PHARMACOLOGICAL EFFECTS OF CAROTENOIDS: A REVIEW

PHARMACOLOGICAL EFFECTS OF CAROTENOIDS: A REVIEW

Sumita S. Kadian*¹ and Munish Garg ²

Guru Jambheshwar University of Science and Technology ¹, Hisar, Haryana, India

Department of Pharmaceutical Sciences, Mahrshi Dayanand University ², Rohtak-124001, Haryana, India

ABSTRACT:Vitamin A is an essential vitamin which is required in the vision process, epithelial maintenance, mucous secretion and reproduction obtained from carotenoids.  Carotenoids have been considered to provide benefits in age-related diseases, against some forms of cancer (in especial lung cancer), strokes, macular degeneration, and cataracts. Till date, more than 600 carotenoids are known and 50 of them are consumed in meals to be transformed into the essential nutrient vitamin A. After their absorption, these carotenoids are metabolized by an oxidative rupture to retinal, retinoic acid and small quantities of breakdown products and are transported by plasma lipoproteins. Carotenes are mainly associated with low-density lipoproteins, while xanthophylls show a uniform distribution between the low- and high-density lipoproteins. The present review provides an insight into the recent status of pharmacological aspects of carotenoids.

Anti-mutagen

INTRODUCTION: Carotenoids are synthesized in plants but not in animals. In nature, more than 600 types of carotenoid have been determined. Carotenoids are localized in sub-cellular organelles (plastids), i.e. chloroplasts and chromoplasts. In chloroplasts, the carotenoids are chiefly associated with proteins and serve as accessory pigments in photosynthesis, whereas in chromoplasts they are deposited in crystalline form or as oily droplets ¹.

Some of the carotenoids such as the xanthophylls are involved in photosynthesis by participating in energy transfer in the presence of chlorophyll in plants ². Studies have shown that carotenoids contribute to the yellow color found in many fruits and vegetables ³. The colors of fruits and vegetables depend on conjugated double bonds and the various functional groups contained in the carotenoid molecule ⁴. A study also reported that the greater the number of conjugated double bonds, the higher the absorption maxima (λmax) ⁵.

As a result, the color ranges from yellow, red to orange in many fruits and vegetables ⁶.

Carotenoids are fat soluble compounds that are associated with the lipid fractions.  Chemically, these are long, aliphatic, conjugated double bond systems, i.e., polyenes. A part of them are hydrocarbons, are usually composed of eight isoprene units and have the molecular formula C40H56 ⁷. The molecule contains four isoprene units, the central two of which are joined tail-to-tail and open chain or ring structures form the ends of this chain.

A great majority of natural carotenes have double bonds in the all- Trans position, where R is an open-chain structure or a ring system. Only a few natural carotenes exhibit a cis-trans configuration. Carotenoids are polyisoprenoid compounds and can be divided into two main groups: (a) hydrocarbon carotenoids also known as carotenes, only composed of carbon and hydrogen atoms and (b) xanthophylls that are oxygenated hydrocarbon derivatives that contain at least one oxygen function such as hydroxyl, keto, epoxy, methoxy or carboxylic acid groups. Their structural characteristic is a conjugated double bond system, which influences their chemical, biochemical and physical properties ⁸.

Carotenoids are a class of yellow or red natural pigments occurs widely in nature.  These occur widely in plants, animals and humans. They are synthesized in plants and in some microorganisms and are only introduced with diet into human and animal organisms, which are incapable of their de novo synthesis but sometimes capable of their structural modification. They are highly physiologically important and fulfill many tasks ⁹.About one half of natural carotenoids are chiral, usually containing one to six chiral centers.

Carotenoid molecules and their products properties have profound effects on the structure and configuration by physico-chemical attacks (light, temperature, oxidants, subsistent etc.) and also on their physico-chemical properties. Trans-cis shifts have especially strong effects on shape of the molecule and thus also on its properties ⁴.  In general, all-trans isomers have lower energy and are more stable than cis-trans and cis isomers. These changes strongly influence spectral and chromatographic properties of carotenoids and also alter their optical activity if a chiral centre is present in the molecule, and thus may adversely affect the reliability of analytical measurements.

Up to now, more than 600 carotenoids have so far been isolated from natural sources. However, only about 40 are present in a typical human diet. About 20 carotenoids have been identified in human blood and tissues ¹⁰.

