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The toxicity of Aloe barbadensis Miller juice is due to the induction of oxidative stress.

Introduction

Aloe barbadensis Miller (Aloe vera) has a long history of use for medicinal and dietary purposes and as a component of many cosmetic preparations. Aloe vera has anti-inflammatory [1, 2], immunostimulatory [3], antibacterial [4] and antiviral activities [5] and accelerates wound healing [6,7]. There have also been reports of toxic effects of Aloe vera juice. Various low molecular weight components are cytotoxic to fibroblasts [8] and normal human skin cells and tumour cells in vitro [9]. The mechanism of toxicity is yet to be determined.

Many active constituents have been isolated from A. barbadensis leaves and their bioactivities studied. The anthraquinones, anthrones and chromones are effective at counteracting various diseases [2, 10]. Aloe emodin has been shown to inhibit lipid peroxidation [11]. Aloin and aloe emodin may act as either pro-oxidants or antioxidants dependent on their concentration [12]. Therefore these compounds may act in either a protective or toxic manner at different concentrations. It has been suggested that the toxicity of Aloe vera components may be due oxidative stress induction [12]. Conversely, at lower concentrations, these same components may act as antioxidants and protect cells from oxidative stress [13].

Oxidative stress is associated with many human diseases including lung cancer, chronic inflammation, atherosclerosis and Alzheimer's disease [14]. Individuals with elevated dietary intakes of nonenzymatic antioxidants such as vitamins A, C and E are less likely to suffer heart and vascular disease, diabetes and some forms of cancer [15]. However, studies into the medicinal effects of antioxidants have proved confusing, with some studies showing therapeutic effects [16, 17], whilst other studies indicate that these antioxidants may themselves be toxic [18]. It has been shown in a variety of human and animal models that the effects of vitamin E and vitamin C are dose dependent with low doses behaving as antioxidants, while high doses themselves induce toxicity through oxidative stress [19].

There is growing interest in the use of natural antioxidants to protect against a variety of diseases. Consumption of beverages such as tea and fruit juices, which are rich in flavanoids, has been associated with decreasing serum cholesterol and systolic blood pressure, thus decreasing coronary vascular disease [20]. Eating the seeds of Garcinia kola, which contains the potent antioxidant kolaviron, has been reported to have anticarcinogenic and hepatoprotective effects [21]. Cloves contain high levels of antioxidant phytochemicals and have been linked with prevention of lung cancer [22]. Even drinking wine is linked with protection against oxidative stress through its antioxidant phytochemicals [20]. This study reports on the ability of Aloe vera juice to induce oxidative stress in Artemia franciscana and the ability of vitamin E and its analogue Trolox to counteract this oxidative stress. Furthermore, this study examines the mechanism of Aloe vera juice toxicity by examining its effect on various biomarker enzymes.

Materials and Methods

1. Chemical reagents

Aloe vera juice was obtained from Aloe Wellness Pty Ltd, Australia and was stored at 4[degrees]C until use. Aloe vera juice was serially diluted in deionised water for use in the A. franciscana bioassay. Vitamin E ([alpha]-tocopherol; Sigma, purity > 96%) was dissolved in 60% methanol to give a 10 mg/ml stock. This stock was serially diluted in deionised water for use in the bioassay. Trolox[TM] (Sigma, purity >97%) was prepared as a 1.5 mg/ml stock in 60% methanol and was diluted in deionised water for use in the bioassay. Vitamin C (L-Ascorbic acid, AR grade, Chem-Supply) was dissolved and diluted in deionised water. Aloe emodin (Sigma, purity >95%) was prepared by dissolving in distilled water to give a concentration of 500 [micro]g/ml and serially diluting in deionised water. All reagents were prepared fresh before use.

2. Reference toxins for biological screening

Potassium dichromate (AR grade, Chem-Supply) was prepared as a 1.6 mg/ml solution in distilled water and was serially diluted in artificial seawater for use in the A. franciscana nauplii bioassay. Mevinphos (2-methoxycarbonyl-1-methylvinyl dimethyl phosphate) was obtained from Sigma-Aldrich with a mixture of cis (76.6%) and trans (23.0%) isomers and prepared as a 4 mg/ml stock in distilled water. The stock was serially diluted in artificial seawater for use in the bioassay.

