Free radicals are molecules with a single unpaired electron
in an outer orbital position - they react vigorously with and modify a
wide range of molecules affecting especially lipids (of cell membranes),
DNA and proteins.
Damage to the lipids, DNA and proteins give rise to:
inflammation
ageing
radiation damage
re-perfusion injury
Some of these free radicals decompose naturally, or removed by anti-oxidants
especially vitamin C and through the action of enzymes such as superoxide
mutase and catalase.
Photoreceptors are susceptible to damage from radicals due to its high
metabolism and constant exposure to light which increase the amount of
free radicals formation. These radicals cause damage by reacting with the
protein, DNA and membrane of the photoreceptors. In addition, the formation
of lipofuschin may be the results of such damage.
Fortunately, a number of protective mechanisms are in place to reduce
such damage. These include:
a. Free radical scavenger such as vitamin E. Vitamin E
inhibit lipid peroxidation which is important in
free
radical formation
b. Enzymes that removed free radicals such as glutathione
peroxidase and superoxide dismutase which are present
in high concentration in the retina.
c. Mechanisms that reduce light damage such as
xanthophyll in the macula and melanin in the retinal
pigment epithelium which absorb the harmful blue
light
and reduce free radical formation.