Department of Botany, North Orissa University, Baripada - 757003, Odisha, India. *Corresponding author: E-mail: [email protected]2 Microalgae: A Potential Source of Bioactive Metabolites S. BHAKTA, E. SAHU AND A.K. BASTIA* ABSTRACT Microalgae, more specifically cyanobacteria, produce a wide array of compounds with biological activity. These include antibiotics, algaecides, toxins, pharmaceutically active compounds and plant growth regulators. Recently, microalgae have become targets for screening programmes in search of novel compounds of potential medicinal value. Secondary metabolite production by microalgae varies with environmental conditions. When these processes are better understood, microalgae might become economic sources of new drugs and other structurally specific chemicals because production can be optimized in controlled culture. Secondary metabolites from microalgae comprise a variety of substances. The chemicals involved are mostly unidentified and needed to explore for human welfare. In this review an attempt has done to through light on important bioactive compounds such as carotenoids, micosporin like aminoacids, phycocolloids, lectins, halogenated compounds, fatty acids, sterols, polyketides and other compounds used frequently for pharmaceuticals. Key words: Microalgae, Cyanobacteria, Bioactive metabolites INTRODUCTION The use of natural bioactive compounds as a source of remedies for treatment of disease till date is well realized despite of the significant contribution of synthetic drugs to modern pharmaceuticals. The growing evidence of acquired resistance by target species with consequent implications for both cost and
Transcript
Department of Botany, North Orissa University, Baripada - 757003, Odisha, India.*Corresponding author: E-mail: [email protected]
2
Microalgae: A Potential Source ofBioactive Metabolites
S. BHAKTA, E. SAHU AND A.K. BASTIA*
ABSTRACT
Microalgae, more specifically cyanobacteria, produce a wide array of compoundswith biological activity. These include antibiotics, algaecides, toxins,pharmaceutically active compounds and plant growth regulators. Recently,microalgae have become targets for screening programmes in search of novelcompounds of potential medicinal value. Secondary metabolite production bymicroalgae varies with environmental conditions. When these processes are betterunderstood, microalgae might become economic sources of new drugs and otherstructurally specific chemicals because production can be optimized in controlledculture. Secondary metabolites from microalgae comprise a variety of substances.The chemicals involved are mostly unidentified and needed to explore for humanwelfare. In this review an attempt has done to through light on important bioactivecompounds such as carotenoids, micosporin like aminoacids, phycocolloids,lectins, halogenated compounds, fatty acids, sterols, polyketides and othercompounds used frequently for pharmaceuticals.
The use of natural bioactive compounds as a source of remedies for treatmentof disease till date is well realized despite of the significant contribution ofsynthetic drugs to modern pharmaceuticals. The growing evidence of acquiredresistance by target species with consequent implications for both cost and
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sustainability, the search and screening for new and natural biologically activecompounds has become important. In recent decades microbial sources especiallythe micro algae and cyanobacteria get focused due to their diverse range ofsecondary metabolite production potentiality. Cyanobacteria and othermicroalgae have the potential to revolutionize biotechnology in a number ofareas including nutrition, agriculture, pharmaceuticals and biofuel. To cope upwith extreme adverse environment, microalgae have to develop defencestrategies that result in tremendous diversity of compounds from differentmetabolic pathways (Cardozo et al., 2007). The current application of biochemicalcompounds isolated from diverse classes of microalgae is enormous. The differentclasses of microalgae produce different bioactive compounds which are diversein their chemical structure and physiological function. Various strains ofmicroalgae are known to produce intracellular and extracellular metaboliteswith diverse biological activities such as antialgal, antifungal, antibacterial andantiviral activity. The exploration of these organisms for pharmaceutical purposeshas revealed important chemical prototypes for the discovery of new agents,stimulating the use of sophisticated physical techniques and synthesis of newcompounds with biomedical application. Chemical and environmental factorsthat influence the production of secondary metabolites in Scytonema ocellatum,which produces tolytoxin (a macrocyclic lactone that depolymerizes actin invivo to disrupt cell division in eukaryotic organisms), has shown that cyanophytescan be manipulated in culture to improve growth and product yields (Pattersonet al., 1994).
