Oxytoxins, bioactive molecules produced by the marine opisthobranch mollusc Oxynoe olivacea from a diet-derived precursor

Bioactive Compounds from Marine Heterobranchs

The natural products of heterobranch molluscs display a huge variability both in structure and in their bioactivity. Despite the considerable lack of information, it can be observed from the recent literature that this group of animals possesses an astonishing arsenal of molecules from different origins that provide the molluscs with potent chemicals that are ecologically and pharmacologically relevant. In this review, we analyze the bioactivity of more than 450 compounds from ca. 400 species of heterobranch molluscs that are useful for the snails to protect themselves in different ways and/or that may be useful to us because of their pharmacological activities. Their ecological activities include predator avoidance, toxicity, antimicrobials, antifouling, trail-following and alarm pheromones, sunscreens and UV protection, tissue regeneration, and others. The most studied ecological activity is predation avoidance, followed by toxicity. Their pharmacological activities consist of cytotoxicity and antitumoral activity; antibiotic, antiparasitic, antiviral, and anti-inflammatory activity; and activity against neurodegenerative diseases and others. The most studied pharmacological activities are cytotoxicity and anticancer activities, followed by antibiotic activity. Overall, it can be observed that heterobranch molluscs are extremely interesting in regard to the study of marine natural products in terms of both chemical ecology and biotechnology studies, providing many leads for further detailed research in these fields in the near future.

Terpenoids in Marine Heterobranch Molluscs

Heterobranch molluscs are rich in natural products. As other marine organisms, these gastropods are still quite unexplored, but they provide a stunning arsenal of compounds with interesting activities. Among their natural products, terpenoids are particularly abundant and diverse, including monoterpenoids, sesquiterpenoids, diterpenoids, sesterterpenoids, triterpenoids, tetraterpenoids, and steroids. This review evaluates the different kinds of terpenoids found in heterobranchs and reports on their bioactivity. It includes more than 330 metabolites isolated from ca. 70 species of heterobranchs. The monoterpenoids reported may be linear or monocyclic, while sesquiterpenoids may include linear, monocyclic, bicyclic, or tricyclic molecules. Diterpenoids in heterobranchs may include linear, monocyclic, bicyclic, tricyclic, or tetracyclic compounds. Sesterterpenoids, instead, are linear, bicyclic, or tetracyclic. Triterpenoids, tetraterpenoids, and steroids are not as abundant as the previously mentioned types. Within heterobranch molluscs, no terpenoids have been described in this period in tylodinoideans, cephalaspideans, or pteropods, and most terpenoids have been found in nudibranchs, anaspideans, and sacoglossans, with very few compounds in pleurobranchoideans and pulmonates. Monoterpenoids are present mostly in anaspidea, and less abundant in sacoglossa. Nudibranchs are especially rich in sesquiterpenes, which are also present in anaspidea, and in less numbers in sacoglossa and pulmonata. Diterpenoids are also very abundant in nudibranchs, present also in anaspidea, and scarce in pleurobranchoidea, sacoglossa, and pulmonata. Sesterterpenoids are only found in nudibranchia, while triterpenoids, carotenoids, and steroids are only reported for nudibranchia, pleurobranchoidea, and anaspidea. Many of these compounds are obtained from their diet, while others are biotransformed, or de novo biosynthesized by the molluscs. Overall, a huge variety of structures is found, indicating that chemodiversity correlates to the amazing biodiversity of this fascinating group of molluscs.

Acetylenic Aquatic Anticancer Agents and Related Compounds

Although acetylenes are common as components of terrestrial plants, it is only within the last 30 years that biologically active polyacetylenes having unusual structural features have been reported from aquatic organisms: cyanobacteria, algae, fungi, invertebrates, and other sources. Naturally occurring aquatic acetylenes are of particular interest since many of them display important biological activities and possess antitumor, antibacterial, antimicrobial, antifouling, antifungal, pesticidal, phototoxic, HIV inhibitory, and immuno-suppressive properties. There is no doubt that they are of great interest, especially for the medicinal and/or pharmaceutical industries. This review presents structures and describes cytotoxic and anticancer activities of more than 230 acetylenic metabolites isolated from aquatic organisms. With the computer program PASS some additional biological activities are also predicted, which point toward possible new applications of these compounds. This review emphasizes the role of aquatic acetylenic compounds as an important source of leads for drug discovery.

Investigation of the electrophilic reactivity of the biologically active marine sesquiterpenoid onchidal and model compounds

The structure of the sesquiterpene onchidal (6), a component of the defensive secretion of the shell-less mollusc Onchidella binneyi, contains a masked α,β-unsaturated 1,4-dialdehyde moiety, the presence of which has been proposed to be the cause of the feeding deterrent activity exhibited by the mollusc. We have found onchidal acts as an electrophile, reacting rapidly with the model nucleophile n-pentylamine forming diastereomeric aminated pyrrole adducts. Somewhat surprisingly, no reaction was observed between onchidal and n-pentanethiol. Structurally simplified n-pentyl 11–13 and cyclohexylmethyl 15–17 analogues of onchidal were prepared and demonstrated similar amine-selective reactivity. Onchidal and analogues reacted with the model protein lysozyme, forming covalent adducts and leading to protein cross-linking. These results provide preliminary evidence supporting the molecular mechanism of biological activity exhibited by onchidal.

