Vitamin carotene dioxygenase

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.

As shown in Figure 11.3, P-carotene and other pro-vitamin A carotenoids are cleaved in the intestinal mucosa by carotene dioxygenase, yielding retinaldehyde, which is reduced to retinol, esterified and secreted in chylomicrons together with esters formed from dietary retinol. 

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. ... 

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-... 

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.

---