3. Biological screening

Toxicity was tested using a modified form of the Artemia nauplii lethality assay developed by Meyer et al. [23] for the screening of active plant constituents. Artemia franciscana Kellog cysts were obtained from North American Brine Shrimp, LLC, USA (harvested from the Great Salt Lake, Utah). Synthetic seawater was prepared using Reef Salt, AZOO Co., USA. Seawater solutions at 34 g/l distilled water were prepared prior to use. 2 g of A. franciscana cysts were incubated in 1 litre synthetic seawater under artificial light at 25[degrees]C, 2000 Lux with continuous aeration. Hatching commenced within 1618 h of incubation. Newly hatched A. franciscana (nauplii) were used within 10 h of hatching. Nauplii were separated from the shells and remaining cysts and were concentrated to a suitable density by placing an artificial light at one end of their incubation vessel and the nauplii rich water closest to the light was removed for biological assays. 400 [micro]l of seawater containing approximately 25 (mean 25.2, n = 286, SD 8.6) nauplii were added to wells of a 48 well plate and immediately used for bioassay. A. barbadensis juice was diluted serially in deionised water. 400 [micro]l of the diluted juice and the reference toxins were transferred to the wells and incubated at 25 [+ or -] 1[degrees]C under artificial light (500 Lux). A negative control (400 [micro]l seawater) was run in at least triplicate for each plate. All concentrations of treatments also were performed in at least triplicate. The wells were checked at regular intervals and the number of dead counted. The nauplii were considered dead if no movement of the appendages was observed within 10 seconds. After 72 h all nauplii were sacrificed and counted to determine the total number per well. The [LC.sub.50] with 95% confidence limits for each treatment was calculated using Probit analysis [24].

4. The effect of antioxidants on Aloe vera induced toxicity

4.1. Antioxidant co-treatment

To determine the ability of antioxidants to block the toxic effect of Aloe vera juice, vitamin C was freshly prepared in deionised water as a 400 [micro]g/ml solution. Vitamin E and Trolox[TM] were each dissolved in 60% methanol and diluted in deionised water to a concentration of 400 [micro]g/ml. Aloe vera juice was diluted in deionised water to give a 24% solution. The 24% juice was added to an equal volume of the relevant antioxidant (400 [micro]g/ml) and mixed well. 400 [micro]l of the juice/antioxidant mixtures was added to wells of a 48 well plate containing 400 [micro]l of seawater containing A. franciscana nauplii, resulting in test concentrations of 6% juice and 100 [micro]g/ml antioxidant in the bioassay. The mortality was monitored at regular intervals. All tests were performed at least three times in triplicate.

4.2. Antioxidant Pre-treatment

Aloe vera juice was prepared as a 24% solution and the antioxidants were prepared as 400 [micro]g/ml solutions as described for the co-treatment experiments. 200 [micro]l of the test antioxidants were added to wells of a 48 well plate containing 400 [micro]l of seawater containing A. franciscana nauplii and incubated at 25[degrees]C for 4 h. Following the 4 h antioxidant pre-treatment, 200 [micro]l of 24% juice was added to the wells resulting in a final concentration of 6% juice and 100 [micro]g/ml in the bioassay. The mortality was monitored at regular intervals. All tests were performed at least three times in triplicate.

5. Effect of Aloe barbadensis Miller juice on oxidative stress biomarkers

5.1. Exposure to oxidative stress

A franciscana nauplii were separated from the shells and remaining cysts and were concentrated to a suitable density by placing an artificial light at one end of their incubation vessel and the nauplii rich water closest to the light was removed for biological assays. For each oxidative stress test, 50 ml of saline containing approximately 10,000 A. franciscana were removed from the stock cultures. Each was treated with the test solutions (1.6 ml Aloe vera juice resulting in a 3% concentration in assay; juice/antioxidant cotreatment consisting of 1.6 ml Aloe vera juice (3% concentration in the assay) and 2.15ml of Vitamin E (100 mg/ml concentration in the assay) respectively). The concentrations of test treatments used were determined in this study to be sub-lethal. The control (no treatment) required no additives. All tests had artificial seawater added to give a final volume of 54 ml. All A. franciscana tests were incubated for 24 h at 25[degrees]C.