Chemical mediation as a structuring element in marine gastropod predator-prey interactions

Some diterpenoid compounds protect the sacoglossansElysiasp. andCyerce nigricansfrom their carnivorous predator the dorid nudibranch,Gymnodorissp., unlike chemically unprotected gastropods that are consumed by this voracious nudibranch (photo Philippe Bourseiller).

Oxytoxin-2, An Algal-Derived Molecule from a Cultured Mollusc Volvatella vigourouxi

A natural caulerpenyne-derived sesquiterpene, oxytoxin-2 (1), was isolated from a cultured mollusc Volvatella vigourouxi. The structure of 1 was elucidated on the basis of spectroscopic analysis and this compound was suggested to be a diet-derived metabolite from the green alga Caulerpa sertularioides. This is the first study on the chemical constituents of V. vigourouxi. Oxytoxin-2 (1) was found to inhibit significantly the expression of the pro-inflammatory inducible nitric oxide synthase (iNOS) protein of lipopolysaccharide (LPS)-stimulated RAW264.7 macrophage cells.

Anticancer activity of natural and synthetic acetylenic lipids

This review is a comprehensive survey of acetylenic lipids and their derivatives, obtained from living organisms, that have anticancer activity. Acetylenic metabolites belong to a class of molecules containing triple bond(s). They are found in plants, fungi, microorganisms, and marine invertebrates. Although acetylenes are common as components of terrestrial plants, fungi, and bacteria, it is only within the last 30 years that biologically active polyacetylenes having unusual structural features have been reported from plants, cyanobacteria, algae, invertebrates, and other sources. Naturally occurring aquatic acetylenes are of particular interest since many of them display important biological activities and possess antitumor, antibacterial, antimicrobial, antifouling, antifungal, pesticidal, phototoxic, HIV-inhibitory, and immunosuppressive properties. There is no doubt that they are of great interest, especially for the medicinal and/or pharmaceutical industries. This review presents structures and describes cytotoxic and anticancer activities only for more than 300 acetylenic lipids and their derivatives isolated from living organisms.

New caulerpenyne-derived metabolites of an Elysia sacoglossan from the South Indian coast

Chemical analysis of the secondary metabolite pattern of the sacoglossan mollusc Elysia cf. expansa, collected along South Indian coasts, showed the presence of the typical Caulerpa-derived sesquiterpene caulerpenyne (1) and two new minor co- occurring metabolites, the compounds dihydrocaulerpenyne (4) and expansinol (5). The chemical characterization of these molecules, structurally related to 1, is reported.

Palatability of Macroalgae that Use Different Types of Chemical Defenses

This study compared algal palatability and chemical defenses from subtropical green algae that may use different types of defense systems that deter feeding by the rock-boring sea urchin Echinometra lucunter. The potential defense systems present include (1) the terpenoid caulerpenyne and its activated products from Caulerpa spp., and (2) dimethylsulfoniopropionate (DMSP)-related defenses in Ulva spp. Secondary metabolites from these chemical groups have been shown to deter feeding by various marine herbivores, including tropical and temperate sea urchins. Live algal multiple-choice feeding assays and assays incorporating algal extracts or isolated metabolites into an artificial diet were conducted. Several green algae, including Ulva lactuca, Caulerpa prolifera, and Cladophora sp., were unpalatable. Nonpolar extracts from U. lactuca deterred feeding, whereas nonpolar extracts from C. prolifera had no effect on feeding. Polar extracts from both species stimulated feeding. Caulerpenyne deterred feeding at approximately 4% dry mass; however, dimethyl sulfide and acrylic acid had no effect at natural and elevated concentrations. E. lucunter is more tolerant than other sea urchins to DMSP-related defenses and less tolerant to caulerpenyne than many reef fish. Understanding the chemical defenses of the algae tested in this study is important because they, and related species, frequently are invasive or form blooms, and can significantly modify marine ecosystems.

Identification of low molecular weight molecules as new components of the nacre organic matrix

Nacre of Pinctada margaritifera displays a number of interesting biological activities on bone, mainly concentrated in a water-soluble organic matrix representing 0.24% of the nacre weight. Dialysis of that matrix through 8 kDa and 1 kDa cut-off membranes showed that 60% of it is made of small molecules of molecular masses below 1 kDa. Reversed-phase high-performance liquid chromatography of the small molecule fractions and subsequent electrospray ionization mass spectrometric analysis of 19 fractions thereof indicated the presence of at least 110 different molecules, in the range 100 Da-700 Da. Evidence for aggregate-forming properties of the small molecules was given. Amino acid analysis revealed that most of the small molecules were not peptides and tandem mass spectrometric gas-phase fragmentations clearly indicated a structural relationship between several molecules. Intriguingly, differences of a single Dalton between mono-charged ions peaks were observed. Further, approximately 40 analytes could be arranged in a ladder-like manner with mass spaces of 57 Da. Some of the water-soluble peptide sequences obtained after MS/MS fragmentation revealed that the 57 Da shift corresponds to the repetition of glycine residues. Furthermore, the exchange of glycine against alanine explains the 14 Da shift observed between some peptides. These data show for the first time that small molecules, especially peptides, are prevalent components of nacre. The molecular species described in this report might have a functional role in nacre.

