Hill, R.C. Synthesis, opiate receptor binding and analgesic activity of enkephalin analogues, Helv. Chim. Acta 1979, 62, 398-411. 

Similarly, a turn mimetic analogue of Leu-enkephalin (Scheme 27) was prepared.1"11 The compound exhibited significant in vivo analgesia despite relatively low affinity in a binding assay. 

It has been suggested that y-turns are present in the solution structures of several peptides, and furthermore implicated in their bioactive conformations 101 including brady-kinin, 111 substance P,1121 cyclic somatostatin analogues, 131 cyclolinopeptide, 141 and the 6-opioid receptor bound conformation of enkephalin. 151 Yet, despite the fact that y-tums are frequently hypothesized to represent important features of secondary structure 161 based upon computational,1171 IR absorption,1181 NMR spectroscopic,119 201 and X-ray diffraction crystallographic determinations,1211 verification of the role of this predicted secondary structural element remains a difficult, but nonetheless critical step. 

Huffman et alJ161 have also incorporated a /ram-alkene y-turn mimetic into a series of enkephalin analogues 14 (Scheme 9) with complete loss of biological activity and into an HIV-1 protease inhibitor replacing the -Asn-Tyr-Pro- sequence in 15 corresponding to the P2-Pi-P( positions of the peptide substrate giving 16[39-401 (Scheme 10). 

An examination of the structure-activity relationship (SAR) of a series of enkephalin analogues and morphine derivatives in competing with the binding of radioisotope labeled DADLE and FK 33-824 or opiates revealed that DADLE binds to an opioid receptor selective to Leu-enkephalin and its analogues. In contrast, FK 33-824, morphine, or naloxone binds to opioid receptors with selectivity in favor of morphine and derivatives simi-... 

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