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* Dept
of Postharvest Science of Fresh Produce, ARO – The Bet *Corresponding
author. Tel. +972-3-9683609, e-mail: vrodov@agri.gov.il Issued
25 July 2008 Abstract Khat
(Catha edulis
Forsk.) is used in Introduction Khat
(Catha edulis
Forsk.,
Celastraceae) is an evergreen shrub or small tree
native to 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 (β-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. Results
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. Discussion 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
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Wu,
X., |