Two novel aromatic valerenane-type sesquiterpenes from the Chinese green alga Caulerpa taxifolia

Caulerpal A (2) and B (3), two novel sesquiterpenes possessing an uncommon aromatic valerenane-type carbon skeleton, along with one known metabolite, caulerpin (4), have been isolated from the Chinese green alga Caulerpa taxifolia (Vahl) C. Agardh. Their structures and relative stereochemistry were elucidated on the basis of extensive spectroscopic analysis. Compounds 2-4 were evaluated for their inhibitory activity against hPTP1B and the result showed that only compound 4 had a strong PTP1B inhibitory activity with an IC50 value of 3.77 microM.

Marine natural products

This review covers the literature published in 2004 for marine natural products, with 693 citations (491 for the period January to December 2004) 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 (716 for 2004), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies (8), and syntheses (80), including those that lead to the revision of structures or stereochemistries, have been included.

Secondary Metabolites from the Marine Alga Caulerpa brownii (Chlorophyta)

The green seaweed Caulerpa brownii (Chlorophyta, Caulerpaceae) occurs in Tasmania in two morphological forms (branched and unbranched) and each form has a different profile of diterpenoid secondary metabolites. Unbranched specimens gave rise to the novel secondary metabolites 11, 13, 14, 17, and 18, the secondary metabolite 8 that has been isolated for the first time as a natural product, as well as the known compounds 1 and 3–7. Branched specimens of C. brownii yielded the novel terpenoid esters 21 and the known compounds 1 and 2.

Comparison of the wound-activated transformation of caulerpenyne by invasive and noninvasive Caulerpa species of the Mediterranean

The invasive green alga, Caulerpa taxifolia, that has spread rapidly after its introduction into the Mediterranean and the North American Pacific, reacts to wounding by transforming its major metabolite caulerpenyne. This wound-activated reaction involves the transformation of the bis-enol acetate moiety of 1, releasing reactive 1,4-dialdehydes. The ability to perform this transformation is found also in both the noninvasive Mediterranean C. prolifera and the invasive C. racemosa. Trapping experiments, as well as transformation of the model substrate geranyl acetate, suggest that all three investigated Caulerpa spp. rely on esterases that act upon wounding of the algae by subsequently removing the three acetate residues of caulerpenyne. The resulting reactive 1,4-dialdehyde oxytoxin 2 can be identified by liquid chromatography-mass spectrometry and is unstable in the wounded tissue. Caulerpenyne transformation occurs rapidly, and severe tissue damage caused degradation of more than 50% of the stored caulerpenyne within 1 min in all three algae. Prevention of the enzymatic reaction before extraction, by shock freezing the tissue with liquid nitrogen, was used for the determination of the caulerpenyne content in intact algae. It gives about twofold higher values compared to an established methanol extraction protocol. The speed and mechanism of the wound-activated transformation, as well as the caulerpenyne content in intact tissue of invasive and noninvasive Caulerpa spp., are comparable. Thus, this enzymatic transformation, despite being fast and efficient, is likely not the key for the success of the investigated invasive species.

Are the Mediterranean ascoglossan molluscs Oxynoe olivacea and Lobiger serradifalci suitable agents for a biological control against the invading tropical alga Caulerpa taxifolia?

The Mediterranean ascoglossan Oxynoe olivacea and Lobiger serradifalci, which are scarce in meadows of their usual food the alga Caulerpa prolifera, have become adapted to feeding on the invading tropical alga Caulerpa taxifolia. Grazing rates are low (O. olivacea destroys only a 5-cm C. taxifolia frond in 3-7 d). The grazing rates are a function of temperature. Despite a high spawning frequency (0.25 egg-mass.day-1) and a large number of eggs released per spawning event (O. olivacea: from 2,545 to 29,128 eggs.spawning-1; L. serradifalci: from 12,112 to 48,448 eggs.spawning-1), the recruitment on Caulerpa meadows is usually low due to the hazardous pelagic development of the larvae. The possible use of these molluscs as agents of biological control against C. taxifolia appears to be possible only through an artificial enhancement of their populations after cultivation of the veligers and release of juveniles during the winter season. This means of control is discussed and compared with the possible use of tropical ascoglossan species.

Volvatellin, caulerpenyne-related product from the sacoglossan volvatella sp

Volvatellin (4) is a highly unstable terpene isolated from the extracts of the Indian opisthobranch mollusk Volvatella sp. The structure and the relative stereochemistry of 4 were determined by NMR methods. The paper also describes a hypothetical biosynthesis of 4 starting from the alga-derived caulerpenyne.