A novel cDNA sequence encoding the precursor of the D-amino acid-containing neuropeptide fulicin and multiple alpha-amidated neuropeptides from Achatina fulica

D-Amino Acids in Peptides from Animals, Including Human: Occurrence, Structure, Bioactivity and Pharmacology

All life forms typically possess homochirality, with rare exceptions. In the case of peptides and proteins, only L-amino acids are known to be encoded by genes. Nevertheless, D-amino acids have been identified in a variety of peptides, synthesized by animal cells. They include neuroexcitatory and neuroprotective peptides, cardioexcitatory peptides, hyperglycemic hormones, opioid peptides, antimicrobial peptides, natriuretic and defensin-like peptides, and fibrinopeptides. This article is a review of their occurrence, structure and bioactivity. It further explores the pharmacology and potential medical applications of some of the peptides.

Neuropeptides predicted from the transcriptome analysis of the gray garden slug Deroceras reticulatum

The gray garden slug, Deroceras reticulatum (Gastropoda: Pulmonata), is one of the most common terrestrial molluscs. Research for this slug has focused mainly on its ecology, biology, and management due to the severe damage it causes on a wide range of vegetables and field crops. However, little is known about neuropeptides and hormonal signalings. This study, therefore, aimed to establish the transcriptome of D. reticulatum and to identify a comprehensive repertoire of neuropeptides in this slug. Illumina high-throughput sequencing of the whole body transcriptome of D. reticulatum generated a total of 5.9 billion raw paired-end reads. De novo assembly by Trinity resulted in 143,575 transcripts and further filtration selected 120,553 unigenes. Gene Ontology (GO) terms were assigned to 30,588 unigenes, composed of biological processes (36.9%), cellular components (30.2%) and molecular functions (32.9%). Functional annotation by BLASTx revealed 39,987 unigenes with hits, which were further categorized into important functional groups based on sequence abundance. Neuropeptides, ion channels, ribosomal proteins, G protein-coupled receptors, detoxification, immunity and cytoskeleton-related sequences were dominant among the transcripts. BLAST searches and PCR amplification were used to identify 65 putative neuropeptide precursor genes from the D. reticulatum transcriptome, which include achatin, AKH, allatostatin A, B and C, allatotropin, APGWamide, CCAP, cerebrin, conopressin, cysteine-knot protein hormones (bursicon alpha/beta and GPA2/GPB5), elevenin, FCAP, FFamide, FVamide (enterin, fulicin, MIP and PRQFVamide), GGNG, GnRH, insulin, NdWFamide, NKY, PKYMDT, PRXamide (myomodulin, pleurin and sCAP), RFamide (CCK/SK, FMRFamide, FxRIamide, LFRFamide, luqin and NPF), and tachykinin. Over 330 putative peptides were encoded by these precursors. Comparative analysis among different molluscan species clearly revealed that, while D. reticulatum neuropeptide sequences are conserved in Mollusca, there are also some unique features distinct from other members of this species. This is the first transcriptome-wide report of neuropeptides in terrestrial slugs. Our results provide comprehensive transcriptome data of the gray garden slug, with a more detailed focus on the rich repertoire of putative neuropeptide sequences, laying the foundation for molecular studies in this terrestrial slug pest.

Characterization of GdFFD, a D-amino acid-containing neuropeptide that functions as an extrinsic modulator of the Aplysia feeding circuit

During eukaryotic translation, peptides/proteins are created using L-amino acids. However, a D-amino acid-containing peptide (DAACP) can be produced through post-translational modification via an isomerase enzyme. General approaches to identify novel DAACPs and investigate their function, particularly in specific neural circuits, are lacking. This is primarily due to the difficulty in characterizing this modification and due to the limited information on neural circuits in most species. We describe a multipronged approach to overcome these limitations using the sea slug Aplysia californica. Based on bioinformatics and homology to known DAACPs in the land snail Achatina fulica, we targeted two predicted peptides in Aplysia, GFFD, similar to achatin-I (GdFAD versus GFAD, where dF stands for D-phenylalanine), and YAEFLa, identical to fulyal (YdAEFLa versus YAEFLa), using stereoselective analytical methods, i.e. MALDI MS fragmentation analysis and LC-MS/MS. Although YAEFLa in Aplysia was detected only in an all L-form, we found that both GFFD and GdFFD were present in the Aplysia CNS. In situ hybridization and immunolabeling of GFFD/GdFFD-positive neurons and fibers suggested that GFFD/GdFFD might act as an extrinsic modulator of the feeding circuit. Consistent with this hypothesis, we found that GdFFD induced robust activity in the feeding circuit and elicited egestive motor patterns. In contrast, the peptide consisting of all L-amino acids, GFFD, was not bioactive. Our data indicate that the modification of an L-amino acid-containing neuropeptide to a DAACP is essential for peptide bioactivity in a motor circuit, and thus it provides a functional significance to this modification.