Generally the bioactive compounds are the secondary metabolites producedby the cells during secondary metabolism. But the secondary metabolism is ofrestricted distribution, while the primary metabolism furnishes intermediatesfor the synthesis of essential macromolecules. Although chemical research onthe algal product is very active, biosynthetic studies have been scanty and mainlyconcerned with secondary metabolism, which present a high structural diversity,due to modifications and combinations of reactions from the primary metabolicpathways (Fig. 1). However, with the emergence of molecular biology tools,metabolic pathways have been clarified, paving the way for generating novelbioactive metabolites in quantity by genetic engineering.
BIOACTIVE COMPOUND PRODUCING ALGAE FROM SPECIALIZEDHABITATS
Due to their ability to endure extreme conditions, the extremophiliccyanobacteria thrive on the edge of temperature, pH, pressure, hypersalinity,dryness and desiccation and posses a great capacity for producing biologicallyactive compounds. According to the investigation, the higher biological activityof terrestrial strains as representatives of extremophiles may present them assignificant bioactive compound producers. Significant biological activity (in-vivoand in-vitro) is found to happen in the case of extracts of cyanobactrial strains
Microalgae: A Potential Source of Bioactive Metabolites 23
isolated from fresh water and marine environments. It appears that extracts ofterrestrial cyanobacteria have been found to possess even greater biologicalactivity (Patterson et al., 1994). Because of their special growing condition,terrestrial cyanobacteria possess survival and adaptation mechanisms not foundin the aquatic species. It is reported that the more harsh and extreme conditionslead to a wider amplitude of metabolic extremities and possibilities, which causethe production of a most diverse range of more or less specific substances. Athermotolerant species of Phormidium produced extracellular anti-microbialmaterial which found inactive when screened against a number of othercyanobacteria but it inhibited the growth of a wide range of Gram-positive andGram-negative heterotrophic bacteria, Candida albicans and Cladosporiumresinae (Fish and Codd, 1994).
Cyanobacteria, in particular those found living in the ocean, are emergingas an important source of unique bioactive secondary metabolites. Geneticstudies on the biosynthetic capacity of these marine cyanobacteria revealedmany novel biochemical features. From extensive studies, it has been shownthat cyanobacterial species, genus Lyngbya are a potent source of unique bioactivenatural molecules. Certain strains of L majuscula are also deemed as “Super”producer of secondary metabolites, represented by different structural classes.An example of L. majuscula strain 19 L collected from the Curacao where atleast six compounds, curacin A (antimitotic), barbamide (molluscicidal),malyngamide H (brine shrimp toxic), antillatoxin (neurotoxic and ichthyotoxic),carmabines A and B, have been reported, each with unique biological properties.Currently there are well over 200 secondary metabolites, mostly nitrogencontaining molecules, being reported from marine cyanobacteria. These naturalproducts represent great structural diversity, belonging to the polyketide
Fig. 1: Main pathways for biosynthesis of some secondary and primary metabolitesbiosynthesis (modified from Burja et al., 2001).
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synthetase (PKS), non-ribosomal polypeptide synthetase (NRPS), as well ashybrid PKS-NRPS structural classes (Gerwick et al., 2001). There is a highpreponderance of compounds containing amino or hydroxyl acids, pyrrolidonerings and heterocyclic moieties such as thaizoline, thiazole, oxazoline and oxazoleflanked by amino or hydroxyl acids. A number of important marinecyanobacterial molecules, including dolastatin 10, cryptophycins and curacin A,have been discovered and these were either in preclinical or clinical testing asanticancer agents (Newman and Cragg, 2004). Marine cyanobacterial moleculeswith potent biological activities are also lead compounds for the development ofsynthetic analogs having increased potency and decreased toxicity. Some marinecyanobacterial molecules are found to target either the polymerization of actin(e.g hectochlorin) or tubulin (e.g. curacin A and dolastain 10). In addition, anumber of neurotoxins act as either blockers or activators of the mammalianvoltage- gated sodium channel (Gerwick et al., 2001). In addition, hectochlorinpossess significant antifungal activity against Candida albicans. Other noteworthy marine cyanobacterial molecules reported recently in the literaturehaving significant biological activities include the apratoxins (Luesch et al.,2001) which is a cytotoxic agent, the lyngbyabellins which is a cytotoxic andantifungal agent and the wewakpeptins (Milligan et al., 2000; Han et al., 2005).Some of these molecules possess exquisite biological properties usually in thelow micromolar or nanomolar range.
