for cysteine by the ASC system is approximately
10 times faster than that for cystine by the xc–
system (Watanabe & Bannai 1987). Thus, DHLA
markedly improves cysteine availability within
the cell resulting in accelerated GSH synthesis
(Hanet al. 1997; Sen et al. 1997b). Both lipoate and
DHLA have remarkable reactive oxygen detoxi-
fying properties (Packer et al. 1995, 1997).
Selenium
Forty years ago, traces of dietary selenium were
observed to prevent nutritional liver necrosis in
vitamin E-deficient rats (Schwarz & Foltz 1957).
At present, selenium is widely used in agricul-
ture to prevent a variety of selenium- and
vitamin E-sensitive conditions in livestock and
poultry (Board on Agriculture 1983). In animal
tissues, selenium is either present as selenocys-
teine in selenoproteins such as GSH peroxidase,
selenoprotein P or thioredoxin reductase. Alter-
natively, selenium may present in animal tissues
as selenomethionine, which is incorporated in
place of methionine in a variety of proteins.
Selenomethionine-containing proteins serve as a
reservoir of selenium that provides selenium to
the organism when the dietary supply of sele-
nium is interrupted. Selenocysteine is the form of
selenium that accounts for its biological activity.
Perhaps the most prominent biological activity
of selenium is that it is an essential cofactor for
the critical hydroperoxide-metabolizing enzyme
GSH peroxidase (Rotruck et al. 1973). It has been
suggested that selenium may also have direct
antioxidant effects in biological systems (Burk
et al. 1980). The selenium content of food sources
may markedly vary depending on the selenium
content of the feed for animals, or selenium
content in the agricultural soil. Organ meats,
seafoods and muscle meat are considerable
sources of selenium. Dairy products, cereal and
grains may also provide significant amounts to
meet the RDA value of 70 and 55mg · day–1calcu-
lated for adult men and women, respectively
(International Programme of Chemical Safety
1987).
Antioxidant deficiency in exercise
The antioxidant deficiency model has been used
to test the significance of various antioxidants
in exercise-induced oxidative stress. Several
studies have consistently indicated that vitamin
E deficiency can lead to enhanced free radical
formation resulting in compromised exercise
performance and increased tissue lipid peroxida-
tion (Dillard et al. 1978; Quintanilha et al. 1982;
Quintanilha & Packer 1983; Salminen et al. 1984;
Gohilet al. 1986; Jackson 1987; Amelink et al.
1991). These studies suggest that inadequate
amounts of dietary vitamin E may decrease
endurance performance by as much as 40% and
lead to enhanced oxidative lipid damage of
several tissues (Dillard et al. 1977; Davies et al.
1982; Gohil et al. 1984, 1986). Also, vitamin E defi-
ciency was associated with increased fragility of
lysosomal membranes and greater haemolysis
of red blood cells (Davies et al. 1982; Gohil et al.
1986). Vitamin E deficiency also decreased oxida-
tive phosphorylation (Quintanilha et al. 1982;
Gohilet al. 1984) in skeletal muscle, liver and
adipose tissues. In female rats, however, vitamin
E deficiency does not appear to influence the
ability to run nor does it enhance tissue lipid per-
oxidation (Tiidus et al. 1993). It has been sug-
gested that female rats may be less susceptible
to free radical damage compared to male rats
because of higher levels of oestrogen, a potential
antioxidant, in the circulation (Davies et al. 1982;
Salminenet al. 1984; Bar & Amelink 1997). The
effects of an ascorbate-depleting diet on run time
were examined in guinea pigs, which do not
synthesize vitamin C. Run time of ascorbate-
depleted guinea pigs was significantly less than
ascorbate-adequate animals (Packer et al. 1986).
Dietary selenium deficiency impairs tissue
antioxidant defences by markedly downregulat-
ing GSH peroxidase activity in tissues such as the
liver and muscle. This effect on the antioxidant
enzyme did not influence endurance to treadmill
run, however. This suggests that muscle GSH
peroxidase activity is not a limiting factor in
physical performance (Lang et al. 1987). Sele-