Amphibians, skin peptides

Kreil, G., Barra, D., Simmaco, M., Erspamer, V., Erspamer, G. F., Negri, L., Severini, C., Corsi, R., Melchiorri, P. Deltorphin, a novel amphibian skin peptide with high selectivity and affinity for S opioid receptors, Eur. J. Pharmacol. 1989, 162, 123-128. 

We started out our studies on amphibian opioid peptides in the wake of the explosion of research on mammalian endogenous ligands for opiate receptors. We asked ourselves whether the never disappointing amphibian skin contained related molecules. The question was reasonably posed in the light of the outcome of our previous, yearlong research showing that amphibian skin peptides often had counterparts in mammalian CNS and gastrointestinal tract [1]. 

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... 

Mangoni ML, Rinaldi AC, Di Giulio A, Mignogna G, Bozzi A, Barra D, Simmaco M (2000) Structure-function relationships of temporins, small antimicrobial peptides from amphibian skin. Eur J Biochem 267 1447-1454... 

With these words Vittorio Erspamer depicted the rational basis of the pharmacological research on peptides and proteins of the amphibian skin. This 10-year period of research has shown that amphibian skin, with its generally large peptide content, offers a rich source of secretory peptide compounds, analogues of mammalian neuropeptides and hormones. 

The first peptide family of amphibian opiates was discovered in 1981 and named dermorphins [2,3], Until the discovery of mammalian endomor-phins by Zadina et al. [4], these peptides represented the most potent and selective mu opiate receptor agonists identified in living organisms. Nine years later, deltorphins were discovered in the amphibian skin. These peptides are still the most potent and selective delta opiate agonists available today [5]. 

Because L-Xaa2-containing peptides have never been found in amphibian skin extracts, the epimerization mechanism probably involves a quantitative inversion of the chirality of the a-carbon of the amino acid residue, rather than a racemization, which would yield an equimolar mixture of L and D isomers [16,17]. Enzymes catalyzing the formation of D amino acids are so far known only in yeast [18]. From Bombina skin secretions Kreil et al. [19] recently purified a 52-kDa glycoprotein which catalyzes the reaction Ile-Ile-Gly to Ile-D-allo-Ile-Gly. The partial conversion of He to D-allo-Ile in peptide linkage proceeds without the addition of cofactors. 

Similar to mammalian prohormones, all opioid peptides in amphibian skin precursors are flanked by paired dibasic amino acids (Lys-Arg). Moreover, the precursor sequence contains an additional Gly residue at its carboxyl terminus this extra residue is required for the carboxamidation of the mature heptapeptide [20]. 

Table 7.13 Opioid Receptor Affinities and Opioid Activity in the GPI and MVD of Peptides from Amphibian Skin ...

The unique feature of these amphibian skin opioid peptides is the sequence between the important aromatic residues. In contrast to the enkephalins and other mammalian opioid peptides that contain the Gly-Gly dipeptide sequence between Tyr and Phe, the amphibian opioid peptides contain a single D-aminoacid (see Fig. 7.41), which apparently arises from a post-translational conversion of the L-amino acid to its d isomer (665). The identification of these unusual opioid peptides expanded our understanding of the structural requirements for interaction with opioid receptors and provided new lead compounds for further modification (see Sections 6.5.1 and... 

Heyl DL, Schullery SE (1997) Developments in the structure-activity relationships for the delta-selective opioid peptides of amphibian skin. Curr Med Chem 4 117-150... 

Zasloff is quick to note that nobody has a clue how a giant squid or an octopus—which have neither antibodies nor white blood cells called lymphocytes—avoids becoming consumed by microbes Over the years, he and his colleagues have uncovered many frog-made peptides that possess potent microbe-killing properties. Such a chemical defense system operates by virtue of the peptides ability to poke holes in the cell membranes that serve to protect bacteria from the outside world. In addition to the peptides, scientists including Zasloff have found hundreds of other types of molecules called alkaloids in amphibian skin. When inside cells, many alkaloids home in on structures called ion channels—tunnel-like assemblies through which important electrolytes pass. These are key cellular fixtures,... 

There has been an explosion of recent interest in antimicrobial peptides, which have been shown to be ubiquitous in nature as evidenced in numerous review articles [155-159]. Frequently, these peptides have been located where microbial control is important, such as in mammalian intestines or trachea, in seminal fluid, on amphibian skin, as plant defense compounds or on insect larvae. Hence, such peptide antibiotics can be seen as a response to ecological conditions, specifically the propensity of microorganisms to thrive wherever they can gain access to a macroorganism. 

Barra, D. Simmaco, M. "Amphibian skin a promising resource for antimicrobial peptides", TIB Tech., 1995, 13, 205-209. 

V. Erspamer, Peptides of amphibian skin active on the gut. II. Bombesin like peptides isolation, structure and basic functions. In Gastrointestinal Hormones (G.B. Jerzy Glass ed.) Raven, New York, 1980, pp. 344-361... 

Melanotropin release inhibiting hormone (MIH), melanostatin a hormone structurally equivalent to the peptide side chain of oxytocin, Pro-Leu-Gly-NH2 (bovine, rat), or to Pro-His-Phe-Arg-Gly-NHj (bovine), MRF stimulates, whereas MIH inhibits synthesis and secretion of melanotropin by the anterior pituitary, leading respectively to darkening or lightening of amphibian skin. [Special section incorporating 4 papers on Gonadotropin-Releasing Hormone Trends Endocrinol. 7 (1996) 55-68]... 

In addition to these well-characterized adaptive immune defenses, amphibians have an array of innate immune defenses. These include complement-mediated defenses [13], Natural Killer cells (NK) [14], phagocytic cells [15], and antimicrobial peptides secreted into the gut and skin mucosa [16, 17],... 

Three of the classes of compounds found from the skin of amphibians have been identified from the skin of bufonids, including Bufo marinus. These are steroids (bufadienolides), biogenic amines (catecholamines, indolylalkylamines and alkaloids) and bioactive peptides and proteins. 

---