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  • Title: Stoichiometric conversion of all trans-beta-carotene to retinal by pig intestinal extract.
    Author: Nagao A, During A, Hoshino C, Terao J, Olson JA.
    Journal: Arch Biochem Biophys; 1996 Apr 01; 328(1):57-63. PubMed ID: 8638938.
    Abstract:
    beta-Carotene and other provitamin A carotenoids are major dietary sources of vitamin A for humans and for many animals throughout the world. Two pathways for this oxidative reaction in mammals are: (1) central cleavage by beta-carotene 15,15'-dioxygenase (EC 1.13.11.21) to yield two molecules of retinal per molecule of beta-carotene consumed and (2) eccentric (or random) cleavage via several beta-apo carotenals to yield one molecule of retinal and various smaller fragments per molecule of beta-carotene oxidized. By the use of improved methods to minimize nonenzymatic reactions and to measure isomers of retinal, the mean experimentally measured molar ratio of retinal formed to beta-carotene consumed by pig intestinal preparations (800 and 10,000g supernatants) was 1.88 +/- 0.08, close to the theoretical value of 2.0 for central cleavage. beta-Apo carotenals, retinol, and retinoic acid were detected, if at all, in trace amounts during incubation. Thus, preparations of pig intestinal mucosa, which is considered a good physiologic model for human intestine, clearly convert beta-carotene to retinal, in large part if not solely, by central cleavage. Whether eccentric cleavage plays a greater role in vivo than it does in vitro is still uncertain.
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