Cis-trans Isomerism and Provitamin A Effect of Carotenoids
The chemical and biological relationships between carotene and vitamin A have been established by the classical investigations of Steenbock (412), Moore (320), Euler (111), Karrer (112), and others. It is now well known that β-carotene whose molecules contain two unsubstituted β-ionone rings is partially converted in the body into vitamin A, according to the schematic expression, C40H56 → C20H29OH. Several other naturally occurring carotenoids such as α-carotene, γ-carotene, and cryptoxanthin undergo analogous cleavage but in these instances only one half of each molecule may theoretically yield the bio- active substance. Although the postulate that at least one unsubstituted β-ionone ring must be present in provitamins A has been rather generally accepted, there are some exceptions. For example, 2,2′-dimethyl-β-carotene is effective in the rat to the extent of one half of the β-carotene potency [Eugster, Trivedi and Karrer (110)].
KeywordsTerminal Double Bond Analogous Cleavage Isomeric Carotenoid Short Wavelength Maximum Altered Configuration
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