Vitamin A activity: retinoids and provitamin A carotenoids

THE CONTEXT

Vitamin A activity comes from preformed retinoids and from plant carotenoids that can serve as vitamin A precursors.

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Retinoids comprise retinol, retinaldehyde, and retinoic acid. Preformed vitamin A is found only in foods of animal origin.

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Plant carotenoids include carotenes and related compounds; many can be cleaved to yield retinaldehyde and subsequently retinol and retinoic acid.

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The quantitatively most important provitamin A carotenoids are α-, β-, and γ-carotenes and cryptoxanthin.

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Carotene dioxygenase cleaves β-carotene and other provitamin A carotenoids in the intestinal mucosa to produce retinaldehyde.

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Retinaldehyde is reduced to retinol, which is esterified and secreted in chylomicrons together with esters formed from dietary retinol.

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Intestinal carotene dioxygenase activity is low, so a relatively large proportion of ingested β-carotene may circulate unchanged.

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One carotene dioxygenase isoenzyme cleaves the central bond of β-carotene.

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The other isoenzyme causes asymmetric cleavage, producing 8′-, 10′-, and 12′-apo-carotenals.

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These apo-carotenals are oxidized to retinoic acid but cannot serve as sources of retinol or retinaldehyde.

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