Following the oxidative stress exposure period the A. franciscana nauplii were sacrificed, dried by rotary evaporation and resuspended in 5 ml of ice cold 0.1 M phosphate buffer pH 7.2 containing 0.1% Triton X-100. The resulting mixture was homogenised by sonication and centrifuged at 15,000 rpm for 5 mins. The supernatant was passed through a 10 cm x 1 cm G25 Sephadex (Sigma) column. The protein containing filtrate was dried by rotary evaporation and resuspended in 5 ml of ice cold 0.1 M phosphate buffer pH 7.2. Aliquots were stored at -10[degrees]C for protein estimation and enzymatic activity determinations.

5.2. Protein estimation

Protein concentrations were estimated by the Bradford [25] protein method adapted to microplate. Bovine serum albumin (Sigma, > 96%) was diluted in deionised water and was used as a standard.

5.3. Glutathione reductase activity

Glutathione reductase (GR) activity was determined by an adaptation of the protocol of Carlberg and Mannervik [26]. Briefly, enzyme activity was quantified spectrophotometrically by monitoring the [DA.sub.340]. Enzymatic activity was expressed as Units of enzyme activity/mg protein where Units of activity are refined as Lmoles of NADPH oxidised/min.

5.4. Glutathione peroxidise activity

Glutathione peroxidase (GPx) activity was determined by an adaption of the method of Smith and Levander [27]. Enzyme activity was quantified spectrophotometrically by monitoring the [DA.sub.340] as oxidised glutathione (GSSG) is reduced back to the reduced form (GSH) by glutathione reductase. Enzymatic activity was expressed as Units of enzyme activity/mg protein where enzyme activity is expressed as nanomoles of NADPH/min/mg protein.

5.5. Thioredoxin reductase activity

Thioredoxin reductase (TrxR) enzyme activity was determined by measuring the NADPH-dependant reduction of DTNB to TNB. This reduction to TNB was then measured by monitoring the change in absorbance at 412 nm for 10 mins. Enzymatic activity was expressed as Units of enzyme activity/mg protein where Units of activity are refined as [micro]moles of DTNB reduced /min.

6. Statistics

The Paired T-Test was used to calculate statistical significance between control and treated groups with a P value < 0.05 considered to be statistically significant.

Results

1. Aloe vera juice toxicity

A 50% dilution of Aloe vera juice was found to induce 100% mortality within 4 h in the A. franciscana bioassay. Neither of the reference toxins, Mevinphos (2000 [micro]g/ml) nor potassium dichromate (800 [micro]g/ml) produced notable mortality compared to the negative controls in 4 h. A time course study was performed to determine the rate at which 50% Aloe vera juice could induce toxicity in the bioassay. As is seen in Figure 1a, the onset of Aloe vera juice toxicity (as defined by mortality) was seen at approximately 120 min, and approximately 240 min was required for 100% mortality. In contrast, both Mevinphos (Figure 1b) and potassium dichromate (Figure 1c) took much longer to exert their effect. The onset of Mevinphos toxicity was approximately 12 h and more than 36 h was required for 100% mortality. Similarly, potassium dichromate toxicity was not evident until 12 h and approximately 24 h was required for 100% mortality. Spontaneous mortality in all seawater controls was < 1% at 24 h (Figure 1d). Due to the rapid toxicity of aloe vera juice, we have reported [LC.sub.50] values for 4 h (Table 1). To enable comparison to the [LC.sub.50] values of the reference toxins the LC50 at 24, 48 and 72 h are also reported.

