STOICHIOMETRIC CONVERSION OF ALL TRANS-BETA-CAROTENE TO RETINAL BY PIG INTESTINAL EXTRACT

beta-Carotene and other provitamin A carotenoids are major dietary sou rces 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 y ield one molecule of retinal and various smaller fragments per molecul e of beta-carotene oxidized. By the use of improved methods to minimiz e nonenzymatic reactions and to measure isomers of retinal, the mean e xperimentally 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 detecte d, if at all, in trace amounts during incubation, Thus, preparations o f pig intestinal mucosa, which is considered a good physiologic model for human intestine, clearly convert beta-carotene to retinal, in larg e part if not solely, by central cleavage. Whether eccentric cleavage plays a greater role in vivo than it does in vitro is still uncertain. (C) 1996 Academic Press, Inc.