Function Of Carotenoids

It is tempting to propose a single universal function for the carotenoids since they are found in such diverse tissues. Failing this, it is tempting to ascribe some function, wherever a carotenoid is found. Although the function of the carotenoids has been proven in some cases, their universal function, if any, remains to be determined. Where the function has been proven, it generally is with some aspect of the light-absorbing property of the carotenoid pigment. The critical role of these pigments in photosynthesis has been the best documented. Phototropism and phototaxis responses of plants seem to be related to the light absorption spectra of the carotenoids. Functions in animals are related to the antioxidant and to the light-absorbing property of these pigments. The function in vitamin A formation has been discussed. Because these pigments are found in many reproductive tissues, it has been suggested that they have a role in reproduction. The results in this area are less convincing than those in other areas.

In the photosynthetic process the carotenoids have been shown to be active in the light-gathering process (eg, they absorb light at wavelengths not absorbed by chlorophyll). In Chlorella, other green algae, and higher plants, the light absorbed by the carotenoids was used at low efficiency. In the diatom and brown algae the energy transfer is comparable to chlorophyll where the main pigment in fucoxan-thin. In the photosynthetic process two photo systems are involved. In general, more carotenes are found in photosystem I and xanthophylls in photosystem II.

The role of the carotenoids in photoprotection of photosynthetic bacteria has been well documented. A mutant, which only produced the colorless polyenes phytoene and phytofloene, was compared with the wild type Rhodopseu-

dowonas sphaeroides. The wild type showed good growth under all conditions of light and oxygen. However, the mutant could only grow under anaerobic-light or aerobic-dark conditions.

Under light with 02, massive killing resulted at room temperature and bacterial chlorophyll was destroyed. If the mutant was grown in the dark with air, chlorophyll disappeared. Where chlorophyll was missing, light and 02 have no effect.

Diphenylamine (DPA) inhibits the formation of colored carotenoids. When the wild type blue-green bacteria is treated with DPA, it is destroyed like the mutant. At low temperatures, killing proceeds in the mutant but bacterial chlorophyll is not destroyed.

Corynebacterium poinsettiae is not photosynthetic, and both the wild type and the carotenoid-less mutant can tolerate high intensities of light in the presence of air. When the exogenous dye, toluidine blue, is added as a photosensitizing pigment, the carotenoid-less mutant is killed. The mechanism involves the simultaneous interaction of visible light, a photosensitizing dye, and 02. Generally, the carotenoids are effective for visible light but have no effect in ultraviolet, gamma, or X radiation.

The reactions are listed as follows:

*CHL + -»■ photosynthesis or 3CHL triplet-excited state intersystem crossing

3CAR 1CAR -> harmless decay

^02 + CAR CAR 02 CAR can be regenerated or 102 + A A 02 photodynamic action

3CAR -» XCAR harmless decay

As can be seen in these reactions, carotenoids may protect photosynthetic bacteria at various levels by quenching the singlet-excited state of 02 or the triplet-excited state of chlorophyll. The ground states of oxygen would be 302 and for CHL the triplet state. The carotenoids may be the preferred substrate for oxidation or may act in quenching reactive species.

Phototropism is a response of higher plants and some fungi that results in the plant turning to the light. Rival claims for either /f-carotene or riboflavin as the active compound have been made based on the action spectra. More recent reports seem to rule out ^-carotene as the mediator in the response.

Phototaxis is a response in which an organism such as Euglena can move toward the light. The action spectra does not match the spectra of the carotenoids.

The involvement of the carotenoids in reproduction appear to be coincidental to the process of sexual reproduction and of no known significance to the process. The major pigment of the brine shrimp is canthaxanthin; however, the female Artemia converts the all-trans canthaxanthin to cis canthaxanthin during the time of sexual activity.

In animals the major function of carotenoids is as a precursor to the formation of vitamin A. It is assumed that in order to have vitamin A activity a molecule must have one-half of the structure similar to that of /^-carotene. Recently it has been shown that astaxanthin can be converted to zeaxanthin in trout where the fish is sufficient in vitamin A. Tritiated astaxanthin was converted to retinol in strips of duodenum or inverted sacks of trout intestines. Astaxanthin, canthaxanthin, and zeaxanthin can be converted to vitamin A and A2 in guppies.

We have become increasingly aware of a mounting body of evidence that suggests that the carotenoids can function in medical applications apart from their role as vitamin A precursors. The symptoms of erythropoietic Protoporphyria can be relieved by large doses of /?-carotene (38). This pigment was deposited in the skin of patients who lacked skin pigmentation.

As of late some carotenoids have been claimed to be effective in the treatment of cancer, particularly cancer of epithelial origin. Other carotenoids such as canthaxanthin and phytoene with no vitamin A activity were, in cases, found to be as effective as /^-carotene.

Some of the first studies were based on epidemiological studies. In one such study it was shown that the participants who consumed diets rich in carotenoids developed fewer lung cancers whether or not they smoked. This study assumed that the active ingredients are the carotenoids contained in fruits and vegetables.

More recently a number of animal studies have shown that /¡¡-carotene or other carotenoids can prevent or slow down the growth of skin cancer and other cancerous tumors. Review have been published on the role of the carotenoids in relation to cancer. Human studies are presently being conducted (39,40).

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