bodies such as carrot, papaya or squash con-
tain mostly b-carotene, a-carotene and b-
cryptoxanthin. Tomatoes are rich in lycopene
because the b-carotene biosynthetic pathway ter-
minates prior to the formation of the terminal
rings. Chemically, carotenoids are highly unsta-
ble and are susceptible to auto-oxidation (Han-
delman et al. 1991). Although most reports
indicate that carotenoids do have effective
antioxidant functions in biological systems,
some studies show that carotenoids may also
show toxic pro-oxidant effects (Burton & Ingold
1984; Andersen & Andersen 1993).
Ubiquinone
Coenzyme Q 10 , also called ubiquinone, is an inte-
gral component of the mitochondrial electron
transport chain. Coenzyme Q 10 is found in the
phospholipid bilayer of plasma membranes, all
intracellular membranes and also in low-density
lipoproteins. The actual mechanism of antioxi-
dant action of ubiquinones is still conjectural.
One possibility is that ubiquinols act indepen-
dently as lipid peroxidation chain-breaking
antioxidants. Alternatively, a redox interaction of
ubiquinol with vitamin E has been suggested in
which ubiquinol mainly acts by regenerating
vitamin E from its oxidized form (Kagan et al.
1996).
Vitamin C
Vitamin C or ascorbate is an excellent water-
soluble antioxidant. Although in most higher
organisms it is synthesized from abundant
glucose precursors, other species, including
humans, solely depend on nutritional supply.
Because of its strong reducing properties, ascor-
bate readily reduces Fe^3 +and Cu^2 +to Fe^2 +and
Cu+, respectively. In this way, ascorbate can con-
tribute to the redox cycling of these metals, gen-
erating transition metal ions that can stimulate
free radical chemistry. Thus, ascorbate may have
pro-oxidant effects in the presence of free metals
(Austet al. 1985; Buettner 1986). Apart from
direct free radical scavenging activity, ascorbate
may also enhance the antioxidant action of
vitamin E. The phenol group of tocopherol,
which is the basis of its antioxidant action,
appears to be located at the water–membrane
interface of biological membranes. Such localiza-
tion facilitates ascorbate–vitamin E interaction
(see Fig. 22.2). Dehydroascorbate, the two-
electron oxidation product of ascorbate, is
reduced to ascorbate by reduced GSH. Thus,
ascorbate plays a central role in the antioxidant
network.Glutathione
Glutathione (l-g-glutamyl-l-cysteinylglycine)
is implicated in the circumvention of cellular
oxidative stress and maintenance of intracellular
thiol redox status (Meister 1992a, 1992b, 1995;
Sen & Hanninen 1994). GSH peroxidase is spe-
cific for its hydrogen donor, reduced GSH, but
may use a wide range of substrates extending
from H 2 O 2 to organic hydroperoxides. The
cytosolic and membrane-bound monomer
GSH phospholipid hydroperoxide-GSH per-
oxidase and the distinct tetramer plasma GSH
peroxidase are able to reduce phospholipid
hydroperoxides without the necessity of prior
hydrolysis by phospholipase A 2. The protective
action of phospholipid hydroperoxide-GSH
peroxidase against membrane-damaging lipid
peroxidation has been directly demonstrated
(Thomaset al. 1990). Reduced GSH is a major
cellular electrophile conjugator as well. GSH
S-transferases catalyse the reaction between
the -SH group of GSH and potential alkylating
agents, thereby neutralizing their electrophilic
sites and rendering them more water-soluble.
GSH S-transferases represent a major group of
phase II detoxification enzymes (Hayes &
Pulford 1995).
Intracellular synthesis of GSH is a tightly
regulated two-step process, both steps being
adenosine triphosphate dependent. g-
Glutamylcysteine synthetase (also referred to as
glutamate-cysteine ligase) catalyses the forma-
tion of the dipeptide g-glutamylcysteine (DeLeve
& Kaplowitz 1990) and subsequently the addi-