Fruits and vegetables containing vitamin C, vitamin E (tocopherols) and carotenoids (α-carotene, β-carotene, β-cryptoxanthin, lutein, zeaxanthin and lycopene) have been suggested as a natural source of antioxidants. Antioxidant functions are associated with decreased DNA damage, diminished lipid peroxidation, maintained immune function and inhibited malignant transformation or proliferation  in vitro that are thought to prevent the development of some diseases ^{11, 12}.

Also, α-carotene, β-carotene and β-cryptoxanthin are considered as pro-vitamin- A carotenoids ¹³. Moreover, it is generally accepted that the significance of vegetable consumption can play an important role in maintaining health and reducing the risk of illness ¹⁴. In particular, increased vegetable consumption helps reduce the risk of cancer ¹⁵. That is why vegetables as well as fruits are widely recommended as healthy food ¹⁶.  Vegetables are also a valuable part of the diet owing to their nutritive values ¹⁷. They are low in energy containing limited amounts of carbohydrates and fats and high in dietary fiber, minerals and vitamins.

Carotenoids, pro-vitamins A or not, have been credited with other beneficial effects to human health: enhancement of the immune response and reduction of the risk of degenerative diseases such as cancer, cardiovascular diseases, cataract and muscular degeneration ^{18, 19}. The carotenoids, action against diseases is due to antioxidant activity, especially to quench oxygen ²⁰. However, other mechanisms have been reported such as modulation of carcinogen metabolism, regulation of cell growth, inhibition of cell proliferation, enhancement of cell differentiation, stimulation of cell-to-cell gap junction communication, retinoid-dependent signaling and filtering of blue light ^{21, 22}.

CHEMICAL STRUCTURE OF SOME COMMON CAROTENOIDS ²²

Pharmacological Effects: Many diseases, such as cancer and strokes, involve oxidative processes mediated by free radicals. Carotenoids, by their antioxidant effect, can show benefits in such diseases; however, this function is not completely demonstrated in vivo ²³. Carotenoids are an integral part of membranes. Carotenes are immersed in membranes, but xanthophylls showed a variable membrane position, polar groups in xanthophylls affect their position and mobility. Consequently, carotenes are able to react efficiently only with radicals generated inside the membrane. While zeaxanthin, with their polar groups aqueous exposed, is able to react with radicals produced in that zone.

Additionally, it has been suggested that carotenoids influence the strength and fluidity of membranes, thus affecting its permeability to oxygen and other molecules. It has been also determined that carotenoids have a remarkable effect in the immune response and in intercellular communication ^24-27.  β- Carotene, canthaxanthin, 4-hydroxy-β-carotene, and the synthetic retro-dehydro-β-carotene show an efficient induction of the gap junctional communication (GJC) in murine fibroblasts.

The GJC stimulation was more than five times, depending on the carotenoid. β-Carotene and retro hydro-β-carotene were the most efficient inducers of GJC, indicating that the presence of a six-member ring is important in the GJC induction; carotenoids with five-member rings showed little activity.

Interestingly, the induction of GJC did not show correlation with the quenching of singlet oxygen. Thus, it was suggested that GJC and antioxidant activities in cancer prevention operate independently of each other ²⁸.

The importance of carotenoids with rings of six members was emphasized by the isolation of a carotenoid-binding protein (CCBP) of 67 kDa from ferret liver. Carotenoids with substituted β-ionone rings, without β-rings, or with an intact β-ring with a shorter lineal chain were not bonded by CCBP. Consequently, it was showed that CCBP binds β-carotene mole per mole with high efficiency and specificity. Thus, CCBP may play a major role in the storage, transport, and targeting of β-carotene in mammalian systems. Additionally, it was suggested that the high affinity between CCBP and β-carotene may protect the carotenoid from degradation, and its antioxidant activity must be better ²⁹.

There exists evidence of the effectiveness of β-carotene in the treatment of certain kinds of cancer, for example, smoking related cervical intraepithelial neoplasia and cervical and stomach cancer ³⁰.  β - Carotene affects the immune response in rats, and by this means tumor growth is inhibited ³¹. It was demonstrated that retinoic acid regulates the γ-interferon (IFN-γ) gene, which has an important role in practically all the stages of the immune and inflammatory responses ³².