BIOACTIVE METABOLITES
Bioactive metabolites are the compounds which enables the production ofstructurally complex molecules which are difficult to synthesize chemically.Several analytical steps have to follow to achieve this goal which has beenillustrated (Fig. 2).
Pigments
Carotenoids are natural pigments derived from five-carbon isoprene units thatare polymerized enzymatically to form regular highly conjugated 40-carbonstructures (with up to 15 conjugated double bonds) (Fig. 3).
At least 600 different carotenoids exercising important biological functionsin bacteria, algae, plants and animals have been identified to date (Polivka andSundström, 2004). For human nutritional purposes, some carotenoids offerprovitamin A activity (Mayne, 1996). Provitamin A carotenoids are generallyconverted to retinal via catalysis by the intestinal enzyme b-carotene 15, 152 -monooxygenase (Lindqvist and Andersson, 2002). Vitamin A deficiency is aproblem that has prevailed in developing countries during the last decades.Many studies have associated high consumption of carotenoids with lower risksof certain pathologies (Tapiero et al., 2004). Carotenoids directly providephotoprotection against UV light photooxidation in the skin (Sies and Stahl,
Microalgae: A Potential Source of Bioactive Metabolites 25
Fig. 2: Flow diagram showing the process followed in the search for bioactive moleculesfrom algae (Borowitzka, 1995).
2004; Aust et al., 2005), while b-carotene was also shown to modulate UV-Ainduced gene expression in human keratinocytes (Wertz et al., 2004, 2005). Theketocarotenoid astaxanthin is believed to play a key role in the amelioration/prevention of several human pathological processes, such as skin UV-mediatedphotooxidation, inflammation, prostate and mammary carcinogenesis, ulcersdue to Helicobacter pylori infection and age-related diseases (Bennedsen et al.,1999; Guerrin et al., 2003).
Mycosporine-like Amino Acids
Mycosporine-like amino acids (MAAs) are a family of intracellular compoundsinvolved in the protection of aquatic organisms against solar radiation. Algae
biosynthesize MAAs while other marine organisms acquire MAAs by diet transfer,symbiotic or bacterial associations (Shick et al., 1992; Stochaj et al., 1994; Carrolland Shick, 1996) (Fig. 4).
Besides their role as a sunscreen in aquatic organisms, it has been suggestedthat some MAAs can act as antioxidants (Dunlap and Yamamoto, 1995). Glycinehas moderate antioxidant activity, providing some protection against photo-oxidative stress induced by ROS. Moreover, 4-deoxygadusol, a precursor of MAAs,has strong antioxidant properties and its retro-biosynthesis through bacterialconversion of algal MAAs has been performed for commercial applications (Masakiet al., 1996; Dunlap and Shick, 1998).
Microalgae: A Potential Source of Bioactive Metabolites 27
Fig. 4: The suggested biosynthesis of mycosporines and MAAs through shikimate pathway.Enzymes: (1) DAHP synthase, (2) DHQ synthase, (3) DHQ dehydratase, (4)shikimate dehydrogenase, (5) shikimate kinase, (6) EPSP synthase, (7) chorismatesynthase. R2: amino acids and amino alcohols characterizing individual MAAs(modified from Shick and Dunlap, 2002).
