Leucine Enkephaline

In the anterior pituitary gland (see Hormones, anteriorpituitaryhormones), both adrenocorticotropic hormones (ACTH) and the endogenous opiate hormone, P-endorphin, are synthesized from a common prohormone (2) (see Opioids,endogenous). In the adrenal medulla, five to seven copies of another opiate hormone, methionine—enkephalin (Met-enkephalin), and one copy of leucine—enkephalin (Leu-enkephalin) are synthesized from each precursor molecule (3). 

When the compounds of interest are fragile and thermally labile, thermospray Ic/ms is a good choice. Figure 5, shows the thermospray spectmm for leucine enkephalin [58822-25-6] a pentapeptide of molecular weight 555. The Ic/ms approach has been very helpful in unraveling the stmcture of large biological molecules (21). 

Delta receptors are relatively selective for two related penta-peptides, methionine enkephalin and leucine enkephalin (met- and leu-enkephalin), which were isolated from porcine brain (Hughes 1975). Both met- and leu-enkephalin inhibit electrically induced contractions of guinea pig ileum, an effect that mimics those effects seen with opioid drugs, and is naloxone reversible. The enkephalins are processed posttranslational ly from proenkephalin, and secreted from central and peripheral neurons and endocrine cells in the adrenal medulla. 

MK Anwer, AF Spatola. An advantageous method for the rapid removal of hydro-genolysable protecting groups under ambient conditions synthesis of leucine-enkephalin. (ammmomum formate) Synthesis 929, 1980. 

M. Kupryszewska, I. Gryczynski, and A. Kawski, Intramolecular donor-acceptor separations in methionine- and leucine-enkephalin estimated by long-range radiationless transfer of singlet excitation energy, Photochem. Photobiol. 36, 499-502 (1982). 

A recent study, however, has shown that aminopeptidase activity is present on the surface of porcine buccal mucosa, and that various aminopeptidase inhibitors, including amastatin and sodium deoxycholate, reduce the mucosal surface degradation of the aminopeptidase substrate, leucine-enkephalin [149], Since the peptidases are present on the surface of the buccal mucosa, they may act as a significant barrier to the permeability of compounds which are substrates for the enzyme. In addition to proteolytic enzymes, there exist some esterases, oxidases, and reductases originating from buccal epithelial cells, as well as phosphatases and carbohydrases present in saliva [154], all of which may potentially be involved in the metabolism of topically applied compounds. 

Faraj et al. [28] studied the effects of different concentrations of leucine enkephalin, peptidase inhibitors, and sodium glycocholate (GC) on the stability and absorption of leucine enkephalin in nasal cavities of rats. Based on the study, the rate and extent of formation of des-tyrosine leucine enkephalin... 

Hussain A, Faraj J, Aramaki Y, Truelove JE (1985) Hydrolysis of leucine enkephalin in the nasal cavity of the rat- a possible factor in the low bioavailability of nasally administered peptides. Biochem Biophys Res Communl33 923-928. 

Faraj JA, Hussain AA, Aramaki Y, Iseki K, Kagoshima M, Dittert LW (1990) Mechanism of nasal absorption of drugs. Ill Nasal absorption of leucine enkephalin. J Pharm Sci 79 698-702. 

Figure 6 shows the signal response in ESI of Leucine Enkephalin, a pentapeptide, as a function of solvent and additive variation. Although the peptide can be ionized in basic media, acidic pH is much more favorable. This result also confirms that formic acid and acetic acid promote ESI signals far more efficiently than TEA. 

Ammonium formate and phosphate buffers CEC-ESI-MS for the analysis of leucine enkephalin and substance E, which are respectively singly and triply charged peptides. The good mass spectra obtained for the peptides in both the volatile and non-volatile buffers (Figures 17 and 18) indicate the non-crystallization of the non-volatile buffer which is further diluted by the sheath liquid. 

The sensitivities obtained with the volatile buffer (ammonium formate) and non-volatile buffer (sodium borate) in CEC-ESI-MS were assessed by comparing the spectra obtained in the two modes using leucine enkephalin and substance P that carry charges of -Fl and -1-3, respectively. 

Figure 17 shows the mass spectra obtained for leucine enkephalin (charged state -Fl at pH 2.8 and —0.5 at pH 9.5) and Figure 18 for substance P (charged state -F3 at pH 2.8, and -F1.5 at pH 9.5). The analytes undergo ionization at the interface to yield the positively charged ions as a result of the addition of the formic acid sheath liquid to the alkaline buffer. 

The spin-spin coupling constants [2-4] of the enkephalins in solution can be interpreted in terms of folded conformations resembling that of morphine in the placement of the residues which appear important for biological activity. X-ray crystallography and theoretical calculations (4-9) have also shown that methionine and leucine enkephalin adopt conformations similar to those concluded from NMR studies. Hence it would appear that opioid peptides can topographically resemble the opiates by assuming preferred, folded, conformations. However, earlier studies from this laboratory (TO) have shown that NMR data can be interpreted in terms of a conformationally flexible structure for methionine enkephalin. 

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. " ... 

Sayani, A.P., I.K. Chnn, and Y.W. Chien, Transmucosal delivery of leucine enkephalin stabilization in rabbit enzyme extracts and enhancement of permeation through mucosae. J Pharm Sci, 1993. 82(11) 1179-85. 

Three families of endogenous opioid peptides have been described in detail the endorphins, the pentapeptide enkephalins methionine-enkephalin (met-enkephalin) and leucine-enkephalin (leu-enkephalin), and the dynorphins. The three families of opioid receptors have overlapping affinities for these endogenous peptides (Table 31-1). 

Sequence Determination of the Brain Peptide Leucine Enkephalin A group of peptides that influence nerve transmission in certain parts of the brain has been isolated from normal brain tissue. These peptides are known as opioids, because they bind to specific receptors that also bind opiate drugs, such as morphine and naloxone. Opioids thus mimic some of the properties of opiates. Some researchers consider these peptides to be the brain s own pain killers. Using the information below, determine the amino acid sequence of the opioid leucine enkephalin. Explain how your structure is consistent with each piece of information. 

Bernkop-Schniirch, A., C. Paikl, and C. Valenta. 1997. Novel bioadhesive chitosan-EDTA conjugate protects leucine enkephalin from degradation by aminopeptidase N. Pharm Res 14 917. 

---