Vitamin A1 (retinol) and vitamin A2 (dehydroretinol) (Figure 5.72) are fat-soluble vitamins found only in animal products, especially eggs, dairy products, and animal livers and kidneys. Fish liver oils, e.g. cod liver oil and halibut liver oil (see Table 3.2) are particularly rich sources. They exist as the free alcohols, or as esters with acetic and palmitic acid. Vitamin A2 has about 40% of the activity of vitamin A^ Carotenoid precursors (provitamins) are widely distributed in plants, and after ingestion, these are subsequently transformed into vitamin A in the liver. Green vegetables and rich plant sources such as carrots help to provide adequate levels. A deficiency of vitamin A leads to vision defects, including impairment at low light levels (night blindness) and a drying and degenerative disease of the cornea. It also is necessary for normal growth of young animals. Retinoids (vitamin A and analogues) are now known to act as signalling molecules which regulate diverse aspects of cell differentiation, embryonic development, growth, and vision. For the processes of vision, retinol needs to be converted first by oxidation into the aldehyde all-trans-retinal, and then by enzymic isomerization to cis-retinal (Figure 5.73). cis-Retinal is then bound to the protein opsin in the retina via a Schiff base linkage to give the red visual pigment rhodopsin, and its sensitivity to light involves isomerization of the cis-retinal portion back to the all-trans form, thus translating the light energy into molecular change, which triggers a nerve impulse to the brain. The absorption of light energy promotes an electron from a n- to a n+-orbital, thus temporarily destroying the double bond character and allowing rotation. A similar cis-trans isomerization affecting cinnamic acids was discussed under coumarins (see page 142). All-trans-retinal is then subsequently released for the process to continue. Vitamin A is relatively unstable, and sensitive to oxidation and light. Antioxidant stabilizers such as vitamin E and vitamin C are sometimes added. It is more stable in oils such as the fish liver oils, which are thus good vehicles for administering the vitamin. Synthetic material is also used. Excessive intake of
(vitamin Ai) (vitamin A2)
(vitamin Ai) (vitamin A2)
The synthetic retinoic acids tretinoin (retinoic acid) and isotretinoin (13-cis-retinoic acid) (Figure 5.72) are retinoids that are used as topical or oral treatments for acne vulgaris, reducing levels of dehydroretinol and modifying skin keratinization. Dehydroretinol levels in the skin become markedly elevated in conditions such as eczema and psoriasis. Acitretin (Figure 5.72) is an aromatic analogue which can give relief in severe cases of psoriasis. All these materials can produce toxic side-effects.
coats of annatto (Bixa orellana; Bixaceae), and bixin is widely used as a natural food colorant. Crocetin, in the form of esters with gentiobiose [D-Glc(pi ^ 6)D-Glc], is the major pigment in stigmas of Crocus sativus (Iridaceae), which comprise the spice saffron.
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