P-Carotene 15,15 -monooxygenase

Interestingly, carotenoids more abundant in the blood plasma than zeaxanthin, such as lycopene, P-carotene, and P-cryptoxanthin, do not accumulate in the retina. RPE cells express p,p-carotene 15,15 -monooxygenase (BCO), formerly known as P-carotene 15,l5 -dioxygcnase, an enzyme that catalyzes the oxidative cleavage of P-carotene into two molecules of all-trans-retinal (Aleman et al., 2001 Bhatti et al., 2003 Chichili et al., 2005 Leuenberger et al., 2001 Lindqvist and Andersson, 2002). Therefore it may be suggested that p -carotene transported into RPE-cells is efficiently cleaved into retinal molecules. BCO cleaves also P-cryptoxanthin (Lindqvist and Andersson, 2002), and its absence in the retina may also be explained by its efficient cleavage to retinoids. However, lycopene, often the most abundant carotenoid in human plasma, cannot serve as a substrate for BCO, and yet it is not detectable in the neural retina (Khachik et al., 2002). 

Design and Synthesis of Enzyme Mimics of P-Carotene 15,15 -monooxygenase 35... 

The enzymatic reaction mechanism was determined by incubating a-carotene 6, a non-symmetric substrate of the enzyme, under a 02 atmosphere in H2 0 followed by isolation and characterization of derivatives of the cleavage product 2 (6). Accordingly, the enzyme cleaving the central double bond of 1 was found to be a non-heme iron monooxygenase (P-carotene 15,15 -monooxygenase) and not dioxygenase as termed earlier (Fig. 3). From the chemical point of view this enzymatic reaction is very unusual for various reasons (i) the reaction... 

Fig. 3. The reaction mechanism of the central cleavage of carotenoids, catalyzed by p-carotene 15,15 -monooxygenase (6).

Fig. 11. Regioselectivity of p-carotene 15,15 -monooxygenase action on non-natural substrates.

Among these carotenoids, there are around 50 carotenoids of provitamin A which could be converted to retinol (vitamin A, 18) by carotenoid 15,15 -oxygenases (CO) such as a P-carotene 15,15 -monooxygenase in their mucosal epithelial cells of the small intestine in human digestive tract (Figure 4) [9, 10]. 

In 2002, a group of Roche Vitamins, Human Nutrition and Health, Carotenoid Group of Switzerland, examined the metabolism of P-carotene (2) by P,P-carotene 15,15 -monooxygenase in rats and ehiekens on the question of whether the preformed vitamin A or P-earotene (2) and its direct metabolites could regulate the enzyme activity in vivo. 

In first central cleavage step, P-carotene (2) was converted to retinal (16) by P,P-carotene 15,15 -monooxygenase, and followed by retinol (18), retinyl esters of the storage forms such as retinyl acetate (22), retinol (18), retinal (16) by each enzyme, and finally retinoic acid (20) by retinal oxidase (Figure 5). 

First, their activity of the intestional P,P-carotene 15,15 -monooxygenase was dose-dependently decreased by the rat oral administration of retinyl acetate (22) (decrease percentage = up to -79%), P-carotene (2) (decrease percentage = up to -79%), P-apo-8 -carotenal (25) (decrease percentage = up to -58%), a -trcms retinoic acid (29) (decrease percentage = up to -88%) and 9-cis retinoic acid (30) (decrease percentage = up to -67%). 

Third, their rat oral administrations of P-apo-12 -carotenal (28) and retinoic acid (20) receptor (RAR) a antagonist Ro 41-5253 (31) (Figure 6) could significantly increase the activity of the intestional P,P-carotene 15,15 -monooxygenase up to 4-55% and +94%, respectively. 

Figure 5. Each vitamin A biotransformation to retinoic acid (20) from provitamin A P-carotene (2) by both central cleavage enzyme P,P-carotene 15,15 -monooxygenase and alternative excentric cleavage enzyme P,P-carotene 9 ,10 -dioxygenase.

Boulanger, A., P. McLemore et al. (2003). Identification of beta-carotene 15,15 -monooxygenase as a peroxisome proliferator-activated receptor target gene. FASEB J. 17(10) 1304—1306. 

Bachmann, H., A. Desbarats, P. Pattison et al. 2002. Feedback regulation of beta,beta-carotene-15, 15 -monooxygenase by retinoic acid in rats and chickens. J Nutr 132(12) 3616-3622. 

Gong, X., S. W. Tsai, B. Yan, and L. P. Rubin. 2006. Cooperation between MEF2 and PPARgamma in human intestinal beta,beta-carotene 15,15 -monooxygenase gene expression. BMC Mol Biol 7 7. 

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. ... 

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