Carotene-15,15 -oxygenase

FIGURE 3.2.2 Metabolic pathways of carotenoids such as p-carotene. CM = chylomicrons. VLDL = very low-density lipoproteins. LDL = low-density lipoproteins. HDL = high-density lipoproteins. BCO = p-carotene 15,15 -oxygenase. BCO2 = p-carotene 9, 10 -oxygenase. LPL = lipoprotein lipase. RBP = retinol binding protein. SR-BI = scavenger receptor class B, type I. 

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. ... 

Lampert, J. M., J. Holzschuh et al. (2003). Provitamin A conversion to retinal via the beta,beta-carotene-15,15 -oxygenase (bcox) is essential for pattern formation and differentiation during zebrafish embryogenesis. Development 130(10) 2173-2186. 

It is well-known that plants do not synthesize vitamin A. Also, animals can only synthesize vitamin A from p-carotene or carotenoids in which one-half of the molecule is like p-carotene. In nature, the vitamin A precursor comes either from plants or microorganisms. The most common sources of vitamin A in citrus are a- and p-carotenes and p-cryptoxanthin. In addition to the above name carotenoids, p-apo-8 -carotenal in citrus peel could be a source of provitamin A. However, the peel is not usually consumed. Provitamin A compounds are cleaved to form vitamin A aldehyde in the intestine by p-carotene 15,15 -oxygenase (Figure 4) (54). Aldehyde reductase reduces the aldehyde to the all trans-vitamin A. p-Carotene is cleaved between the 15,15 carbon... 

Figure 4. Vitamin A biotransfoimation to retinal (16) and relates from provitamin A P-carotene (2) by cleavage enzyme P-carotene-15,15 -oxygenase.

Kim, Y. K., L. Wassef, S. Chung, et al. 2011. P-Carotene and Its Cleavage Enzyme P-Carotene-15,15 -Oxygenase (Cmol) Affect Retinoid Metabolism in Developing Tissues. Faseb J 25, no 5 1641-52. 

Kruse, S. W., K. Suino-Powell, X. E. Zhou, et al. 2008. Identification of Coup-Tfii Orphan Nuclear Receptor as a Retinoic Acid-Activated Receptor. Plos Biol 6, no 9 2002-15. Lampert, J. M., J. Holzschuh, S. Hessel, et al. 2003. Provitamin A Conversion to Retinal Via the P,P-Carotene-15,15 -Oxygenase (Bcox) Is Essential for Pattern Formation and Differentiation During Zehrafish Emhryogenesis. Development 130, no 10 2173-86. 

Wyss, A., Carotene oxygenases a new family of double bond cleavage enzymes, J. Nutr. 134, 246S, 2004. 

Juttner, F. (1988). Carotene oxygenase in microcystis. Meth. Enzymol. 167 336-341. 

P-Carotene oxygenase (crt/-type) crtO Eubacteria Deinococcus radiodurans [92]... 

LOTAN T and HIRSCHBERG J (1995) Cloning and expression in E. coli of the gene encoding 3-C-4-oxygenase, that converts (3-carotene to the ketocarotenoid canthaxanthin in Haematococcus pluvialis , FEBS Lett, 364, 125-8. 

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. 

Obermuller-Jevic, UC. Schlegel, B. Flaccus, A., and Biesalski, HK. 2001. The effect of beta-carotene on the expression of interleukin-6 and heme oxygenase-1 in UV-irradiated human skin fibroblasts in vitro. FEBS Lett 509 186-190. 

Comparison within the family of carotenoid oxygenases show that the four histidines are strictly conserved as well as their close environment. Presumably, all members of this family, including the protein catalyzing central cleavage of P-carotene, share a common chain fold, possess similar active centers and follow a similar reaction mechanism, vide infra. 

In the intestinal mucosal cells, /3-carotene is cleaved via an oxygenase (an enzyme that introduces molecular 02 into organic compounds) to frans-retinal (aldehyde form of trans-retinol, as shown in Table 6.2), which in turn is reduced to frans-retinol, vitamin Av Retinol is then esterified with a fatty acid, becomes incorporated into chylomicrons, and eventually enters the liver, where it is stored in the ester form until it is required elsewhere in the organism. The ester is then hydrolyzed, and vitamin Ax is transported to its target tissue bound to retinol-binding protein (RBP). Since RBP has a molecular weight of only 20,000 and would be easily cleared by the kidneys, it is associated in the bloodstream with another plasma protein, prealbumin. 

The provitamin A, /8-carotene, is cleaved in the enterocyte by a soluble oxygenase w hich requires bile salts for its activity [90]. The product, retinal, is reduced by another soluble enzyme to retinol which is esterified chiefly with palmitic acid by a microsomal enzyme, acyl-CoA retinol acyltransferase which is inhibited by taurocholate in vitro this enzyme is very similar to ACAT [91]. Since j8-carotene is taken up by the intestine and rather efficiently converted to retinyl esters which appear in lymph, it must be inferred that the cytosolic oxygenase is exposed to sufficiently high concentrations of bile salts for the cleavage to occur. 

---