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Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA 50011
3To whom correspondence should be addressed.
ß-Carotene might be converted oxidatively to vitamin A– active products in animals by the following three possible routes: 1) central cleavage, 2) sequential excentric cleavage or 3) random cleavage. Central cleavage is strongly favored by stoichiometric studies with tissue homogenates in vitro. To examine the relative importance of these pathways in rats in vivo, an oral dose (5.6 µmol) of all-trans ß-carotene in oil was given to vitamin A–deficient (-A) and to vitamin A–sufficient (+A) adult female Sprague-Dawley rats. Serum and several tissues were analyzed before and 3 h after dosing. The primary products of ß-carotene found in the intestine, serum and liver were retinol, retinyl esters and retinoic acid. Two minor oxidation products of ß-carotene, namely, 5,6-epoxy-ß-carotene and a partially characterized hydroxy-ß-carotene, were present in the stomach and its contents as well as in intestinal preparations. In the intestine, including its contents, of -A rats, very minor amounts of 5,6-epoxyretinyl palmitate and of ß-apocarotenals (8', 10', 12', 14') were identified. The total amount of the ß-apocarotenoids, however, was <5% of the retinoids formed in the intestine from ß-carotene during the same period. Another ß-carotene derivative, with a spectrum similar to that of semi-ß-carotenone, citranaxanthin and ß-apo-6'-carotenal, was also found in the intestinal extract of a -A rat. ß-Apocarotenals, ß-apocarotenols, ß-apocarotenyl esters and ß-apocarotenoic acids were not detected in tissues of +A rats nor in other tissues of -A rats. These findings agree with the view that central cleavage is by far the major pathway for the formation of vitamin A from ß-carotene in healthy rats in vivo.
KEY WORDS: • ß-carotene • retinoids • ß-apocarotenals • rats • central cleavage
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