Retinol
Retinol, retinaldehyde and retinoic acid are collectively known as vitamin A. Retinol is not obtained directly from the diet, but rather is generated in the intestine from the enzymatic cleavage of beta-carotene, alpha-carotene and beta-cryptoxanthin, which are obtained from fruits and vegetables. Animal products such as egg yolk, milk, liver and fat contain retinyl esters which are hydrolyzed to retinol in the intestine. Retinol is transported in serum to target cells complexed with a specific retinol-binding protein and transthyretin, a thyroxin-binding protein. In the target cells, retinol is oxidized to retinal and retinoic acid. Retinal is utilized in the synthesis of rhodopsin in the cones, and is necessary for night vision. Steroid-hormone nuclear receptors for all-trans and 9-cis retinoic acid have been identified in cells, and form complexes with retinoic acid. These complexes bind to the promoter regions of specialized genes and promote transcription, especially during embryonic development.14
Vitamin A has been shown to enhance gap junction communication in a dose-dependent manner, and may promote normal cell growth.51 Vitamin A is necessary for maintenance of healthy epithelial tissue and can prevent the inception or progress of skin cancers by stimulating normal cell differentiation.14 Vitamin A therapy has been useful for inhibiting or suppressing tumor growth in the mouth,3 cervix,7 breast,8 lung,13 skin,14 and bladder.15 Other functions include mucous production and normal bone growth.28 This vitamin also plays a role in male and female fertility.
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