Plicatamide: A lead to the biosynthetic origins of the tunichromes?

Natural Products Diversity of Marine Ascidians (Tunicates; Ascidiacea) and Successful Drugs in Clinical Development

This present study reviewed the chemical diversity of marine ascidians and their pharmacological applications, challenges and recent developments in marine drug discovery reported during 1994-2014, highlighting the structural activity of compounds produced by these specimens. Till date only 5% of living ascidian species were studied from <3000 species, this study represented from family didemnidae (32%), polyclinidae (22%), styelidae and polycitoridae (11-12%) exhibiting the highest number of promising MNPs. Close to 580 compound structures are here discussed in terms of their occurrence, structural type and reported biological activity. Anti-cancer drugs are the main area of interest in the screening of MNPs from ascidians (64%), followed by anti-malarial (6%) and remaining others. FDA approved ascidian compounds mechanism of action along with other compounds status of clinical trials (phase 1 to phase 3) are discussed here in. This review highlights recent developments in the area of natural products chemistry and biotechnological approaches are emphasized.

Synthesis and biological evaluation of the ascidian blood-pigment halocyamine A

The first synthesis of the (Z)-indolic enamide-containing antibacterial marine natural product halocyamine A is reported.

Structure, biosynthesis and possible function of tunichromes and related compounds

Several species of ascidians (phylum Chordata, subphylum Urochordata) contain a group of oligopeptides called "tunichromes" in their blood cells. These peptides have been implicated in (a) metal chelation and accumulation/sequestration of vanadium or iron; (b) crosslinking of structural fibers in tunic formation, (c) wound healing and (d) defense reactions. However, their biosynthesis, metabolism, and biological function remain largely un-elucidated due to their extreme instability and high reactivity. Tunichromes and related compounds uniquely possess dehydrodopamine moieties, all originating from post-translational modification of peptidyl tyrosine. It is conceivable that the presence of such novel post-translationally modified groups provide attributes that are crucial for their biological roles. Therefore, we examined the chemistry and reactivity of tunichromes in light of the available knowledge of the biochemistry of simple monomeric dehydro-N-acyldopamine units. Based on the reactivity of such simple compounds, the potential biological activities of tunichromes are predicted. Their possible biosynthetic route from peptidyl tyrosine is critically evaluated to provide a better basis for unraveling their biological functions. Prevalence of dehydro-N-acyldopamine units in different tunichromes, some marine antibiotic compounds, insect cuticular sclerotizing precursors and some bioadhesive marine proteins may aid in the de novo design of unique biomaterials with potential antibiotic/adhesive properties.

Antimicrobial peptides from marine invertebrates: challenges and perspectives in marine antimicrobial peptide discovery

The emergence of pathogenic bacteria resistance to conventional antibiotics calls for an increased focus on the purification and characterization of antimicrobials with new mechanisms of actions. Antimicrobial peptides are promising candidates, because their initial interaction with microbes is through binding to lipids. The interference with such a fundamental cell structure is assumed to hamper resistance development. In the present review we discuss antimicrobial peptides isolated from marine invertebrates, emphasizing the isolation and activity of these natural antibiotics. The marine environment is relatively poorly explored in terms of potential pharmaceuticals, and it contains a tremendous species diversity which evolved in close proximity to microorganisms. As invertebrates rely purely on innate immunity, including antimicrobial peptides, to combat infectious agents, it is believed that immune effectors from these animals are efficient and rapid inhibitors of microbial growth.