Phycocolloids
Phycocolloids are polysaccharides of high molecular weight composed of polymersof sugars units. They are the main structural components of seaweed cell wallsand may be involved in recognition mechanisms between seaweeds andpathogens (Potin et al., 1999). Although polysaccharides have been describedwith antioxidant, antiviral, antitumoral and anticoagulant activities (Mayer andLehmann, 2001; Smith, 2004), the most extracted polysaccharides from seaweedsare agar, carrageenan and alginate, due to their extensive use in food andcosmetic industries. Agar and carrageenan are sulfated polysaccharides mainlyextracted from rhodophyceae while alginate, a binary polyuronide made up ofmannuronic acid and guluronic acid, is extracted from Phaeophyceae. Although
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little commercial exploitation of agar occurs outside the hydrocolloid industry,they have been recently employed in medicinal and pharmaceutical areas suchas in a therapy against cancer cells since it can induce the apoptosis of thesecells in vitro (Chen et al., 2004). Several potential pharmaceutical uses ofcarrageenans, including antitumor, antiviral, anticoagulant andimmunomodulation activities, have been recently described (Schaeffer andKrylov, 2000; Zhou et al., 2004, 2005). Carrageenan is effective in scrofula,alimentary canal irritation, diarrhea and dysentery.
Lectins
Among the different classes of compounds with remarkable biochemical activity,it is important to emphasize lectins. Lectins or agglutinins are carbohydrate-binding proteins. In basic and medical sciences, lectins are useful for detectionof disease-related alterations of glycan synthesis, blood group typing anddefinition of secretor status, quantification of aberrations of cell surface glycanpresentation e.g., in malignancy, cell markers for diagnostic purposes includinginfectious agents (viruses, bacteria, fungi, parasites) (Rudiger and Gabius, 2001).They may also be used to target therapeutic agents for different gut componentsor even for different cells due to their property of increasing micro particleadherence to the intestinal epithelium and of enhancing penetration of drugs(Chowdary and Rao, 2004). Moreover, lectins are useful as bio-adhesives thatbind to mucosal surfaces, to deliver vaccines across mucosal surfaces, amongother pharmaceutical utilities (Jepson et al., 2004). Studies have demonstratedthat algae can be good sources of novel lectins (Chu et al., 2004; Sato et al.,2000). Most of the studies screen for haemagglutinin activity when focusing onlectins. However, there are very few studies of the isolation, characterizationand, more importantly, the biological properties of these proteins in algae,making it an open field for new research.
Halogenated Compounds
Halogenated compounds are dispersed in several different classes of primaryand secondary metabolites, including indoles, terpenes, acetogenins, phenols,fatty acids and volatile halogenated hydrocarbons (e.g., bromoform, chloroform,dibromomethane) (Dembitsky and Srebnik, 2002; Butler and Carter-Franklin,2004). In many cases, they possess biological activities of pharmacologicalinterest, including antibacterial (Vairappan et al., 2001) and antitumoral activities(Fuller et al., 1992). The most notable producers of the halogenated compoundsin the marine environment belong to the genus Laurencia (Rhodophyta)(Faulkner, 2001; Wright et al., 2003). Recently, antitumor properties ofcyanobacterial extracts, Anabaena cylindrica and Anabaena variabilis, wereattributed to brominated fatty acids, although their structures were notelucidated (Suzuki et al., 1999). In summary, halogenated compounds are presentin several different classes of primary and secondary metabolites. Halogenation
Microalgae: A Potential Source of Bioactive Metabolites 29
can increase the biological activity or induce activity. Continuing investigationof halogenated compounds is necessary to understand their biosynthesis andtheir biological effects.
Polyketides
Polyketides are an important class of secondary metabolite with an enormousimpact in the pharmaceutical industry due to their high commercial value.Phycochemical studies showed the ability of algae to produce and storepolyketides as polycyclic ether macrolides and open chain polyketides. Themajority of these compounds showed strong toxic effects and cannot be appliedin human therapy. Correlations of algae toxicogenicity with the presence ofpolyketides have been the focus of controversy. Although macrolides producedby terrestrial microorganisms have been used for a long time in humantherapeutics, microlides from microalgae were recently included in a patent.Amphidinolide B is a classical example of a marine macrolide. This class ofcompounds has varied lactone ring sizes, most of which showed high cytotoxicand antitumor activity (Kobayashi and Ishibashi, 1993).
