P-Carotene-15,15 -dioxygenase

During, A. et al., P-Carotene 15,15-Dioxygenase activity in human tissues and cells evidence of an iron dependency, J. Nutr. Biochem., 12, 640, 2001. 

During, A. et al., Assay of P-carotene 15,15-dioxygenase activity by reverse phase high-pressure liquid chromatography, Anal. Biochem., 241, 199, 1996. 

Milk fat supplies the diet with a substantial proportion of its daily P-carotene and vitamin A requirements. Dietary p-carotene is converted to retinal in the intestinal epithelium and in the liver by the enzyme p-carotene-15-15 -dioxygenase. The retinal formed is further metabolized to retinoic acid (vitamin A). 

Carotenoids, also in micellular form, are absorbed into the duodenal mucosal cells by passive diffusion. The efficiency of absorption of carotenoids is much lower than for vitamin A, between 9% and 22%, and is subject to a large number of variables, including carotenoid type, the amount in the meal, matrix properties, nutrient status, and genetic factors. Once inside the mucosal cell, (3-carotene is principally converted to retinal by the enzyme p-carotene-15,15 -dioxygenase, the retinal being converted by retinal reductase to retindl and esterified, though it can also be cleaved eccen-... 

Effects of Dietary Fat and Antioxidant on p-Carotene-15,15 -Dioxygenase Activity... 

Devery J, Milborrow BV (1994) P-Carotene-15,15 -dioxygenase (EC 1.13.11.21) isolation reaction mechanism and an improved assay procedure. Brit J Nutr 72 397-414... 

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... 

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. 

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. ... 

The Reaction Specificity of Carotene Dioxygenase Whereasthe principal site of carotene dioxygenase attack is the 15,15 -central bond of p-carotene, there is evidence that asymmetric cleavage also occurs, leading to formation of 8 -, 10 -, and 12 -apo-carotenals, as shown in Figure 2.4. These apo-carotenals are metabolized by oxidation to apo-carotenoic acids, which are substrates for /3-oxidation to retinoic acid and a number of other metabolites. 

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.

The major natural sources of vitamin A in the diet are certain plant carotenoid pigments, such as p-carotene, and the long-chain retinyl esters found in animal tissues. p-Carotene is converted to vitamin A primarily in the intestinal mucosa. The biosynthetic process involves two soluble enzymes p-carotene 15,15 -diox-ygenase and retinaldehyde reductase. The first enzyme catalyzes the cleavage of P-carotene at the central double bond by a dioxygenase mechanism to yield two molecules of retinaldehyde (Cl) the aldehyde is then reduced to retinol (Al). 

Carotene and other carotenoids with retinol potential that are not cleaved in the enterocyte may be cleaved in the hver, kidney and possibly other tissues that have the necessary enzyme, 15,15 -p-carotenoid dioxygenase (EC 1.13.11.21), and there is some debate as to the relative contribution of the different possible sites of cleavage. 

The metabolism of a provitamin A carotenoid such as P-carotene is depicted in Figure 2. The major pathway involves cleavage by a 15,15 -central dioxygenase that results in the formation of retinal, which can either be reduced to retinol or oxidized to retinoic acid (see Napoli, this volume). This process occurs in the cytoplasm of many cell types, and produces two... 

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