In the process of senile macular degeneration, retinol is related to the induction of a gene cascade permitting the phagocytosis of damaged cell in retina; this process is critical for the photoreceptor survival ³³. Also, it has been established that retinoid affect many biological process, such as cellular proliferation, differentiation, and morphogenesis. Moreover, retinoid have been used in treatments of certain kinds of cancers and some dermatological activities. Additionally, it is mentioned that a diet with deficiencies in vitamin A or supplemented with an excess of retinoic acid could induce teratogenesis ³⁴.

Differentiation is a complex process, during development of the monoblastic cell line U-937, the activation of both the retinoic acid receptor (RAR) and the 9-cis-RA receptor (RXR) are required. Thus, it was suggested that different combinations of retinoids produced different pharmacological responses in the specificity and potency in cancer therapy ^35-37. Another found that retinoic acid induces the response of proteins associated with damage by ultraviolet light in F9 and NIH3T3 cells.

Another research showed that retinoic acid conduces to differentiation of F9 cells by an increment of cellular communication. It was suggested that this process is mediated by the protein connexin 43 that induces the expression of important molecules for cellular adhesion ³⁸.

Ang studied mice embryogenesis and found that retinoic acid has an important role during the gastrula ion process ³⁹. In that stage, alcohol dehydrogenate class IV was identified, and it was proposed that negative effects of alcohol consumption during fetus development could be caused by the inhibition in retinoic acid synthesis, catalyzed by alcohol dehydrogenate, which conduces to a failure in function of retinoic acid receptor (necessary for normal development).

Retinoic acid (RA) has also been related to the aging process. RA affects the gene expression levels through its interaction with triiodothyronine receptor (TR) and RAR. Interestingly, when the TR and RAR mRNA levels were analyzed in the brain of young, adult, or aged rats, lower values were found. Additionally, it was showed that RA supplementation increased the TR and RAR mRNA levels. Also, the transglutaminase activity was reduced in aged rats and recovered after RA treatment. Transglutaminase has involved the memory process ⁴⁰.

Prostaglandins are substances that have been involved in several physiological processes, and in particular prostaglandin D2 has been involved in the endogenous sleep promotion, modulation of several central actions (regulation of body temperature, release of the luteinizing hormone), etc. Recently, it was shown that prostaglandin D (PGD) synthase binds retinoic acid and retinol. In addition, it was demonstrated that all-trans-retinoic acid inhibits its enzymatic action but not retinol.

With the generated information, it was suggested that retinoids may regulate the synthesis of PGD2, and that PGD synthase may be a transporter of retinoids to the place where they are required. Thus, it was suggested that PGD synthase plays a critical role in regulating the development of neurons by the regulation of the transfer of all-trans or 9-cis-retinoic acid to RAR or RXR in the immature nerve cell.

Additionally, it has been mentioned that the over expression of the cyclooxygenase gene (a key enzyme in the formation of prostaglandins) is an early and central event in colon carcinogenesis ²³.

Carpenter observed that mixtures of canthaxanthin with low-density lipoproteins (LDL) inhibited macrophage formation from human monocytes. However, if canthaxanthin and LDL were added simultaneously (but without previous mixture) to cellular medium, it was not observed to any effect. The same was noted with β-carotene, but with zeaxanthin the opposite was found ⁴⁰. It was explained that all evaluated carotenoids show good antioxidant activity, and observed differences are caused because they act at different levels: zeaxanthin can quench radicals in the aqueous phase, while β-carotene inhibits lipid peroxidation; on the other hand canthaxanthin was the most potent agent in the inhibition of methyl linoleate, and it was concluded that antioxidant activity depends on the particular system: radical, carotenoid, microenvironment, etc.

Thus, diets with carotenoid mixtures are recommended instead of having just one particular carotenoid, because in vivo a great variability of radicals and microenvironments take place. Additionally, in some studies β-carotene was supplied to smokers, and it was found that cancer mortality indexes were higher in smokers than in their respective controls ^42-44. It has been signaled that a combined supply of β-carotene, α-tocopherol, and selenium reduces stomach cancer mortality, and it was pointed out that the consumption of marine algae (especially Phaeophyta) diminished the risks of being affected by certain types of cancer.