Fatty Acids
Fatty acids with two or more methylene interrupted double bonds are essentialfor normal cell function and have entered the biomedical and nutraceuticalareas for their biological role in certain clinical conditions like obesity andcardiovascular diseases (Sayanova and Napier, 2004). Moreover, polysaturatedfatty acids (PUFAs) play key roles in cellular and tissue metabolism (Funk,2001). There is increasing interest in a typical polyunsaturated fatty acid family(-3) named eicosapentaenoic acid. EPA is a fatty acid 20 carbons in length withfive double bonds from the carboxyl terminus or with the last double bondlocated at the third carbon from the methyl [] terminus (Nettleton, 1995). Thebiosynthesis of EPA occurs through a series of reactions that can be dividedinto two distinct steps. First is the de novo synthesis of oleic acid (18:1 -9)from acetate, followed by conversion to linoleic acid (18:2 -6) and -linolenicacid (18:3 -3). The subsequent stepwise desaturation and elongation steps forman -3 PUFA (Fig. 5). Inside the cell, EPA is normally esterified (bycyclooxygenase and lipooxygenase activities) to form complex lipid moleculesand plays an important role in higher animals and humans as the precursor ofa group of eicosanoids, hormone-like substances such as prostaglandins,thromboxanes and leucotrienes that are crucial in regulating developmentaland regulatory physiology (Wen and Chen, 2003). Eicosapentanoic acid (EPA)and docosahexaenoic acid from microalgae have therapeutic importance. EPAis used in the treatment of heart and inflammatory disease. Omega 3-polyunsaturated fatty acids are also effective against rheumatoid arthritis andimmunodeficiency disease. The annual worldwide demand of EPA is 300 tons.EPA has been found in a wide variety of marine microalgal classes. However,
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Fig. 5: A simplified biosynthesis scheme of eicosapentaenoic acid and eicosanoid(Prostaglandins, Thromboxanes, Leukotrienes) (Modified from Sayanova andNapier, 2004).
only a few microalgal species have demonstrated industrial production potential.This is found in fish oil and microalgae. In microalgae it is found in the classesof Bacillariophyceae (diatoms), Chlorophyceae, Chrysophyceae, Cryptophyceae,Eustigamatophyceae and Prasinophyceae. This product from algae is superiorover fish oil in not having off flavors, is more pure, has low cholesterol contentand is inexpensive (Belarbi et al., 2000). Although some microalgal species arecultivated as sources of these fatty acids, transgenic algae engineered to produceEPA, like transgenic oilseed crops, could provide an alternative sustainablesource of oil for human consumption (Abbadi et al., 2001).
Sterols
Sterols are one of the most important chemical constituents of microalgae anda major nutritional component in the diet of aquacultured organisms.Consequently, the qualitative and quantitative variability of the sterol
Microalgae: A Potential Source of Bioactive Metabolites 31
composition of microalgae (Fig. 6) used in hatcheries has direct implications forphytosterol and for the cholesterol composition of bivalve larvae and can affectgrowth performance.
Fig. 6: Some sterols found in marine and freshwater microalgae (modified fromPonomarenko et al., 2004).
Enzyme Inhibitors
Many screening programmes focused on examination of enzyme inhibitors fromcyanobacteria. Extract of cyanobacteria have been proved to inhibit variousproteases, including trypsins, chymotrypsin, thrombin, plasmin, elastase andcollagenase (Patterson, 1996). The inhibitor of thrombin has a potential tobecome useful agent for the treatment of stroke and coronary artery-occlusion.By inhibiting the thrombin activity, conversion of fibrinogen to fibrin wasprevented, thereby it ceases the blood clotting process. A trypsin inhibitorsubstance, A90720A, a depsipeptide, found to interact like a substrate, forminga stable non-covalent complex, that resists dissociation, leads to inactivation of
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the enzyme function. LU-3 is another trypsin inhibitor produced by Nostoc sp.CALU893. The inhibitor is accumulated in the cells and is released in to themedium only after lysis. There are many other compounds reported to haveenzyme inhibitory functions, namely aeruginsin 98A, aeruginosin 298-A,micropeptin 90, microviridin B, oscillapeptin etc.
Protease inhibitors
Five classes of protease inhibitors have been reported so far from several toxicgenera of cyanobacteria: they are micropeptins, aerugenosins, microginins,anabaenopeptins and microverdins. Serine protease inhibitors of micropeptintype are the most common inhibitors from cyanobacteria with more than fiftycompounds. Some cyanopeptolins are specific inhibitors of serine proteases,including elastase, which is of critical importance in a number of diseases likelungs emphysema, which is mediated by excessive action of elastase.Furthermore, it has been proposed that un-physiologically high levels of elastaseactivity are involved in myocardial damage and may cause a particular form ofpsoriasis.
