Table

Flavonoid Found in Echinacea Leaf and Corresponding Affinity to Prevent LDL Oxidationa

Inhibition Percentage at Flavonoid 100 |iM

Luteolin 71.1±1.2

Kaempferol 77.8±2.1

Quercetin 73.5±5.4

Luteolin-7-glucoside 78.8±1.9

a Expressed as percent inhibition of cupric ion-induced LDL. Values represent the percent decrease in mobility of human LDL migration on agarose gel electrophoresis. (From CU and DDK, unpublished results, 1998.)

(Afanas'ev et al., 1989; Wu et al., 1995). Potential pro-oxidant activity that will occur between plant phenolics and free transition metal ions must also be considered when evaluating the antioxidant activity of flavonoids. The characteristic redox potential of flavonoids acts to reduce the transition metal ion to a lower valence form that is favorable for the Fenton reaction, and in turn accelerates a pro-oxidant reaction. This property has been reported for both a complex extract (Hu et al., 2000) and a purified catechin, such as epigallocatechin gallate (Hu and Kitts, 2001), where free transition metal ions were present.

Other studies have evaluated the antioxidant activity of these flavonoids using other models, such as the methyl linoleate micelle. It is clear from various studies that flavonoids represent moderate chain-breaking agents that scavenge lipid alkoxyl radicals and peroxyl radicals by acting as a chain-breaking electron donor (Rice-Evans, 1995; Roginsky et al., 1996). The rate constant of flavonoids reacting with peroxyl radical has been estimated to be 1 x 107M-1s-1 (Bors et al., 1994; Belyakov et al., 1995). Specifically, the B-ring catechol structure along with the 2, 3-double bond and 3, 5-hydroxyl groups on the flavonoid backbone is closely related to antioxidant activity (Roginsky et al., 1996). These chemical moieties and structural components of flavonoids furthermore determine the extent of partition between the hydrophilic and hydrophobic phase in a heterogeneous system (Foti et al., 1996).

Another model used to assess antioxidant activity of plant phenolics common in Echinacea is the phospholipid bilayer. Flavonoids such as quercetin are more effective than a-tocopherol, which is mainly responsible for chain-breaking activity against lipid peroxidation products that are exposed to water-soluble peroxyl radicals (Terao et al., 1994). Not withstanding this, however, is the noteworthy finding that the reducing capacity of flavonoids contributes also to indirect properties of antioxidant activity by regenerating a-tocopherol (Mukai et al., 1996). This characteristic explains the unique disappearance rate of a-tocopherol and quercetin if the chain initiation occurs within the membrane (Terao and Piskula, 1998). This model has also shown that quercetin is not as effective as a-tocopherol at scavenging chain-propagating, lipid peroxyl radicals in a hydrophilic phase. This observation is largely due to the localization of quercetin in the aqueous phase, and thus lower affinity to interact with lipid peroxyl radicals that are residing with a-tocopherol in hydrophilic zones of the suspension.

Further evidence of characteristic bioactive properties of different flavonoids can be seen in cytotoxicity studies assessed in vitro with various cell lines. In the example shown below, luteolin exhibited the strongest cytotoxicity against caco-2 cells compared to luteolin-7-glucoside and other flavonoids (Figure 6.4). It is noteworthy that a pattern for relative antioxidant activity and cytotox-icity is present for the various flavonoids.

Sample Concentration (|g/ml)

FIGURE 6.4 Cell viability of Caco-2 cell incubated with different flavonoid found in Echinacea. ♦ = luteolin-7-glucoside, ■ = luteolin, ▲ = quercetin, x = rutin. (CU and DDK, 1999.)

Sample Concentration (|g/ml)

FIGURE 6.4 Cell viability of Caco-2 cell incubated with different flavonoid found in Echinacea. ♦ = luteolin-7-glucoside, ■ = luteolin, ▲ = quercetin, x = rutin. (CU and DDK, 1999.)

