Seaweeds have recently attracted much attention as a source of antioxidant compounds due to, at least in part, to epidemiological research that strikethrough has supported the existence of an inverse correlation between the incidence of different diseases and consumption of seaweeds.^1-4 Additionally, different compounds such as carotenoids, mycosporine-related amino acids and terpenoids together with phenolic compounds have been identified among the main entities responsible for these properties in marine algae.^5,6

Halimeda genus has been investigated over the past years by our group as a source of natural antioxidants, neuro- and hepato-protectant compounds. Indeed, several lines of results have documented the ability of this natural product to target free-radical- mediated processes on in vitro and in vivo experimental models.^9-15 Thus, current results support the notion that marine seaweeds Halimeda spp are a source of hydrophilic antioxidants, which could be further recommended for the prevention of oxidative stress-related disturbances, and can be of usefulness either as dietetic supplements or as food ingredients.

Keywords: seaweeds, antioxidants, halimeda, natural products, phytodrugs

Oxidative stress and its clinical implications may be considered relatively innovative research. From the 1970s there has appeared an intensified work on this topic, which is interesting because as early as in 1956 Hardman suggested the toxicity of free radicals and their possible consequences in the genesis of different diseases, this hypothesis was confirmed in 1962 by Gerschamn. This process is closely linked with a variety of pathologies which include AIDS, atherosclerosis, diabetes, neurodegenerative diseases, chemical carcinogenesis, intoxication with certain xenobiotics and even natural aging and apoptosis processes.^16,17

Oxidative stress is currently defined as the imbalance that occurs between prooxidants reactions (producers of free radical) and antioxidant mechanisms (processes to inactivate the free radical), generally caused by increased production of free radical, so it has been postulated that one way to stop this process is through the supply of antioxidant substances.¹⁸

The protection of mammals against oxidative stress occurs in different ways.¹⁹

1. Indirect antioxidants that are intended to prevent the formation of new free radicals, enzymes and cytosolic proteins that catalyze the removal of ROS.
2. Different authors have remarked that the presence of phenolic compounds in seaweeds confers them with a heavy metal chelating capacity, which is also related to OH. radical scavenging.²⁰ Diaz & Vidal et al.^15,21 demonstrated that the antioxidant properties of Halimeda spp could be explained by the Fe3+ chelating capacity of polyphenolic compounds such as phenolic acids. Halimeda spp have a high content of polyphenolic compounds.^10,11
3. Direct antioxidants that have the function of eliminating formed free radicals before they can start a specific damage, and this is done with endogenous antioxidant biomolecules located in the cell membrane and in the cytosol or exogenous compounds provided by the diet.Halimeda spp extracts exhibited a high free radical scavenging activity which could be explained by its polyphenolic compounds content.^7-15,21
4. Others compounds modulate positively the cellular capacity for confronting the generation of levels high ROS and RNS and repair cellular structures damaged by the attack of the free radical. Different molecules, including enzymes, modulate positively the cellular capacity of repairing cellular structures damaged by the attack of the free radical. Catalase, superoxide-dismutase and glutathione-peroxidase enzymes are considered to be fundamental antioxidant defense system in mammals, and it was demonstrated that oxidative stress significantly reduced the activities of these enzymes. We observed that the treatment with the seaweed Halimeda spp led to a significant increase in the activity of these enzymes, which in turn resulted in an enhanced antioxidant defense. Mancini-Filho et al.¹² reported an over-expression of catalase genes by treatment with FPA from Halimeda monile, while de Oliveira e Silva et al.¹⁴ by RT/PCR analysis showed an increase in the catalase gene expression in the group treated with free phenolic acid (FPA) fractions from Halimeda opuntia, suggesting the inductor effect of these compounds on the enzyme genes. Accordingly, Vidal et al.²¹ demonstrated similar effect with Halimeda incrassata.

Actually it has been demonstrated that seaweeds contribute significantly to this purpose. Since ancient times, seaweed has been used as a diet component in several parts of the world, mainly in Asian countries. Additionally seaweeds exhibit different phyototherapeutic properties, including antioxidant activity and, consequently, during the last years, the interest on the study of seaweeds as sources of bioactive compounds has increased.^22-23 A correlation has also been found between the consumption of phenolic compounds in general, and seaweeds in particular, and the incidence of cardiovascular diseases.² Epidemiological research has been also suggested their positive effects on human health with an inverse correlation between the incidence of different diseases, related to oxidative stress and the consumption of seaweeds.²⁴ Others studies have also demonstrated the different therapeutic properties of these marine algae, which were verified in vitro as well as in vivo.^3,4,25,26

It is very interesting to note that although algae have traditionally formed part of the diet of different countries, they have been less used as phytopharmaceutical or traditional medicine.^27-28 The interest on the study of seaweeds as sources of antioxidants compounds has increased in the last years. Seaweed extracts have the ability to inhibit lipid peroxidation or to scavenge free radicals.²⁹ In vivo studies have confirmed the antioxidant abilities of seaweed extracts, as well,^3,5,6,30 which has suggested their possible uses as phytopharmaceutical and/or nutraceuticals.