Molecular cloning of two distinct precursor genes of NdWFamide, a d-tryptophan-containing neuropeptide of the sea hare, Aplysia kurodai

NdWFamide (NdWFa) is a D-tryptophan-containing cardioexcitatory neuropeptide in gastropod mollusks, such as Aplysia kurodai and Lymanea stagnalis. In this study, we have cloned two cDNA encoding distinct precursors for NdWFa from the abdominal ganglion of A. kurodai. One of the predicted precursor proteins consisted of 90 amino acids (NWF90), and the other consisted of 87 amino acids (NWF87). Both of the predicted precursor proteins have one NWFGKR sequence preceded by the N-terminal signal peptide. Sequential double staining by in situ hybridization (ISH) and immunostaining with anti-NdWFa antibody suggested that NdWFa-precursor and NdWFa peptide co-exist in neurons located in the right-upper quadrant region of the abdominal ganglion. In ISH, NWF90-specific signal and NWF87-specific one were found in different subsets of neurons in the abdominal ganglia of Aplysia. The expression level of NWF90 gene estimated by RT-PCR is much higher than that of NWF87 gene. These results suggest that NWF90 precursor is the major source of NdWFa in Aplysia ganglia.

Neurohormones and neuropeptides encoded by the genome of Lottia gigantea, with reference to other mollusks and insects

The Lottia gigantea genome was prospected for the presence of genes coding neuropeptides and neurohormones. Four genes code insulin-related peptides: two genes code molluscan insulin-like growth hormones, one gene an insulin very similar to vertebrate insulin, and the fourth a peptide related to drosophila insulin-like peptide 7. Four other genes encode the cysteine-knot proteins GPA2/GPB5 and bursicon/parabursicon. Another 37 genes code for precursors of the following neuropeptides: achatin, APGWamide, allatostatin C, allatotropin, buccalin (perhaps an allatostatin A homolog), cerebrin, CCAP, conopressin, elevenin (the predicted neuropeptide made by abdominal neuron 11 in Aplysia), egg laying hormone (two genes), enterin, feeding circuit activating neuropeptide (FCAP), FFamide, FMRFamide, GGNG, a GnRH-like peptide, the newly discovered LASGLVamide, LFRFamide, LFRYamide, LRNFVamide, luqin, lymnokinin, myomodulin (two genes), the newly discovered NKY, NPY, pedal peptide (three genes), PKYMDT, pleurin, PXFVamide, small cardioactive peptides, tachykinins (two genes) and WWamide (an allatostatin B homolog). One gene was found to encode FWISamide, while about 20 closely related genes were found to encode WWFamide. These small neuropeptides appear homologous to the NdWFamide, which contains d-Trp; these genes are similar to the Aplysia gene encoding NWFamide. Some of these peptides had not been previously identified from mollusks, such as the predicted hormones similar to Drosophila and vertebrate insulins, bursicon, the putative proctolin homolog PKYMDT and allatostatin C. Together with neuropeptides which are likely homologs of other insect neuropeptides, such as cerebrin and WWamide, this shows that despite significant differences the molluscan and arthropod neuropeptidomes are more similar than generally recognized.