Also, it was established that the main antitumor agent is not β-carotene, but other components (carotenoids) that are present in algae. In this sense, mixtures of carotenoids α-carotene, fucoxanthin, and halocintiaxanthin) have shown a higher inhibitory activity than β-carotene in proliferation of human neuroblastoma cells. In addition, α-carotene showed higher antitumorigenic activity than β-carotene in rat cancer induced by glycerol, and it was mentioned that carotenoids with a ε-ring (absent in β-carotene) have higher inhibitory activity ⁴⁵. The antimutagenicity of carotenoids in Mexican green peppers (Capsicum annuum) were studied the antimutagenicity inhibition by nitroarenes was higher than 90%. Pepper carotenoids were more efficient antimutagens than pure β-carotene, suggesting that other carotenoids (e.g., lutein, zeaxanthin) in the pepper extracts showed a synergistic effect with β-carotene. Also, it was mentioned that the antimutagen activity might be from blocking the entrance of toxic compounds into the cell or by their antioxidant activity ⁴⁶.

Also, the antimutagenicity activity of carotenoids of Aztec marigold (Tagetes erecta) was evaluated. It was concluded that lutein was the compound with a higher activity on marigold extracts, but similar to the observation mentioned for the pepper extracts, the mixture of carotenoids in the marigold extract had higher antimutagenicity activity. In addition, it was suggested that lutein and 1-nitropyrene (mutagen) formed an extra-cellular complex that limits the bioavailability of 1-nitropyrene and consequently its mutagenicity.

Böhm indicated that carotenoids, α-tocopherol radicals, and ascorbic acid develop their function by diminishing the content of harmful nitrogenous compounds. Also, a synergistic effect between β-carotene and vitamins E and Complainant was observed in cellular protection ⁴⁷. It was explained that β-carotene not only destroys oxyradicals but repairs tocopherol radicals produced when α-tocopherol destroys oxy-radicals. Additionally, it was suggested that low antioxidant levels (e.g., ascorbic acid) in smokers, in contrast with non-smokers, could be related with an apparent failure in the recycling of α-tocopherol by β-carotene.

Carotenoids protect lab animals of UV-induced inflammation and certain type of cancers. Historically, carotenoid supplementation has been used in the treatment of diseases produced by light sensitivity, which are usually hereditary: 84% of patients with erythropoietic protoporphyria, consuming diets supplemented with β-carotene, increased by a factor of 3 their ability to resist sunlight exposition without presenting symptoms. Also, carotenoids have been used in other photosensitivity diseases: congenital porphyria, sideroblastic anemia, and have shown only a limited success in treatment of polymorphic light eruption, solar urticaria Hydroa vacciforme, Porphyria variegata, Porphyria cutanea tarda, oractinic reticuloid ⁴⁷. Lutein and zeaxanthin have been considered as protective agents against aging macular degeneration and senile cataracts ⁴⁴.

Also, it has been suggested that β-carotene suppress the increment of hormones related to stress
syndrome ⁴⁷.

CONCLUSION: The health benefits of carotenoids in the human diet are becoming increasingly apparent in the past few years. Being potent antioxidants, thus preventing from several major health disorders, higher dietary intake of carotenoids also help to rejuvenate the body by promoting the growth of healthy cells and inhibiting the growth of unhealthy ones. Thus, greater incorporation of carotenoids in pharmaceutical, food and nutraceutical products is highly recommended for maximum health benefits.

REFERENCES:

1. Bartley, G.E.; Scolnik, P.A. Plant carotenoids: Pigments for photoprotection, visual attraction, and human health. Plant Cell 1995, 7, 1027–1038.
2. Janik, E.; Grudziński, W.; Gruszecki, W.I.; Krupa, Z. The xanthophyll cycle pigments in Secale cereale leaves under combined Cd and high light stress conditions. J. Photochem. Photobiol. B 2008, 90, 47–52.
3. Lancaster, J.E.; Lister, C.E.; Reay, P.F.; Triggs, C.M. Influence of pigment composition on skin color in a wide range of fruit and vegetables. J. Am. Soc. Hortic. Sci. 1997, 122, 594–598.
4. Rodriguez-Amaya, D.B.; Kimura, M. HarvestPlus Handbook for Carotenoid Analysis. In Harvest plus Technical Monograph, Series 2; International Food Policy Research Institute and International Center for Tropical Agriculture: Washington, DC, USA, 2004.
5. Rodriguez-Amaya, D.B. A Guide to Carotenoid Analysis in Foods; International Life Sciences Institute, ILSI Press: Washington, DC, USA, 2001.
6. Hornero-Méndez, D.; Mínguez-Mosquera, M.I. Xanthophyll esterification accompanying carotenoid overaccumulation in chromoplast of Capsicum annuum ripening fruits is a constitutive process and useful for ripeness index. J. Agric. Food Chem. 2000, 48, 1617–1622.
7. Russel R M. The multifunctional carotenoids: insights into their behavior. J Nutr, 2006; 136:2690S-2692S.
8. F. Delgado-Vargas, A.R. Jimenez and O. Paredes-Lopez: Natural pigments: Carotenoids, Anthocyanins, and Betanins-Characteristics, Biosynthesis, Processing and stability. Cric .Rev. Food Sci. and Nutrition, 2000; 40(3):173-289.
9. Van Het Hof KH, West CE, Weststrate JA, Hautvast JG: Dietary factors that affect the bioavailability of carotenoids. J Nutr, 2000; 130:503-506.
10. Yeum KJ, Russell RM: Carotenoid bioavailability and bioconversion. Annul Rev Nutr.2002; 22: 483-504.
11. Gerster H: The potential role of lycopene for human health. J Am Coll Nutr.1997; 16: 109-126.
12. Rao A V, Rao L G: Carotenoids and human health. Pharmacological Research. 2007; 55: 207-216.
13. Sies H, Stahl W: Vitamin E and C β- carotene and other carotenoids as antioxidants. Am J Clin Nut .1995; 62: 1315-1321.
14. Raju M, Varakumar S, Lakshminarayana R, Krishnakantha T P, Baskaran V: Carotenoid composition and vitamin A activity of medicinally important green leafy vegetables. Food Chem. 2007; 101: 1598-1605.
15. Kalt W: Effects of production and processing factors on major fruit and vegetable antioxidants. J Food Sci. 2005; 70: 11-19.
16. Marinova D, Rbarova F: HPLC determination of carotenoids in Bulgarian berries. J Food Composit Anal. 2007; 20: 370-374.
17. Azevado C H-de, Rodriguez-Amaya D B: Carotenoid composition of kale as influenced by maturity season and minimal processing. J Sci. Food Agric. 2005; 85: 591-597.
18. Ramesh MN: The performance evaluation of a continuous vegetable cooker. Int J Food Sci. Techno .2000; 35: 377-384.
19. Rodriguez-Bernaldo de Quiros A, Costa H S: Analysis of carotenoids in vegetable and plasma samples: A review. J Food Composit Anal. 2006; 19: 97-111.
20. Krinsky NI: Actions of carotenoids in biological systems. Annul Rev Nut 1993; 13: 561-587.
21. Palozza P, Krinsky N I: Antioxidant effects of carotenoids in vivo and in vitro: An overview. Meth Enzymol .1992; 213: 403-420.
22. Krinsky NI, Johnson E J: Carotenoid actions and their relation to health and disease. Molecular Aspects Med .2005; 26: 459-516.
23. Astorg P: Food carotenoids and cancer prevention: An overview of current research. Trends Food Sci. Technol.1997; 8: 406-413.
24. International Agency for Research on Cancer. IARC Handbook of Cancer Prevention: Carotenoids. Lyon: International Agency for Research on Cancer; 1998.
25.  Francisco Delgado-Vargas Octavio Paredes-López: Natural Colorants for Food and Nutraceutical Uses. CRC Press. First Edition 2003.
26. Britton G: Structure and properties of carotenoids in relation to function .FASEB J.  1995; 9: 1551–1558.
27. Clairmont A, Tessmann D, Sies H: Analysis for connexin 43 gene expression induced by retinoic acid in F9 teratocarcinoma cells. FEBS Lett.1996; 397: 22–24.
28. Charleux J L: Beta-carotene vitamin C and vitamin E: the protective micronutrients. Nut. Rev. 1996; 4(11): 109–S114.
