Stoichiometric conversion of all trans-beta-carotene to retinal by pig intestinal extract

• Published in: Archives of biochemistry and biophysics 1996-04, Vol.328 (1), p.57-63
• Main Authors: Nagao, A, During, A, Hoshino, C, Terao, J, Olson, J A
• Format: Article
• Language: English
• Subjects: Animals; beta Carotene; Carotenoids - metabolism; Chromatography, High Pressure Liquid; Female; Intestines - enzymology; Intestines - metabolism; Isomerism; Oxidation-Reduction; Retinaldehyde - biosynthesis; Swine; Vitamin A - metabolism
• ISSN: 0003-9861
• DOI: 10.1006/abbi.1996.0142

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.