Carotenoids are always of plant origin. Chemically, they are tetraterpenes, consisting of eight symmetrically arranged isoprene units (A). Their behavior is determined by their long hydrocarbon chains, which contain many conjugated double bonds: they are lipophilic and colored.
Several hundred different carotenoids have been found in plants, but only 40 of them have provitamin A activity. Quantitatively of the greatest importance to humans are the p-carotenes, which are responsible for the red color in carrots; lycopene, which, as opposed to p-carotenes, has no aromatic components, is the main pigment in tomatoes and red bell peppers. Xanthophylls form through insertion of hydroxy groups into the carotenoid rings. Among others, they cause the yellow color of leaves during fall (mostly lutein, also called leaf xanthophyll). Zeaxan-thin gives corn its yellow color. Egg yolk contains a mixture of several xantho-phylls and p-carotenes; its color depends on the feed composition.
Since p-carotenes are fat-soluble, they are absorbed in the small intestine together with the other lipids (B). The absorption rate of p-carotenes from plant sources ranges between 10 and 50%, with large individual differences. Inside the mucosal cell, p-carotenes are split into two molecules of retinal (vitamin A) at the central double bond. The splitting enzyme, 15,15-dideoxygenase, is regulated: the better the vitamin A supply, the lower the activity of the enzyme. Asymmetrical splitting yields intermediates with uneven side chains. The longer fragments, p-apocarotinals, can be converted into vitamin A. This conversion seems to be dependent on the respective cell's vitamin A needs. Finally, it appears that p-carotene may interfere with oxidative processes as soon as it enters the mucosa cells (see p. 138).
For purposes of transportation, some of the p-carotenes, as well as the esterified retinol, are packaged in chylomicrons and taken to the blood via lymphatic fluid. When endothelial LPL degrades the chylomicrons, p-carotenes are taken up by the cells as well. The remnants are taken to the liver, where p-carotene is also converted to vitamin A. Excess p-carotene is packaged in LDL and VLDL and thus made available to extrahepatic tissues. Polar vitamin A metabolites such as retinoic acid can leave the mucosal cells directly with portal blood.
p-carotene is stored predominantly in fatty tissue and to a lesser extent in the liver, with concentrations in both tissues subject to large individual variations. The high storage capacity of fatty tissue and liver and/or the concentration-dependent absorption and concentration processes are thought to be responsible for the benign effects of high dosages of p-carotene as opposed to similar amounts of vitamin A: p-car-otene is nontoxic and nonteratogenic.
B. ß-Carotene Metabolism
ß-Carotene Bile h
Retinoic acid and polar ■ metabolites
Asymmetrical ' cleavage i
Mucosa cell cleavage o0 o
Extrahepatic Vitamin A tissues
Extrahepatic Vitamin A tissues
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