Agriculture Reference
In-Depth Information
free radicals, reactive oxygen species or superoxides. These reactive groups
have the capacity to attack and damage many vital biochemical systems,
including DNA molecules. Such damage is thought to initiate many
degenerative diseases, including arterial disease, and a proportion of such DNA
damage can lead to loss of control over cell division, resulting in a proliferation
of cancer cells. The ageing process is also regarded as being largely a result of
cumulative damage from free radical attack.
Alliums, in common with other vegetables, are a rich source of antioxidants
that are capable of reacting with free radicals and chemically neutralizing them.
In particular, quercetin, of which onion is the richest vegetable source, is a potent
antioxidant that can raise the overall antioxidant level in blood plasma following
ingestion. Twenty to 50% of ingested quercetin glycosides are absorbed,
depending on the glycoside type (see Fig. 8.9), and this also affects absorption
time, which ranges from 0.5 to 9.0 h. Subsequent excretion of quercetin is slow,
with a half-life of about 24 h. Ability to trap certain types of free radicals has also
been demonstrated in some compounds derived from garlic - for example, diallyl
disulfide (DADS). The more antioxidant present in the system the greater the
proportion of free radicals that can be neutralized before they cause cellular
damage.
In addition to ingested antioxidants, cells produce their own enzymic
systems for neutralizing free radicals and certain mineral elements - notably,
selenium, iron, manganese and copper - are essential cofactors for these
enzymes. Hence the mineral content - in particular, the selenium content of
vegetables - is important to this aspect of cellular defence against free radicals
and their cancerous potential.
It is inevitable that some carcinogens enter the cell biochemistry.
Compounds from alliums have been shown to suppress enzymes involved in
carcinogen activation (termed the Phase I enzymes) and to induce or up-
regulate enzymes involved in detoxification of carcinogens (termed the Phase II
enzymes). For example, allicin and diallyl disulfide inactivates the human
cytochrome P450 involved in carcinogen activation
in vitro
. Furthermore, rats
fed garlic powder had decreased activation of the carcinogen aflatoxin and
increased levels of the enzymes involved in its detoxification in their livers, and
showed less genetic damage as a result of ingesting carcinogens than controls
(Guyonet
et al.
, 2002).
The Phase II enzymes combine carcinogens with biochemical groups that
favour their excretion in urine. Most Phase II enzymes - including glutathione
reductase and quinone reductase - are up-regulated by feeding diallyl
disulfide, which is found in garlic or garlic oil. Garlic or garlic oil and onion oil
have been shown to decrease the number and rate of development of tumours
in mice and to suppress the growth of leukaemia cell cultures (Griffiths
et al.
,
2002).