Anthocyanins

Anthocyanins are one of many distinct groups in the flavonoid family that have been identified as having various health benefits. Flavonoids account for a large proportion of phenolic phytochemicals in the human diet from such sources as tea, vegetables, and fruits, and derivatives (Cook and Samman, 1996). The consumption of particular flavonoids, such as catechin, typically varies by age and gender (Arts et al., 2001) and is partially explained by dietary habits. Epidemiological studies have shown a negative relationship between chronic exposure to flavonoids and incidence of coronary heart disease and ischemic heart disease (Hertog et al., 1997a, 1997b). For example, moderate wine consumption has been linked to the antioxidant properties of anthocyanins and reduced risk of cardiovascular disease (Cao et al., 1998; Wollin and Jones, 2001). In a 4-week clinical trial, human subjects with regular tea consumption exhibited a significantly prolonged LDL oxidation ex vivo compared to the placebo (Ishikawa et al., 1997). Anthocyanin has also been detected in both human and animal blood after consumption, thereby indicating the absorption and possible metabolism of these compounds (Tsuda et al., 1999; Cao et al., 2001). Grape juice, a good source of both anthocyanin and proanthocyanin, has also been shown to extend the lag phase for human LDL oxidation and increase flow-mediated vasodilation compared to controls (Stein et al., 1999).

Anthocyanins provide in large part the plant pigments found in the Echinacea flower (Cheminta et al., 1989). The principal anthocyanins present in the Echinacea flower are cyanidin-3-O-b-glucopyanoside and cyanidin-3-O-6-malonyl-b-D-glucopyranoside. Anthocyanins are also abundant in berries, fruits, and grapes (Cliford, 2000). The antioxidant property of cyaniding-3-glucoside has been demonstrated in various test model systems (Tsuda et al., 1994). Using the conditions outlined in the legend in Figure 6.5, cyanindin-3-glucoside derived from a blackberry source suppresses DNA damage that is mediated by peroxyl radicals. The potential benefit of anthocyanin from Echinacea remains to be determined due to the fact that flowers are less used in this herbal preparation.

FIGURE 6.5 Effect of freeze-dried and frozen blackberry extracts on preventing peroxyl radical-induced supercoiled DNA from nicking. S = supercoiled DNA; N = nicked DNA strand; lane 1 = DNA + PBS; lane 2 = DNA + peroxyl radical + PBS; lane 3 = DNA + peroxyl radical + 0.05 mg/mL freeze-dried blackberry extract; lane 4 = DNA + peroxyl radical + 0.05 mg/mL frozen blackberry extract.

FIGURE 6.5 Effect of freeze-dried and frozen blackberry extracts on preventing peroxyl radical-induced supercoiled DNA from nicking. S = supercoiled DNA; N = nicked DNA strand; lane 1 = DNA + PBS; lane 2 = DNA + peroxyl radical + PBS; lane 3 = DNA + peroxyl radical + 0.05 mg/mL freeze-dried blackberry extract; lane 4 = DNA + peroxyl radical + 0.05 mg/mL frozen blackberry extract.

effects of ECHINACEA extracts on nitrogen radicals

Nitric oxide (NO) with an unpaired electron reacts as a free radical. The production of nitric oxide in mammalian cells by the oxidation of L-arginine by nitric oxide synthase (NOS) includes both constitutional (cNOS) and inducible NOS (iNOS) forms (Nathan and Hibbs, 1991). The NO level in a normal physiological condition is low until the expression of iNOS occurs, which leads to increased amounts of NO production. The level of iNOS expression is determined partially by the rate of transcription, which is dependent on NF-kB activation (Xie et al., 1994). Activation of cells by appropriate stimuli results in the phosphorylation, uniquination, and degradation of IkB, which liberates NF-kB to translocate into nuclei and interact with a kB motif on the promoter of target genes such as iNOS (Han et al., 2001). Oxidative stress was found to be partially responsible for protein phosphorylation (Ushio-Fukai et al., 1998), NF-kB activation (Schreck et al., 1991; Meyer et al., 1993), and oxidative stress gene expression (Lee and Corry, 1998). Pro-inflammatory agents such as bacterial lipopolysaccharide (LPS) or IL-1P, TNF, and IFN-g will stimulate the expression of iNOS. In mouse macrophages, LPS-induced expression of iNOS depends on the activation of NF-kB heterodimer p50/c-rel and p50/Rel A (Xie et al., 1994), and expression of iNOS leads to the production of massive amounts of NO. The reaction between NO and superoxide anion results in the generation of a highly reactive nitrogen species peroxynitrite (ONOO-), bringing the NO into the category of a pro-oxidant (Violi et al., 1999). The reason for high oxidative activity of ONOO- lies with the weak strength of the O-O bond and spontaneous decomposition to form hydroxyl radical and nitrogen dioxide (Koppenol et al., 1992).