Scyptolin
These are cyclic desipeptides with elastase inhibiting activity, isolated fromterrestrial cyanobacterium Scytonema hofmanni PCC 7110. These metabolitessignificantly inhibited porcine pancreatic elastase in in-vitro assays. Scytonemajulianum has been reported as a potent inhibitor of platelet activating factor-induced platelet aggregation. Structural studies of this fraction indicated theexistence of a phosphoglyco-analog of acyl-sphingosine. Two fractions identifiedas phosphoglycolipids include phosphoglyco-analog of acyl-acetylated sphingosineand the second one as a glyco-analog of phosphatidylglycerol (Antonopoulou etal., 2005). Natural elastase inhibitors might serve as valuable lead structuresin pharmaceutical research dedicated to the development of more effective drugs.Three new protease inhibitors, such as planktopeptin BL1125, planktopeptinBL843 and planktopeptin BL1061, were isolated from Planktothrix rubescens.They are micropeptin type serine protease inhibitors. They were also found tobe elastase and chymotrypsin inhibitors (Grach-Pogrebinsky et al., 2003).
OTHER PHARMACEUTICAL ACTIVITIES
Pharmaceutical compounds containing BGA as the active ingredient produceaccelerated cicatrisation of wound. There are also reports concerning thetreatment of external wounds in human by many cyanobacterial activecompounds. There are also reports concerning the treatment of externalwounds in human and fat lowering effects of Spirulina pills (Backer et al.,
Microalgae: A Potential Source of Bioactive Metabolites 33
1986). Many BGA have cholesterol lowering effect in animals and human. Itwas found that adopohepatosis caused by a high cholesterol diet was cured bya diet supplemented with BGA. This was due to the activity of lipoproteinlipase, an enzyme for metabolism of triglyceride rich lipoprotein.Apbanizomenon flos-aquae also shows hypo-cholesterolemic effect due tochlorophyll content, which stimulate the liver function and decreases bloodcholesterol. Apbanizomenon flos-aquae accelerate recovery from mild traumaticbrain injury. It has been reported that -Linolenic acid produced by manycyanobacterial species have wide therapeutic properties. It is involved inprostaglandin synthesis and metabolism. Prostaglandin being associated withmany essential tasks in the body including regulation of blood pressure,inflammation and cell proliferation. In addition, test on children have alsoshown that -Linolenic acid is of benefit in treating atopic eczema, while inwomen it appears to reduce the severity of premenstrual syndrome. Linolenicacid has also been claimed to have a positive effect in heart disease, perkinson’sdisease and multiple sclerosis. The iodine content of many cyanobacterialspecies like Spirulina has a positive effect towards the thyroid stimulation.The cyanobacterial pigment, phycocyanin and -carotene were suggested toreduce certain cancer risk when injected in suitable amount. Other than these,BGA also synthesize vitamin B12 and some growth regulators.
Cyanovirin-N
Cyanovirin-N (CV-N) is a unique, 101 amino acid long, 11 kDa protein. It wasdiscovered as a constituent of a cultured cyanobacterium, Nostocellipsosporum, and both the sequence and the 3-D structure of CV-N areunprecedented. CV-N potently and irreversibly inactivates diverse primarystrains of HIV-1, including M-tropic forms involved in sexual transmission ofHIV. It also blocks cell to cell transmission of HIV infection. It is directlyvirucidal (Burja et al., 2001).
Borophycin
Borophycin is a boron containing metabolites isolated from marine strains ofcyanobacteria Nostoc linckia and Nostoc spongiaeforme. It exhibits potentcytotoxicity against human epidermoid carcinoma and human colorectaladenocarcinoma cell lines and has been found to exhibit antimicrobial activity(Burja et al., 2001).
