Abstract
The absorption and metabolism of β-carotene by rat inverted small intestine were studied in situ. First, an assay system of β-carotene absorption and retinol production was established. Simultaneous use of NADH and NA in the incubation mixture was required for good β-carotene absorption. The pH optima of both β-carotene absorption and retinol production were the same, 7.7. Ten millimolar Na-cholate was required as a β-carotene solubilizing agent. Saponification was necessary in this assay system, because retinol ester was a major product, followed by transportation from the intestine to the incubation mixture. When new parameters, total β-carotene absorbed and total retinol produced, were introduced, the concentration of the substrate β-carotene had a great influence on the absorption rate of β-carotene (38-69nmol/g intestine·h) and also the production rate of retinol (14-36nmol/g intestine·h). The absorption ratio decreased with increasing concentration of the substrate β-carotene, and the ratios were 10.8%, 5.7% and 4.3% at 15, 30 and 70μM β-carotene, respectively. The production ratio of retinol from β-carotene absorbed increased from 15% to 75%, depending on the amount of β-carotene absorbed. These results indicate that a major proportion of β-carotene absorbed is rapidly cleaved, followed by production of retinol through retinal, suggesting the possibility that β-carotene is cleaved at the center site of the isoprenoid structure.