Recently, echinacoside isolated from Cistranche deserticola stem was found to suppress the generation of nitric oxide in J774.1 macrophage cells cultured with lipopolysaccharide and mouse perifoneal macrophage stimulated with LPS and IFN-g (Xiong et al., 2000). No inhibition on iNOS mRNA expression was found; consequently, neither were the iNOS protein found nor the iNOS activity in lipopolysaccharide-stimulated macrophage enhanced. Therefore, the inhibition of the generation of nitric oxide was attributed to the direct scavenging of nitric oxide. Evidence of such direct scavenging was observed in a system with PAPA NONOate, which generates nitric oxide radical by spontaneous dissociation (Xiong et al., 2000). Therefore, the authors concluded that phenyethanoids, including echinacoside, were unlikely to inhibit NF-kB activation; a reaction which differs from the antioxidant (-)-epigallocatechin-3-gallate from green tea, known to suppress nitric oxide in a manner of inhibition of NF-kB activation (Lin and Lin 1997).

Crude extracts of Echinacea (e.g., E. purpurea) that contain cichoric acid, polysaccharide, and alkylamide have been reported to reduce the nitric oxide release from rat alveolar macrophages that were stimulated with LPS. Of the three chemical identities, alkylamide was the most effective, and also increased the production of TNF-a in alveolar macrophage cells in a concentration-dependent manner. These results support the in vivo evidence that alkylamide from Echinacea extract can be an effective nonspecific immunomodulatory agent (Goel et al., 2002). Echinacea extracts chemically standardized to phenolic acid or echinacoside content and fresh pressed juice preparations were found to display antiinflammatory and antioxidant properties in varying degrees, as shown in the suppression of prostaglandin E2 in mouse macrophage RAW264.7 cell treated with IFN-a (Rininger et al., 2000).

Similar to the evidence shown with reactive oxygen species, flavonoids play an important role in suppressing nitric oxide production. Kim et al. (1999) found that the structural features in favor of strong activity to reduce nitric oxide include the C-2, 3-double bond and 5, 7-dihydroxyl groups in the A-ring. The 3-hydroxyl moiety in the C-ring will actually reduce the activity. For example, luteolin reduces the iNOS enzyme expression in LPS-activated RAW264.7 cell in a concentration-dependent manner without inhibition of enzyme activity itself. In terms of transcription, flavonoid-rich Ginkgo biloba (EGb 761) and its major flavonoid, quercetin, were found to inhibit p38 mitogen-activated protein kinase (MAPKs) activity, which is necessary for the iNOS expression in LPS-

stimulated RAW264.7 macrophages. However, quercetin had no effect on LPS-induced activation of NF-kB (Wadsworth and Koop, 2001).

effects of ECHINACEA extracts on oxidative enzymes

Polyphenol oxidase (PPO, EC 1.14.18.1) catalyzes both the hydroxylation of monophenols to o-diphenols and the oxidation of o-diphenols to o-quinones. These reactions lead to the generation of brown color, termed enzymatic browning. This browning reaction occurs in vegetables, fruits, and herbs during postharvest handling and results in a loss of quality that adversely affects acceptability by consumers (Martinez and Whitaker, 1995). Wolfgang et al. (2000) purified PPO from E. purpurea, which has a high affinity for caffeic, cichoric, and rosmarinic acids that represent enzyme substrates. It was of interest to note that PPO from E. purpurea also possesses diphenolase activity in addition to monophenolase activity. As a matter of fact, Nusslein et al. (2000) found the existence of polyphenol peroxidase catalyzed cichoric acid degradation in E. purpurea preparation, suggesting the necessity of increasing ethanol concentration in order to inhibit enzyme activity in processing. Similarly, Kim et al. (2000a) demonstrated the decline of caffeic acid derivatives in E. purpurea flower during drying processing. Taking these findings together, it is clear that caffeic acid derivatives, such as cichoric acid from Echinacea, are sensitive to environmental and herbal processing and handling, therefore extra measures are required for the preparation of this herb. For example, use of low temperature, elevated ethanol concentration, and storage in a low-humidity environment reduces the loss of both alkamide and cichoric acid from E. purpurea (Stuart and Wills, 2000; Wills and Stuart, 2000). Application of metal-chelating agents to deactivate cupric ions in the active site of PPO (Wolfgang et al., 2000) was also found to be useful in order to maximize the retention of cichoric acid in the preparation.

