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conjugation and excretion reactions

Figure 2 Schematic of principal reactions of vitamin A metabolism.

critical for vision, absorbs light maximally at ^365 nm when in organic solvent, but when coupled with a protein, such as opsin, its peak absorptivity is shifted into the visible range of the electromagnetic spectrum (see 'Vision'). In its all-trans isomeric form, retinal is a transient intermediate in the bioconversion of retinol to retinoic acid. Retinoic acid exists in several isomeric forms, two of which (all-trans-retinoic acid and 9-czs-retinoic acid) interact specifically with nuclear receptor proteins.

Numerous metabolites of retinol or retinoic acid are more polar than retinol or retinoic acid due to additional oxidation of the cyclohexenyl ring, often on carbon 4. Some retinoids, particularly retinoic acid and 4-keto-retinoic acid, may be conjugated with glucuronic acid, forming retinyl- or retinoyl-/3-glucuronide (R5); these metabolites are water-soluble and therefore readily excreted. While some polar and water-soluble retinoids possess bioactiv-ity, most show reduced, or no, activity compared to their precursors.

Carotenoids

Carotenoids are produced only by plants and a few microalgae. In plants, they function as accessory light-gathering pigments that enhance the efficiency of photosynthesis. Of the 600 or so carotenoids found in nature, only ^-carotene, a-carotene, and /3-cryptoxanthin have the structural features necessary for vitamin A activity. Beta-carotene is a hydrocarbon (C40H56, molecular mass 536) with two ^-ionone rings, a polyene chain, and structural symmetry around the central 15,15' double bond (Figure 1B). The oxidative cleavage of this bond releases two molecules of retinal, which can be reduced to form vitamin A (retinol). Other isomers of ^-carotene with potential nutritional activity include 9-cis-^-carotene produced by certain microalgae. Other common carotenoids found in fruits and vegetables, such as lycopene, lutein, and zeaxanthin, are absorbable but they lack structural features essential for vitamin A activity.

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