J L Napoli, University of California, Berkeley, CA, USA
© 2005 Elsevier Ltd. All rights reserved.
In 1913, E.V. McCollum isolated a yellow, fat-soluble substance from egg yolks that was critical for animal growth. He called it fat-soluble A, indicating the first isolated of several dietary microconsti-tuents emerging as obligatory for vertebrate life and health. Later, fat-soluble A was renamed vitamin A, derived from the terminology 'vital amine,' coined by Casmir Funk to describe these obligatory micronutrients.
Currently, the term vitamin A refers to the specific organic compound all-trans-retinol (atROH). atROH, however, does not have biological activity in its own right. Rather, it serves as a circulating substrate for metabolism into the compounds that fulfill the biological functions attributed to vitamin A. These metabolites include, but may not be limited to, 11-ds-retinal (11cROH) and all-trans--retinoic acid (atRA). The term 'retinoids' describes all compounds that support vitamin A activity, both naturally occurring and synthetic, including atROH. Figure 1 illustrates the structures of key carotenoids and retinoids.
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