Cryptophycin
Cryptophycin first isolated from Nostoc sp. ATCC 53789 is a potent fungicide. Itwas also found to be very toxic and disregarded as natural product. It has alsobeen isolated from Nostic sp. GSV 224 and has exhibited potent cytotoxicity
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against human tumor cell lines. It shows good activity against a broad spectrumdrug-sensitive and drug resistant murine and human solid tumor (Burja et al.,2001). Structure function study leads to cryptophycin, a semi- synthetic analoguewith greater therapeutic efficiency and lower toxicity.
Lipopeptides
The natural products of many marine cyanobacteria contain an amino-acidderived fragment linked to fatty acid derived portion, forming compounds knownas lipopeptides. Lipopeptides are interesting and biochemically active, havingcytotoxic, anticancer, antibiotic, enzyme inhibitor, antiviral and antifungalactivities (Burja et al., 2001). Hapalosin, a cyclic desipeptide isolated from thecyanobacteria, Hapalosiphon welwitschii, has a reversing activity against MDR(multi drug resistance) derived from P-glycoprotein. Lipopeptides also have anaffinity for liposomes and cell membranes and due to their low molecular weightthey have an ability to pass through blood tissue and blood brain barrier leadingto direct application as a drug delivery system (Burja et al., 2001).
BIOMODULATORY ACTIVITIES
Antimicrobial active compounds of cyanobacteria are of great interest becausethe discovery of new antibiotics is necessary due to the resistance of somemicrobes to common antibiotics. They are Kawaguchipeptins A and B,Hapalindoles, Hierridin A and B and Laxaphycins etc. Since (1981), Pattersonand others (1994) cultured and prepares lipophilic and hydrophilic extracts frommore than 1500 strains representing some 400 species of blue green algae whichwere investigated for their antibiotic activities against several organisms.Antibiotic production by Nostoc has been reported by Bloor and England (1989).
Antibacterial Activity
Antibacterial activities of cyanobacterial extract have been proved by manyexperiments. Among the aqueous extracts, Synechocystis sp.LP10a,Phormidium sp.LP3, Lyngbya sp. Cob1 and Myxosarcina sp. LP5 were foundhighly active against B. substilis, where as Phormidium sp. LP3, Nostoc sp.LP11a and Microcystis sp. B1d1 were found to have high bactericidal potencyagainst E. coli. Staphylococcus aureus was found highly susceptible to extractsof Synechocystic sp. LP10aa, Trichodesmium sp. Cy3, Nostoc sp.LP11a,Myxosarcina sp.LP5 and moderately susceptible to Phormidium sp. LP3 andLyngbya sp. Cob1. Against S. typhi, some aquous extracts viz. Synechocystissp. LP10a, Phormidium sp. LP3, Lyngbya sp. Cob1 and Nostoc sp. LP11a hadshown potent antibacterial activity. Moore et al. (1987) reported an alkaloidpossessing antibacterial and antimycotic action in Hapalosiphon fontinalisisolated from soil. They described the structure of hepalindole A, a novel chlorine
Microalgae: A Potential Source of Bioactive Metabolites 35
and isonitrile containing indole alkaloid which is responsible for most of theantibacterial and antimycotic activities.
Antiviral Activity
Cyanobacteria appear to be a rich source of antiviral compounds. US NationalCancer Institute has demonstrated antiviral activities in approximately 10 % ofthe extracts tested against Human Immunodeficiency Virus (HIV-1). An extractfrom two marine cyanobacteria Phormidium tenue BDU 20571 andPseudoanabaena schmidlei BDU 30313 are reported to have antiviral activitiesagainst Hepatitis B by HBsAg binding assay. Boyd et al. (1997) discovered anovel 11-kDa virucidal protein, named Cyanovirin-N (CV-N) from Nostocellipsosporum which has the ability to irreversibly inactive diverse T-lymphocyte-tropic (T-tropic), laboratory adapted strains of HIV-1, HIV-2 and SimianImmunodeficiency Virus (SIV) as well as prevent in-vitro fusion and transmissionof HIV-1 between infected and uninfected cells. A new class of HIV inhibitorcalled sulfonic acid containing glycolipid was isolated from the extract of bluegreen algae and the compounds were found to be active against the HIV virus.Another compound a sulphated polysaccharide, calcium spirulan (Ca-SP) isolatedfrom Spirulina platensis, was found to have antiviral property. This compoundselectively inhibits the entry of enveloped virus (Herpes simplex, humancytomegalovirus, measles virus etc.) in to the cell.