Figure 2 shows the effect of Aloe vera juice dose on mortality in the A. franciscana bioassay. A. franciscana nauplii were exposed to dilutions of Aloe vera juice for 6 h and the% mortality determined. The induction of toxicity was evident when the A. franciscana were exposed to approximately 4% Aloe vera juice. 6% juice was the lowest dose capable of inducing 100% mortality within the 4 h period. The [LC.sub.50] at 4 h for Aloe vera juice was 5.4% [+ or -] 0.3 (Table 1).

2. Toxicity of antioxidants

To determine whether vitamin C and E and the water soluble vitamin E analogue Trolox[TM] were toxic in the A. franciscana nauplii bioassay, high doses of these antioxidants were tested in the bioassay (vitamin C, 1250 [micro]g/ml; vitamin E, 320 [micro]g/ml; Trolox[TM], 500 [micro]g/ml). Vitamin C induced toxicity inside 90 min with 100% mortality seen in 180 min (Figure 3a). Both vitamin E (Figure 3b) and Trolox[TM] (Figure 3c) were slower in inducing toxicity. For vitamin E and Trolox[TM], the onset of toxicity was seen within 240 min, however only low mortality was seen at this time. The induction of 100 % mortality took much longer to achieve. Trolox[TM] took 48 h to induce 100% mortality (96.4% [+ or -] 3.6). Vitamin E was even slower at inducing mortality, with approximately 80% mortality at the 72 h test period.

The effect of antioxidant concentration on mortality was measured at various times. Table 1 shows that the [LC.sub.50] of the vitamin C (203.1 [micro]g/ml [+ or -] 11.3) was substantially lower than the [LC.sub.50] of the reference toxin Mevinphos (1336 [micro]g/ml [+ or -] 70) at 24 h, demonstrating its relative toxicity. However, potassium dichromate had an [LC.sub.50] of 73 [micro]g/ml [+ or -] 4.0 at 24 h, nearly three fold more toxic than the vitamin C. Interestingly, whilst the toxicity of vitamin C was observed very rapidly, it reached its maximum lethality within 24 h. Only small decreases in LC50 were seen over the next 2 days. In contrast, while both Mevinphos and potassium dichromate took longer to exert their effects, the mortality due to these toxins continued to increase over time.

Trolox[TM] was capable of inducing toxicity rapidly with a 4 h [LC.sub.50] of 453.2 [micro]g/ml [+ or -] 25.2. Trolox[TM] reached its maximum toxicity within 24 h and only small decreases in [LC.sub.50] were seen over the next 2 days. Vitamin E treatment resulted in low A. franciscana mortalities at all doses tested within the first 48 h. Accordingly, only the 72 h [LC.sub.50] for vitamin E treatment is reported here (Table 1). Vitamin E had a 72 h [LC.sub.50] in the Artemia bioassay of 96 [micro]g/ml [+ or -] 6.3.

3. Effects of antioxidant co-treatment on Aloe vera juice toxicity

If the toxic effects of Aloe vera juice to A. franciscana are at least in part due to the induction of oxidative stress, then treatment of the A. franciscana nauplii with sub lethal doses of antioxidants would be expected to decrease the toxicity of the juice. The effects of simultaneous exposure of Aloe vera juice and various antioxidants are shown in Figure 4. Vitamin C did not decrease the toxicity of Aloe vera juice in the A. franciscana bioassay. In fact, vitamin C appeared to enhance the toxicity of the juice. Studies within this laboratory have shown that vitamin C speeds up the toxic effect of the juice (unpublished data). In contrast, vitamin E was capable of decreasing Aloe vera juice toxicity by 23.3% ([+ or -] 8.1%). The water soluble vitamin E analogue, Trolox[TM], also decreased Aloe vera juice toxicity, although to a lesser degree (15.3% [+ or -] 5.4).

