Polysaccharides

Presently sulfated polysaccharides are the group which is identified as economically most important among other ingredients found in algae that have been extensively used in the industry for food and medicinal purposes. Red algae and brown algae are the classes that produce these polysaccharides of interest in higher concentrations. These polysacchar-ides act as dietary fiber as they are not digested in the upper digestive tract but may be degraded by the colonic bacteria to some extent in the colon. Direct comparisons show that, in most of the seaweeds dietary, fiber amounts are similar or slightly elevated than the levels of total fiber in terrestrial foodstuffs. Edible seaweed contain 33-62% total fibers on a dry weight basis, which is higher than the levels found in higher plants, and these fibers are rich in soluble fractions (Dawczynski et al., 2007; Lahaye, 1991). A growing interest can be seen among researchers to study the roles of polysaccharides in the human body particularly how they prevent the occurrence of certain diseases.

Carrageenans are generally identified as carbohydrate antigens and has the potency to promote the growth of connective tissues. Antiviral properties of few algal species have been studied extensively including Chondrus crispus and Gelidium cartilagineum, the species produce agar and carrageenan in higher concentrations. Researchers have concluded that this property is attributed to the galactan units available in agar and carrageenan of these algal species. Current research develops strong evidences to promote carrageenan as an useful antiviral agent that blocks the transmission of the HIV virus as well as other STD viruses such as gonorrhea, genital warts, and the herpes simplex virus (Buck et al., 2006). Carrageenan is also studied extensively in ulcer therapy, and it has been concluded that carrageenan is involved in developing protective layer by interacting with the mucoid lining of the stomach and thereby preventing enzymes and acid secretion (Emerson and Kerkut, 1974). Agar has similar structural and functional properties as carrageenans. Both agar and car-rageenan have the ability to exert effects in modifying the adhesion and proliferation of normal and tumoral human colonic cells thereby affecting the process of metastasis (Zhou et al., 2006).

Other important polysaccharide, alginic acid, is present naturally in seaweeds as calcium or magnesium salts which are insoluble in water. Algins/alginates are extracted from brown seaweeds and are available in both acid and salt forms. Commercially alginic acid is extracted mainly from brown seaweeds as soluble sodium alginate. Sodium alginate is reported to serve as a coadjutor in immunization against strain-specific influenza virus. Sodium alginate also has tried in the treatment of esoph-agitis and urolithiasis. It has the ability to function as a haemostatic agent which is capable of clotting blood in situ. Alginates have the capability to act like fibers and help clearing the digestive system to protect surface membranes of the stomach and intestine from potential carcinogens. Further, this feature has a link with its ability to prevent proliferation of implanted cancer cells in the stomach. Moreover, alginic acid and its derivatives are used for the production of drugs in the treatment of gastritis and gastroduodenal ulcers, as well as alginates are used as antiulcer remedies. The mechanism action of these materials has a link to its ability to effectively suppress postprandial acidic refluxes and binding of bile acids. Alginates are capable of reducing hypertension through several mechanisms including physical binding of sodium in the gastrointestinal tract and calcium channel blocker activity (Draget and Taylor, 2011).

The polysaccharide laminarin is commercially extracted mainly from kelp and fucoids and is a main form of food storage of brown algae. Sulfated laminarins have antilipidemic activities and capable of reducing serum cholesterol levels and total serum lipids (Kiriyama et al., 1969). The anticoagulant activity of this material is attributed to its antithrombotic property, and laminarin only shows anticoagulant activity after structural modifications such as sulfation, reduction, or oxidation (Miao et al., 1999). Structural similarity of laminarin to barley, which is potent prebiotic, has prompted the study of laminarin as a prebiotic. Studies have proved that laminarin provides a substrate for prebiotic bacteria and promotes their growth and function in human (Deville et al., 2004). Further, laminarin has proved to be involved in modulating the gut environment and act as an immunostimulant. Further studies have revealed the potential of lami-narin as a cancer therapeutic and as a tumor inhibiting agent (Miao et al., 1999).

Fucoidan, another polysaccharide of brown algae, is not found in other algae or in higher plants. Fucoidan has shown promising antiviral, immu-nomodulating, and antibacterial activities. Fucoidan inhibits the angio-genesis and promotes apoptosis in human cancer cells. Further, it inhibits the proliferation of tumor cells and thereby reduces the growth and the size of the tumor. Further, this compound has proven its capability to act as anti-inflammatory and anticoagulant agents. Further, fucoidan preparations have been proposed as an alternative to the injectable anticoagulant heparin considering its safety being free of viruses as they originate from plant matter and exert protective effects through direct inhibition of viral replication against HIV, hepatitis, and herpes viruses. Further, fucoi-dan has reputed for its ability to stimulate the immune system by acting as an immunomodulator directly on macrophage (Li et al., 2008).

Ulvan is a water soluble polysaccharide obtained from members of the Ulvales. Bioactive properties of ulvan such as cytoxicity against colonic cancer cells through modification of the adhesion and proliferation of tumoral human colonic cells and modulating the expression of transforming growth factors related to cellular differentiation are reported. Further, there are reports to confirm that ulvan acts as an antiviral and antibacterial agent (Lahaye and Robic, 2007).

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