β-Carotene and Aryl Hydrocarbon Hydroxylase in the Rat: An Effect of β-Carotene Independent of Vitamin A Activity

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Abstract
Animal models demonstrate a cancer-protective effect of vitamin A. However, human epidemiologic studies correlate the intake of the precursor, β-carotene, rather than active vitamin A, to a reduced risk for certain cancers. This suggests that β-carotene may have cancer-protective properties independent of its vitamin A activity. In the present rat study, effects of β-carotene or active vitamin A on carcinogen metabolizing enzyme activity were evaluated. The activity of intestinal aryl hydrocarbon hydroxylase (AHH, EC 1.14.14.1) was higher in rats fed a purified diet supplemented with β-carotene than in rats fed the control diet containing adequate vitamin A as retinyl palmitate (165 ± 30 vs. 90 ± 18 pmol/min × mg), P < (0.05). Supplementing the control diet with retinyl acetate had no effect. This AHH-enhancing effect of β-carotene on the activity of the intestinal mucosal enzyme was not seen on the hepatic enzyme, which is consistent with the nearly complete conversion of β-carotene to vitamin A prior to reaching the liver. These results demonstrate an effect of β-carotene on carcinogen metabolism which is independent of its vitamin A activity. This may help explain human epidemiologic data, and may lead to further work which would allow for prudent dietary recommendations toward a reduction in cancer risk.