Superlative carotenoids

A selection of carotenoids beyond normal appearance or properties has been presented at the 16(th) International Symposium on Carotenoids. Some of the exceptional compounds shown at the conference cannot be reproduced in this proceeding since they have not yet been published. In addition, editorial space limitation does not allow illustrating all of the previously mentioned carotenoids.     

Bacterial carotenoids

Zeaxanthin dirhamnoside is the major carotenoid of Corynebacterium autotrophicum. The absolute configuration 3R,3R followed from CD-properties of its hexaacetate, -L-assignment and ¹C₄ conformation were concluded from ¹H NMR data by comparison with model compounds.No. XLIX: Acta chem. scand. (in press)Dedicated to Professor R. Y. Stanier on his 60th birthday in appreciation of his studies on the function and biosynthesis of carotenoids and influence on the scientific development of his co-workers     

Fungal carotenoids

The Botanical Review -     

Photoprotection by carotenoids

Carotenoid pigments have been found to have a protective function against photosensitization in green plants. This protective ability has been exploited in the administration of high doses of beta-carotene to patients with erythropoietic protoporphyria to ameliorate the photosensitivity associated with this disease. The carotenoids seem to exert their light-protective function by quenching excited species such as singlet oxygen and free radicals.     

Algal carotenoids 52; secondary carotenoids of algae 3; carotenoids in a natural bloom of Euglena sanguinea

Quantitative carotenoid analysis of a natural bloom of Euglena sanguinea Ehrenberg revealed the presence of β,β-carotene (1% of total carotenoids), monoesters of adonirubin (3%), diesters of (3S, 3′R)-adonixanthin (13%), diesters of (3S, 3′S)-astaxanthin (75%), 19-monoester of (3R, 3′R, 6R)-loroxanthin (1%), (3R, 3′R)-diatoxanthin (6%), diadinoxanthin (1%) and neoxanthin (traces). The carotenoid content amounted to 0.7% of the dry wt. Methods employed included TLC, HPLC, VIS, MS, CD and H NMR (400 and 500 MHz). The high content of ketocarotenoids is characteristic of secondary carotenoids produced under stressed growth conditions. Previously secondary carotenoids were associated with green algae (Chlorophyceae), but have now been encountered in Euglenophyceae.     

Antioxidant activity of carotenoids

Carotenoids are pigments which play a major role in the protection of plants against photooxidative processes. They are efficient antioxidants scavenging singlet molecular oxygen and peroxyl radicals. In the human organism, carotenoids are part of the antioxidant defense system. They interact synergistically with other antioxidants; mixtures of carotenoids are more effective than single compounds. According to their structure most carotenoids exhibit absorption maxima at around 450 nm. Filtering of blue light has been proposed as a mechanism protecting the macula lutea against photooxidative damage. There is increasing evidence from human studies that carotenoids protect the skin against photooxidative damage.     

Non-antioxidant properties of carotenoids

Dietary antioxidants such as carotenoids, tocopherols, vitamin C or flavonoids exhibit biological activities that are not directly related to their antioxidant properties. The parent compounds and/or their metabolites have impact on cellular signaling pathways, influence the expression of certain genes or act as inhibitors of regulatory enzymes. Thus, they reveal additional biological effects which might be of importance in context with the prevention of degenerative diseases related to the consumption of a diet rich in antioxidants. This review focuses on known non-antioxidant properties of carotenoids, including retinoid-dependent signaling, stimulation of gap junctional communications, impact on the regulation of cell growth and induction of detoxifying enzymes, such as cytochrome P450-dependent monooxygenases.     

Cancer prevention by carotenoids

Various natural carotenoids seem to be valuable for cancer prevention, and these carotenoids may be more suitable in combinational use, rather than in single use. In fact, we have proven that combinational use of natural carotenoids resulted in significant suppression of liver cancer. Patients of viral hepatitis with cirrhosis were administered with β-cryptoxanthin-enriched Mandarin orange juice, in addition to capsules of carotenoids mixture. Cumulative incidence of hepatocellular carcinoma development was compared with that in the group treated with carotenoids mixture capsules alone, or in the group without treatment (control group). In the data analysis at year 2.5, cumulative incidence of liver cancer in β-cryptoxanthin-enriched orange juice with carotenoids mixture capsules-treated group was lower than that in the control group (p = 0.05). Cumulative incidence of liver cancer in the group treated with carotenoids mixture capsules alone was also lower than that in the control group, but not statistically significant.     

Biological actions of carotenoids

Of 600 carotenoids from natural sources that have been characterized, fewer than 10% serve as precursors of vitamin A. Many dietary carotenoids, both with and without provitamin A activity, are found in the blood and tissues of humans. beta-Carotene, the most nutritionally active carotenoid, comprises 15-30% of total serum carotenoids. Vitamin A is formed primarily by the oxygen-dependent central cleavage of beta-carotene and other provitamin A carotenoids. Several carotenoids show enhancement of the immune response, inhibition of mutagenesis, reduction of induced nuclear damage, and protection from various neoplastic events in cells, tissues, and whole animals. Carotenoids also protect against photo-induced tissue damage. Some carotenoids, including beta-carotene, quench highly reactive singlet oxygen under certain conditions and can block free radical-mediated reactions. In epidemiological studies, the intake of carotenoid-rich fruits and vegetables has been correlated with protection from some forms of cancer, particularly lung cancer. Similarly, serum beta-carotene levels have been associated with a decreased chance of developing lung cancer. It must be stressed, however, that these epidemiological associations do not show cause and effect. In this regard, long-term intervention trials with beta-carotene supplements are in progress. Whatever the results of these trials, carotenoids clearly show biological actions in animals distinct from their function as precursors of vitamin A.     

Algal carotenoids and chemotaxonomy

A qualitative and quantitative examination of the carotenoids of pure cultures of four marine microalgae, including Chroomonas salina, Vaucheria sessilis and Coccolithus huxleyi is reported. The latter contained a new, major carotenoid, and not fucoxanthin.     

Antioxidant functions of carotenoids

Carotenoid pigments, including hydrocarbons such as β-carotene or xanthophylls such as lutein and zeaxanthin, are very widely distributed in nature, where they play an important role in protecting cells and organisms against the harmful effects of light, air, and sensitizer pigments. This process has been demonstrated in bacteria, algae, plants, animals, and even in humans in the light-sensitive disease, erythropoietic protoporphyria. The primary mechanism of action of this phenomenon appears to be the ability of carotenoids to quench excited sensitizer molecules as well as quench ¹O₂.

In addition to this protection, and potentially of even greater biological importance, is the fact that carotenoids can also serve as antioxidants under conditions other than photosensitization. This review presents the data available indicating the extent of this important function. Antioxidant action can be documented in both enzymic and nonenzymic systems, and has been reported in subcellular, cellular, and animal studies. In fact, the many reports indicating that carotenoids may possess some anticarcinogenic properties may well be related to their ability to interact with and quench various radical species that can be generated within cells.