These properties could be explained on the basis of their chemical components. Among the main entities responsible for these properties in marine algae, different compounds have been identified such as mycosporine-related aminoacids, carotenoids, and terpenoids. However, several authors consider polyphenols as such phenolic acids, phlorotannins, and bromophenols the most relevant compounds to explain the antioxidant properties of seaweeds.^5,6 In our previous work, Vidal et al.¹⁰ identified 8 phenolic acids in Halimeda opuntia and H. monile (Chlrophyta) respectively. They reported that salicylic, cinnamic, gallic, pirogalic and cafeic acids were the principal polyphenolic compounds in both seaweeds. In Halimeda incrassata, it was identified that there were major polyphenolic compounds of salicylic and ferulic acids, and they suggested that their levels were related to the antioxidant activity of the seaweed¹¹ while in Halimeda monile the main components were salicylic, cinnamic, gallic, and caffeic acids. Yoshie et al. identificated caffeic acid and flavonoids in Halimeda macroloba.³¹

In general, Halimeda genus are green seaweeds that grows in shallow waters of tropical regions. They are harmless, very easy to collect, and a potential source of phytomedicines. These organisms are largely exposed to a combination of sunlight and oxygen that leads to the formation of free radicals. However, the absence of oxidative damage on the structural components of seaweeds and their stability to oxidation during storage indicate that their cells should have potent protective antioxidative defense systems.³²

Some species from the genus Halimeda have been demonstrated to be phytopharmaceuticals. Over the last few years, the genus Halimeda has been studied for different pharmacological properties as antimicrobial activity, induction of apoptosis, anti-trichomonal, and antiinflammatory activity.^33-40 The antitumour effect was detected by induction of apoptosis in leukemia cells, using extracts from H. discoidea.⁴¹ Moreover, the genus Halimeda has been studied for antioxidant properties; animal studies carried out in our laboratory have shown that Halimeda spp effectively attenuates oxidative stress exhibiting neuroprotective and hepatoprotective activities.^{8,9,12-14,21,37,39,42} In the quest for more potent antioxidants from natural sources, our group has been especially interested in studying the beneficial properties of seaweed from the Halimeda genus for an application in biomedicine in hepato-, neuro- and athero-protection.

It has been shown that Halimeda spp has a high phenolic content^10-13 together with low amounts of other antioxidants, such as ascorbate, β-carotene, chlorophylls, and selenium; taken together the ensemble of these compounds can explain its antioxidant properties. In our laboratory, we have identified and quantitated phenolic acids as major component of Halimeda spp such as salicylic, cinamic, galic pyrogalic, ferulic and caffeic acids and it has been suggested that the significant antioxidant activity displayed by Halimeda extract is at least partly related to its high levels of total phenolic compounds and particularly due to the phenolic acids content.^10-12

Halimeda spp has been investigated over the past years by our group as a source of natural antioxidants and hepato- neuro-protectant compounds. A summary of this research is shown in Table I. In several works it has been demonstrated the ability of Halimeda to capture free radicals and inhibit lipoperoxidation. These biological activities were confirmed in different models with excellent results. Indeed, several lines of results have documented the ability of aqueous extract from these seaweeds to target free-radical mediated processes in vitro cell culture models such as GT1–7 mice hypothalamic immortalized and Vero cells.^7-15,21

Significant antioxidant activity of Halimeda extracts has been also observed in animal models, indicating great potential for phytotherapeutic, nutraceutical, or both applications. Animal studies (CCl4-induced oxidative damage in rat liver, Gerbil model of global isquemia and rats intoxicated with Methyl- mercury) carried out in our laboratory have shown that Halimeda genus effectively attenuates oxidative damage and consequently it produces neuro- and hepato-protection.

In previous in vitrostudies, Halimeda spp seaweeds have been described as having a relationship between antioxidant activity and antiatherogenic properties. Zaldivar et al.⁴³ have indicated an antiatherogenic effect of the Halimeda incrassata in atherosclerosis progression in apo E-/- mice and Costa-Mugica et al.,^44,45 adds evidence to a potential atheroprotective application of H. incrassata considering its antioxidant action and its high activity for targeting LDL oxidation and smooth muscle cell migration.

In summary, the green (no explained in above) seaweeds Halimeda spp displays a potent antioxidant activity, which could be at least partially explained by the presence of several phenolic acids, although the involvement of other phenolic compounds is not excluded. Thus, current results support the notion that marine’s algae Halimeda is a potential source of hydrophilic antioxidants, which could be further recommended for the prevention of oxidative stress-related disturbances, and can be of usefulness either as dietetic supplements, drugs or as food ingredients.

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