Carotenes cleavage

Carotene cleavage enzymes — Two pathways have been described for P-carotene conversion to vitamin A (central and eccentric cleavage pathways) and reviewed recently. The major pathway is the central cleavage catalyzed by a cytosolic enzyme, p-carotene 15,15-oxygenase (BCO EC 1.13.1.21 or EC 1.14.99.36), which cleaves p-carotene at its central double bond (15,15 ) to form retinal. Two enzymatic mechanisms have been proposed (1) a dioxygenase reaction (EC 1.13.11.21) that requires O2 and yields a dioxetane as an intermediate and (2) a monooxygenase reaction (EC 1.14.99.36) that requires two oxygen atoms from two different sources (O2 and H2O) and yields an epoxide as an intermediate. ... 

Another study showed that a mixture of oxidative metabolites of P-carotene, but not P-carotene, was able to increase the binding of benzo[a]pyrene to DNA. Other mixtures of P-carotene cleavage products have been shown to induce oxidative stress in vitro,exert cytotoxic and genotoxic effects, and inhibit gap junction intercellular communications. It has been suggested that these detrimental effects could possibly occur in vivo following the intake of high doses of carotenoids. 

Augustin, W. et al.. Beta-carotene cleavage products induce oxidative stress by impairing mitochondrial functions brain mitochondria are more sensitive than liver mitochondria, Free Rad. Biol. Med., 33, S326, 2002. 

Siems, W, Sommerburg, O, Schild, L, Augustin, W, Langhans, CD, and Wiswedel, I, 2002. Beta-carotene cleavage products induce oxidative stress in vitro by impairing mitochondrial respiration. Faseb J 16,... 

Sommerburg, O, Langhans, CD, Amhold, J, Leichsenring, M, Salerno, C, Crifo, C, Hoffmann, GF, Debatin, KM, and Siems, WG, 2003. Beta-carotene cleavage products after oxidation mediated by hypochlorous acid—A model for neutrophil-derived degradation. Free Radic Biol Med 35, 1480-1490. 

T. van Vliet, F. van Schaik, W. H. Schreurs, and H. van den Berg, In vitro measurement of beta-carotene cleavage activity Methodological considerations and the effect of other carotenoids on beta-carotene cleavage, Int. J. Vitam. Nutr. Res. 66 (1996) 77-85. 

Parvin, S. G. and B. Sivakumar. 2000. Nutritional status affects intestinal carotene cleavage activity and carotene conversion to vitamin A in rats. J Nutr 130(3) 573-577. 

The mechanism of p—carotene cleavage is still unproven, but probably involves oxidative 15,15 cleavage to retinal. The sequence involves the loss of a C2 unit, a reduction of the 11,11 double bond and a series of oxidations. The nature of the proposed metabolic grid, with the different steps in the two different mating types, explains why trisporic acid is only synthesised when the two thalli are in diffusion contact i.e. the intermediates (and/or enzymes) can then diffuse from one cell to the other to complete the metabolic pathway. 

Hickenbottom SJ, Limke SL, Dueker SR, Y, Follett JR, Carkeet C, Buchholz BA, Vogel JS, Clilfford AJ. Dual isotope test for assessing beta-carotene cleavage to vitamin A in humans. Eur J Nutr 2002 41(4) 141—147. 

The presence of other carotenoids can affect the absorption of carotenoids into intestinal mucosal cells, since carotenoids can compete for absorption or facilitate the absorption of another. Data on carotenoid interactions are not clear. Human studies show that /3-carotene decreases lutein absorption, while lutein has either no effect or a lowering effect on /3-carotene absorption. Although not confirmed in humans, the inhibitory effect of lutein on /3-carotene absorption might be partly attributed to the inhibition of the /3-carotene cleavage enzyme by lutein shown in rats. Beta-carotene also seemed to lower absorption of canthaxanthin, whereas canthaxanthin did not inhibit /3-carotene absorption. Studies showed that /3-carotene increased lycopene absorption, although lycopene had no effect on /3-carotene. Alpha-carotene and cryptoxanthin show high serum responses to dietary intake compared to lutein. In addition, cis isomers of lycopene seem to be more bioavailable than the -trans, and selective intestinal absorption of a)X-trans /3-carotene occurs, as well as conversion of the 9-cis isomer to sW-trans /3-carotene. It is clear, then, that selective absorption of carotenoids takes place into the intestinal mucosal cell. 

Vitamins are classified by their solubiUty characteristics iato fat-soluble and water-soluble groups. The fat-soluble vitamins A, E, and K result from the isoprenoid biosynthetic pathway. Vitamin A is derived by enzymic cleavage of the symmetrical C q beta-carotene, also known as pro-vitamin A. Vitamins E and K result from condensations of phytyldiphosphate (C2q) with aromatic components derived from shikimic acid. Vitamin D results from photochemical ring opening of 7-dehydrocholesterol, itself derived from squalene (C q). 

