There is accumulating evidence that oxidative
damage to key biological sites is implicated in
the ageing process, and in the development of
chronic age-related disease, including Alzheimer's
disease, cancer, cardiovascular disease and cataracts. Oxidative damage is caused by reactive
species, most of which are of endogenous origin,
and many of which are unavoidable. Reactive
species damage protein thiol groups, polyunsaturated fatty acids (PUFA) and nucleoside bases, with consequent changes in the associated
structures, e.g. proteins, LDL, cell membranes
and DNA. There are many intra- and extracellular
antioxidant defences to oppose oxidative
damage. Defences include antioxidant enzymes,
such as superoxide dismutase and glutathione
peroxidase, metal binding proteins, such as
transferrin and caeruloplasmin, and dietary
derived scavenging or chain breaking antioxidants,
such as ascorbic acid (vitamin C) and
a-tocopherol (vitamin E). A relative or absolute
deficiency of antioxidants causes a pro-oxidant
shift in oxidant/antioxidant imbalance, and
results in increased oxidative stress. Although
inter-relationships are currently unclear, assessing
oxidative stress and antioxidant status may help
identify individuals at particularly high risk of
developing age-related disease. Optimising balance
and lowering oxidative stress, through dietary
and other strategies, may promote longevity
and healthy ageing. Owing to the variety of targets
for oxidative damage, and to the number and
diversity of antioxidants, there is no single test
of oxidative stress, nor is there one definitive test
of antioxidant defence. Rather, an array of
biomarkers is used, each assessing one aspect of
damage or defence. In general biomarkers can be
grouped into those which detect or measure: 1)

oxidative damage to key biological sites such as
DNA or lipid; 2)degradation products of oxidized
lipid or DNA; 3)resistance of key structures to
oxidant challenge; 4)individual antioxidants; 5)
overall('total') antioxidant capacity. A variety
of sensitive biomonitoring tools is needed to
measure these, and results are used to build a
composite picture of overall oxidant/antioxidant
balance, and to assess the level of oxidative
stress. Tools and biomarkers used currently
include: the Ferric Reducing/Antioxidant Power
(FRAP) assay for 'total antioxidant activity'
(Benzie & Strain, US patented); a modification
of this assay (FRASC) for the simultaneous
measurement of ascorbic acid and antioxidant
power; HPLC measurement of plasma atocopherol,
allantoin (the non-enzymatic-oxidative
product of uric acid) and malondialdehyde (a
degradation product of oxidized PUFA); a
probe-assisted flow cytometric method for lipid
peroxidation in membranes of living cells; the
single cell gel electrophoresis, or comet, assay for
DNA damage, mutation and repair. Measurement
of F2 isoprostanes (products of oxidized
arachadonic acid), and 8-oxodG (an oxidized
base spliced out from damaged DNA during
repair) can also be performed using ELISA,
HPLC or GC-MS techniques. It is important
to understand the rationale behind these
biomarkers in the assessment of oxidative stress,
and to appreciate their value, limitations and
inter-relationships in order to be able to interpret
their meaning in age-related research. This
paper will describe the rationale behind
selection of biomarkers of oxidative stress,
explain in brief detail how they can be
measured, and discuss the information they
may reveal regarding oxidant/antioxidant
balance in ageing and age-related disease.