β-Carotene 15,15′-Dioxygenase activity in human tissues and cells: evidence of an iron dependency

The two objectives of this study were to investigate β-carotene 15,15′-dioxygenase activity in human tissues and to determine the effect of desferrioxamine on the dioxygenase activity. Two human in vitro models were used: the TC7 clone of the intestinal cell line Caco-2 and small intestinal mucosa preparations. β-Carotene 15,15′-dioxygenase activity in the small intestinal 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 was detected in adult stomach tissue. We report for the first time the dioxygenase 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 β-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 β-carotene/retinal cleavage ratio of 1:2. The dioxygenase activity was decreased up to 80% with increasing desferrioxamine concentrations in the two in vitro models. Desferrioxamine was characterized as a noncompetitive inhibitor. In TC7 cells, the inhibitory effect of desferrioxamine was reversed by iron addition, suggesting that this effect was related to the ability of desferrioxamine to chelate iron, purported to be an obligate cofactor of the enzyme. In conclusion, these data report the presence of β-carotene 15,15′-dioxygenase activity in human small intestine and liver and demonstrate that desferrioxamine efficiently inhibits intestinal β-carotene cleavage in human tissues and cells.