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Acetate, by condensation and decarboxylation reactions, is converted to isopentenyl pyrophosphate, which condenses to form C20 terpenol pyrophosphates. These latter compounds condense to yield the initial C40 carotenoid precursor which is presumably phytoene. By a series of dehydrogenation, cyclization, isomerization, and hydration reactions, various acyclic and alicyclic carotenoids are formed. Subsequently, hydroxylation, epoxidation, and oxidation–reduction reactions may occur. Several carotenoids and β-apocarotenals may be converted into retinol in mammalian tissues. Thereafter, retinol may be esterified, oxidized to retinal and retinoic acid, isomerized, and further metabolized. This review has stressed the route by which these transformations occur and the characteristics of the enzymes involved rather than nutritional, functional, or chemical aspects of carotenoids.