Vitamins C and E, as well as ubiquinones (Fig. 3) and
derivatives of the nonmetallic element selenium, together
with sulfur-containing proteins, all participate in an elaborate
antioxidant system. This system protects us against
many of the adverse effects of reduced oxygen compounds
such as hydrogen peroxide (H2
), superoxide (O2
hydroxyl (OH) radicals. The system is quite complex and
not fully understood. However, ascorbic acid, which can
itself form free radicals readily, appears to be a key player.
Water-soluble and present in a high concentration, its role
seems to be to keep many cellular components reduced.
The tocopherols (vitamin E), in their various isomeric
forms, scavenge free radicals formed from oxidation of
unsaturated fatty acids within cell membranes. Vitamin E
is especially effective in removing organic peroxide radicals.
Supplementation with dietary vitamin E is being
tested for prevention or amelioration of a variety of diseases
of aging including atherosclerosis and Parkinson’s
and Alzheimer’s diseases. The resulting tocopherol radicals
are rereduced by ascorbate in the aqueous phase.
Ascorbate can also donate electrons to ubiquinone radicals
present in the membranes of the mitochondria. It is
within the mitochondria that many damaging radicals are
thought to arise as side products of the reduction of O2
that occurs there.
In addition to its antioxidant role, ascorbic acid functions
to keep various metallic ions in catalytic centers in
their reduced forms. For example, some oxygenases require
iron or copper in their Fe2+
states of oxidation.
If these protein-bound ions are accidentally left
in a more oxidized state they may need to be reduced
by ascorbate ions. While this is a protectant role, there
are some enzymes for which ascorbate has become a
cosubstrate. An example is dopamine β-hydroxylase,
which converts dopamine to the neurotransmitter noradrenaline.
The enzyme contains copper which cycles
, as it incorporates one atom of
oxygen from O2
into its substrate. Ascorbate supplies the
electrons for reduction of the second atom of the O2
O. A recent report describes another distinct function
for ascorbate ion. It apparently acts as a basic catalytic
group for proton abstraction from a water molecule during
the action of a glycosyltransferase enzyme, becoming
part of the active site of that enzyme.