In nature, vitamin A aldehyde is produced by the oxidative cleavage of P-carotene by 15,15 - P-carotene dioxygenase. Alternatively, retinal is produced by oxidative cleavage of P-carotene to P-apo-S -carotenal followed by cleavage at the 15,15 -double bond to vitamin A aldehyde (47). Carotenoid biosynthesis and fermentation have been extensively studied both ia academic as well as ia iadustrial laboratories. On the commercial side, the focus of these iavestigations has been to iacrease fermentation titers by both classical and recombinant means. 

Figure 45-1. P-Carotene and the major vitamin A vitamers. Shows the site of cleavage of P-carotene into two molecules of retinaldehyde by carotene dioxygenase.

A small but variable proportion of the carotenoids with one or two P-ionone rings (mainly P-carotene) are cleaved in the enterocytes to produce retinol (vitamin A). This process is very tightly controlled, so that too much vitamin A is not produced, although the control mechanism is not clear. Some cleavage of P-carotene can also occur in the liver, but this does not account for the turnover of P-carotene in the body. Small amounts of carotenoids are subject to enterohepatic circulation, but this does not account for losses. 

VAN VLIET T, SCHREURS w H and VAN DEN BERG H (1995) Intestinal beta-carotene absorption and cleavage in men response to beta-carotene and retinyl esters in the triglyceride-rich lipoprotein fi action after a single oral dose of beta-carotene. Am J Clin Nutr 62(1) 110-16. 

The second pathway is the eccentric cleavage that occurs at double bonds other than the central 15,15 -double bond of the P-carotene molecule to produce different products called P-apocarotenals with various chain lengths. Because only trace amounts of apocarotenals were detected in vivo from tissues of animals fed P-carotene and these compounds can be formed non-enzymatically from P-carotene auto-oxidation, the existence of this pathway was controversial until recently. The identification of P-carotene 9, 10 -oxygenase (BC02), which acts specifically at the 9, 10 double bond of P-carotene to produce P-apo-lO -carotenal and P-ionone, provided clear evidence of the eccentric cleavage pathway in vivo. Lycopene was also reported as a substrate for BC02 activity. 

Olson, J.A. and Hayaishi, O., The enzymatic cleavage of beta-carotene into vitamin A by soluble enzymes of rat liver and intestine, Proc. Natl Acad. Sci. USA, 54,1364,1965. 

Leuenberger, M.G., Engeloch-Jarret, C., and Woggon, W.-D., The reaction mechanism of the enzyme-catalyzed central cleavage of P-carotene to retinal, Angew. Chem. Int. Ed, 40, 2613, 2001. 

Yeum, K.J. et al., The effect of a-tocopherol on the oxidative cleavage of P-carotene,... 

The speed of autoxidation was compared for different carotenoids in an aqueous model system in which the carotenoids were adsorbed onto a C-18 solid phase and exposed to a continnons flow of water saturated with oxygen at 30°C. Major products of P-carotene were identified as (Z)-isomers, 13-(Z), 9-(Z), and a di-(Z) isomer cleavage prodncts were P-apo-13-carotenone and p-apo-14 -carotenal, and also P-carotene 5,8-epoxide and P-carotene 5,8-endoperoxide. The degradation of all the carotenoids followed zero-order reaction kinetics with the following relative rates lycopene > P-cryptoxanthin > (E)-P-carotene > 9-(Z)-p-carotene. 

Stndies of the antoxidation of carotenoids in liposomal suspensions have also been performed since liposomes can mimic the environment of carotenoids in vivo. Kim et al. stndied the antoxidation of lycopene," P-carotene," and phytofluene" " in liposomal snspensions and identified oxidative cleavage compounds. Stabilities to oxidation at room temperature of various carotenoids incorporated in pig liver microsomes have also been studied." The model took into account membrane dynamics. After 3 hr of reactions, P-carotene and lycopene had completely degraded, whereas xanthophylls tested were shown to be more stable. 

As described in the preceding paragraphs, oxidation products of carotenoids can be formed in vitro as a result of their antioxidant or prooxidant actions or after their autoxidation by molecular oxygen. They can also be found in nature, possibly as metabolites of carotenoids. Frequently encountered products are the monoepoxide in 5,6- or 5, 6 -positions and the diepoxide in 5,6 5, 6 positions or rearrangement products creating furanoid cycles in the 5,8 or 5, 8 positions and 5,8 5, 8 positions, respectively. Products like apo-carotenals and apo-carotenones issued from oxidative cleavages are also common oxidation products of carotenoids also found in nature. When the fission occurs on a cyclic bond, the C-40 carbon skeleton is retained and the products are called seco-carotenoids. 

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