Bioactive dehydrotyrosyl and dehydrodopyl compounds of marine origin

The amino acid, tyrosine, and its hydroxylated product, 3,4-dihydroxyphenylalanine (dopa), plays an important role in the biogenesis of a number of potentially important bioactive molecules in marine organisms. Interestingly, several of these tyrosyl and dopa-containing compounds possess dehydro groups in their side chains. Examples span the range from simple dehydrotyrosine and dehydrodopamines to complex metabolic products, including peptides and polycyclic alkaloids. Based on structural information, these compounds can be subdivided into five categories: (a) Simple dehydrotyrosine and dehydrotyramine containing molecules; (b) simple dehydrodopa derivatives; (c) peptidyl dehydrotyrosine and dehydrodopa derivatives; (d) multiple dehydrodopa containing compounds; and (e) polycyclic condensed dehydrodopa derivatives. These molecules possess a wide range of biological activities that include (but are not limited to) antitumor activity, antibiotic activity, cytotoxicity, antioxidant activity, multidrug resistance reversal, cell division inhibition, immunomodulatory activity, HIV-integrase inhibition, anti-viral, and anti-feeding (or feeding deterrent) activity. This review summarizes the structure, distribution, possible biosynthetic origin, and biological activity, of the five categories of dehydrotyrosine and dehydrodopa containing compounds.

Halocyntin and papillosin, two new antimicrobial peptides isolated from hemocytes of the solitary tunicate, Halocynthia papillosa

We report here the screening of five marine invertebrate species from two taxa (tunicates and echinoderms) for the presence of cationic antimicrobial peptides (AMP) in defence cells (hemocytes). Antimicrobial activities were detected only in the two tunicates Microcosmus sabatieri and Halocynthia papillosa. In addition, we report the isolation and characterization of two novel peptides from H. papillosa hemocytes. These molecules display antibacterial activity against Gram-positive and Gram-negative bacteria. Complete peptide characterization was obtained by a combination of Edman degradation and mass spectrometry. The mature molecules, named halocyntin and papillosin, comprise 26 and 34 amino acid residues, respectively. Their primary structure display no significant similarities with previously described AMP.

Utilizing the Power of Microbial Genetics to Bridge the Gap Between the Promise and the Application of Marine Natural Products

Marine organisms are a rich source of secondary metabolites. They have yielded thousands of compounds with a broad range of biomedical applications. Thus far, samples required for preclinical and clinical studies have been obtained by collection from the wild, by mariculture, and by total chemical synthesis. However, for a number of complex marine metabolites, none of these options is feasible for either economic or environmental reasons. In order to proceed with the development of many of these promising therapeutic compounds, a reliable and renewable source must be found. Over the last twenty years, the study of microbial secondary metabolites has greatly advanced our understanding of how nature utilizes simple starting materials to yield complex small molecules. Much of this work has focused on polyketides and nonribosomal peptides, two classes of molecules that are prevalent in marine micro- and macroorganisms. The lessons learned from the study of terrestrial metabolite biosynthesis are now being applied to the marine world. As techniques for cloning and heterologous expression of biosynthetic pathways continue to improve, they may provide our greatest hope for bridging the gap between the promise and application of many marine natural products.

Marine natural products

This review covers the literature published in 2003 for marine natural products, with 619 citations (413 for the period January to December 2003) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (656 for 2003), together with their relevant biological activities, source organisms and country or origin. Biosynthetic studies or syntheses that lead to the revision of structures or stereochemistries have been included (78), including any first total syntheses of a marine natural product.

Plicatamide, an antimicrobial octapeptide from Styela plicata hemocytes

Plicatamide (Phe-Phe-His-Leu-His-Phe-His-dc Delta DOPA), where dc Delta DOPA represents decarboxy-(E)-alpha,beta-dehydro-3,4-dihydroxyphenylalanine, is a potently antimicrobial octapeptide from the blood cells of the solitary tunicate, Styela plicata. Wild type and methicillin-resistant Staphylococcus aureus (MRSA) responded to plicatamide exposure with a massive potassium efflux that began within seconds. Soon thereafter, treated bacteria largely ceased consuming oxygen, and most became nonviable. Native plicatamide also formed cation-selective channels in model lipid bilayers composed of bacterial lipids. Methicillin-resistant S. aureus treated with plicatamide for 5 min contained prominent mesosomes as well as multiple, small dome-shaped surface protrusions that suggested the involvement of osmotic forces in its antimicrobial effects. To ascertain the contribution of the C-terminal dc Delta DOPA residue to antimicrobial activity, we synthesized several analogues of plicatamide that lacked it. One of these peptides, PL-101 (Phe-Phe-His-Leu-His-Phe-His-Tyr-amide), closely resembled native plicatamide in its antimicrobial activity and its ability to induce potassium efflux. Plicatamide was potently hemolytic for human red blood cells but did not lyse ovine erythrocytes. The small size, rapid action, and potent anti-staphylococcal activity of plicatamide and PL-101 make them intriguing subjects for future antimicrobial peptide design.