[FIGURE 1 OMITTED]

4. Effects of antioxidant pre-treatment on Aloe vera juice toxicity

Pre-treating A. franciscana nauplii with the vitamin antioxidants proved more effective at blocking the toxicity of Aloe vera juice than simultaneous treatment. Figure 5 shows the decrease in Aloe vera juice induced toxicity when A. franciscana nauplii were pre-treated with the antioxidants for 4 h prior to addition of Aloe vera juice. As with the antioxidant co-treatment, vitamin C was ineffective at blocking Aloe vera juice toxicity. Indeed, vitamin C accelerated the onset of toxicity of Aloe vera juice (unpublished data) to A. franciscana nauplii. Vitamin E proved extremely effective at blocking Aloe vera juice toxicity at the doses tested. Pre-treatment of A. franciscana nauplii with vitamin E resulted in a 90.4% [+ or -] 2.1 decrease in Aloe vera juice toxicity. Trolox[TM] was also effective at blocking toxicity, reducing A. franciscana mortality by 52.8% [+ or -] 0.9.

5. Effect of Aloe vera juice on oxidative stress biomarkers

5.1. Thioredoxin Reductase Activity

Exposure of A. franciscana nauplii to 3% Aloe vera juice resulted in a significant decrease in thioredoxin reductase activity as shown in figure 6. Thioredoxin reductase activity decreased from 3.8 [+ or -] 0.53 Units/mg total protein in the untreated control protein extracts to 2.5 [+ or -] 0.15 Units/mg total protein, in the A. franciscana nauplii exposed to Aloe vera juice. This represents an approximate 34% decrease in enzymatic activity. This inhibitory effect of the juice could be partially counteracted by the coexposure of juice in the presence of the antioxidant vitamin E. As shown in Figure 6, the co-exposure of Artemia nauplii to vitamin E along with the Aloe vera juice resulted in increase in thioredoxin reductase activity, closer to the control values. Vitamin E treatment did not completely overcome the effect of Aloe vera juice. Indeed, even with coexposure with vitamin E, the thioredoxin reductase activity is still only approximately 78% of the control value.

5.2. Glutathione Reductase Activity

Glutathione reductase activity was similarly affected by exposure to 3% Aloe vera juice (Figure 7). Indeed, Aloe vera juice treatment resulted in a more dramatic decrease in glutathione reductase enzymatic activity than seen for thioredoxin reductase. Glutathione reductase activity decreased from 4.25 [+ or -] 0.53 Units/mg total protein in the untreated control protein extracts to 0.91 [+ or -] 0.15 Units/mg total protein. This represents an approximate 79% decrease in enzymatic activity on exposure of the A. franciscana nauplii to Aloe vera juice. As with thioredoxin reductase activity, this inhibitory effect could be partially counteracted by the co-exposure of juice in the presence of the antioxidant vitamin E. The co-exposure of A. franciscana nauplii to vitamin E along with the Aloe vera juice resulted in an increase in glutathione reductase activity, closer to the control values (Figure 7). Vitamin E treatment did not completely overcome the effect of Aloe vera juice. In extracts from A. franciscana co-exposed to vitamin E and Aloe vera juice, the glutathione reductase activity increases to approximately 44% of the control value.

5.3. Glutathione Peroxidase Activity

Exposure of A. franciscana nauplii to 3% Aloe vera juice resulted in a significant decrease in glutathione peroxidise enzymatic activity as shown in Figure 8. Glutathione peroxidase activity decreased from 23.4 [+ or -] 2.5 Units/mg total protein in the untreated control protein extracts to 2.3 [+ or -] 0.4 Units/mg total protein when the A. franciscana were exposed to 3% Aloe vera juice. This represents an approximate 90% decrease in enzymatic activity. This inhibitory effect of the juice could be partially counteracted by the co-exposure of juice in the presence of the antioxidant vitamin E. Co-exposure of Artemia franciscana nauplii to vitamin E along with the Aloe vera juice resulted in increase in glutathione peroxidise activity, closer to the control values (Figure 8). Vitamin E treatment only partially overcame the effect of Aloe vera juice. Indeed, even with co-exposure to vitamin E, the thioredoxin reductase activity is still only approximately 32% of the control value.

