Antioxidant effects

Oxidative stress is a primary cause for development of various human chronic diseases such as cardiovascular disease, cancers, and neurode-generative diseases. Therefore, wide range of potential metabolites has been evaluated against oxygen-induced damage and hence lowers the risk of human chronic diseases. Among them, brown algae serve as an important bioresource of antioxidative phlorotannins with significant

OH

Phlorofucofuroeckol A

Phlorofucoduroeckol B

Phlorofucofuroeckol A

Phlorofucoduroeckol B

O OH

8,8 Bieckol

O OH

OH OH

Dioxinodehydroeckol

8,8 Bieckol

OH

trihydroxyphenoxy)-2,4,9-trihydroxybenzo-1,4-

dioxin

OO OH

trihydroxyphenoxy)-2,4,9-trihydroxybenzo-1,4-

dioxin

O OH

OO OH

Diphlorethohydroxycarmalol

FIGURE 8.1 Structures of phlorotannins isolated from marine brown algae.

pharmaceutical potential. Algae as intertidal organisms require an endogenous antioxidant capacity to withstand UV irradiation and the effects of desiccation from daily tidal fluctuations (Yuan and Walsh, 2006). In brown algae, this antioxidant protection encompasses mainly by

phlorotannins (Yan et al., 1996). Therefore, they have received the greatest attention and have been investigated extensively since they are high free radical scavengers in nature and less toxic than synthetic antioxidants such as BHA and BHT (Jung et al., 2008).

1. Free radical scavenging ability

Unregulated production of free radicals in the cellular systems is responsible in causing cellular damage by oxidizing macromolecules, DNA, proteins, and lipids in the cell. Therefore, antioxidative therapeutics have great demand to act against free radicals. In this aspect, phlorotan-nins serve as one of the most promising natural antioxidants. Numbers of studies have shown the radical scavenging potential of phlorotannins isolated from marine brown algae. These phlorotannins have shown significant scavenging ability toward hydroxyl, superoxide, alkyl, and DPPH radicals. As shown in Table 8.1, most of the phlorotannin compounds have shown more potent antioxidant activities than commercially available antioxidants a-tocopherol, butylated hydroxyl-anisole (BHA), butylated hydroxytoluene (BHT). And also phlorotannins are investigated for their ability in scavenging reactive oxygen species (ROS) in cellular systems. Diphlorethohydroxycarmalol and 6,6-bieckol isolated from Ishige okamurae (Zou et al., 2008); fucodiphloroethol G and dieckol isolated from Ecklonia cava (Li et al., 2009) have shown of 77.2%, 78.9%, 75.6%, and 84.3% ROS inhibition at 50 mM, respectively, in H2O2-induced RAW264.7 cells. Moreover, Kang et al. (2004) have shown strong ROS inhibitory activity of phlorotannins; eckol (IC50 4.04 p.M) and phlorofuco-furoeckol A (IC50 3.8 mM) isolated from Ecklonia stolonifera on rat kidney homogenates of freshly killed male Wistar rats.

2. Photoprotective ability

Ultraviolet B (UVB; 280-320 nm) radiation of the sun is highly oxidative and directly triggers photodamage of the skin cells. This UVB radiation induces the overproduction of ROS which interacts with cellular DNA, proteins, and lipids to alter their cellular functions (Heo et al., 2010). Regular intake of antioxidants would be an useful strategy to combat UVB induced photodamage. Yan et al. (1997) and Hupel et al. (2011) have shown that brown algae are more tolerant to UVB irradiation due to the presence of phlorotannins. Therefore, these phlorotannins have been isolated from algae and investigated for their possible application in reducing the photodamage of the skin. Dieckol was found to be the most potent phlorotannin to protect the cells from UVB irradiation (50 mJ cm"2). Heo et al. (2009) have found that dieckol (100 mM) isolated from E. cava increase the cell survival up to 77.1% in UVB irradiated human dermal fibroblasts (HDF). And also they have shown that the DNA damage due to UVB irradiation was also inhibited by 57.8% with

TABLE 8.1 Radical scavenging activities of phlorotannins from brown algae

Radical scavenging activity IC50 (mM)

TABLE 8.1 Radical scavenging activities of phlorotannins from brown algae

Radical scavenging activity IC50 (mM)

Compound

Hydroxyl

Superoxide

Alkyl

DPPH

Source

Reference

Phloroglucinol

392.5

115.2

128.9

NA

Eclonia cava

Li et al. (2009)

Eckol

51.8

26.5

28.4

22.89

Fucodiphloroethol G

33.5

18.6

18.1

14.72

Phlorofucofuroeckol A

39.2

21.6

21.4

17.66

7-phloroeckol

39.6

21.9

22.7

18.64

dieckol

28.6

16.2

14.5

8.28

6,6'-bieckol

29.7

15.9

17.1

8.69

Diphlorethohydroxycarmalol

28.7

15.4

17.3

9.1

Ishige okamurae

Zou et al. (2008)

8,8'-bieckol

NA

NA

NA

59

Ecklonia arborea

Sugiura et al. (2008a)

Phlorofucofuroeckol B

NA

NA

NA

81

Catechin

NA

NA

NA

10

Kang et al. (2003)

BHA

NA

NA

NA

34

BHT

NA

NA

NA

144

A-tocopherol

NA

NA

NA

17

NA, not analysed.

the treatment of 50 pM dieckol. Moreover, Ko et al. (2011) have also studied on the photoprotective effect of dieckol from E. cava using human epithelial keratinocytes (HaCat) and have found that dieckol treatment (100 pM) on UVB irradiated cells induces the cell survival up to 88.42%. Further, they have studied this promising photoprotective effect of dieckol using zebrafish as an alternate animal model system. Their results indicate that the ROS formation in UVB irradiated zebra fish embryos was significantly inhibited by the dieckol treatment (50 pM). ROS levels were measured using image analysis and fluorescence micrographs, and dieckol has also shown a typical fluorescence micrograph of the nonirradiated zebrafish. Diphlorethohydroxycarmalol isolated from I. okamurae has also been studied for its photoprotective ability in UVB (50 mJ cm"2) irradiated HDF cells. It has shown 45.57% ROS scavenging ability and 49.33% inhibition of DNA damage at 250 pM concentrations (Heo et al., 2010).

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