Antifungal Activity
The cyanobacterial extracts perform antifungal activities. The cryptophycincomprises the largest class of cyanobacterial depsipeptides in which cryptophycin-1 was first isolated from Nostoc species as an antifungal agent. The scytophycincompounds are also found to be potent antifungal agent. Tjipanazoles, N-glycosides of indolo-[2,3-a] Tolypothrix tjipanensis exhibited appreciable fungicidalactivities in test against phyto pathogenic fungi. Scytophycin are highly cytotoxicand fungicidal macrolides. The laxaphycins are a large family of cyclic undecaand dodecapeptides are responsible for the antifungal activities of the crudeextract of Anabaena laxa FK-1-2.
Anti Algal Activity
Hapalosiphon intricatus has been reported to produce an extracellular substancethat inhibits the growth of another blue green alga, and Anabaena species. Thefilamentous cyanobacterium Scytonema hofmanni was reported to inhibit thegrowth of other cyanobacteria and some eukaryotic algae when the organismswere cultured together in petridishes. These metabolites named cyanobacterinis toxic to most cyanobacteria. A strain of cyanobacterium Nostoc has produceda new antibiotic, cyanobacterin LU-2 which is active against a wide range of
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cyanobacteria, but has low activity against green algae and is inactive towardsfungi and bacteria. An algaecidal product from Oscillatoria species abolishedphotosystem II reaction and eventually bleached and detoxified thecyanobacterium, Microcystis. These effects could be duplicated in normalcondition, implying utility of the natural algaecidal in control of toxiccyanobacteria.
Anti-inflammatory Activity
Blue green algae contain significant amount of carotenoids (b-carotene, lycopene,lutein) having antioxidant properties. By the quenching action on the reactiveoxygen species, these carotenoids also have anti-inflammatory activities. Theanti-inflammatory activity of BGA is also due to phycocyanin, a photo harvestingpigment. Phycocyanin is a free radical scavenger and has a significanthepatoprotective effect. The anti-imflammatory effect seemed to be a result ofphycocyanin inhibiting the formation of leucotriene, an inflammatory metaboliteof arachidonic acid. Aphanizomenon flos-aquae contain significant amounts ofomega-3-alpha linolenic acid which inhibit the formation of inflammatorypostaglandins and arachidonate metabolite.
CONCLUSIONS
Major advances in key areas such as health and nutrition over the last centurywere made possible by understanding and exploiting the properties of bioactivecompounds. In particular, a significant part of the increase both in life expectancyand quality of life was made possible through the understanding andadministration of vitamins, the identification of natural toxins and also thediscovery of natural-product derived drugs. Natural substances always playedkey role in drug discovery. The current applications of chemical compoundsisolated from diverse classes of algae are enormous. Focusing on bi-products,recent trends in drug research from natural resources suggest that algae arepromising group to furnish novel biochemically active substances (Burja et al.,2001; Singh et al., 2005; Blunt et al., 2005). The mining of algae diversity fromthe extreme habitat where organisms were adapted to different stress and strainwill be indispensible as these inhabitants synthesised different structuralchemicals through different metabolic pathways. The exploration of theseorganisms for pharmaceutical purposes has revealed important chemicalprototypes for the discovery of new agents, stimulating the use of sophisticatedphysical technologies and new synthesis of compounds with biomedicalapplication. Moreover, algae are promising organisms for providing both novelbiologically active substances and essential compounds for human nutrition.Therefore, an increasing supply of algal extracts, fractions or pure compoundsfor the economical sector is needed. In this regard, both secondary and primarymetabolisms have been studied as a prelude to future economic exploitation.The search for bioactive secondary metabolites from prokaryotic (cyanobacteria)
Microalgae: A Potential Source of Bioactive Metabolites 37
and eukaryotic microalgae will continually provide novel useful and structurallyspecific compounds. In order to develop production of valuable secondarymetabolites in cultures of these microorganisms, increased understanding ofthe biochemical and molecular basis of the biosynthesis involved must bepromoted through scientific research.
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