tive stress are cigarette smoking, alcoholism and
high altitude (Moller et al. 1996). Each puff of a
cigarette is estimated to contain approximately
1014 free radicals in the tar phase and approxi-
mately 10^15 in the gas phase (Duthie & Arthur
1994). The metabolism of ethanol produces
acetaldehyde that is known to consume the key
physiological antioxidant, glutathione (GSH)
(Videla & Valenzuela 1982). Ingestion of ethanol
is associated with enhanced lipid peroxidation
(Nadigeret al. 1988). Increased levels of lipid
peroxidation by-products were observed in the
alcohol-administered rat cerebral cortex, cerebel-
lum and brain stem (Nadiger et al. 1986). Several
factors, including hypoxia, altered mitochon-
drial respiration and exposure to UV radiation,
are known contribute to oxidative stress at high
altitude (Simon-Schnass 1994; Moller et al. 1996).
It is also evident that exercise can induce changes
in biochemical parameters that are indicative of
oxidative stress in the fit horse and that this is
exacerbated during exercise at high temperature
and humidity (Mills et al. 1996).
Evidence
Multiple unsaturation points in polyunsaturated
fatty acids (PUFA) make them highly susceptible
to ROS attack and oxidative damage. Uncon-
trolled and autocatalytic oxidative destruction of
PUFA, commonly referred to as lipid peroxida-
tion, is initiated when a ROS having sufficient
energy to abstract a H-atom of a methylene
(-CH 2 ) group (of the PUFA backbone) reacts with
a PUFA (Alessio 1994). Peroxyl radicals thus
formed are particularly dangerous because they
are capable of propagating oxidative damage.
These ROS are carried by the blood to distant
targets where fresh oxidative damage may be
initiated. Membrane lipid peroxidation may alter
fluidity and permeability, and compromise the
integrity of the barrier. Hence, the study of lipid
peroxidation to estimate oxidative stress is a
popular practice. In 1978, Dillard et al. first
reported that in humans physical exercise at 75%
V
.
o2max.increased the level of pentane, a possible
by-product of oxidative lipid damage or lipid
peroxidation, by 1.8-fold in the expired air com-
pared with resting subjects. Since then, consider-
able evidence has accumulated showing that
physical exercise may trigger lipid peroxidation
in several tissues including skeletal muscles,
heart, liver, erythrocytes and plasma (Sen 1995).
In a human study, serum lipid peroxidation was
measured by three different methods during
physical exercise of different duration with the
aim of uncovering the significance of each
method in measuring oxidative stress after
physical exercise (Vasankari et al. 1995).
Oxidative protein damage is widespread
within the body at rest. It has been estimated
that, at rest, 0.9% of the total oxygen consumed
by a cell contributes to protein oxidation (Floyd
1995). Most of this damage is irreparable, and by-
products of such damage are either stored or
degraded. Proteins that have been damaged by
reactive oxygen are highly susceptible to prote-
olytic cleavage. The amount of oxidized protein
in various tissues increases with age (Levine &
Stadtman 1996). Certain components of protein
such as tyrosine, methionine, tryptophan, histi-
dine, and sulfhydryl residues are highly suscep-
tible to oxidative damage. Following reactive
oxygen attack, amino acid residues are converted
to carbonyl derivatives. Alternatively, reducing
sugars linked with the eamino group of Lys
residues can be oxidized. As a result, protein
carbonyl formation is widely used as an index of
oxidative protein damage. Other specific
markers of oxidative amino acid modification are
dityrosine crosslinking and formation of disul-
phide bridges (-S-S-) and mixed disulphides in
cysteine residues. For example, in dystrophic
muscle the protein disulphide to sulphydryl
(SS/SH) ratio has been observed to be increased,
suggesting the possible involvement of oxidative
damage (Kondo & Itokawa 1994). Oxidative
modification of proteins may cause receptor
modification, disturbance in intracellular ionic
homeostasis, and altered signal transduction,
and may also influence other fundamental cell-
regulatory processes. Reznick et al. (1992) have
reported that exhaustive exercise triggers skele-
tal muscle protein oxidation in rats. In another