Measuring D-amino acid-containing neuropeptides with capillary electrophoresis

Neuropeptides are heavily posttranslationally modified (PTM) gene products that are often characterized by a variety of mass spectrometric approaches. Recently, the occurrence of amino acids in the D-form has been documented in several neuropeptides. As this modification has no associated mass shift, this particular PTM is difficult to evaluate using mass spectrometry (MS) alone. Here we demonstrate several approaches using capillary electrophoresis (CE) with absorbance and laser-induced fluorescence (LIF) for the separation of native and derivatized molluscan peptides containing D-amino acids. The combination of peptide derivatization followed by CE/LIF is well suited for single cell measurements because of its ability to characterize the peptides in such small samples. In order to verify this approach, the D-Trp-containing peptide NdWFa (NH2-Asn-D-Trp-Phe-CONH2), present in individual neurons from the marine mollusk Aplysia californica, has been characterized. The mass spectra show that NdWFa and/or NWFa are present in specific neurons; CE/LIF analysis of these cells demonstrates that NdWFa is the dominant form of the peptide.

Identification of GYRKPPFNGSIFamide (crustacean-SIFamide) in the crayfish Procambarus clarkii by topological mass spectrometry analysis

A new concept relating to the purification protocol for biological proteins and peptides has been designed as "topological mass spectrometry analysis," in combination with MALDI-TOF MS using slices of tissues, chromatographic purification from the extract of tissues, molecular cloning for the determination of the precursor structure, and capillary LC-MS/MS analysis for elucidation of its posttranslational modifications. In an actual application, we identified an alpha-amidated neuropeptide from the red swamp crayfish (Procambarus clarkii) brain. Initially, an MS number of around m/z 1382 was found by the direct MALDI-TOF MS analysis with slices of the accessory lobe of the brain. After two steps of reversed-phase HPLC separation with brain extract, the structure of a 1381 Da peptide was sequenced to the GYRKPPFNGSIFamide (named crustacean-SIFamide). Subsequently, the cDNA has been characterized and encodes a 76 amino acid precursor protein that contains a signal sequence, one copy of GYRKPPFNGSIFG and one additional peptide. The RT-PCR analysis implied that the mRNA of the neuropeptide was expressed throughout the nervous system of the crayfish. Furthermore, immunostaining demonstrated that the neuropeptide is distributed in the olfactory lobe, accessory lobe, olfactory globular tract, and olfactory lobe cells. In addition, database searches revealed that there are homologous sequences of the AYRKPPFNGSIFamide in the genome library of fruitfly Drosophila melanogaster and AYRKPPFNGSLFamide isolated from the grey fleshfly Neobellieria bullata, and GYRKPPFNGSIFamide isolated from the giant tiger prawn Penaeus monodon. These results suggested that the neuropeptide family might be widely distributed in arthropods and plays a significant role in the nervous system.

Distribution and function of an Aplysia cardioexcitatory peptide, NdWFamide, in pulmonate snails

The distribution and function of an Aplysia cardioexcitatory peptide, NdWFamide, were examined in the nervous system of pulmonate snails. We chemically identified the authentic NdWFamide from a land snail (Euhadra congenita) and a freshwater snail (Lymnaea stagnalis). NdWFamide potentiated the heartbeat of those snails. Immunohistochemistry using anti-NdWFamide antibody demonstrated the distribution of NdWFamide-containing neurons and fibers in the central nervous system, as well as peripheral tissues, such as the cardiovascular region and accessory sex organs. These results suggest that NdWFamide is a neuropeptide mediating the neural regulation of the activity of the cardiovascular and reproductive systems of snails.

Cell biology of peroxisomes and their characteristics in aquatic organisms

The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.

Fulicin regulates the female reproductive organs of the snail, Achatina fulica

Fulicin is a D-amino acid-containing neuropeptide that has been thought to control male copulatory behavior in the land snail, Achatina fulica. In the present study, we demonstrated that the vagina and the oviduct of Achatina were densely innervated by fulicin-like immunoreactive neuronal fibers. We confirmed that fulicin was actually present in the vagina by mass spectrometry. Furthermore, fulicin showed a profound excitatory effect on contractions of the vagina and the oviduct. These results suggest that fulicin controls female egg-laying behavior as an excitatory neuropeptide regulating the female reproductive organs of the snail.