Discussion

Many studies have reported on the antioxidant and pro-oxidant potential of A. barbadensis extracts [13; 28]. Aloe emodin in particular has high inhibitory free radical scavenging activity [11] and can inhibit linoleic acid peroxidation [2]. In contrast, other studies have also reported on the toxic effects of Aloe vera components [8, 9]. The current study demonstrated the ability of Aloe vera juice to induce mortality in A. franciscana. Aloe vera juice exposure resulted in acute toxicity, being capable of inducing mortality at dilutions as low as 4% juice, with an [LC.sub.50] at 24 h of 4.6% [+ or -] 0.3. At 6% dilutions, Aloe vera juice was capable of causing 100% mortality within 4 h of exposure to A. franciscana.

These seemingly conflicting reports on the protective/toxic effects of Aloe vera components may be due to the different concentrations used in these various studies. Tian and Hua [12] have reported on the concentration dependence effects of two common Aloe vera components. Aloin has a pro-oxidant effect at low concentrations and had an antioxidant effect at higher concentrations. In contrast, aloe emodin was shown to function as a pro-oxidant only at high concentrations. Thus, the acute toxicity induced by Aloe vera juice in the current study may be due to a relatively high level of aloe emodin and/or aloin present in the juice. Interactions between the various components within the crude juice may also play a role in converting otherwise antioxidant molecules into pro-oxidants in the juice.

Similarly, other well studied antioxidants have also been shown to have opposing effects at different concentrations. Previous studies have shown the therapeutic effect of many vitamin antioxidants [16, 29], whilst other work indicated that these antioxidants may be toxic [18]. As for the Aloe vera active phenolics, the antioxidant/pro-oxidant effects of these vitamins also seem to be dependent on their concentrations. Tafazoli et al. [16] have shown that a variety of vitamin E analogues, including atocopherol and Trolox[TM], behave as antioxidants at low concentrations and convert to pro-oxidants as the concentration increases. Trolox[TM] has also been shown to have direct therapeutic effects in Lumbriculus variegatus being capable of blocking copper toxicity at low concentrations whilst itself being toxic at high doses [29].

The current study demonstrates the toxicity of high doses vitamin C and vitamin E (and its analogue Trolox[TM] in A. franciscana nauplii. Exposure to high doses of vitamin C was particularly effective at inducing A. franciscana nauplii mortality. This may be due to causes other than the conversion of vitamin C from a free radical scavenger to an electron donor. Studies within this laboratory have shown A. franciscana nauplii to be sensitive to pH changes (unpublished data). The addition of vitamin C in the doses used in these experiments resulted in a pH decrease of up to 2 pH units. Thus, it was likely that the mortality induced by vitamin C in these experiments was due to the pH decrease associated with its addition in this system.

Whilst vitamin E exposure did not induce the level of mortality seen for vitamin C, vitamin E was itself toxic to A. franciscana nauplii at high concentrations (320 [micro]g/ml). This lethality was slow in its onset, taking 72 h to become evident (compared to mortality for 1250 [micro]g/ml vitamin C becoming evident within 2 h). Concentrations of vitamin E below 100 [micro]g/ml were not lethal to A. franciscana nauplii, even at these extended times. The toxic effect seen for vitamin E was likely due to a conversion of vitamin E from a free radical scavenger to an electron donor. Vitamin E is the major lipid soluble antioxidant of biomembranes. Its antioxidant activity is dependent upon its ability to donate hydrogen from a hydroxyl group on its chromone ring to free radicals [30], thus reducing membrane lipid peroxidation. However, under some conditions (eg. at high concentrations) vitamin E may convert to pro-oxidant activities and itself induce oxidative stress [16].

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Trolox[TM] was also toxic to A. franciscana nauplii in the higher doses tested. Trolox[TM] is a phenolic antioxidant originally designed as a food preservative due to its free radical trapping capability [31]. Its structure is similar to that of a-tocopherol but lacks the hydrophobic tail, making it the more hydrophilic than other vitamin E analogues. Like vitamin E, the ability of Trolox[TM] to function as either an antioxidant or a pro-oxidant is concentration dependent, with higher concentrations favouring a pro-oxidant function [31]. In their 2005 study, Tafazoli et al. [16] revealed Trolox[TM] to be a substantially more effective pro-oxidant than vitamin E. The results reported in the current paper support Trolox[TM] inducing oxidative stress more effectively than vitamin E. Trolox[TM] was capable of inducing mortality in A. franciscana nauplii in a much shorter time (4h) compared to vitamin E (72h) at the same concentration.

