2. What are free radicals? How do they cause damage to retinal 
    photoreceptors? What protective mechanisms are present in 
    the normal eye? 
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.

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