Identification of orcokinin gene-related peptides in the brain of the crayfish Procambarus clarkii by the combination of MALDI-TOF and on-line capillary HPLC/Q-Tof mass spectrometries and molecular cloning

We developed a strategy for the exploration of brain peptides in the red swamp crayfish, Procambarus clarkii, utilizing the combined techniques of matrix-assisted laser desorption/ionization with time-of-flight mass spectrometry (MALDI-TOF MS), molecular cloning, and on-line capillary reversed-phase HPLC/quadrupole orthogonal acceleration time-of-flight (Q-Tof)-MS. We initially performed direct MALDI-TOF MS analysis with slices of the brain. The MS spectra from a slice of the olfactory lobe indicated that an orcokinin (NFDEIDRSGFGFN) occurs in this species. Subsequently, its occurrence was confirmed by molecular cloning of the cDNAs encoding the precursor protein of orcokinin. The deduced amino acid sequences indicated that there are two different types of preproorcokinins. Preproorcokinin A (251 residues long) contains not only seven copies of orcokinin but also two copies of NFDEIDRSGFGFV and one copy each of NFDEIDRSGFGFA, NFDEIDRTGFGFH, and FDAFTTGFGHS. The former three peptides were previously isolated from another crayfish, Orconectes limosus, and/or the shore crab, Carcinus maenas, and the latter two were novel. Preproorcokinin B (266) harbors one additional orcokinin. All sequences of the peptides are flanked by dibasic sequences which are the consensus signal for processing. Moreover, brain extract was subjected to Sephadex G-25 and, subsequently, to on-line capillary reversed-phase HPLC/Q-Tof MS analysis. From the LC-MS analysis, the molecular weights of orcokinin, NFDEIDRSGFGFV, NFDEIDRSGFGFA, NFDEIDRTGFGFH, and FDAFTTGFGHS were identified as the doubly charged ions at m/z 759.37, 751.92, 737.86, 777.90, and 593. 78, respectively. In addition, the sequences were assigned by the collision-induced dissociation spectra using the doubly charged ions in the LC-MS/MS analysis. These data suggest that orcokinin and its related peptides are especially abundant in the olfactory lobe and are synthesized and processed from the two types of preproorcokinins in the crayfish brain.

Seasonal variation of peroxisomal enzyme activities and peroxisomal structure in mussels Mytilus galloprovincialis and its relationship with the lipid content

Variations in structure and function of peroxisomes in digestive tissues of marine mussels have been proposed to be valid biomarkers of environmental contamination by organic xenobiotics. The aim of the present work was to study the seasonal variations in peroxisomal enzyme activities, catalase, palmitoyl-CoA oxidase (AOX) and D-amino acid oxidase (DAOX) and peroxisomal structure in mussels. Peroxisomal changes were related to the seasonal variation in the contents of neutral lipids in the digestive gland as the lipid metabolism in mussels is subjected to seasonal variations linked to the reproductive cycle and food intake. Significant higher catalase activities were recorded from April to June when compared to the rest of the year and AOX activity was also markedly induced during the late winter and spring (February to May) with maximal activities in April. DAOX activity did not vary seasonally, the highest activities being measured in November, February and May. Stereological studies of peroxisomes in digestive tubule cells revealed significantly higher volume, surface and numerical densities in animals collected in spring and summer. Changes in peroxisomal volume density were found to be significantly and positively correlated with changes in AOX and catalase activities. The peroxisomal structure and enzyme activities were negatively correlated with the lipid contents of digestive tubules. In April, the volume density occupied by neutral lipids was higher in duct epithelia while previously it was higher in digestive tubule epithelia. This trend was maintained during the period in which peroxisomes were more abundant until July. In the following September, digestive tubules recovered their lipid load. It is concluded that seasonal changes related to food intake and reproductive cycle induce changes in peroxisomal parameters that can be compared to typical peroxisome proliferation, with a 25-fold increase in AOX activity and an 8-fold increase in the peroxisomal numerical density in early spring.