Tunichrome sp-1: new pentapeptide tunichrome from the hemocytes of Styela plicata

A modified pentapeptide has been isolated from the hemocytes of the ascidian Styela plicata. The structure of the peptide was determined by Edman sequence analysis, mass spectrometry, and NMR spectroscopy with the stereochemistry assigned by acid hydrolysis followed by both (a) GC-MS of the volatile amino acid derivatives on a chiral column and (b) ultrasensitive detection of fluorescent diasteromeric derivatives of the component amino acids after reversed-phase HPLC. The peptide L-DOPA-L-DOPA-Gly-L-Pro-dcdeltaDOPA (where DOPA = 3,4-dihydroxyphenylalanine and dcdeltaDOPA = decarboxy-(E)-alpha,beta-dehydro-3,4-dihydroxyphenylalanine) we designate as tunichrome Sp-1.

Fragmentation of a novel marine peptide, plicatamide, involves an unusual gas-phase intramolecular rearrangement

During our characterization of plicatamide 1, a modified octapeptide: Phe-Phe-His-Leu-His-Phe-His-dc deltaDOPA (where dc deltaDOPA = decarboxy-(E)-alpha,beta-dehydro-3,4-dihydroxyphenylalanine) from the blood cells of the ascidian Styela plicata, we noted a series of fragment ions from the [M + H]+ ion which could not be assigned. There was no evidence in the 1H NMR spectrum to support an alternative molecular structure and the series of fragment ions were not present in the tandem mass spectrometry analysis of the [M + Na]+ ion. In addition, there was no evidence that the sample was a mixture of isobaric compounds. We propose that an unusual C-terminal to N-terminal rearrangement is responsible for the series of fragment ions from the [M + H]+ ion. This rearrangement was not observed in peptide analogs of plicatamide which did not contain the dc deltaDOPA at the C-terminus suggesting that this moiety is critical for the rearrangement. The proposed reaction is analogous to that recently reported by Vachet et al. involving a fragment ion formed from leucine enkephalin.

Styelin D, an extensively modified antimicrobial peptide from ascidian hemocytes

We isolated styelin D, a 32-residue, C-terminally amidated antimicrobial peptide, from the blood cells (hemocytes) of the solitary ascidian, Styela clava. Styelin D had remarkably extensive post-translational modifications, containing two novel amino acids, dihydroxyarginine and dihydroxylysine, and two distinctly unusual ones, 6-bromotryptophan and 3,4-dihydroxyphenylalanine. In addition, the peptide exhibited microheterogeneity because of differential mono- and dihydroxylation of several lysine residues. The primary sequence of one variant was: GW(*)LR(**)K(**)AAK(**)SVGK(**)FY(*)Y(*)K(**)HK(*)Y(*) Y(*)IK(*)AAWQIG KHAL-NH(2), where W(*) is 6-bromotryptophan, R(**) is dihydroxyarginine, Y(*) is 3,4-dihydroxyphenylalanine, K(*) is 5-hydroxylysine, and K(**) is dihydroxylysine. Styelin D exhibited activity against Gram-negative and Gram-positive bacteria, and this activity was retained in 200 mm NaCl. The role of the extensive modifications may be to preserve activity at low pH and/or high salinity because, under these conditions, the native peptide was considerably more active against the Gram-positive bacterial strains than its unmodified synthetic analogue. The peptide was also hemolytic and quite cytotoxic to eukaryotic cells. These broad ranging activities, combined with its relative abundance in ascidian hemocytes, suggest that styelin D plays a significant role in the innate immune mechanisms of S. clava.