Examples of Carotenoid Biotransformations

Solely on the basis of structural considerations, the types of metabolism to be expected of carotenoids include cleavage (both polyene chain and ionone ring) and chain shortening, hydroxylation/oxidation, and dehydration (of xanthophylls) (195). Cleavage of the polyene chain, either symmetrical or asymmetrical, is well known; opening of ionone rings has not been reported. Esterification of xanthophylls might be expected (by analogy with retinol and cholesterol) but has not been definitively proven to occur.

Cleavage of provitamin A carotenoids such as ^-carotene is discussed in detail elsewhere in this volume. In the current context, it should be remembered that retinoic acid (as well as retinal) can be produced by cleavage of ^-carotene in some tissues (196).

Khachik et al. have proposed that metabolism of lycopene in human tissues might involve formation of epoxides, specifically at the 1,2- and 5,6 double bonds, with subsequent rearrangement of the strained three-carbon epoxide rings to five-membered rings (197). To what extent these are enzymatically catalyzed reactions within human tissues and to what extent they occur nonenzymatically during processing of tomato products is not yet clear.

Anhydrolutein, a dehydration product of lutein, has been found in human plasma (198). Subsequently the structural configuration of the major anhydrolutein product in human plasma was identified as (3R,6'R)-3-hydroxy-3',4'-didehydro-b,y-carotene, with lower concentrations of (3R,6'R)-3-hydroxy-2',3'-didehydro-b,y-carotene (199). To date these compounds have been detected in few foodstuffs (200,201); thus it is assumed that the amounts found in human plasma are due to transformation from dietary lutein, perhaps catalyzed in the acid milieu of the stomach (199). Savithry et al. found that anhydrolutein is converted to 3-dehydroretinol (vitamin A2) in rat intestines (202). Vitamin A2 is found in human tissues also, but its source in humans has not been determined; we suggest that it may arise from this process as well as preformed from diet.

Evidence for biotransformation of carotenoids in humans is mostly indirect at best. This is in large part because of lack of appropriate labeled compounds. However, there is evidence of biotransformation in other species, in particular, in birds. McGraw et al. have demonstrated that cardinals (Cardinalis cardinalis) are capable of transforming dietary yellow carotenoids to red carotenoids for plumage coloration (203), and also suggested that the zebra finch (Taeniopygia guttata) is capable of transforming dietary lutein to anhydrolutein (204). As in the cardinal, the American goldfinch (Carduelis tristis) also shows gender-specific biotransformation of carotenoids (205). Selective distribution of carotenoids among different tissues and organs has been shown in avian species. In the free-living gull (Larus fucus), liver contained the highest carotenoid concentrations, and b-carotene was the most prominent carotenoid in liver; in all other tissues studied, lutein was the predominant carotenoid, with zeaxanthin, canthaxanthin, b-cryptoxanthin, echinenone, and b-carotene present in lesser concentrations (206). Surai et al. found that b-carotene was the predominant carotenoid in the egg yolks of common moorhen (Gallinula chloropus), American coot (Fulica americana), and lesser black-backed gull (Larus fuscus), in contrast to the domestic chicken (Gallus domesticus) in whose yolk xanthophylls are predominant (207). In the newly hatched gull, proportions of lutein and zeaxanthin were high in heart and muscle (but low in liver) when compared to the yolk; proportions of canthaxanthin, echinenone, and b-carotene were lower in heart and muscle than in yolk. In the newly hatched coot and moorhen, the liver was relatively enriched in b-cryptoxanthin and b-carotene (as well as echinenone in the moorhen) compared to the egg yolks. Plumage coloration in wild house finches (Carpodacus mexicanus) is related to dietary carotenoid intake (208). Red coloration of male barn swallows (Hirundo rustica) correlated with plasma concentrations of lutein (209).

Khachik et al. (179) studied carotenoid concentrations in the eyes of several species and identified nondietary carotenoids; they concluded that biotransformations of xanthophylls involve oxidation-reduction reactions and double-bond isomerizations. Similar carotenoid profiles were found in humans, frogs (179), and quail (187), suggesting the presence of similar biotransformation pathways.

Esters of xanthophylls are generally not detectable in human tissues [except when vary large quantities are fed (169)]. This suggests that intact esters are not absorbed at usual dietary intake levels, and that either (a) they must be hydrolyzed in the intestinal lumen or (b) they are generally not available for absorption. A variety of evidence indicates that xanthophyll esters are bioavailable and that they are hydrolyzed by esterases in the intestinal lumen. However, some animals, such as the lobster, can accumulate appreciable quantities (as much as 80% of total carotenoids) of xanthophyll esters (210), and lutein esters have been detected in tissues of the chicken (211).

Tyckowski and Hamilton found that canthaxanthin is reduced to an alcohol in chickens (212), but similar metabolism has not yet been described in humans.

Enzymatic cleavage of carotenoids is discussed elsewhere in this volume. Although the quantitatively greater pathway is that of central cleavage, there is evidence for excentric cleavage to retinoids and apocarotenoids in animal and human tissues [reviewed in (213)]. After rats had been fed ¡-apo-8'-carotenal, shorter chain apocarotenoic acids could be identified in their intestinal mucosa (214); it is not yet known whether this is due to activity of a carotenoid cleavage enzyme, or to ¡3 oxidation of the polyene chain, as is seen with chain shortening of «-tocopherol (215,216).

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Responses

  • kimberly costello
    What is anhydrolutein in the intestins?
    15 days ago

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