Characterization of a cDNA encoding a precursor polypeptide of a D-amino acid-containing peptide, achatin-I and localized expression of the achatin-I and fulicin genes

Achatin-I and fulicin, isolated from the ganglia and atria of the giant land snail Achatina fulica, are a tetrapeptide and pentapeptide containing a d-Phe and d-Asn at position 2, respectively. We succeeded in cloning a cDNA encoding a precursor of achatin-I from the Achatina ganglia, revealing that the d-Phe present in achatin-I is coded by a common l-Phe codon, TTT or TTC. The deduced polypeptide was found to comprise seven repeats of the achatin sequence GFAD and one analog GFGD flanked on both sides by the typical endoproteolytic site KR. Northern blot analysis of transcripts and Southern blot analysis of reverse transcription (RT)-PCR products demonstrated that achatin-I mRNA was localized in the subesophageal ganglia, whereas expression of fulicin mRNA was detected in the atrium as well as in the subesophageal ganglia. Furthermore, localization of the achatin gene transcript in the right and left pedal ganglia compartments was shown by in situ hybridization on sections of subesophageal ganglia, whereas the fulicin transcript was observed in the right and left parietal ganglia. These data suggested that achatin-I plays an essential role in the regulation of the heart as a neurotransmitter or neurohormone through production in the pedal ganglia and transport to the atrium, whereas fulicin serves not only as a neurotransmitter or neurohormone but also as a novel atrial hormone.

Biosynthesis of D-amino acid-containing peptides: exploring the role of peptide isomerases

The discovery of D-amino acid residues in a growing number of gene-encoded peptides suggests that such biochemical modifications are more common than initially thought. In fact, the extent to which D-amino acids are incorporated into peptides by multicellular organisms probably has not been fully realized, since routine Edman sequencing does not provide the absolute stereochemistry of amino acid residues. Unless both the D and L isomers of a particular peptide sequence are isolated, D-amino acid-containing peptides are often identified only after synthesis of naturally-occurring peptide fails to yield the desired activity. To date, D-amino acid residues (e.g., alanine, methionine, leucine, isoleucine, phenyl alanine, asparagine, tryptophan and serine) have been identified in peptides from a variety of species, including frogs, snails, clams, lobsters and spiders. While most have a single D-amino acid residue located near their N-termini, an exception is found with omega-Aga IVB. The examples highlighted in this chapter are the result of a unique strategy of multicellular organisms to circumvent stereochemical limitations imposed by the genetic code in an effort to increase molecular diversity. The presence of D-amino acids permits the generation of novel tertiary structure that could not be accessed from L-amino acids alone. Moreover, advantages of increased potency and protease stability are often observed. Our understanding of the biosynthesis of these D-amino acid-containing peptides is still in its infancy. Nevertheless, the discovery of a novel peptide isomerase from the venom of the Agelenopsis aperta spider provides some important clues to explain the incorporation of single D-amino acid residues within a peptide chain. Given its high homology with other serine proteases, the isomerase may represent an opportune mutation in response to evolutionary pressures. Yet, is the isomerase a unique exception or simply the first in a class of enzymes of varying substrate specificity capable of synthesizing D-amino acid-containing peptides? To be sure, much more remains to be explored about the precise timing and mechanism of the isomerization process, in addition to obtaining further structural data on the enzyme itself. Therein lies the continuation of this fascinating story in enzyme biochemistry.

Demonstration of a cell-specific isomerization of invertebrate neuropeptides

Neurohemal organs of the lobster Homarus americanus contain isoforms of the crustacean hyperglycemic hormone, which differ by the third amino acid (phenylalanyl) residue that is either in the L- or in the D-configuration. Polyclonal antisera have been raised in rabbit against synthetic octapeptides with the sequence corresponding to the N-terminal part of the L- or D-phenylalanine-containing isoforms. Their specificity was shown by immunoassays, indicating that they discriminate the isoforms of the lobster hyperglycemic neuropeptides. It was demonstrated that the two major forms of the crayfish Orconectes limosus hyperglycemic hormone also correspond to peptide isomers containing the L- or D-phenylalanyl residue. The cellular distribution of the isoforms among the neurosecreting cells of the major neuroendocrine complex in lobster and crayfish has been studied by immunohistochemistry. Every hyperglycemic hormone-containing cell was labelled with the anti-L antisera while only some of them were visualized with the anti-D antisera. These results constitute the first observation of peptide isomerization at the cellular level and suggest that the isomerization process occurs in specialized neuroendocrine cells.