Although both vitamin E and Trolox[TM] were toxic to A. franciscana nauplii at high concentrations, this study also demonstrated the ability of vitamin E and Trolox[TM] to reduce or protect A. franciscana nauplii against Aloe vera induced oxidative stress at lower concentrations. Both vitamin E and Trolox[TM] were able to block Aloe vera juice induced toxicity when they were added to the A. franciscana nauplii simultaneously with the juice. This ability to block the juice induced oxidative stress indicated the ability of vitamin E (or Trolox[TM]) to scavenge free radicals in/induced by the juice. Vitamin E provided a more effective protectant than Trolox[TM] at the dose tested (100 [micro]g/ml), being capable of blocking 23.3% ([+ or -] 8.1%) mortality. The water soluble vitamin E analogue, Trolox[TM] (100 [micro]g/ml), also decreased Aloe vera juice toxicity, although to a lesser degree (15.3% [+ or -] 5.4). Whether this difference in levels of protection of the vitamin E analogues was due to differences in their mechanisms of action was not established in these tests. It was possible that structural differences may make Trolox[TM] more susceptible to conversion from an antioxidant to a pro-oxidant and thus less effective at blocking lethality.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

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[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Vitamin E and Trolox[TM] proved more effective at blocking oxidative stress induced mortality when the A. franciscana nauplii were exposed to the antioxidant for 4 h prior to addition of the Aloe vera juice. Vitamin E pre-treatment resulted in a 90.4% [+ or -] 2.1 decrease in mortality (compared to a 23.3% [+ or -] 8.1 decrease for simultaneous treatment). Similarly, Trolox[TM] pre-treatment blocked 52.8% [+ or -] 0.9 mortality, a significant decrease in mortality from the 15.3% ([+ or -] 5.4) decrease seen for the simultaneous treatment experiment. Presumably the antioxidants are more effective when given time to enter the cells. This is not surprising, given the role of vitamin E analogues in blocking cell membrane lipid peroxidation. The more hydrophobic nature of vitamin E compared to Trolox[TM] may mean that it may more effectively interact with membrane lipids, accounting for its greater effectiveness at blocking Aloe vera juice induced oxidative stress.

The current studies used the marine crustacean Artemia franciscana to examine the effects on antioxidant enzyme activities of Aloe vera juice exposure. Little literature exists on the antioxidant defence mechanisms in Artemia species. The only study found that examines antioxidant enzymes uses a different Artemia species (Artemia parthogenica) [32]. That study highlighted the difficulties encountered in studying Artemia antioxidant enzyme activities. Specifically, the authors state that antioxidant enzymatic activities in Artemia can be very different to activities of similar enzymes in vertebrate cells. These authors state the activities determined in their studies for thioredoxin reductase, glutathione reductase and glutathione peroxidise to be substantially lower than the corresponding activities in vertebrate cells. Indeed, one of the difficulties encountered in our studies was the relatively low enzyme activities, in the cases of thioredoxin reductase and glutathione reductase being just above baseline levels. Conversely, the same study report superoxide dismutase levels to be higher than those seen for vertebrate cells [32]. Whilst these authors use a different test organism (Artemia parthogenica compared to Artemia franciscana used in our studies) it is likely a similar situation exists in A. franciscana. Similarly low antioxidant enzyme activities have been reported for other marine crustaceans such as Aristeus antennatus [33] and freshwater crustaceans such as Daphnia magna [34].