An in vitro screening test widely used to determine the antiinflammatory activity of Echinacea is the inhibition of cyclooxygenase and 5-lipoxygenase (Celotti and Laufer, 2001; Bernrezzouk et al., 2001). These two enzymes are central to the pathway producing thromboxanes, prostaglandins, and leukotrienes (Borchers et al., 2000). Non-heme iron-centered lipooxygenase exists in both animal and plant tissues. The enzyme catalyzes the oxidation of polyunsaturated fatty acid with conjugated diene substructure, such as linoleic acid and arachidonic acid. One mechanism of lipoxygenase is the oxidation and deprotonation of diene to generate a pentodienyl radical. When oxygen is subsequently added to form a fatty acid peroxyl radical, the ferrous ion is regenerated back to a ferric form (de Groot et al., 1975). Alternatively, the diene substrate can be deprotonated and coordinated with ferric ion to form a o-organometallic complex, where di-oxygen, when inserted to break down the Fe-C bond, resulted in fatty acid hydroperoxide and the regeneration of enzymes (Corey and Nagata, 1987). Both cyclooxygenase and 5-lipoxygenase are critical for the arachidonic acid metabolism and associated with the formation of leukotrienes and prostaglan-dins. The alkamide fractions from both E. purpurea and E. angustifolia inhibit 5-lipoxygenase activity (Wagner et al., 1989). Specifically, eight alkamides isolated from E. angustifolia have shown different inhibitory activities on both enzymes in vitro. Specifically, pentadeca-2E, 9Z-diene-12, 14-diynoic acid isobutylamide, and dodeca-2E, 4E, 8Z, 10E/Z-tetraenoic acid isobutylamide exhibit the highest inhibitory activities against cyclooxygenase and lipoxygenase activities, respectively. The mechanism underlying this inhibition has been suggested to involve the enzymatic competition between structurally similar alkamides and arachidonic acid in the reaction. Moreover, possible redox-inhibitory properties or radical scavenging capacities may also be involved (Muller-Jakic et al., 1994). Significant consideration should also be given to dodeca-2E, 4E, 8Z, 10E/Z-tetraenoic acid isobutylamide activity, since both account for the predominant alkamide in the lipophilic fraction from E. angustifolia (root), E. purpurea, and E. pallida (Bauer and Remiger, 1989). E. tennesseensis contains only low quantities of dodeca-2E, 4E, 8Z, 10E/Z-tetraenoic acid isobutylamide (Bauer et al., 1990). Taking the antioxygenase activity into account, it is necessary to emphasize the importance of postharvest procedures in order to maximize the retention of biologically relevant alkamides. For example, Kim et al. (2000b) observed that freeze-dried Echinacea root resulted in the best retention of dodeca-2E, 4E, 8Z, 10E/Z-tetraenoic acid isobutylamide and other individual alkamides, compared with conventional air drying. Perry et al. (1997) analyzed the distribution of alkamide in different parts of E. purpurea, and noted that dodeca-2E, 4E, 8Z, 10E/Z-tetraenoic acid isobutylamide was most abundant in the vegetative stem, albeit this part accounts for only 2% of the whole plant.

final comments

The literature reviewed above describes the potent antioxidant and free radical scavenging activities of Echinacea extracts and individual components, albeit many of these studies were conducted in vitro. It is not clear if these same components from Echinacea possess similar bioactivity in vivo, and whether or not the antioxidant properties claimed potentiate the reported health benefits of Echinacea, such as antiinflammation and general cold relief. Pharmacokinetic data are currently unavailable for the major antioxidant components present from Echinacea. It is important that this information be obtained in order to understand how antioxidant components derived from Echinacea work in vivo to trigger other related protein expressions. For example, isoflavones genistein and daidzein significantly increased the expression of antioxidant protein metallothionein in human intestinal Caco-2 cells (Kameoka et al., 1999). This effect was decreased by the treatment of quercetin (Kuo et al., 1998). In fact, catalase and Cu/Zn superoxide dismutase in Caco-2 cells were not affected by exposure to 100 mM of flavonoids, thereby suggesting that the effects of flavonoids on the antioxidant protein expression are possibly related to the specific structure of the compound. Similar studies are required with specific Echinacea phytochemicals.

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