Molluscan neuropeptides

Achatin-I, fulicin, fulyal, Mytilus-FFRFamide and Helix CCAP-RP-III are D-amino acid-containing neuropeptides from molluscs. Achatin-I, fulicin and fulyal from Achatina showed excitatory and/or modulatory actions on the penis retractor, radula retractor or ventricular muscles and neurons, though their L isomers were devoid of activity. On the other hand, both Mytilus-FFRFamide and its L isomer showed excitatory effects on the anterior byssus retractor muscle. Moreover, in contrast to Achatina neuropeptides, Helix CCAP-RP-III exhibited no remarkable activities on any of the muscles tested; instead, its L isomer possessed various excitatory effects. The molecular structures of these short peptides would be affected by the L-->D conversion and could influence activity. Molecular biological studies on the fulicin precursor suggest that fulicin, fulyal and related peptides are produced in Achatina ganglia and heart by processing of the ribosomally made precursor, and that L-isomeric fulicin and fulyal further undergo epimerization to yield the D-isomers.

A novel D-amino acid-containing peptide, fulyal, coexists with fulicin gene-related peptides in Achatina atria

Fulicin (Phe-D-Asn-Glu-Phe-Val-NH2) is a neuropeptide from ganglia of the African giant snail (Achatina fulica). Previously, the sequences of nine fulicin gene-related peptides (FGRP-1 to -9) have been predicted from the cDNA encoding the ganglia fulicin precursor and the transcripts have been detectable in the heart. We synthesized twenty peptides related to fulicin and FGRPs containing either an L- or a D-amino acid at position 2 and used them to identify FGRPs in atrial extracts. We identified ten alpha-amidated peptides, including fulicin and confirmed their structures as follows: Tyr-Ala-Glu-Phe-Leu-NH2 (FGRP-9), [D-Ala2]FGRP-9 (fulyal), [L-Asn2]fulicin, fulicin, Ser-Tyr-Asp-Phe-Val-NH2 (FGRP-2), Thr-Tyr-Asp-Phe-Leu-NH2 (FGRP-3), Tyr-Asp-Phe-Ile-NH2 (FGRP-5), Ser-Pro-Tyr-Asp-Phe-Ile-NH2 (FGRP-6), Asn-Tyr-Asp-Phe-Val-NH2 (FGRP-7) and Ser-Pro-Tyr-Asp-Phe-Val-NH2 (FGRP-8). We analyzed the biological activities of synthetic FGRPs using the snail penis retractor muscle. The results revealed that fulyal remarkably potentiated the tetanic contraction at concentrations as low as 10(-12) M. FGRP-9 was about 10,000-fold less potent. Fulyal, like fulicin, seems to undergo preferential maturation to participate in the penis retractor muscle contraction as a neuropeptide containing a D-amino acid.

D-amino acids in animal peptides

D-amino acids have been detected in a variety of peptides synthesized by animal cells. These include opiate and antimicrobial peptides from amphibian skin, neuropeptides from snail ganglia, a hormone from crustaceans, and a constituent of a spider venom. cDNA cloning has shown that at those positions where a D-amino acid is found in the end-product, a normal codon for the corresponding L-amino acid is present. This implies that the D-residues are formed from L-amino acids by a posttranslational reaction. A prototype enzyme catalyzing such a reaction has recently been isolated from the venom of the funnel web spider.

Contryphan is a D-tryptophan-containing Conus peptide

In this report, we document for the first time the occurrence of D-tryptophan in a normally translated polypeptide, contryphan. The peptide, isolated from the venom of the fish-hunting marine snail Conus radiatus, produces the "stiff-tail" syndrome in mice. Characterization of the octapeptide gave the following sequence, Gly-Cys-Hyp-D-Trp-Glu-Pro-Trp-Cys-NH2 where Hyp = 4-trans-hydroxyproline. The presence of D-tryptophan in position 4 of contryphan was confirmed by chemical synthesis. The post-translational epimerization in all other D-amino acid-containing small peptides characterized previously from vertebrates and molluscan systems is in position 2.