beta-carotene 15,15 '-dioxygenase activity in human tissues and cells: evidence of an iron dependency

The two objectives of this study were to investigate beta -carotene 15,15'- dioxygenase activity in human tissues and to determine the effect of desfer rioxamine on the dioxygenase activity. Two human in vitro models were used: the TC7 clone of the intestinal cell line Caco-2 and small intestinal muco sa preparations. beta -Carotene 15,15'-dioxygenase activity in the small in testinal mucosa was (mean +/- SD) 97.4 +/- 39.8 pmol/h.mg protein for five adults (44-89 y) and 20 pmol/h.mg for an infant (17 months). No activity wa s detected in adult stomach tissue. We report for the first time the dioxyg enase activity in human liver: 62 pmol/h.mg for a normal adult liver and 7 pmol/h.mg for a liver exhibiting gross pathology. The maximum capacity of b eta -carotene cleavage in an adult was estimated to be 12 mg/day (one fifth by small intestine and four fifths by liver), assuming an optimal beta -ca rotene/retinal cleavage ratio of 1:2. The dioxygenase activity was decrease d up to 80% with increasing desferrioxamine concentrations in the two in vi tro models. Desferrioxamine was characterized as a noncompetitive inhibitor . In TC7 cells, the inhibitory effect of desferrioxamine was reversed by ir on addition, suggesting that this effect was related to the ability of desf errioxamine to chelate iron, purported to be an obligate cofactor of the en zyme. In conclusion, these data report the presence of beta -carotene 15,15 '-dioxygenase activity in human small intestine and liver and demonstrate t hat desferrioxamine efficiently inhibits intestinal beta -carotene cleavage in human tissues and cells. (C) 2001 Elsevier Science Inc. All rights rese rved.