Ethnobotanical Leaflets 12: 557-64. 2008.
Hydrophilic and Lipophilic Antioxidant Capacity and Content of Phenolic Compounds in Fresh Khat Leaves (Catha edulis Forsk.)
Yakov Vinokur, Azri Levi, Oleg Feygenberg, and Victor Rodov*
of Postharvest Science of Fresh Produce, ARO – The
*Corresponding author. Tel. +972-3-9683609, e-mail: firstname.lastname@example.org
Issued 25 July 2008
Forsk.) is used in
Celastraceae) is an evergreen shrub or small tree
The legal status of khat is controversial; it is considered a controlled drug in some countries (e.g.
The stimulant effect of the plant is attributed to the presence of amphetamine-like phenylpropylamino alkaloids, cathinone and to a lesser extent cathine (Szendrei, 1980; Krizevki et al., 2007). Cathedulins are another group of khat alkaloids with poorly investigated biological effects (Al-hebshi and Skaug, 2005). In addition, khat leaves are rich in phenolic compounds including flavonoid glycosides and condensed tannins releasing flavonols kaempferol, quercetin and myricetin after acid hydrolysis (El Sissi and Abd Alla, 1966). High content of ascorbic acid (vitamin C) was reported in shoot tips and especially in leaves of khat (Mustard, 1952). The leaves also contain other vitamins (thiamin, niacin and riboflavin), carotenoids (-carotene), amino acids, minerals (calcium, iron) and fiber (Duke, 1992).
The information about prooxidant/antioxidant effects of khat is equivocal. Individuals regularly using khat, especially in combination with tobacco smoking, showed higher level of post-meal plasma lipid peroxidation as compared to control group not following these habits. At the same time, the lipid peroxidation level decreased during the khat chewing session following the meal consumption, suggesting certain antioxidant activity (Al-Zubairi et al., 2003). The authors associated these antioxidant properties with the presence of polyphenolic compounds in the khat leaves. Oral administration of total aqueous khat extract or of its alkaloid fraction exacerbated the oxidative stress in restrained rats due to the decreased activity of antioxidant enzymes, i.e. superoxide dismutase, catalase, glutathione-S-transferase (Al-Qirim et al., 2002). Similarly, khat induced an increase in reactive oxygen species and a depletion of intracellular glutathione in the cell cultures of human keratinocytes and fibroblasts, the reactions that could be opposed by addition of exogenous antioxidants (Lukandu et al., 2008). But on the other hand, the flavonoid fraction of the khat enhanced the activity of the antioxidant enzymes in rats and thus could provide a protection against the oxidative stress (Al-Qirim et al., 2002). In addition, this fraction demonstrated an anti-inflammatory activity (Al-Meshal et al., 1986). Methanolic extract of dried khat leaves showed in vitro high scavenging activity towards artificial free radical 1,1-diphenyl-2-picryldrazyl (DPPH) (Dudai et al., 2008). Thus, it seems that khat leaves simultaneously possess prooxidant (alkaloid) and antioxidant (phenolic) fractions.
In the present work, we have investigated the content of phenolic compounds and the activity of hydrophilic and lipophilic antioxidants extracted from fresh khat leaves and stems as affected by age and plant genotype.
Materials and Methods
Shoots of the two distinct
genetic forms of Catha edulis known in
The following samples were
prepared from the shoots of each genetic form as shown in Fig. 1: (1) upper
Hydrophilic and lipophilic antioxidant fractions were isolated from the
fresh samples using a stepwise
extraction of the plant material with acetate buffer, acetone and hexane and
repeated partition of water-soluble and water-insoluble portions as described
previously (Vinokur and Rodov,
2006). The radical-scavenging activity of each fraction was measured by our
modification of the TEAC (Trolox Equivalent
Antioxidant Capacity) method based on decolorization
of 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulphonic acid) radical cation (ABTS+·) generated in acidified ethanol
medium (Vinokur and Rodov,
2006). The method was further modified by using
Figure 1. The appearance of "red" and "green" khat shoots and location of samples taken.
The khat leaf extracts demonstrated very high hydrophilic antioxidant capacity, close to 300 µM Trolox equivalents per gram fresh weight. No significant difference in this parameter was found between the leaves of red and green khat forms, as well as between the leaves of different age (distance from the shoot tip). The hydrophilic activity in stems was significantly lower than that in leaves, although still rather high, close to 200 µM TE per gram fresh weight (Fig. 2).
The content of total phenolic compounds followed a pattern similar to that of the hydrophilic antioxidant capacity, especially in the “red khat” samples (Fig. 3). The content of phenolic compounds in the “green khat” leaves tended to be somewhat lower than in the red form. This difference was especially evident in the young (upper) leaves and tended to reduce in the leaves of older ages (medium and lower locations). On the contrary, the content of phenolic compounds in the stems of the “green khat” was slightly higher than that in the red form, both significantly than in the lower leaves.
Figure 2. Activity of hydrophilic antioxidants in the leaves of ‘red’ and
‘green’ khat of the three age groups.
‘red’ and ‘green’ khat of the three age groups.
The values of lipophilic antioxidant capacity in the khat samples were much lower than those of the hydrophilic one, in agreement with the typically observed situation (Wu et al., 2004). The youngest upper leaves tended to have less lipophilic antioxidants than the older ones. Even lower lipophilic activity was found in the stems, especially in the stems of the “red khat”. At the same time, the mature leaves of the “red khat” demonstrated the highest lipophilic antioxidant activity (Fig. 4).
Figure 4. Activity of lipophilic antioxidants in the leaves of
‘red’ and ‘green’ khat of the three age groups.
Our study has shown that fresh leaves of khat possess very high hydrophilic radical-scavenging capacity, comparable to that of the tea leaves (Chan et al., 2007) and correlating with the high content of phenolic compounds. This finding corroborates the recent report of Dudai et al. (2008) concerning the dried khat leaves. In addition, khat contains a noteworthy but not outstanding amount of lipophilic antioxidants, which at least partially can be attributed to the leaf carotenoids.
Similar amounts of phenolic antioxidants were found in the leaves of the
three age groups tested. Lack of clear gradient of the antioxidant activity
along the top
The analysis of the information available indicates that most negative health consequences of the khat chewing (including the prooxidant and cytotoxic effects) are related to the alkaloids, primarily cathinon, while the phenolic compounds (e.g., flavonoids) are responsible for its positive antioxidant, anti-inflammatory and antimicrobial effects (Al-Meshal et al., 1986; Al-Qirim et al., 2002; Al-Zubairi et al., 2003; Al-hebshi et al., 2006). Indeed, alkaloids do not contribute to the radical-scavenging activity of the khat leaves (Dudai et al., 2008). Accordingly, the overall health effect of khat would depend on the balance between alkaloid and phenolic fractions.
Although nowadays chewing is
the major way of consuming khat, its leaves were
also used in the past for brewing a tea (Getahun
and Krikorian, 1973; Al-hebshi
and Skaug, 2005). Moreover, Paris and Moyse (1958) described the organoleptic
quality of the ‘Abyssinian tea’ prepared from the dried leaves of Catha edulis (5-
We suppose that utilizing leaves from the upper part of the khat shoot (combination of the three types of leaves used in this study) for brewing a tea would result in a beverage with high antioxidant activity and reduced alkaloid content. Such tea would be mildly stimulant and fully realize the health-promoting antioxidant potential of the khat with minimized adverse effects. Both “red” and “green” forms of khat can be used for preparing such tea.
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