Carotene asymmetric cleavage

While the principal site of carotene dioxygenase attack is the central bond of P-carotene, asymmetric cleavage may also occur, leading to the formation of 8 -, 10 - and 12 -apo-carotenals, which are oxidized to retinoic acid but cannot be used as sources of retinol or retinaldehyde. 

Ziouzenkova, O., G. Orasanu, G. Sukhova et al. 2007b. Asymmetric cleavage of beta-carotene yields a transcriptional repressor of retinoid X receptor and peroxisome proliferator-activated receptor responses. Mol Endocrinol 21( 1) 77—88. 

In addition to oxidation of retinol, retinoic acid may be formed by the /3-oxidation of apo-carotenals arising from the asymmetric cleavage of /3-carotene (Section 2.2.2.1). 

Asymmetric cleavage p-carotene has been reported by several groups, and this controversy also seems to be solved as a result of the cloning of a cDNA encoding an enzyme that asym-... 

In second alternative excentric cleavage step of asymmetric cleavage, P-carotene (2) was converted to P-apo-lO -carotenal (23) and P-ionone (24) by P,P-carotene 9 ,10 -dioxygenase. These P-apo-carotenalscould be subsequently oxidized to the P-apo-carotenoic acids such as P-apo-10 -carotenoic acid (27) and P-apo-12 -carotenoic acid (28). Finally, these P-apo-carotenoic acids were further shortened to retinoic acid (20) (Figure 5). 

Figure 2.4. Potential products arising from enzymic or nonenzymic symmetrical (a) or asymmetric (b to d) oxidative cleavage of, 6-carotene. Apocarotenals can undergo side chain oxidation to yield retinoic acid, but cannot form retinaldehyde or retinol.

In contrast to [3-carotene, a-carotene, or P-cryptoxanthin, lycopene is not a provitamin A carotenoid the absence of the P-ionone rings at the two ends of the molecule prevents its acceptance as a substrate by the 15,15 central cleavage enzyme to generate retinal. A mammalian enzyme that catalyzes asymmetrical oxidative cleavage of P-carotene at the 9, 10 position has recently been identified this cleaved lycopene to form apolycopenales... 

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