Carotenoids

Carotenoids, a collective designation for a series of pigments of yellow, orange, or red color, characterized by their ability to dissolve in the same solvents as fats and constituting the main part of so-called lipochromes. They are widely distributed in the plant world. The most important representatives of carotenoids are carotene and xanthophyll, which always accompany chlorophyll in chloroplasts, and the former has been found even in etiolated plants. Both are also found in algae, fungi, and bacteria. Rhodoxanthin has been found in thuja, juniper, and others. Lycopin participates in the coloring of fruits. Fucoxanthin has been isolated from brown algae. Similar or related carotenoids are also found in the animal world. In the blood serum of cattle and horses, a pigment similar to plant carotene has been discovered. A similar compound has been found in the corpus luteum of cows. An isomer of xanthophyll has been isolated from egg yolk. A pigment similar to lycopin has been isolated from the scales of goldfish. Carotenoids, mainly xanthophyll, participate in the coloring of feathers in birds. Among the skin pigments of many fish and amphibians (frogs, salamanders), carotenoids are found, but their nature has not been closely studied. The incompleteness, and sometimes uncertainty, of information about the individuality of particular carotenoids is due to both their small content in the objects studied and their easy oxidizability. Carotene and xanthophyll have been more closely studied chemically. The first is an individual chemical substance and has the formula C₄₀H₆₆. The latter is attributed the formula C₄₀H₅₆O₆, and it should be kept in mind that xanthophyll apparently represents a mixture of four pigments. The structure of carotene and xanthophylls is unknown, although they give well-expressed crystals. Other carotenoids, such as lycopin and fucoxanthin, also crystallize well. Regarding other carotenoids, it should be noted that lycopin is apparently isomeric with carotene, rhodoxanthin is close to xanthophyll, and fucoxanthin to carotene. The physical properties of carotenoids, especially their spectroscopic properties, have been studied in somewhat more detail. Thus, for a solution of 5 mg of carotene in 1 liter of methyl alcohol with a layer thickness of 5 mm, two absorption bands are characteristic for wavelengths: 492-478 mμ and 459-446 mμ, and for an identical solution of xanthophyll—three: 484-472 mμ, 454-441 mμ and 419 mμ. To obtain carotene and xanthophyll for spectral analysis, for example from leaves, the latter are dried, crushed, steeped with acetone, and both pigments are separated from chlorophyll by extracting them with methyl alcohol; then xanthophyll is separated from carotene with the help of petroleum ether. The physiological role of carotenoids is generally unclear. It is assumed that in plants they are regulators of oxidation processes, participate in the photosynthesis of carbohydrates, and possibly protect cellular enzymes from the action of light. The role of carotenoids in animals is even less clear. Euler attributes to carotenoids the properties of vitamin A (growth vitamin). Carotene and lycopin cause growth in animals in which it has ceased due to the removal of vitamin A from food, in the presence of vitamins B, C, and D. These carotenoids give the characteristic reaction of vitamin A: coloring in a chloroform solution upon the addition of antimony trichloride. The effect of carotene is manifested with a daily dose of 0.015 mg. In connection with this function, it is interesting to note the high content of carotenoids in the yolk, which serves as the main reserve of nutrients for the developing embryo, i.e., during the period of most active growth of the organism. It is assumed that isoprene serves as the material for the formation of carotenoids.