29. Hong W K, Sporn M B: Recent advances in chemoprevention of cancer. Science .1997;278: 1073–1077.
30. Stahl, W., Nicolai, S., Briviba, K., Hanusch, M.,Broszeit, G., Peters, M., Martin, H. D., and Sies, H., Biological activities of natural and synthetic carotenoids: induction of gap junctional communication and singlet oxygen quenching. Carcinogenesis. 1997; 89-92.
31. Rao M N, Ghosh P, Lakshman M R: Purification and partial characterization of a cellular carotenoid binding protein from ferret liver. J Biol Chem. 1997;272(39): 24455-24460.
32. Siefer E, Rettura G, Levenson S H: Carotenoids and cell-mediated immune responses. In: Charalambous G, Inglett, G, Eds. TheQuality of Foods and Beverages. Vol. 2: Chemistry and Technology, Academic Press, New York.1981 335–347.
33. Cippitelli  M, Ye J, Viggiano V, Sica A, Ghosh, P, Gulino A, Santoni A,Young H A:Retinoic acid-induced transcriptional modulation of the human interferon-γ promoter. J Biol. Chem.1996;27:143-147.
34. Ershov A V, Lukin W J, Bazan N G: Selective transcription factor induction in retinol pigment epithelial cells during photoreceptor phagocytosis. J Biol. Chem.1996;271(45): 28458–28462.
35. Means A L, Gudas L J: The roles of retinoids in vertebrate development. Ann Rev Biochem. 1995; 64: 201–233.
36. Botling J, Castro D S, Öberg, F, Nilsson K, Perimann T: Retinoic acid receptor/retinoid X receptor heterodimers can be activated through both subunits providing a basis for synergistic transactivation and cellular differentiation. J Biol. Chem.1997;272(14): 9443–9449.
37. Guérin E, Ludwig M G, Basset P, Anglard P: Stromelysin-3 induction and interstitial collagenase repression by retinoic acid. J Biol. Chem.1997; 272(17): 11088–11095.
38. Eichele, G: Retinoids from hindbrain patterning to Parkinson disease. Trends Genet 1997;13: 343–345.
39. Clairmont A, Tessmann D, Sies H : Analysis of connexin 43 gene expression induced by retinoic acid in F9 teratocarcinoma cells. FEBS Lett. 1996; 397(1):22-4
40. Ang H L, Deltour L, Hayamizu T F, ZgombicKnight M, Duester G: Retinoic acid synthesis in mouse embryos during gastrulation and craniofacial development linked to class IV alcohol dehydrogenase gene expression. J Biol. Chem. 1996; 27: 9526–9534.
41. Radlwimmer F B, Yokoyama S: Cloning and expression of the red visual pigment of goat (Caprahircus). Gene.1997;198: 211–215.
42. Carpenter KL, van der Veen C, Hird R, Dennis IF, Ding T, Mitchinson MJ.The carotenoids beta-carotene, canthaxanthin and zeaxanthin inhibit macrophage-mediated LDL oxidation. FEBS Lett. 1997; 401:262-266.
43. Armstrong G A, Hearst J E: Genetics and molecular biology of carotenoid pigment biosynthesis. FASEB J.1996:10: 228–237.
44. Olson J A, Krinsky N I: Introduction The colorful, fascinating world of the carotenoids: important physiologic modulators. FASEB J.1995;9: 1547–1550.
45. Taylor-Mayne S: Beta-carotene carotenoids and disease prevention in humans. FASEB J.1996; 10:609-611.
46. Murakami A, Ohigashi H, Koshimizu K: Antitumor promotion with food phytochemicals: A strategy for cancer chemoprevention. Biosci.  Biotechnol. Biochem. 1996;60(1): 1–8.
47. Quintanar-Hernández J A, Loarca-Piña M G F, González-de-Mejía, E: Efecto de los carotenoides presentes en chile verde (Capsicum annuum spp.) de mayor consumo contra tóxicos en alimentos, Tecnología Alimentaria.1996;31(6): 15–21.
48. Böhm F, Edge R, Land E J, McGarvey D J,Truscott T G:Carotenoids enhance vitamin E antioxidant efficiency. J Am Chem. Soc. 1997;119:621–622.
49. Mathews-Roth, M. M: Carotenoids in erythropoietic protoporphyria and other photosensitivity disease. In: Canfield L M, Krinsky N I, Dunastable J A, Eds., Carotenoids in Human Health. Annals of the New York Academy of Sciences, New York, Vol.691, 1993, 127–138.
50. Hasegawa T: Anti-stress effect of β-carotene. In: Canfield L M, Krinsky N I, Dunastable J A,Eds. Carotenoids in Human Health. Annals of the New York Academy of Sciences, New York. 1993. Vol. 691: 281–283.
51. Sun N K, Huang S L, Lin-Chao S, and Chao CC K: Induction not associated with host-cell reactivation of damaged plasmid. DNA, of damaged-DNA-recognition proteins by retinoic acid and dibutyryl cyclic AMP in mammalian cells. Biochem J 1996; 13: 441–445.
    

    ---