The levels of enzyme inhibition seen when Artemia nauplii were exposed to Aloe vera juice (34%, 79% and 90% for thioredoxin reductase, glutathione reductase and glutathione peroxidise respectively), are indicative of a mechanism of oxidative stress induction after acute exposure of the nauplii to the toxin. A consistent pattern was observed where the Artemia franciscana responded to oxidative stress by decreasing the overall activity of redox related enzymes. Similarly apparent was the trend whereby the co-exposure of the nauplii to vitamin E counteracted this effect. For each of the biomarker enzymes tested, vitamin E co-exposure resulted in enzyme activities closer to the control value. Vitamin E addition helps Artemia franciscana nauplii to overcome/block the juice induced oxidative stress.

The current study used passive diffusion through the gills as the predominant entrance route for the test compounds, which would be the natural route for uptake in marine and freshwater crustaceans. However, the gills may act as selective barriers to toxin or vitamin E uptake and therefore may affect the response to their exposure. It is likely that the uptake of the relatively insoluble vitamin E may be more dramatically affected than that of the more soluble phenolic components of the Aloe vera juice, accounting for the only partial restoration of enzymatic activity seen with vitamin E co-exposure.

Conclusions

In conclusion, the current study demonstrated the toxicity of Aloe vera juice towards A. franciscana nauplii. The identity of the toxic components of the juice are not reported here but previous studies in this laboratory [35] indicated that a number of toxic compounds, including aloe emodin, may be responsible for this toxicity. This work provide further support that these toxic components exert an effect through the induction of oxidative stress, which could be blocked/counteracted by antioxidants.

Acknowledgments

The authors wish to thank John Gorringe of Aloe Wellness Australia Pty Ltd for the gift of Aloe barbadensis Miller juice used during these experiments. Financial support for this work was provided by the School of Biomolecular and Physical Sciences, Griffith University, Australia.

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Corresponding Author

I.E. Cock, Biomolecular and Physical Sciences, Nathan Campus, Griffith University, 170 Kessels Rd, Nathan, Brisbane, Queensland 4111, Australia Tel.: +61 7 37357637; Fax: +61 7 37355282. E-mail: I.Cock@griffith.edu.au

(1,2) Ian E. Cock and (1) Joseph Sirdaarta

(1) Biomolecular and Physical Sciences, Nathan Campus, Griffith University, 170 Kessels Rd, Nathan, Brisbane, Queensland 4111, Australia

(2) Genomics Research Centre, Gold Coast Campus, Griffith University, Parklands Drive, Southport, Queensland 4222, Australia

Ian E. Cock and Joseph Sirdaarta; The toxicity of Aloe barbadensis Miller juice is due to the induction of oxidative stress
Table 1: Brine shrimp larvicidal activity of Aloe vera juice,
reference toxins Mevinphos and potassium dichromate and the
antioxidants vitamin C, vitamin E and Trolox[TM].

                       [LC.sub.50] value in [micro]g/ml * (95 %
                       confidence interval) at time (h)

Treatment              4                       24

Aloe vera juice        5.4 [+ or -] 0.3 % *    4.6 [+ or -] 0.3 % *
Mevinphos              --                      1336 [+ or -] 70
Potassium Dichromate   --                      73 [+ or -] 4
Vitamin C              608.3 [+ or -] 39.7     203.1 [+ or -] 11.3
Vitamin E              --                      --
Trolox[TM]             453.2 [+ or -] 25.2     283.3 [+ or -] 25.8

                       [LC.sub.50] value in [micro]g/ml * (95 %
                       confidence interval) at time (h)

Treatment              48                      72

Aloe vera juice        4.4 [+ or -] 0.4 % *    4.3 [+ or -] 0.2 % *
Mevinphos              501 [+ or -] 33         109 [+ or -] 12
Potassium Dichromate   12 [+ or -] 4           3.7 [+ or -] 0.3
Vitamin C              196.9 [+ or -] 17.9     182.0 [+ or -] 10.1
Vitamin E              --                      96.0 [+ or -] 6.3
Trolox[TM]             279.8 [+ or -] 18.2     275.0 [+ or -] 15.3

* Values are expressed as mean ([micro]g/ml unless otherwise stated)
[+ or -] S.D.
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Title Annotation:Original Article
Author:Cock, Ian E.; Sirdaarta, Joseph
Publication:Advances in Environmental Biology
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2011
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