According to Wald (1935) the characteristic pigments of the rods and cones in the retina are rhodopsin and iodopsin. They differ only in respect of the protein moieties. The specific pigment common to both is a cis-isomer of retinene. Vitamin A alcohol is oxidized to vitamin A aldehyde in the epithelium of the rods by alcohol dehydrogenase in the presence of NAD. The protein opsin reacts with retinene to form rhodopsin. Light initiates a series of photochemical changes in rhodopsin, beginning with bleaching of the purple pigment and ending with formation of all–Trans-retinene and its isomerisation. The resynthesis of rhodopsin is isomer specific, requiring the 11-cis-isomer of retinene.
Vitamin A and epithelial tissues:Vitamin A is essential for the integrity of the mucous secreting cells of epithelial tissues. In vitamin Adeficiency, the epithelial tissues are keratinized. The tissues affected are salivary glands, respiratory tract, eyes, skin and sex organs.
Mucopolysaccharides and sulphate metabolism:Vitamin A takes part in the incorporation of inorganic sulphate in mucopolysaccharide synthesis.
Vitamin A and mucoprotein synthesis:Vitamin A is essential for the synthesis of mucoproteins and glycoproteins.
Vitamin A and reproduction: In vitamin Adeficiency, reproduction does not take place due to (I) infertility in the male and (II) failure of the female to conceive, if conceived resorption or abortion of the fetus occurs.